Ardipithecus Unveiled

Transcription

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AAAS is here.
Evolution
In America today, 1 in 3 individuals do not accept evolution.1 That’s why AAAS continues to play an important
role in the effort to protect the integrity of science education. AAAS is hard at work ensuring that evolution
continues to be taught in science classrooms, but we need your help.
Join us. Together we can make a difference. aaas.org/plusyou/evolution
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Ardipithecus ramidus
Contents
Introduction and Author List
4 Light on the Origin of Man
Editorial
7 Understanding Human Origins
Bruce Alberts
News Focus
8 A New Kind of Ancestor: Ardipithecus Unveiled
12 Habitat for Humanity
13 The View From Afar
Authors’ Summaries
16 Ardipithecus ramidus and the Paleobiology of Early Hominids
Tim D. White et al.
17 The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of
Giday WoldeGabriel et al.
18 Taphonomic, Avian, and Small-Vertebrate Indicators of Ardipithecus ramidus Habitat
Antoine Louchart et al.
19 Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus
Tim D. White et al.
20 The Ardipithecus ramidus Skull and Its Implications for Hominid Origins
Gen Suwa et al.
21 Paleobiological Implications of the Ardipithecus ramidus Dentition
Gen Suwa et al.
22 Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive
C. Owen Lovejoy et al.
23 The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking
24 Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus
25 The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with
African Apes
26 Reexamining Human Origins in Lightof Ardipithecus ramidus
C. Owen Lovejoy
Credit: copyright T. White, 2008
See also related video, Science Podcast at
www.sciencemag.org/ardipithecus/
introduction
Light on the Origin of Man
Charles Darwin’s seminal work On the Origin of Species,
published 150 years ago next month, contains just one understated
sentence on the implications of his theory for human evolution:
“Light will be thrown on the origin of man and his history.” As
Darwin implied in his introduction to The Descent of Man, he felt
that those implications were obvious; he appreciated, as events
quickly showed, that it would be only natural to look at evolution
foremost from our human perspective and contemplate what makes
us unique among other primates—our large brains and ability to
communicate, to create, and to understand and investigate our history and nature; our culture, society, and religion; the ability to run
fast on two legs and manipulate tools; and more innovations that
separate us from our primate relatives.
Tracing our evolution and how we came to acquire these skills
and traits, however, has been difficult. Genetic data now confirm
that our closest living primate relative is the chimpanzee. We shared
and evolved from a common ancestor some 6 million or more years
ago. But identifying our unique genes and other genetic differences
between us and our primate cousins does not reveal the nature of
that ancestor, nor what factors led to the genetic changes that underlie our divergent evolutionary paths. That requires a fossil record
and enough parts of past species to assess key anatomical details.
It also requires knowing the habitat of early humans well, to determine their diet and evaluate what factors may have influenced their
evolution through time. Many early human fossils have been found,
but with a few exceptions, these are all less than 4 million years old.
The key first several million years of human evolution have been
poorly sampled or revealed.
This issue presents 11 papers authored by a diverse international
team (see following pages) describing an early hominid species,
Ardipithecus ramidus, and its environment. The hominid fossils
are 4.4 million years old, within this critical early part of human
evolution, and represent 36 or more individuals, including much of
the skull, pelvis, lower arms, and feet from one female. The papers
represent three broad themes. Five focus on different parts of the
anatomy that are revealing for human evolution. These show that
Ardipithecus was at home both moving along trees on its palms and
walking upright on the ground. Three characterize Ardipithecus’s
habitat in detail, through analysis of the hosting rocks and thousands
of fossils of small and large animals and plants. These show that
Ardipithecus lived and ate in woodlands, not grasslands. The first
paper presents an overview, and it and the last two papers trace early
human evolution and synthesize a new view of our last common ancestor with chimps. One conclusion is that chimps have specialized
greatly since then and thus are poor models for that ancestor and for
understanding human innovations such as our ability to walk.
These papers synthesize an enormous amount of data collected
and analyzed over decades by the authors. Because of the scope of
these papers and the special broad interest in the topic of human
evolution, we have expanded our usual format for papers and coverage. The papers include larger figures, tables, and discussions, and
the overview and two concluding papers provide extended introductions and analyses.
In addition, to aid understanding and introduce the main results
of each paper, the authors provide a one-page summary of each
paper, with an explanatory figure aimed at the general reader. Our
News Focus section, written by Ann Gibbons, provides further
analysis and coverage, and it includes maps and a portrait of
the meticulous and at times grueling field research behind the
discoveries. Available online are a video interview and a podcast
with further explanations.
To accommodate this material and allow the full papers, this print
issue presents an Editorial, News coverage, the authors’ summaries,
and four papers in full: the overview paper and one key paper from
each thematic group above. The other research papers, and of course
all content, are fully available online. In addition, a special online
page (www.sciencemag.org/Ardipithecus/) links to several print and
download packages of this material for AAAS members, researchers, educators, and other readers.
This collection, essentially an extra issue of Science in length,
reflects efforts by many behind the scenes. Every expert reviewer
evaluated, and improved, multiple papers, and several commented
on all 11 of them. The authors provided the summaries on top of
an already large writing and revision effort. Paula Kiberstis helped
in their editing. The figures and art were drafted and improved by
J. H. Matternes, Henry Gilbert, Kyle Brudvik, and Josh Carlson,
as well as Holly Bishop, Nathalie Cary, and Yael Kats at Science.
Numerous other Science copyediting, proofreading, and production
staff processed this content on top of their regular loads. Finally,
special thanks go to the people of Ethiopia for supporting and facilitating this and other research into human origins over many years,
and for curating Ardipithecus ramidus for future research and for all
of us to admire.
Ardipithecus ramidus thus helps us bridge the better-known,
more recent part of human evolution, which has a better fossil
record, with the scarcer early human fossils and older ape fossils
that precede our last common ancestor. Ardipithecus ramidus is a
reminder of Darwin’s conclusion of The Origin:
There is grandeur in this view of life, with its several powers, having
been originally breathed into a few forms or into one; and that,
whilst this planet has gone cycling on according to the fixed law of
gravity, from so simple a beginning endless forms most beautiful
and most wonderful have been, and are being, evolved.
– Brooks Hanson
4
www.sciencemag.org SCIENCE VOL 326 2 October 2009 Published by AAAS
The Authors
Giday WoldeGabriel
Earth Environmental
Sciences Division,
Los Alamos National
Laboratory, Los Alamos,
NM 87545, USA.
Antoine Louchart
UMR 5125 PEPS CNRS,
France, Université Lyon 1,
69622 Villeurbanne Cedex,
France, and Institut de
Génomique Fonctionnelle
de Lyon, Université de Lyon, Université
Lyon 1, CNRS, INRA, Ecole Normale
Supérieure de Lyon, France.
Gen Suwa
The University Museum,
the University of Tokyo,
Hongo, Bunkyo-ku, Tokyo
113-0033, Japan.
C. Owen Lovejoy
Department of
Anthropology, School
of Biomedical Sciences,
Kent State University,
Kent, OH 44240–0001, USA.
Stanley H. Ambrose
Department of
Anthropology, University
of Illinois, Urbana, IL
61801, USA.
Berhane Asfaw
Rift Valley Research
Service, P.O. Box 5717,
Addis Ababa, Ethiopia.
Doris Barboni
CEREGE (UMR6635
CNRS/Université
Aix-Marseille), BP80,
F-13545 Aix-en-Provence
Cedex 4, France.
Raymond L. Bernor
National Science
Foundation,
GEO:EAR:SEPS
Sedimentary Geology
and Paleobiology Program,
Arlington, VA 22230, and College of Medicine, Department of Anatomy, Laboratory
of Evolutionary Biology, Howard University,
520 W St., Washington, DC 20059, USA.
Michel Brunet
Collège de France, Chaire
de Paléontologie Humaine,
3 Rue d’Ulm, F-75231
Paris Cedex 05, France.
Brian Currie
Department of Geology,
Miami University, Oxford,
OH 45056, USA.
Yonas Beyene
Department of Anthropology
and Archaeology,
Authority for Research and
Conservation of the Cultural
Heritage, Ministry of Youth,
Sports and Culture, P.O. Box 6686, Addis
Ababa, Ethiopia.
Michael T. Black
Phoebe A. Hearst Museum
of Anthropology,
103 Kroeber Hall, no. 3712,
University of California
Berkeley, Berkeley, CA
94720–3712, USA.
Robert J. Blumenschine
Center for Human
Evolutionary Studies,
Department of
Anthropology, Rutgers
University, 131 George St.,
New Brunswick, NJ 08901–1414, USA.
Jean-Renaud Boisserie
Paléobiodiversité et
Paléoenvironnements, UMR
CNRS 5143, USM 0203,
Muséum National d’Histoire
Naturelle, 8 Rue Buffon, CP
38, 75231 Paris Cedex 05, France, and Institut
de Paléoprimatologie et Paléontologie
Humaine, Évolution et Paléoenvironnements,
UMR CNRS 6046, Université de Poitiers,
40 Avenue du Recteur-Pineau, 86022 Poitiers
Cedex, France.
Mesfin Asnake
Ministry of Mines and
Energy, P.O. Box 486,
Addis Ababa, Ethiopia.
62
Laurent Bremond
Center for Bio-Archaeology
and Ecology (UMR5059
CNRS/Université
Montpellier 2/EPHE),
Institut de Botanique,
F-34090 Montpellier, France.
2 OCTOBER 2009
Raymonde Bonnefille
CEREGE (UMR6635
CNRS/Université AixMarseille), BP80, F-13545
Aix-en-Provence Cedex 4,
France.
VOL 326
SCIENCE
David DeGusta
Department of
Anthropology, Stanford
University, Stanford, CA
94305–2034, USA.
Eric Delson
Department of
Anthropology, Lehman
College/CUNY, Bronx, NY
10468; NYCEP; and
Department of Vertebrate
Paleontology, American Museum of Natural
History; New York, NY 10024, USA.
Stephen Frost
Department of
Anthropology, University
of Oregon, Eugene, OR,
97403–1218, USA.
Nuria Garcia
Dept. Paleontología,
Universidad Complutense
de Madrid & Centro
de Evolución y
Comportamiento Humanos,
ISCIII, C/ Sinesio Delgado 4, Pabellón 14,
28029 Madrid, Spain.
Ioannis X. Giaourtsakis
Ludwig Maximilians
University of Munich,
Department of Geo- and
Environmental Sciences,
Section of Paleontology.
Richard-Wagner-Strasse 10, D-80333
Munich, Germany.
CREDITS: PHOTOS COURTESY OF THE AUTHORS
Tim D. White
Human Evolution Research
Center and Department
of Integrative Biology,
3101 Valley Life Sciences
Building, University
of California at Berkeley, Berkeley, CA
94720, USA.
www.sciencemag.org
Published by AAAS
5
SPECIALSECTION
Yohannes Haile-Selassie
Department of Physical
Anthropology, Cleveland
Museum of Natural History,
1 Wade Oval Drive,
Cleveland, OH 44106, USA.
Thomas Lehmann
Senckenberg
Forschungsinstitut,
Senckenberganlage 25,
D-60325 Frankfurt am Main,
Germany.
Gina Semprebon
Science and Mathematics,
Bay Path College,
588 Longmeadow St.,
Longmeadow, MA 01106,
USA.
William K. Hart
Department of Geology,
Miami University, Oxford,
OH 45056, USA.
Andossa Likius
Département de
Paléontologie, Université
de N’Djamena, BP 1117,
N’Djamena, Chad.
Scott W. Simpson
Department of Anatomy,
Case Western Reserve
University School of
Medicine, Cleveland, OH
44106–4930, USA.
Jay H. Matternes
4328 Ashford Lane, Fairfax,
VA 22032, USA.
Linda Spurlock
Cleveland Museum of
Natural History, Cleveland,
OH 44106–4930, USA.
Alison M. Murray
Department of Biological
Sciences, University of
Alberta, Edmonton AB
T6G2E9, Canada.
Kathlyn M. Stewart
Paleobiology, Canadian
Museum of Nature, Ottawa,
K1P 6P4, Canada.
Leslea J. Hlusko
Center and Department of
Integrative Biology,
University of California at
Berkeley, 3010 Valley Life
Sciences Building, Berkeley, CA, 94720, USA.
F. Clark Howell
Anthropology, 3101 Valley
Life Sciences Building,
Berkeley, Berkeley, CA 94720, USA
(deceased).
M. C. Jolly-Saad
Université Paris-Ouest La
Défense, Centre Henri Elhaï,
200 Avenue de la
République, 92001 Nanterre,
France.
Reiko T. Kono
Department of Anthropology,
National Museum of Nature
and Science, Hyakunincho,
Shinjuku-ku, Tokyo,
169-0073, Japan.
CREDITS: PHOTOS COURTESY OF THE AUTHORS
Daisuke Kubo
Department of Biological
Sciences, Graduate School
of Science, the University
of Tokyo, Tokyo, 113-0033,
Japan.
Jackson K. Njau
Integrative Biology,
Berkeley, 3010 Valley Life
Sciences Building, Berkeley, CA, 94720, USA.
Cesur Pehlevan
University of Yuzuncu
Yil, Department of
Anthropology, The Faculty
of Science and Letters, Zeve
Yerlesimi 65080 Van, Turkey.
Denise F. Su
Department of
Anthropology,
The Pennsylvania State
University, University Park,
PA 16802, USA.
Mark Teaford
Center for Functional
Anatomy and Evolution,
Johns Hopkins University
School of Medicine, 1830 E.
Monument St., Room 303,
Baltimore, MD 21205.
Paul R. Renne
Berkeley Geochronology
Center, 2455 Ridge Road,
Berkeley, CA 94709, and
Department of Earth
and Planetary Science,
University of California at Berkeley, Berkeley,
CA 94720, USA.
Bruce Latimer
Department of Anatomy,
Case Western Reserve
University School of
Medicine, Cleveland, OH
44106–4930, USA.
www.sciencemag.org
Haruo Saegusa
Institute of Natural and
Environmental Sciences,
University of Hyogo,
Yayoigaoka,
Sanda 669-1546, Japan.
SCIENCE
VOL 326
Elisabeth Vrba
Department of Geology and
Geophysics, Yale University,
New Haven, CT 06520,
USA.
Henry Wesselman
P.O. Box 369, Captain Cook,
Hawaii, 96704, USA.
2 OCTOBER 2009
Published by AAAS
6
63
EDITORIAL
Understanding Human Origins
RESPONDING TO A QUESTION ABOUT HIS SOON-TO-BE-PUBLISHED ON THE ORIGIN OF SPECIES,
CREDITS: (TOP) TOM KOCHEL; (RIGHT) ISTOCKPHOTO.COM
Bruce Alberts is Editorin-Chief of Science.
Charles Darwin wrote in 1857 to Alfred Russel Wallace, “You ask whether I shall discuss
‘man’; I think I shall avoid the whole subject, as so surrounded with prejudices, though
I freely admit that it is the highest and most interesting problem for the naturalist.” Only
some 14 years later, in The Descent of Man, did Darwin address this “highest problem”
head-on: There, he presciently remarked in his introduction that “It has often and confidently been asserted, that man’s origin can never be known: but . . . it is those who know
little, and not those who know much, who so positively assert that this or that problem will
never be solved by science.”
Darwin was certainly right. The intervening years provide conclusive evidence that it is
very unwise to predict limits for what can be discovered through science. In fact, it now seems
likely that, through synergistic advances in many disciplines, scientists will eventually decipher a substantial portion of the detailed evolutionary history of our
own species at both the morphological and molecular levels.
First, what can we expect from paleoanthropology? In this 200th
anniversary year of Darwin’s birth, Science is pleased to publish the
results of many years of scientific research that suggest an unexpected form for our last common ancestor with chimpanzees. This
issue contains 11 Research Articles involving more than 40 authors,
plus News articles that describe the life and times of Ardipithecus
ramidus, a hominid species that lived 4.4 million years ago in the
Afar Rift region of northeastern Ethiopia. This region exposes a total
depth of 300 meters of sediments that were deposited in rivers, lakes,
and floodplains between about 5.5 and 3.8 million years ago. Even
considering only this one site (there are many others), it is staggering
to reflect on the huge number of hominid remains that can in principle be discovered, given sufficient time and effort. Moreover, the
history of science assures us that powerful new techniques will be developed in the coming
years to accelerate such research, as they have been in the past. We can thus be certain that
scientists will eventually obtain a rather detailed record showing how the anatomy of the
human body evolved over many millions of years.
What can we expect from a combination of genetics, genomics, biochemistry, and comparative organismal biology? We will want to interpret the history of the morphological transformations in the humanoid skeleton and musculature in terms of the molecular changes in the
DNA that caused them. Genes and their regulatory regions control the morphology of animals
through very complex biochemical processes that affect cell behavior during embryonic development. Nevertheless, experimental studies of model organisms such as fruit flies, worms, fish,
and mice are advancing our understanding of the molecular mechanisms involved. New inexpensive methods for deciphering the complete genome sequence of any organism will soon
accelerate this process, allowing scientists to analyze the recurring evolutionary morphological
transformations that have been identified by organismal biologists,* so as to determine the specific DNA changes involved. And the DNA sequences that have changed most rapidly during
recent human evolution are being cataloged, providing a new tool for finding important molecular differences that distinguish us from chimpanzees.†
The majesty of the discoveries already made represents a major triumph of the human
intellect. And, as emphasized here, there will be many more discoveries to come. Darwin’s
summary of his own efforts to understand human evolution is thus still relevant today:
“Man may be excused for feeling some pride at having risen, though not through his own
exertions, to the very summit of the organic scale; and the fact of his having thus risen,
instead of having been aboriginally placed there, may give him hope for a still higher destiny
– Bruce Alberts
in the distant future.”
10.1126/science.1182387
*R. L. Mueller et al., Proc. Natl. Acad. Sci. U.S.A. 101, 3820 (2004). †S. Prabhakar et al., Science 314, 786 (2006).
www.sciencemag.org SCIENCE VOL 326 2 OCTOBER 2009
17
Published by AAAS
7
NEWSFOCUS
A New Kind of Ancestor:
Ardipithecus Unveiled
The oldest known hominin skeleton reveals the body plan of our very
early ancestors and the upright origins of humankind
From the inside out. Artist’s reconstructions show how Ardi’s
skeleton, muscles, and body looked and how she would have
moved on top of branches.
36
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2 OCTOBER 2009
VOL 326
SCIENCE
www.sciencemag.org
by AAAS2 October 2009 Published by AAAS
www.sciencemag.org SCIENCE Published
VOL 326 CREDITS: ILLUSTRATIONS © 2009, J. H. MATTERNES
Every day, scientists add new pages to the story of human evolution by deciphering clues to our past in everything from the DNA in our genes to the bones and
artifacts of thousands of our ancestors. But perhaps once each generation, a
spectacular fossil reveals a whole chapter of our prehistory all at once. In 1974,
it was the famous 3.2-million-year-old skeleton “Lucy,” who proved in one
stroke that our ancestors walked upright before they evolved big brains.
Ever since Lucy’s discovery, researchers have wondered what came before
her. Did the earliest members of the human family walk upright like Lucy or on
their knuckles like chimpanzees and gorillas? Did they swing through the trees
or venture into open grasslands? Researchers have had only partial, fleeting
glimpses of Lucy’s own ancestors—the earliest hominins, members of the group
that includes humans and our ancestors (and are sometimes called hominids).
Now, in a special section beginning on page 60 and online, a multidisciplinary
international team presents the oldest known skeleton of a potential human
ancestor, 4.4-million-year-old Ardipithecus ramidus from Aramis, Ethiopia.
This remarkably rare skeleton is not the oldest putative hominin, but it is
by far the most complete of the earliest specimens. It includes most of the
skull and teeth, as well as the pelvis, hands, and feet—parts that the
authors say reveal an “intermediate” form of upright walking, consid-
Ardipithecus
Ardipithecus
ramidus
ramidusNEWSFOCUS
NEWSFOCUS
ered ered
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Tim White
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In 11Inpapers
11 papers
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this issue
and online,
and online,
the team
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the village
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the polished
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Ar. ramidus
looked
looked
and moved.
and moved.
The skeleThe skelesurface
surface
of a of
tooth
a tooth
root,root,
and he
andimmediately
he immediately
ton, ton,
nicknamed
nicknamed
“Ardi,”
“Ardi,”
is from
is from
a female
a female
who who
livedlived
in a in
woodland
a woodland
knewknew
it wasit awas
hominin
a hominin
molar.
molar.
OverOver
the next
the next
few days,
few days,
the team
the team
scoured
scoured
(see (see
sidebar,
sidebar,
p. 40),
p. 40),
stoodstood
aboutabout
120 centimeters
120 centimeters
tall, tall,
and weighed
and weighed
the area
the area
on hands
on hands
and knees,
and knees,
as they
as they
do whenever
do whenever
an important
an important
piecepiece
aboutabout
50 kilograms.
50 kilograms.
She was
She thus
was thus
as big
asas
biga chimpanzee
as a chimpanzee
and had
and ahad of
a hominin
of hominin
is found
is found
(see story,
(see story,
p. 41),
p. and
41), collected
and collected
the lower
the lower
jaw of
jaw
a of a
brainbrain
size size
to match.
to match.
But she
But did
shenot
didknuckle-walk
not knuckle-walk
or swing
or swing
through
through
childchild
with with
the milk
the milk
molarmolar
still attached.
still attached.
The molar
The molar
was so
was
primitive
so primitive
that that
the trees
the trees
like living
like living
apes.apes.
Instead,
Instead,
she walked
she walked
upright,
upright,
planting
planting
her her
the team
the team
knewknew
they they
had found
had found
a hominin
a hominin
both both
olderolder
and more
and more
primitive
primitive
feet flat
feeton
flatthe
onground,
the ground,
perhaps
perhaps
eating
eating
nuts,nuts,
insects,
insects,
and small
and small
mammamthan than
Lucy.Lucy.
Yet the
Yetjaw
thealso
jaw had
alsoderived
had derived
traits—novel
traits—novel
evolutionary
evolutionary
char-charmalsmals
in theinwoods.
the woods.
acters—shared
acters—shared
withwith
Lucy’s
Lucy’s
species,
species,
Au. afarensis,
Au. afarensis,
suchsuch
as anasupper
an upper
She was
She awas
“facultative”
a “facultative”
biped,
biped,
say the
sayauthors,
the authors,
still living
still living
in both
in both
canine
canine
shaped
shaped
like alike
diamond
a diamond
in side
in view.
side view.
worlds—upright
worlds—upright
on the
onground
the ground
but also
but also
able able
to move
to move
on allonfours
all fours
on on The The
teamteam
reported
reported
15 years
15 years
ago in
agoNature
in Nature
that that
the fragmentary
the fragmentary
top of
top
branches
of branches
in theintrees,
the trees,
with with
an opposable
an opposable
big toe
bigtotoe
grasp
to grasp
limbs.
limbs.
fossils
fossils
belonged
belonged
to the
to“long-sought
the “long-sought
potential
potential
root root
species
species
for the
for the
“These
“These
things
things
werewere
veryvery
odd odd
creatures,”
creatures,”
says says
paleoanthropologist
paleoanthropologist
Hominidae.”
Hominidae.”
(They
(They
first first
called
called
it Au.it ramidus,
Au. ramidus,
then,then,
afterafter
finding
finding
AlanAlan
Walker
Walker
of Pennsylvania
of Pennsylvania
StateState
University,
University,
University
University
Park.Park.
“You“You
partsparts
of the
ofskeleton,
the skeleton,
changed
changed
it to it
Ar.toramidus—for
Ar. ramidus—for
the Afar
the Afar
words
words
knowknow
whatwhat
Tim Tim
[White]
[White]
onceonce
said:said:
If you
If wanted
you wanted
to find
to find
something
something
for “root”
for “root”
and “ground.”)
and “ground.”)
In response
In response
to comments
to comments
that he
thatneeded
he needed
leg leg
that moved
that moved
like these
like these
things,
things,
you’dyou’d
havehave
to gototogo
thetobar
theinbar
Star
in Wars.”
Star Wars.”
bones
bones
to prove
to prove
Ar. ramidus
Ar. ramidus
was an
wasupright
an upright
hominin,
hominin,
White
White
jokedjoked
that that
MostMost
researchers,
researchers,
who who
havehave
waited
waited
15 years
15 years
for the
forpublication
the publication
of of
he would
he would
be delighted
be delighted
withwith
moremore
parts,parts,
specifically
specifically
a thigh
a thigh
and an
and an
this find,
this find,
agreeagree
that Ardi
that Ardi
is indeed
is indeed
an early
an early
hominin.
hominin.
TheyThey
praise
praise
the the
intactintact
skull,skull,
as though
as though
placing
placing
an order.
an order.
detailed
detailed
reconstructions
reconstructions
needed
needed
to piece
to piece
together
together
the crushed
the crushed
bones.
bones. Within
Within
2 months,
2 months,
the team
the team
delivered.
delivered.
In November
In November
1994,1994,
as theasfosthe fos“This“This
is anisextraordinarily
an extraordinarily
impressive
impressive
workwork
of reconstruction
of reconstruction
and and
sil hunters
sil hunters
crawled
crawled
up anupembankment,
an embankment,
Berkeley
Berkeley
graduate
graduate
student
student
description,
description,
well well
worth
worth
waiting
waiting
for,” for,”
says says
paleoanthropologist
paleoanthropologist
David
David
Yohannes
Yohannes
Haile-Selassie
Haile-Selassie
of Ethiopia,
of Ethiopia,
now now
a paleoanthropologist
a paleoanthropologist
at theat the
Pilbeam
Pilbeam
of Harvard
of Harvard
University.
University.
“They
“They
did this
did job
thisvery,
job very,
veryvery
well,”
well,”
Cleveland
Cleveland
Museum
Museum
of Natural
of Natural
History
History
in Ohio,
in Ohio,
spotted
spotted
two pieces
two pieces
of a of a
agrees
agrees
neurobiologist
neurobiologist
Christoph
Christoph
Zollikofer
Zollikofer
of the
of University
the University
of of
bonebone
fromfrom
the palm
the palm
of a hand.
of a hand.
ThatThat
was soon
was soon
followed
followed
by pieces
by pieces
of a of a
Zurich
Zurich
in Switzerland.
in Switzerland.
pelvis;
pelvis;
leg, ankle,
leg, ankle,
and foot
and foot
bones;
bones;
manymany
of theofbones
the bones
of theofhand
the hand
and and
But not
Buteveryone
not everyone
agrees
agrees
withwith
the team’s
the team’s
interpretations
interpretations
aboutabout
how how
arm;arm;
a lower
a lower
jaw with
jaw with
teeth—and
teeth—and
a cranium.
a cranium.
By January
By January
1995,1995,
it wasit was
Ar. ramidus
Ar. ramidus
walked
walked
upright
upright
and what
and what
it reveals
it reveals
aboutabout
our ancestors.
our ancestors.
apparent
apparent
that they
that they
had made
had made
the rarest
the rarest
of rare
of finds,
rare finds,
a partial
a partial
skeleton.
skeleton.
CREDITS: (LEFT) C. O. LOVEJOY ET AL., SCIENCE; (TOP) G. SUWA ET AL., SCIENCE; (BOTTOM) C. O. LOVEJOY ET AL., SCIENCE; (RIGHT) C. O. LOVEJOY ET AL., SCIENCE
Online
Online
Unexpected
Unexpected
anatomy.
anatomy.
Ardi has
Ardianhas
opposable
an opposable
toe (left)
toe (left)
and flexible
and flexible
hand hand
(right);
(right);
her canines
her canines
(top center)
(top center)
are sized
are sized
between
between
thosethose
of a human
of a human
(top left)
(topand
left)chimp
and chimp
(top right);
(top right);
and the
andblades
the blades
of herofpelvis
her pelvis
(lower(lower
left) are
left)broad
are broad
like Lucy’s
like Lucy’s
(yellow).
(yellow).
www.sciencemag.org
www.sciencemag.org
SCIENCE
SCIENCE
VOLVOL
326 326
2 OCTOBER
2 OCTOBER
20092009
Published
Published
by326 AAAS
by AAAS
www.sciencemag.org SCIENCE VOL
2 October 2009 Published by AAAS
37 37
9
NEWSFOCUS
NEWSFOCUS Ardipithecus
Ardipithecusramidus
ramidus
FOSSILS
FOSSILSOFOFTHE
THEHUMAN
HUMANFAMILY
FAMILY
H. H.
floresiensis
floresiensis
HOMO
HOMO
Indonesia
Indonesia
H. H.
heidelbergensis
heidelbergensis
Europe
Europe
Kenyanthropus
Kenyanthropus
platyops?
platyops?
Kenya
Kenya
H. H.
habilis
habilis H. H.
erectus
erectus
Sub-Saharan
Sub-Saharan Africa
Africa
andand
AsiaAsia
Africa
Africa
SAHELANTHROPUS
SAHELANTHROPUS
AUSTRALOPITHECUS
AUSTRALOPITHECUS
Ar.Ar.
ramidus
ramidus
Ethiopia
Ethiopia
Ardi
Ardi
Ethiopia,
Ethiopia,
Kenya
Kenya
Ethiopia
Ethiopia
Kenya,
Kenya,
Ethiopia
Ethiopia
Au.Au.
africanus
africanus Au.Au.
robustus
robustus
Taung
Taung
Child
Child
South
South
Africa
Africa
South
South
Africa
Africa
Au.Au.
bahrelghazali
bahrelghazali
? ?
O. O.
tugenensis
tugenensis
Au.Au.
boisei
boisei
Eastern
Eastern
Africa
Africa
2
2
4
4
5
5
3
3
Abel
Abel
Chad
Chad Au.Au.
aethiopicus
aethiopicus
Eastern
Eastern
Africa
Africa
Millennium
Millennium
Man
Man
Kenya
Kenya
6
6
Worldwide
Worldwide
Eastern
Eastern
Africa
Africa
Au.Au.
afarensis
afarensis
Lucy
Lucy
Ethiopia,
Ethiopia,
Tanzania
Tanzania
ORRORIN
ORRORIN
Pliocene
Pliocene
Epoch
Epoch
H. H.
sapiens
sapiens
? ?
Au.Au.
rudolfensis
rudolfensis
Pleistocene
Pleistocene
Epoch
Epoch
Today
Today
Ar.Ar.
kadabba
kadabba
Toumaï
Toumaï
Chad
Chad
7 Million Years Ago
7 Million Years Ago
S. tchadensis
S. tchadensis
Miocene
Miocene
Epoch
Epoch
Au.Au.
garhi
garhi
Au.Au.
anamensis
anamensis
Europe
Europe
andand
AsiaAsia
1 Million Years Ago
1 Million Years Ago
ARDIPITHECUS
ARDIPITHECUS
H. H.
neanderthalensis
neanderthalensis
Holocene
Holocene
Epoch
Epoch
CREDITS: (TIMELINE LEFT TO RIGHT) L. PÉRON/WIKIPEDIA, B. G. RICHMOND ET AL., SCIENCE 319, 1662 (2008); © T. WHITE 2008; WIKIPEDIA; TIM WHITE; TIM WHITE; (PHOTO) D. BRILL
It It
is is
one
one
ofof
only
only
a half-dozen
a half-dozen
such
such
skeletons
skeletons
known
known
from
from
more
more
than
than onon
thethe
task.
task.
“You
“You
gogo
piece
piece
byby
piece.”
piece.”
1 million
1 million
years
years
ago,
ago,
and
and
thethe
only
only
published
published
one
one
older
older
than
than
Lucy.
Lucy.
Once
Once
hehe
had
had
reassembled
reassembled
thethe
pieces
pieces
in in
a digital
a digital
reconstruction,
reconstruction,
hehe
It was
It was
thethe
find
find
ofof
a lifetime.
a lifetime.
But
But
thethe
team’s
team’s
excitement
excitement
was
was
tempered
tempered and
and
paleoanthropologist
paleoanthropologist
Berhane
Berhane
Asfaw
Asfaw
ofof
thethe
Rift
Rift
Valley
Valley
Research
Research
byby
thethe
skeleton’s
skeleton’s
terrible
terrible
condition.
condition.
The
The
bones
bones
literally
literally
crumbled
crumbled
when
when Service
Service
in in
Addis
Addis
Ababa
Ababa
compared
compared
thethe
skull
skull
with
with
those
those
ofof
ancient
ancient
and
and
touched.
touched.
White
White
called
called
it road
it road
kill.
kill.
And
And
parts
parts
ofof
thethe
skeleton
skeleton
had
had
been
been living
living
primates
primates
in in
museums
museums
worldwide.
worldwide.
ByBy
March
March
ofof
this
this
year,
year,
Suwa
Suwa
trampled
trampled
and
and
scattered
scattered
into
into
more
more
than
than
100
100
fragments;
fragments;
thethe
skull
skull
was
was was
was
satisfied
satisfied
with
with
hishis
10th
10th
reconstruction.
reconstruction.
Meanwhile
Meanwhile
in in
Ohio,
Ohio,
crushed
crushed
to to
4 centimeters
4 centimeters
in in
height.
height.
The
The
researchers
researchers
decided
decided
to to
remove
remove Lovejoy
Lovejoy
made
made
physical
physical
models
models
ofof
thethe
pelvic
pelvic
pieces
pieces
based
based
onon
thethe
origorigentire
entire
blocks
blocks
ofof
sediment,
sediment,
covering
covering
thethe
blocks
blocks
in in
plaster
plaster
and
and
moving
moving inal
inal
fossil
fossil
and
and
thethe
CTCT
scans,
scans,
working
working
closely
closely
with
with
Suwa.
Suwa.
HeHe
is also
is also
satsatthem
them
to to
thethe
National
National
Museum
Museum
ofof
isfied
isfied
that
that
thethe
14th
14th
version
version
ofof
thethe
Ethiopia
Ethiopia
in in
Addis
Addis
Ababa
Ababa
to to
finish
finish
pelvis
pelvis
is is
accurate.
accurate.
“There
“There
was
was
anan
excavating
excavating
thethe
fossils.
fossils.
Ardipithecus
Ardipithecus
that
that
looked
looked
just
just
like
like
It It
took
took
three
three
field
field
seasons
seasons
to to
that,”
that,”
hehe
says,
says,
holding
holding
upup
thethe
final
final
uncover
uncover
and
and
extract
extract
thethe
skeleton,
skeleton,
model
model
in in
hishis
lab.
lab.
repeatedly
repeatedly
crawling
crawling
thethe
site
site
toto
Putting
Putting
their
their
heads
heads
together
together
gather
gather
100%
100%
ofof
thethe
fossils
fossils
prespresent.
ent.
AtAt
last
last
count,
count,
thethe
team
team
had
had
AsAs
they
they
examined
examined
Ardi’s
Ardi’s
skull,
skull,
cataloged
cataloged
more
more
than
than
110
110
specispeciSuwa
Suwa
and
and
Asfaw
Asfaw
noted
noted
a number
a number
mens
mens
ofof
Ar.Ar.
ramidus,
ramidus,
notnot
to to
menmenofof
characteristics.
characteristics.
Her
Her
lower
lower
face
face
tion
tion
150,000
150,000
specimens
specimens
ofof
fossil
fossil
had
had
a muzzle
a muzzle
that
that
juts
juts
outout
less
less
than
than
plants
plants
and
and
animals.
animals.
“This
“This
team
team
a chimpanzee’s.
a chimpanzee’s.
The
The
cranial
cranial
base
base
is is
seems
seems
to to
suck
suck
fossils
fossils
outout
ofof
thethe
short
short
from
from
front
front
to to
back,
back,
indicatindicatearth,”
earth,”
says
says
anatomist
anatomist
C.C.
Owen
Owen
inging
that
that
herher
head
head
balanced
balanced
atop
atop
thethe
Lovejoy
Lovejoy
ofof
Kent
Kent
State
State
University
University
spine
spine
asas
in in
later
later
upright
upright
walkers,
walkers,
in in
Ohio,
Ohio,
who
who
analyzed
analyzed
thethe
postpost- Fossil
rather
than
than
to to
thethe
front
front
ofof
thethe
spine,
spine,
Fossil
finders.
finders.
TimTim
White
White
and
and
local
local
Afar
Afar
fossil
fossil
hunters
hunters
pool
pool
their
their
finds
finds
after
after rather
scouring
thethe
hillside
hillside
at Aramis.
at Aramis.
cranial
cranial
bones
bones
butbut
didn’t
didn’t
work
work
in in scouring
asas
in in
quadrupedal
quadrupedal
apes.
apes.
Her
Her
face
face
is is
thethe
f ield.
f ield.
InIn
thethe
lab,
lab,
hehe
gently
gently
in in
a more
a more
vertical
vertical
position
position
than
than
in in
unveils
unveils
a cast
a cast
ofof
a tiny,
a tiny,
pea-sized
pea-sized
sesamoid
sesamoid
bone
bone
forfor
effect.
effect.
“Their
“Their chimpanzees.
chimpanzees.
And
And
herher
teeth,
teeth,
like
like
those
those
ofof
allall
later
later
hominins,
hominins,
lack
lack
thethe
obsessiveness
obsessiveness
gives
gives
you—this!”
you—this!”
daggerlike
daggerlike
sharpened
sharpened
upper
upper
canines
canines
seen
seen
in in
chimpanzees.
chimpanzees.
The
The
team
team
White
White
himself
himself
spent
spent
years
years
removing
removing
thethe
silty
silty
clay
clay
from
from
thethe
fragile
fragile realized
realized
that
that
this
this
combination
combination
ofof
traits
traits
matches
matches
those
those
ofof
anan
even
even
older
older
fossils
fossils
at at
thethe
National
National
Museum
Museum
in in
Addis
Addis
Ababa,
Ababa,
using
using
brushes,
brushes, skull,
skull,
6-million
6-million
to to
7-million-year-old
7-million-year-old
Sahelanthropus
Sahelanthropus
tchadensis,
tchadensis,
syringes,
syringes,
and
and
dental
dental
tools,
tools,
usually
usually
under
under
a microscope.
a microscope.
Museum
Museum
techtech- found
found
byby
Brunet’s
Brunet’s
team
team
in in
Chad.
Chad.
They
They
conclude
conclude
that
that
both
both
represent
represent
anan
nician
nician
Alemu
Alemu
Ademassu
Ademassu
made
made
a precise
a precise
cast
cast
ofof
each
each
piece,
piece,
and
and
thethe early
early
stage
stage
ofof
human
human
evolution,
evolution,
distinct
distinct
from
from
both
both
Australopithecus
Australopithecus
and
and
team
team
assembled
assembled
them
them
into
into
a skeleton.
a skeleton.
chimpanzees.
chimpanzees.
“Similarities
“Similarities
with
with
Sahelanthropus
Sahelanthropus
areare
striking,
striking,
in in
that
that
it it
Meanwhile
Meanwhile
in in
Tokyo
Tokyo
and
and
Ohio,
Ohio,
Suwa
Suwa
and
and
Lovejoy
Lovejoy
made
made
virtual
virtual also
also
represents
represents
a first-grade
a first-grade
hominid,”
hominid,”
agrees
agrees
Zollikofer,
Zollikofer,
who
who
diddid
aa
reconstructions
reconstructions
ofof
thethe
crushed
crushed
skull
skull
and
and
pelvis.
pelvis.
Certain
Certain
fossils
fossils
were
were three-dimensional
three-dimensional
reconstruction
reconstruction
ofof
that
that
skull.
skull.
taken
taken
briefly
briefly
to to
Tokyo
Tokyo
and
and
scanned
scanned
with
with
a custom
a custom
micro–computed
micro–computed
Another,
Another,
earlier
earlier
species
species
ofof
Ardipithecus—Ar.
Ardipithecus—Ar.
kadabba,
kadabba,
dated
dated
tomography
tomography
(CT)
(CT)
scanner
scanner
that
that
could
could
reveal
reveal
what
what
was
was
hidden
hidden
inside
inside
thethe from
from
5.55.5
million
million
to to
5.85.8
million
million
years
years
ago
ago
butbut
known
known
only
only
from
from
teeth
teeth
bones
bones
and
and
teeth.
teeth.
Suwa
Suwa
spent
spent
9 years
9 years
mastering
mastering
thethe
technology
technology
to to and
and
bits
bits
and
and
pieces
pieces
ofof
skeletal
skeletal
bones—is
bones—is
part
part
ofof
that
that
grade,
grade,
too.
too.
And
And
reassemble
reassemble
thethe
fragments
fragments
ofof
thethe
cranium
cranium
into
into
a virtual
a virtual
skull.
skull.
“I “I
used
used
6565 Ar.Ar.
kadabba’s
kadabba’s
canines
canines
and
and
other
other
teeth
teeth
seem
seem
to to
match
match
those
those
ofof
a third
a third
pieces
pieces
ofof
thethe
cranium,”
cranium,”
says
says
Suwa,
Suwa,
who
who
estimates
estimates
hehe
spent
spent
1000
1000
hours
hours very
very
ancient
ancient
specimen,
specimen,
6-million-year-old
6-million-year-old
Orrorin
Orrorin
tugenensis
tugenensis
from
from
3838
10
2 OCTOBER
2 OCTOBER
2009
2009 VOL
VOL
326
326 SCIENCE
SCIENCE www.sciencemag.org
www.sciencemag.org
Published
by by
AAAS
AAAS
VOL
326 2 October 2009 Published by AAAS
CREDITS: (TIMELINE LEFT TO RIGHT) L. PÉRON/WIKIPEDIA, B. G. RICHMOND ET AL., SCIENCE 319, 1662 (2008); © T. WHITE 2008; WIKIPEDIA; TIM WHITE; TIM WHITE; (PHOTO) D. BRILL
Filling
Filling
a gap.
a gap.
Ardipithecus
Ardipithecus
provides
provides
a link
a link
between
between
earlier
earlier
andand
later
later
hominins,
hominins,
as as
seen
seen
in in
thisthis
timeline
timeline
showing
showing
important
important
hominin
hominin
fossils
fossils
andand
taxa.
taxa.
Ardipithecus
Ardipithecusramidus
ramidus NEWSFOCUS
NEWSFOCUS
Kenya,
ered awhich
hallmark
alsoofhas
hominins.
a
“We thought Lucy was the find of the “The authors … are framing the debate that will inevitably follow,”
thighbone
century but,
thatinappears
retrospect, it isn’t,” says paleoanthropologist Andrew because the description and interpretation of the finds are entwined,
to Hill
have
of been
Yale University.
used for “It’s worth the wait.”
says Pilbeam. “My first reaction is to be skeptical about some of the
upright
Towalking
some researchers’
(Science, surprise, the female skeleton doesn’t look conclusions,” including that human ancestors never went through a
21 much
Marchlike
2008,
a chimpanzee,
p. 1599). gorilla, or any of our closest living primate chimpanzee-like phase. Other researchers are focusing intently on
So,relatives.
“this raises
Even though
the in-this species probably lived soon after the dawn the lower skeleton, where some of the anatomy is so primitive that
triguing
of humankind,
possibility
it was
that not transitional between African apes and they are beginning to argue over just what it means to be “bipedal.”
we’re
humans.
looking
“We
at have
the same
seen the ancestor, and it is not a chimpanzee,” says The pelvis, for example, offers only “circumstantial” evidence for
genus”
paleoanthropologist
for specimens
Tim White of the University of California, Berke- upright walking, says Walker. But however the debate about Ardi’s
now
ley,put
co-director
in three of
genera,
the Middle Awash research group, which discovered locomotion and identity evolves, she provides the first hard evidence
says
andPilbeam.
analyzed the
But
fossils.
the
that will inform and constrain future ideas
discoverers
Instead, of
theO.
skeleton
tuge- and pieces of at least 35 additional individuals
about the ancient hominin bauplan.
nensis
of Ar.
aren’t
ramidus
so sure.
reveal
“Asafor
new
Ardi
type
andofOrrorin
early hominin
being thethat
same
was
genus,
neither
Digging it
no,chimpanzee
I don’t thinknor
this
human.
is possible,
Although
unless
theone
team
really
suspects
wantsthat
to accept
Ar. ramidus
an
sciencemag.org
unusual
may have
amount
given
of variability”
rise to Lucy’s
within
genus,
a taxon,
Australopithecus,
says geologistthe
Martin
fossils
The first glimpse of this strange creature came
Podcast interview
Pickford
“show of
forthe
theCollege
first time
dethat
France,
therewho
is some
found
new
Orrorin
evolutionary
with Brigitte
grade of
on 17 December 1992 when a former graduate
with author
Senut
hominid
of thethat
National
is not Australopithecus,
Museum of Natural
thatHistory
is not Homo,”
in Paris.says paleontolstudent of White’s, Gen Suwa, saw a glint
Ann Gibbons on
Ardipithecus and
ogist
Whatever
Michel
theBrunet
taxonomy
of theofCollege
Ardipithecus
de France
and in
theParis.
other very ancient
among the pebbles of the desert pavement
fieldwork in the Afar.
hominins,
In 11they
papers
represent
published
“an enormous
in this issue
jumpand
to Australopithecus,”
online, the team the
of 47
near the village of Aramis. It was the polished
next
researchers
hominin indescribes
line (see timeline,
how Ar. ramidus
p. 38), says
looked
australopithecine
and moved. The
expert
skelesurface of a tooth root, and he immediately
William
ton, nicknamed
Kimbel of“Ardi,”
ArizonaisState
fromUniversity,
a female who
Tempe.
livedFor
in example,
a woodland knew it was a hominin molar. Over the next few days, the team scoured
although
(see sidebar,
Lucy’s p.
brain
40),isstood
only aabout
little 120
larger
centimeters
than that oftall,
Ardipithecus,
and weighed the area on hands and knees, as they do whenever an important piece
Lucy’s
aboutspecies,
50 kilograms.
Au. afarensis,
She was
was
thus
an as
adept
big as
biped.
a chimpanzee
It walked upright
and had a of hominin is found (see story, p. 41), and collected the lower jaw of a
like
brain
humans,
size toventuring
match. Butincreasingly
she did not knuckle-walk
into more diverse
or swing
habitats,
through child with the milk molar still attached. The molar was so primitive that
including
the treesgrassy
like living
savannas.
apes.And
Instead,
it hadshe
lost
walked
its opposable
upright,big
planting
toe, asher the team knew they had found a hominin both older and more primitive
seen
feet
in flat
3.7-million-year-old
on the ground, perhaps
footprints
eating
at Laetoli,
nuts, insects,
Tanzania,
and reflecting
small mam- than Lucy. Yet the jaw also had derived traits—novel evolutionary charan mals
irreversible
in the woods.
commitment to life on the ground.
acters—shared with Lucy’s species, Au. afarensis, such as an upper
Lucy’s
She was
direct
a “facultative”
ancestor is widely
biped,considered
say the authors,
to be Au.
still anamensis,
living in both canine shaped like a diamond in side view.
a hominin
worlds—upright
whose skeleton
on the ground
is poorly
but known,
also ablealthough
to move its
on shinbone
all fours on
The team reported 15 years ago in Nature that the fragmentary
suggests
top of branches
it walkedinupright
the trees,
3.9with
million
an opposable
to 4.2 million
big toeyears
to grasp
agolimbs.
in Dream
fossils
belonged
thein“long-sought
root
species
team.
Gen Suwato
(left)
Tokyo focused onpotential
the skull; C.
Owen
Lovejoyfor
(topthe
in Kent, Ohio,(They
studiedfirst
postcranial
Yohannesthen,
Haile-Selassie
and
Kenya
“These
andthings
Ethiopia.
wereArdipithecus
very odd creatures,”
is the current
says leading
paleoanthropologist
candidate right)
Hominidae.”
calledbones;
it Au.and
ramidus,
after finding
and analyzed
key itfossils
in ramidus—for
Ethiopia.
forAlan
Au. anamensis’s
Walker of Pennsylvania
ancestor, ifState
onlyUniversity,
because it’sUniversity
the only putative
Park. “You Berhane
partsAsfaw
of thefound
skeleton,
changed
to Ar.
the Afar words
hominin
know what
in evidence
Tim [White]
between
once
5.8said:
million
If you
andwanted
4.4 million
to find
years
something
ago.
for “root” and “ground.”) In response to comments that he needed leg
Indeed,
that moved
Au. anamensis
like these things,
fossils you’d
appear
have
in the
to goMiddle
to the bar
Awash
in Star
region
Wars.” climber
probably
moved on
flatanhands
andhominin,
feet on top
of branches
bonesthat
to prove
Ar. ramidus
was
upright
White
joked that
just 200,000
Most researchers,
years after who
Ardi.have waited 15 years for the publication of in he
thewould
midcanopy,
a type of
locomotion
known
as palmigrady.
For an
be delighted
with
more parts,
specifically
a thigh and
this find, agree that Ardi is indeed an early hominin. They praise the example,
four bones
in theplacing
wrist ofanAr.
ramidus gave it a more flexible
intact skull,
as though
order.
Making
strides
detailed
reconstructions needed to piece together the crushed bones. hand that
could
be bent the
backward
at the wrist.
This is in contrast
thefosWithin
2 months,
team delivered.
In November
1994, astothe
But“This
the team
not connectingimpressive
the dots between
Au.reconstruction
anamensis andand hands
is anisextraordinarily
work of
of knuckle-walking
chimpanzees
and Berkeley
gorillas, which
havestudent
stiff
sil hunters
crawled up an
embankment,
graduate
Ar.description,
ramidus justwell
yet, worth
awaiting
morefor,”
fossils.
now they are focusing
waiting
saysFor
paleoanthropologist
David wrists
that absorb
forces on of
their
knuckles.
Yohannes
Haile-Selassie
Ethiopia,
now a paleoanthropologist at the
on Pilbeam
the anatomy
of ArdiUniversity.
and how she
moved
world.
of Harvard
“They
didthrough
this jobthe
very,
veryHer
well,” Cleveland
However, Museum
several researchers
soin
sure
about
thesetwo
inferences.
of Naturalaren’t
History
Ohio,
spotted
pieces of a
foot
agrees
is primitive,
neurobiologist
with an opposable
Christophbig
Zollikofer
toe like that
of the
used
University
by living of Some
skeptical
that of
thea crushed
pelvis
the by
anatomical
bonearefrom
the palm
hand. That
wasreally
soonshows
followed
pieces of a
apes
to grasp
branches. But the bases of the four other toe bones details
Zurich
in Switzerland.
needed
to demonstrate
bipedality.
Theofpelvis
is “suggestive”
pelvis;
leg, ankle,
and foot bones;
many
the bones
of the handofand
were oriented
so that they
reinforced
forefoot
into a more
rigid
But not everyone
agrees
with the the
team’s
interpretations
about
how bipedality
but notjaw
conclusive,
says paleoanthropologist
Carol Ward
of itthewas
arm; a lower
with teeth—and
a cranium. By January
1995,
lever
she pushed
off.upright
In contrast,
the toes
of a chimpanzee
curve University
Ar. as
ramidus
walked
and what
it reveals
about our ancestors.
Missouri,
Columbia.
“does
not appear
to
apparentofthat
they had
made theAlso,
rarestAr.oframidus
rare finds,
a partial
skeleton.
as flexibly as those in their hands, say Lovejoy and co-author have had its knee placed over the ankle, which means that when walkBruce Latimer of Case Western Reserve University in Cleveland. ing bipedally, it would have had to shift its weight to the side,” she says.
Ar. ramidus “developed a pretty good bipedal foot while at the same Paleoanthropologist William Jungers of Stony Brook University in
time keeping an opposable first toe,” says Lovejoy.
New York state is also not sure that the skeleton was bipedal. “Believe
The upper blades of Ardi’s pelvis are shorter and broader than in me, it’s a unique form of bipedalism,” he says. “The postcranium alone
apes. They would have lowered the trunk’s center of mass, so she could would not unequivocally signal hominin status, in my opinion.” Paleobalance on one leg at a time while walking, says Lovejoy. He also anthropologist Bernard Wood of George Washington University in
infers from the pelvis that her spine was long and curved like a Washington, D.C., agrees. Looking at the skeleton as a whole, he says,
human’s rather than short and stiff like a chimpanzee’s. These “I think the head is consistent with it being a hominin, … but the rest of
changes suggest to him that Ar. ramidus “has been bipedal for a very the body is much more questionable.”
long time.”
All this underscores how difficult it may be to recognize and
Yet the lower pelvis is still quite large and primitive, similar to define bipedality in the earliest hominins as they began to shift from
African apes rather than hominins. Taken with the opposable big toe, trees to ground. One thing does seem clear, though: The absence of
and primitive traits in the hand and foot, this indicates that Ar. ramidus many specialized traits found in African apes suggests that our
didn’t walk like Lucy and was still spending
a lot of
time inArdi
thehas
trees.
ancestors
never
knuckle-walked.
Unexpected
anatomy.
an opposable
toe (left)
and
flexible hand (right);
But it wasn’t suspending its body beneath
branches
African
apes
That ofthrows
a monkey
wrench
her canines
(toplike
center)
are sized
between those
a human
(top left) and
chimpinto a hypothesis about the last
or climbing vertically, says Lovejoy. (top
Instead,
wasthea blades
slow, of
careful
of living
apes and humans. Ever since Darwin
right);itand
her pelvis common
(lower left) ancestor
are broad like
Lucy’s (yellow).
CREDITS (TOP TO BOTTOM): TIM WHITE; BOB CHRISTY/NEWS AND INFORMATION, KENT STATE UNIVERSITY; TIM WHITE
Online
www.sciencemag.org
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SCIENCEVOL
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3263262 OCTOBER
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Published
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VOL
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11
Habitat for Humanity
ARAMIS, ETHIOPIA—A long cairn of black stones
marks the spot where a skeleton of Ardipithecus
ramidus was found, its bones broken and scattered
on a barren hillside. Erected as a monument to an
ancient ancestor in the style of an Afar tribesman’s
grave, the cairn is a solitary marker in an almost
sterile zone, devoid of life except for a few spindly
acacia trees and piles of sifted sediment.
That’s because the Middle Awash research team
sucked up everything in sight at this spot, hunting
for every bit of fossil bone as well as clues to the
landscape 4.4 million years ago, when Ardipithecus died here. “Literally, we crawled every square
inch of this locality,” recalls team co-leader Tim
White of the University of California, Berkeley.
“You crawl on your hands and knees, collecting
every piece of bone, every piece of wood, every
seed, every snail, every scrap. It was 100% collection.” The heaps of sediment are all that’s left
behind from that fossil-mining operation, which
yielded one of the most important fossils in human evolution (see main text,
p. 36), as well as thousands of clues to its ecology and environment.
The team collected more than 150,000 specimens of fossilized plants and
animals from nearby localities of the same age, from elephants to songbirds
to millipedes, including fossilized wood, pollen, snails, and larvae. “We have
crates of bone splinters,” says White.
A team of interdisciplinary researchers then used these fossils and
thousands of geological and isotopic samples to reconstruct Ar. ramidus’s
Pliocene world, as described in companion papers in this issue (see p. 66
and 87). From these specimens, they conclude that Ardi lived in a woodland, climbing among hackberry, fig, and palm trees and coexisting with
monkeys, kudu antelopes, and peafowl. Doves and parrots flew overhead.
All these creatures prefer woodlands, not the open, grassy terrain often
conjured for our ancestors.
The team suggests that Ar. ramidus was “more omnivorous” than chimpanzees, based on the size, shape, and enamel distribution of its teeth. It
probably supplemented woodland plants such as fruits, nuts, and tubers
with the occasional insects, small mammals, or bird eggs. Carbon-isotope
studies of teeth from five individuals show that Ar. ramidus ate mostly
woodland, rather than grassland, plants. Although Ar. ramidus probably ate
suggested in 1871 that our ancestors arose in Africa, researchers have
debated whether our forebears passed through a great-ape stage in
which they looked like proto-chimpanzees (Science, 21 November
1969, p. 953). This “troglodytian” model for early human behavior
(named for the common chimpanzee, Pan troglodytes) suggests that
the last common ancestor of the African apes and humans once had
short backs, arms adapted for swinging, and a pelvis and limbs
adapted for knuckle walking. Then our ancestors lost these traits,
while chimpanzees and gorillas kept them. But this view has been
uninformed by fossil evidence because there are almost no fossils of
early chimpanzees and gorillas.
Some researchers have thought that the ancient African ape bauplan was more primitive, lately citing clues from fragmentary fossils
of apes that lived from 8 million to 18 million years ago. “There’s
been growing evidence from the Miocene apes that the common
ancestor may have been more primitive,” says Ward. Now
Ar. ramidus strongly supports that notion. The authors repeatedly
40
12
2 OCTOBER 2009
VOL 326
Past and present. Ardipithecus’s woodland was more like Kenya’s Kibwezi Forest
(left) than Aramis today.
figs and other fruit when ripe, it didn’t consume as much fruit as chimpanzees do today.
This new evidence overwhelmingly refutes the once-favored but now
moribund hypothesis that upright-walking hominins arose in open grasslands. “There’s so much good data here that people aren’t going to be able to
question whether early hominins were living in woodlands,” says paleoanthropologist Andrew Hill of Yale University. “Savannas had nothing to do
with upright walking.”
Geological studies indicate that most of the fossils were buried within a
relatively short window of time, a few thousand to, at most, 100,000 years
ago, says geologist and team co-leader Giday WoldeGabriel of the Los Alamos
National Laboratory in New Mexico. During that sliver of time, Aramis was not
a dense tropical rainforest with a thick canopy but a humid, cooler woodland.
The best modern analog is the Kibwezi Forest in Kenya, kept wet by groundwater, according to isotope expert Stanley Ambrose of the University of
Illinois, Urbana-Champaign. These woods have open stands of trees, some
20 meters high, that let the sun reach shrubs and grasses on the ground.
Judging from the remains of at least 36 Ardipithecus individuals found so
far at Aramis, this was prime feeding ground for a generalized early biped. “It
was the habitat they preferred,” says White.
–A.G.
note the many ways that Ar. ramidus differs from chimpanzees and
gorillas, bolstering the argument that it was those apes that changed
the most from the primitive form.
But the problem with a more “generalized model” of an arboreal
ape is that “it is easier to say what it wasn’t than what it was,” says
Ward. And if the last common ancestor, which according to genetic
studies lived 5 million to 7 million years ago, didn’t look like a
chimp, then chimpanzees and gorillas evolved their numerous similarities independently, after gorillas diverged from the chimp/human
line. “I find [that] hard to believe,” says Pilbeam.
As debate over the implications of Ar. ramidus begins, the one thing
that all can agree on is that the new papers provide a wealth of data to
frame the issues for years. “No matter what side of the arguments you
come down on, it’s going to be food for thought for generations of
graduate students,” says Jungers. Or, as Walker says: “It would have
been very boring if it had looked half-chimp.”
–ANN GIBBONS
SCIENCE
www.sciencemag.org
VOL 326 CREDITS (LEFT TO RIGHT): TIM WHITE; ANN GIBBONS
NEWSFOCUS
The crawl. Researchers hunt
down every fossil at Aramis.
PALEOANTHROPOLOGY
The View From Afar
CREDITS (TOP TO BOTTOM): HENRY GILBERT; SOURCE: TIM WHITE
How do you find priceless hominin fossils in a hostile desert? Build a strong team and obsess over the details
MIDDLE AWASH VALLEY, THE AFAR DEPRESSION, ETHIOPIA—It’s of how upright walking evolved and how our earliest ancestors difabout 10 a.m. on a hot morning in December, and Tim White is fered from chimpanzees (see overview, p. 60, and main Focus text,
watching a 30-year-old farmer inch his way up a slippery hill on his p. 36). But Aramis is just one of 300 localities in the Middle Awash,
knees, picking through mouse-colored rubble for a bit of gray bone. which is the only place in the world to yield fossils that span the entire
The sun is already bleaching the scrubby badlands, making it diffi- saga of hominid evolution. At last count, this team had gathered
cult to distinguish a fragment of bone in the washed-out beige and 19,000 vertebrate fossils over the past 19 years. These include about
gray terrain. The only shade in this parched gully is from a small, 300 specimens from seven species of hominins, from some of the
thorny acacia tree, so the fossil hunters have draped their heads with first members of the human family, such as 5.8-million-year-old
kerchiefs that hang out from under their “Cal” and “Obama for Pres- Ar. ramidus kadabba, to the earliest members of our own species,
ident” baseball caps, making them look like a strange tribe of Berke- Homo sapiens, which lived here about 160,000 years ago.
ley Bedouins. If there are fossils here,
As they work in different places in the
White is conf ident that the slender WESTERN AFAR RIFT, ETHIOPIA
valley, the team members travel back and
farmer, Kampiro Kayrento, will f ind
forth in time. Today, this core group is
HADAR
GONA
them. “Kampiro is the best person in the
working in the western foothills near the
world for f inding little pieces of fosBurka catchment, where an ancient river
silized human bone,” says White, 59, a
laid down sediments 3 million to 2 milpaleoanthropologist at the University of
lion years ago and where the team has
California, Berkeley, who has collected
found specimens of Australopithecus
Awash
fossils in this region since 1981.
garhi, a species they suspect may have
River
Watching Kayrento is a sort of spectagiven rise to the f irst members of our
Afar
tor sport, because he scores so often. Just
genus, Homo.
Rift
Aramis
minutes earlier, he had walked over the
This season, after a rough start, the 25
crest of a small hill, singing softly to himscientists, students, cooks, and Ethiopian
Addis Ababa
self, and had spotted the fossilized core of
and Afar officials and guards in camp are
ETHIOPIA
a horn from an ancient bovid, or antelope.
working well together. Their tented camp is
Bouri
Peninsula hours from any town, graded road, or fresh
Then he picked up a flat piece of gray bone
Middle Awash
Yardi Lake
Hominid Localities
nearby and showed the fossil to Ethiopian
water. (They dug their own well to get
Burka
Ardipithecus
paleoanthropologist Berhane Asfaw, askwater.) “The 1st week, it’s like an engine
Australopithecus
Homo
ing, “Bovid?” Asfaw, 55, who hired
that’s running but not running smoothly,”
Kayrento when he was a boy hanging out
says White, who, with Asfaw, runs a wellat fossil sites in southern Ethiopia, looked Ancestral territory. The area where Ardi was found is rich in organized camp where every tool, map, and
over the slightly curved piece of bone the hominin fossil sites, including these worked by the Middle shower bag has its proper place. “By the
size of a silver dollar and suggested, “Mon- Awash research team.
3rd week, people know their jobs.”
key?” as he handed it to White. White
The 1st week, White and a paleontoloturned it over gently in his hands, then said: “Check that, Berhane. We gist were sick, and White is still fighting a harsh cough that keeps
just found a hominid cranium. Niiiice.”
him awake at night. The 2nd week, some aggressive Alisera tribesAs word spreads that Kayrento found a hominin, or a member of men who live near the Ar. ramidus site threatened to kill White and
the taxon that includes humans and our ancestors, the other fossil Asfaw, making it difficult to return there. (That’s one reason the
hunters tease him: “Homo bovid! Homo bovid! Niiiice.”
team travels with six Afar policemen armed with AK-47s and
The Middle Awash project, which includes 70 scientists from 18 Obama caps, dubbed “The Obama Police.”) The day before, a stunations, is best known for its discovery of the 4.4-million-year-old dent had awakened with a high fever and abdominal pain and had to
partial skeleton of Ardipithecus ramidus at Aramis, about 34 kilo- be driven 4 hours to the nearest clinic, where he was diagnosed with
meters north of here. That skeleton is now dramatically revising ideas a urinary tract infection, probably from drinking too little water in
www.sciencemag.org
SCIENCE
VOL 326
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by AAAS
VOL 326 2 October 2009 Published by AAAS
41
13
the 35˚C heat. “The best laid plans change every day,” says White,
who has dealt with poisonous snakes, scorpions, malarial mosquitoes, lions, hyenas, flash floods, dust tornadoes, warring tribesmen,
and contaminated food and water over the years. “Nothing in the
field comes easy.”
Calling the “A” team
Nothing in the Afar, for that matter, comes easy. We are reminded of
that as we drive across the dusty Saragata plain to the target fossil site
at 8 a.m., making giant circles in the dust with the Toyota Land Cruiser
so we can find our tracks home at the end of the day. Men clad in plaid
wraps, with AK-47s slung over their shoulders, flag us down seeking
help. They bring over a woman who looks to be in her 70s but is probably much younger. Her finger is bleeding, and the men tell White and
Asfaw, in Afar, that a puff adder bit her the night before while she was
gathering wood. A quick-thinking boy had sliced her finger with a
knife, releasing the venom and probably saving her life. White gets out
a first-aid kit, removes a crude poultice, and cleans and bandages the
wound, putting on an antibiotic cream. “It’s good she survived the
night,” he says as we drive off. “The danger now is infection.”
After inching down the sandy bank of a dry river, we reach the socalled Chairman’s site. This is one of dozens of fossil localities discovered in the Burka area since 2005: exposed hillsides that were
spotted in satellite and aerial photos, then laboriously explored on
foot. The plan was to search for animal fossils to help date a hominid
jawbone discovered last year. But in the 1st hour, with Kayrento’s discovery, they’re already on the trail of another individual instead.
As soon as White identifies the bit of skull bone, he swings into
action. With his wiry frame and deep voice, he is a commanding
presence, and it soon becomes clear how he earned his nickname,
“The General.” In his field uniform—a suede Australian army hat
with a rattlesnake band, blue jeans, and driving gloves without
fingers—he uses a fossil pick to delineate the zones in the sandstone
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CREDITS (TOP TO BOTTOM): DAVID BRILL; TIM WHITE; DAVID BRILL
Division of labor. Kampiro Kayrento (top left) homes in on fossils; he and others
sweep the surface, and Giday WoldeGabriel dates sediments.
where he wants the crew
deployed. “Get everybody
out of the area,” he calls to the
15 people already fanned out
over the gully, scanning for
fossils. “I want the ‘A’ team.”
He singles out Kayrento and
three others and hands them
yellow pin-flags, saying, “Go
back to the bottom.” As he
watches them move up the
slope, he warns: “Go slowly.
You’re moving too fast. …
Don’t squash the slope. Move
like a cat, not a cow.”
By looking at the relatively
fresh fractured edge of the
bone fragment, White knows
that it comes from a larger piece of skull that broke after it was exposed,
not while it was buried. As Kayrento and the others find other bits of
bone, they place yellow pin-flags at those spots. “This process establishes the distributional cone,” White explains. The top flag marks the
highest point on the surface where the skull came out of the ground; the
bottom boundary marks the farthest point where a fragment might
finally have come to rest, following the fall line down the slope.
This discovery also illustrates one reason why the team comes to
the field right after the rainy season. If they’re lucky, rain and floods
will cut into the ancient sediments, exposing fossils. But they have to
get there before the fossils disintegrate as they are exposed to the elements or are trampled by the Afar’s goats, sheep, and cattle. Timing is
everything, and this season they’re a bit late. “The ideal situation is to
find a fossil just as it is eroding out of the bank,” says White.
As they crawl the entire length of the gully, they turn over every
rock, mud clod, and piece of carbonate rubble to make sure it doesn’t
contain a fossil fragment. “Not good,” says Kayrento. “This is yucky,”
agrees Asfaw, co-director of the team and the first Ethiopian scientist
to join it, in 1979 when he was invited to earn his Ph.D. at Berkeley
(Science, 29 August 2003, p. 1178).
After 2 hours, the team has collected a few more pieces of skull
around the temple, forehead, and ear. “It’s getting bigger by the
minute,” White says. “If we’re lucky, we’ll find it buried right in here.”
The team has to wait until the next day to find out just how lucky.
At 9:45 a.m. Thursday, they return with reinforcements: Asfaw has
hired two Afar men to help with the heavy lifting of buckets of dirt.
With a button-down Oxford cloth shirt and a pistol stuck in the waistband of his khakis, Asfaw commands respect, and he is the best at
negotiating with the Afar. In this case, he settles an argument by letting
clan leaders select which men, among a large group, will get jobs.
At the site, White sets up a perimeter of blue pin-flags that look
like a mini slalom course, outlining the gully that he calls the “Hot
Zone” where fossil pieces are most likely to be buried. The plan is to
excavate all the rock and dirt around those flags, down to the original layers of sediment. White explains that the ancient landscape
would have been flatter and more verdant before tectonic movements of Earth’s crust cracked and tilted the sediment layers. But the
original soil is still there, a red-brown layer of clay beneath a gray
veneer of sandstone. “Throw every piece of stone out of the channel,” he orders. “If you see a hominid, I need to know right away!”
White and Kayrento literally sweep off the gray lag with a push
broom and then scrape back the layers of time with a trowel to the
ancient surface underneath. “Once we brush out the slopes, we’ll be
CREDITS (TOP TO BOTTOM): TIM WHITE; HANK WESSELMAN; TIM WHITE
sure no fossil is left in place,”
says White. In case they miss a
fragment, the loose sediment is
carried to giant sieves where the
crew sifts it for bits of bone or
teeth. The sifted rubble is taken
to a circle of workers who then
empty it into small aluminum
pans, in which they examine
every single, tiny piece—a job
that gives new meaning to the
word tedium. “Sieving 101,”
observes Asfaw, who supervises
sieving and picking today.
By 11:10 a.m., the pace of discovery has slowed. When the A
team tells White it’s “not good,” he tries to infuse them with some of
his energy, reminding everyone to stay focused, to keep going, to not
step on fossils. But by midday, White is grumbling, too, because
they’ve scoured the Hot Zone and it’s clear the skull is not there.
“We’ve eliminated every hope of finding it in situ.”
NEWSFOCUS
Intensive care. Tim White uses
dental tools and a gluelike adhesive
to extract fragile fossils from
matrix.
Time travel
It’s a good time to take a walk with the four geologists, who are combing the terrain, hoping to find sediments with volcanic minerals to
Luckily, the fossil hunters
help them date the locality and its fossils precisely. While fossil
have found a pig known to
hunters move slowly, stooped at the waist and focused on the ground,
have lived about 2.6 million
the geologists move fast, heads up, scanning the next horizon for a
to 2.7 million years ago,
rock face with a layer cake of sediments, like those exposed in road
which suggests that the sedicuts. The 6-million-year record of Middle Awash sediments is not ments and the new discovery are also that old.
stacked neatly in one place, with oldest rocks on the bottom and
At 9 a.m. Friday, 12 December, we’re back at the Chairman’s site
youngest on top. (If it were, the stack would be 1 kilometer thick.) for a 3rd day, this time with a film crew from Sweden. After White
Instead, the rocks are faulted and tilted into different ridges. By trac- and Kayrento jokingly reenact the discovery of the skull bone for the
ing a once-horizontal layer from ridge to ridge, sometimes for kilo- film crew, they resume sweeping and sifting, exactly where they left
meters, the geologists can link the layers and place different snap- off. At first, there’s little return. Berkeley postdoc Cesur Pehlevan
shots of time into a sequence.
from Ankara hands White a piece of bone: “Nope, tough luck. Right
Today, Ethiopian geologist Giday WoldeGabriel of the Los color, right thickness. Nope, sorry.”
Alamos National Laboratory in New Mexico, also a co-leader of the
Finally, someone hands White something special. “Oh nice, frontal
team (he joined in 1992), is searching for a familiar-looking motif— bone with frontal sinus. This is getting better. That’s what we’re after,”
a distinct layer of volcanic tuff called the SHT (Sidiha Koma Tuff), says White. “If we can get that brow ridge, we can match it with the
previously dated to 3.4 million years ago by radiometric methods.
known species.” He turns over the new piece of frontal bone in his
So far, the team has found just one species of hominin— hand, examining it like a diamond dealer assessing a gemstone.
Au. garhi—that lived at this time in the Middle Awash (Science,
By the end of 3 days, the team of 20 will have collected a dozen
23 April 1999, p. 629), although a more robust
pieces of one skull, an average yield for this region.
species, Au. aethiopicus, appears 2.6 million years “Nothing in the field Taken together, says White, those pieces show that
ago in southern Ethiopia and Kenya. That’s also when
“It’s an Australopithecus because it has a small braincomes easy.”
the earliest stone tools appear in Gona, Ethiopia,
case, small chewing apparatus.” There’s still not
–TIM WHITE, UNIVERSITY enough to identify the species, though White thinks it
100 kilometers north of here. The earliest fossils of our
genus Homo come a bit later—at 2.3 million years ago OF CALIFORNIA, BERKELEY is Au. garhi. He notes that “it’s a big boy, big for an
at Hadar, near Gona, also with stone tools. That’s why
australopithecine.” If it is Au. garhi, that would be
it is important to date Au. garhi precisely: Was it the maker of the one more bit of evidence to suggest that Au. afarensis gave rise to
stone tools left in the Afar? The team thinks Au. garhi is the direct Au. garhi; males are bigger than females in Au. afarensis—and so
descendant of the more primitive Au. afarensis, best known as the perhaps in Au. garhi, too.
species that includes the famous 3.2-million-year-old skeleton of
For now, White and Asfaw are pleased with the new snapshot
Lucy, also from Hadar. But did Au. garhi then evolve into early they’re getting of Au. garhi. On our way back to camp, White stops
Homo? They need better dates—and more fossils—to find out.
so we can take a photo of the moon rising over Yardi Lake in front
“Now that we have the SHT as a reference point here, we have of us, the sun setting behind us. The landscape has changed since
to try to trace it to where the new fossils are,” says WoldeGabriel. the australopithecines were here. But one thing’s been constant in
The only problem is that the SHT is several ridges and basins over the Middle Awash, he notes: “Hominids have been right here lookfrom the excavation; linking the two will be difficult if not impos- ing at the moon rising over water for millions of years.”
–ANN GIBBONS
sible. The team will also use other methods to date the new fossils.
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AUTHORS’SUMMARIES
Ardipithecus ramidus and the Paleobiology
of Early Hominids
Tim D. White, Berhane Asfaw, Yonas Beyene, Yohannes Haile-Selassie, C. Owen Lovejoy, Gen Suwa,
Giday WoldeGabriel
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CREDIT: ILLUSTRATION OF AR. RAMIDUS: COPYRIGHT J. H. MATTERNES
C
harles Darwin and Thomas
Huxley were forced to
ponder human origins
and evolution without a relevant
fossil record. With only a few
Pan
Pan
Neanderthal fossils available to
troglo
paniscus
Gorilla
dytes
g
o
r
i
l
a
l
supplement their limited knowlCLCA
edge of living apes, they specu• Partially arboreal
• Striding terrestrial biped
• Palmigrade
lated about how quintessentially
• Enlarged brain
• Facultative biped
• Postcanine megadontia
GLCA
arborealist
human features such as upright
• Dentognathic reduction
• Feminized canine
• Pan-African
• Dimorphic
canines
• Technology-reliant
• Woodland omnivore
• Wide niche
walking, small canines, dexterous
• Forest
A us
• Old World range
A
hands, and our special intelligence
r
tralo
dipi
frugivore/
Ho
pithe
thecu
mo (
s (~ 6 to 4 Ma)
cus (
omnivore
had evolved through natural selec<
~ 4 to
1 Ma) ~ 2.5 Ma
)
tion to provide us with our complex way of life. Today we know of
early Homo from >2.0 million Evolution of hominids and African apes since the gorilla/chimp+human (GLCA) and chimp/human (CLCA) last
common ancestors. Pedestals on the left show separate lineages leading to the extant apes (gorilla, and chimp and
years ago (Ma) and have a record bonobo); text indicates key differences among adaptive plateaus occupied by the three hominid genera.
of stone tools and animal butchery
that reaches back to 2.6 Ma. These demonstrate just how deeply tech- probably was more omnivorous than chimpanzees (ripe fruit specialnology is embedded in our natural history.
ists) and likely fed both in trees and on the ground. It apparently conAustralopithecus, a predecessor of Homo that lived about 1 to 4 Ma sumed only small amounts of open-environment resources, arguing
(see figure), was discovered in South Africa in 1924. Although slow to against the idea that an inhabitation of grasslands was the driving force
gain acceptance as a human ancestor, it is now recognized to represent in the origin of upright walking.
an ancestral group from which Homo evolved. Even after the discovAr. ramidus, first described in 1994 from teeth and jaw fragments,
eries of the partial skeleton (“Lucy”) and fossilized footprints is now represented by 110 specimens, including a partial female
(Laetoli) of Au. afarensis, and other fossils that extended the antiquity skeleton rescued from erosional degradation. This individual weighed
of Australopithecus to ~3.7 Ma, the hominid fossil record before about 50 kg and stood about 120 cm tall. In the context of the many
Australopithecus was blank. What connected the small-brained, small- other recovered individuals of this species, this suggests little body
canined, upright-walking Australopithecus to the last common ances- size difference between males and females. Brain size was as small as
tor that we shared with chimpanzees some time earlier than 6 Ma?
in living chimpanzees. The numerous recovered teeth and a largely
The 11 papers in this issue, representing the work of a large inter- complete skull show that Ar. ramidus had a small face and a reduced
national team with diverse areas of expertise, describe Ardipithecus canine/premolar complex, indicative of minimal social aggression.
ramidus, a hominid species dated to 4.4 Ma, and the habitat in which Its hands, arms, feet, pelvis, and legs collectively reveal that it moved
it lived in the Afar Rift region of northeastern Ethiopia. This species, capably in the trees, supported on its feet and palms (palmigrade
substantially more primitive than Australopithecus, resolves many clambering), but lacked any characteristics typical of the suspenuncertainties about early human evolution, including the nature of the sion, vertical climbing, or knuckle-walking of modern gorillas and
last common ancestor that we shared with the line leading to living chimps. Terrestrially, it engaged in a form of bipedality more primchimpanzees and bonobos. The Ardipithecus remains were recovered itive than that of Australopithecus, and it lacked adaptation to
from a sedimentary horizon representing a short span of time (within “heavy” chewing related to open environments (seen in later
100 to 10,000 years). This has enabled us to assess available and pre- Australopithecus). Ar. ramidus thus indicates that the last common
ferred habitats for the early hominids by systematic and repeated ancestors of humans and African apes were not chimpanzee-like and
sampling of the hominid-bearing strata.
that both hominids and extant African apes are each highly specialBy collecting and classifying thousands of vertebrate, invertebrate, ized, but through very different evolutionary pathways.
and plant fossils, and characterizing the isotopic composition of soil
samples and teeth, we have learned that Ar. ramidus was a denizen of See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
woodland with small patches of forest. We have also learned that it When citing, please refer to the full paper, available at DOI 10.1126/science.1175802.
AUTHORS’SUMMARIES
Authors’Summaries
The Geological, Isotopic, Botanical,
Invertebrate, and Lower Vertebrate
Surroundings of Ardipithecus ramidus
Giday WoldeGabriel, Stanley H. Ambrose, Doris Barboni, Raymonde Bonnefille, Laurent Bremond, Brian Currie,
David DeGusta, William K. Hart, Alison M. Murray, Paul R. Renne, M. C. Jolly-Saad, Kathlyn M. Stewart, Tim D. White
A
rdipithecus ramidus was found in exposed sediments flanking the Awash River, Ethiopia. The
local geology and associated fossils provide
critical information about its age and habitat.
Most of Africa’s surface is nondepositional and/or
covered by forests. This explains why so many discoveries related to early hominid evolution have been
made within eastern Africa’s relatively dry, narrow,
active rift system. Here the Arabian and African tectonic plates have been pulling apart for millions of
years, and lakes and rivers have accumulated variably
fossil-rich sediments in the Afar Triangle, which lies
at the intersection of the Red Sea, Gulf of Aden, and
Main Ethiopian Rifts (see map). Some of these deposits
were subsequently uplifted by the rift tectonics and are
now eroding. In addition, volcanoes associated with Map showing the Middle Awash area (star) and rift locations (red lines). Photo shows the
this rifting have left many widespread deposits that we 4.4 Ma volcanic marker horizon (yellow bed) atop the locality where the skeleton and holocan use to determine the age of these fossils using type teeth of Ar. ramidus were discovered. Also shown are some of the fossil seeds.
modern radioisotopic methods.
Several of the most important hominid fossils have been found near between two key volcanic markers, each dated to 4.4 Ma. Their simithe Afar’s western margin, north and west of the Awash River (star on lar ages and sedimentology imply that the fossils themselves date to
map), including Hadar (the “Lucy” site), Gona [known for the world’s 4.4 Ma and were all deposited within a relatively narrow time interval
oldest stone tools at 2.6 million years ago (Ma)], and the Middle Awash lasting anywhere from 100 to 10,000 years. Today the unit is exposed
(including Aramis). Cumulatively, these and nearby study areas in across a 9-km arc that represents a fortuitous transect through the
Ethiopia have provided an unparalleled record of hominid evolution.
ancient landscape. The western exposure, in particular, preserves a
Fossil-bearing rocks in the Middle Awash are intermittently rich assemblage of plant and animal fossils and ancient soils.
exposed and measure more than 1 km in thickness. Volcanic rocks
Fossilized wood, seeds, and phytoliths (hard silica parts from
near the base of this regional succession are dated to more than 6 Ma. plants) confirm the presence of hackberry, fig, and palm trees. There
Its uppermost sediments document the appearance of anatomically is no evidence of a humid closed-canopy tropical rainforest, nor of
near-modern humans 155,000 years ago. As is the case for many river the subdesertic vegetation that characterizes the area today.
and lake deposits, fossil accumulation rates here have been highly Invertebrate fossils are abundant and include insect larvae, broodvariable, and the distribution and preservation of the fossils are balls and nests of dung beetles, diverse gastropods, and millipedes.
uneven. Alterations of the fossils caused by erosion and other factors The terrestrial gastropods best match those seen in modern groundfurther complicate interpretation of past environments. To meet this water forests such as the Kibwezi in Kenya. Aquatic lower vertechallenge, beginning in 1981, our research team of more than 70 sci- brates are relatively rare and probably arrived episodically during
entists has collected 2000 geological samples, thousands of lithic flooding of a river distal to the Aramis area. The most abundant fish
artifacts (e.g., stone tools), and tens of thousands of plant and animal is catfish, probably introduced during overbank flooding and/or by
fossils. The emergent picture developed from the many Middle predatory birds roosting in local trees.
Awash rock units and their contents represents a series of snapshots
Our combined evidence indicates that Ar. ramidus did not live in
taken through time, rather than a continuous record of deposition.
the open savanna that was once envisioned to be the predominant
Ar. ramidus was recovered from one such geological unit, 3 to 6 m habitat of the earliest hominids, but rather in an environment that
thick, centered within the study area. Here, the Aramis and adjacent was humid and cooler than it is today, containing habitats ranging
drainage basins expose a total thickness of 300 m of sediments largely from woodland to forest patches.
deposited in rivers and lakes, and on floodplains, between ~5.5 and
3.8 Ma. Within this succession, the Ar. ramidus–bearing rock unit See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
comprises silt and clay beds deposited on a floodplain. It is bracketed When citing, please refer to the full paper, available at DOI 10.1126/science.1175817.
www.sciencemag.org
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Authors’Summaries
AUTHORS’SUMMARIES
Taphonomic, Avian, and Small-Vertebrate
Indicators of Ardipithecus ramidus Habitat
Antoine Louchart, Henry Wesselman, Robert J. Blumenschine, Leslea J. Hlusko, Jackson K. Njau,
Michael T. Black, Mesfin Asnake, Tim D. White
T
Coliidae <1% (2) Bucorvus <1% (2)
he stratigraphic unit conOtididae 2% (3)
Apodidae <1% (2)
taining Ardipithecus rami6 other taxa (1 each)
Coturnix <1% (2)
dus was probably deposited
Anatidae
3% (8)
rapidly, thus providing a transect
Numididae
through a 4.4-million-year-old
5% (7)
landscape. To help reconstruct
Falconiformes
and understand its biological
6% (7)
setting as thoroughly as possible,
Columbidae
Psittacidae
we recovered an assemblage of
6% (13)
36% (22)
>150,000 plant and animal fossils.
More than 6000 vertebrate speciPasseriformes
Pavo
6% (4)
mens were identified at the family
15% (16) Tyto
8% (7)
level or below. These specimens
Abundance of birds (left) associated with Ar. ramidus.
represent animals ranging in size
Francolinus
These distributions are consistent with a mostly wood8% (10)
from shrews to elephants and
land habitat. (Above) An example of the many small
mammal and bird bones.
include abundant birds and small
mammals that are usually rare in
hominid-bearing assemblages. Many of these birds and small mam- damage patterns of the fossils representing small mammals and birds
mals are highly sensitive to environmental conditions and thus are par- suggest that they are derived from undigested material regurgitated
ticularly helpful in reconstructing the environment.
by owls (owl pellets). Because of their fragility and size, bird bones
Accurate interpretation of fossil assemblages can be challenging. have been rare or absent at most other eastern African fossil assemEven fossils from one layer can represent artificial amalgamations blages that included early hominids. However, we cataloged 370
that might have originated thousands of years apart. Moreover, the avian fossils; these represent 29 species, several new to science. Most
remains of animals living in different habitats can be artificially of the birds are terrestrial rather than aquatic, and small species such
mixed by flowing water or by shifting lake and river margins. as doves, lovebirds, mousebirds, passerines, and swifts are abundant.
Ecological fidelity can be further biased by unsystematic recovery if, Open-country species are rare. Eagles and hawks/kites are present,
for example, only the more complete, identifiable, or rare specimens but the assemblage is dominated by parrots and the peafowl Pavo, an
are collected. Thus, interpreting the Ardipithecus-bearing sediments ecological indicator of wooded conditions.
requires that we deduce the physical and biological conditions under
The small-mammal assemblage includes up to 20 new species,
which the fossils accumulated and the degree to which these biases including shrews, bats, rodents, hares, and carnivores. Extant counoperated at the time of deposition—a practice called “taphonomy.”
terparts live in a variety of habitats, but their relative abundance in
Both the large- and small-mammal assemblages at Aramis lack the fossil assemblage indicates that Ardipithecus lived in a wooded
the damage that would result from transport and sorting by water, a area. Avian predators most probably procured the much rarer squirfinding consistent with the fine-grained sediments in which the rels and gerbils from drier scrub or arid settings at a distance. Most
bones were originally embedded. Many of the limb bone fragments of the bat, shrew, porcupine, and other rodent specimens are compatof large mammals show traces of rodent gnawing and carnivore ible with a relatively moist environmental setting, as are the abunchewing at a time when the bones were still fresh. These bones were dant fossils of monkeys and spiral-horned antelopes.
most probably damaged by hyenas, which in modern times are known
The combination of geological and taphonomic evidence, the
to destroy most of the limb bones and consume their marrow. The assemblage of small-mammal and avian fossils, and the taxonomic
actions of hyenas and other carnivores that actively competed for and isotopic compositions of remains from larger mammals indicate
these remains largely explain why the fossil assemblage at Aramis that Aramis was predominantly a woodland habitat during Ar.
contains an overrepresentation of teeth, jaws, and limb bone shaft ramidus times. The anatomical and isotopic evidence of Ar. ramidus
splinters (versus skulls or limb bone ends).
itself also suggests that the species was adapted to such a habitat.
As a result of this bone destruction, whole skeletons are extremely
rare at Aramis, with one fortunate exception: the partial skeleton of See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
Ar. ramidus excavated at ARA-VP-6/500. The relative abundance and When citing, please refer to the full paper, available at DOI 10.1126/science.1175823.
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AUTHORS’SUMMARIES
Authors’Summaries
Macrovertebrate Paleontology and the
Pliocene Habitat of Ardipithecus ramidus
Tim D. White, Stanley H. Ambrose, Gen Suwa, Denise F. Su, David DeGusta, Raymond L. Bernor,
Jean-Renaud Boisserie, Michel Brunet, Eric Delson, Stephen Frost, Nuria Garcia, Ioannis X. Giaourtsakis,
Yohannes Haile-Selassie, F. Clark Howell, Thomas Lehmann, Andossa Likius, Cesur Pehlevan, Haruo
Saegusa, Gina Semprebon, Mark Teaford, Elisabeth Vrba
E
ver since Darwin, scholars have
(red crosses in figure) account for
speculated about the role that
nearly a third of the entire large mamenvironment may have played in
mal collection. Leaf-eating colobines
human origins, evolution, and adaptatoday exhibit strong preferences for
tion. Given that all living great apes live
arboreal habitats, and the carbon isoand feed in trees, it has been assumed
tope compositions of the fossil teeth are
that the last common ancestor we
consistent with dense to open forest
shared with these forms was also a forarboreal feeding (see figure).
est dweller. In 1925, Raymond Dart
The other dominant large mammal
described the first Australopithecus, a
associated with Ar. ramidus is the
child’s skull, at Taung, South Africa.
spiral-horned antelope, Tragelaphus
Its occurrence among other fossils
(the kudu, green circle). Today, these
indicative of a grassland environment
antelopes are browsers (eating mostly
prompted speculation that the open
leaves), and they prefer bushy to
grasslands of Africa were exploited by
wooded habitats. The dental morpholearly hominids and were therefore
ogy, wear, and enamel isotopic comsomehow integrally involved with the
position of the Aramis kudu species
origins of upright walking.
are all consistent with such placeThe Ardipithecus-bearing sediments
ment. In contrast, grazing antelopes
at Aramis now provide fresh evidence
(which eat mostly grass) are rare in
that Ar. ramidus lived in a predomithe Aramis assemblage.
nantly woodland setting. This and corThe large-mammal assemblage
roborative evidence from fossil assemshows a preponderance of browsers
blages of avian and small mammals
and fruit eaters. This evidence is conimply that a grassland environment was
sistent with indications from birds,
not a major force driving evolution of
small mammals, soil isotopes, plants,
the earliest hominids. A diverse assemand invertebrate remains. The emerblage of large mammals (>5 kg body
gent picture of the Aramis landscape
weight) collected alongside Ardipithe- Carbon and oxygen isotope analyses of teeth from the Ar. ramidus during Ar. ramidus times is one of a
cus provides further support for this localities. Species are listed in order of abundance, and isotopic woodland setting with small forest
conclusion. Carbon isotopes from data separate species by what they ate and their environment.
patches. This woodland graded into
tooth enamel yield dietary information
nearby habitats that were more open
because different isotope signatures reflect different photosynthetic and are devoid of fossils of Ardipithecus and other forest-to-woodland–
pathways of plants consumed during enamel development. Therefore, community mammals. Finally, the carbon isotopic composition of
animals that feed on tropical open-environment grasses (or on grass-eat- Ar. ramidus teeth is similar to that of the predominantly arboreal, small,
ing animals) have different isotopic compositions from those feeding on baboon-like Pliopapio and the woodland browser Tragelaphus, indibrowse, seeds, or fruit from shrubs or trees. Moreover, oxygen isotopes cating little dietary intake of grass or grass-eating animals. It is therehelp deduce relative humidity and evaporation in the environment.
fore unlikely that Ar. ramidus was feeding much in open grasslands.
The larger-mammal assemblage associated with Ardipithecus was
These data suggest that the anatomy and behavior of early
systematically collected across a ~9 km transect of eroding sediments hominids did not evolve in response to open savanna or mosaic setsandwiched between two volcanic horizons each dated to 4.4 million tings. Rather, hominids appear to have originated and persisted
years ago. It consists of ~4000 cataloged specimens assigned to within more closed, wooded habitats until the emergence of more
~40 species in 34 genera of 16 families.
ecologically aggressive Australopithecus.
There are only three primates in this assemblage, and the rarest is
Ardipithecus, represented by 110 specimens (a minimum of 36 individ- See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
uals). Conversely, colobine monkeys and a small baboon-like monkey When citing, please refer to the full paper, available at DOI 10.1126/science.1175822.
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Authors’Summaries
AUTHORS’SUMMARIES
The Ardipithecus ramidus Skull and
Its Implications for Hominid Origins
Gen Suwa, Berhane Asfaw, Reiko T. Kono, Daisuke Kubo, C. Owen Lovejoy, Tim D. White
T
he key feature that distintus and are known as “robust”
Australopithecus afarensis
guishes Homo sapiens from
Australopithecus.
Pan troglodytes
other primates is our unusuAr. ramidus had a small brain
ally large brain, which allows us
(300 to 350 cm3), similar to that of
200cc 300
400
500
600
bonobos and female chimpanzees
to communicate, make tools, plan,
and smaller than that of Australoand modify our environment. Unpithecus. The Ar. ramidus face is
derstanding how and when our
14mm16
18
20
22
24
also small and lacks the large
cognitive ability evolved has been
cheeks of “heavy chewing” Ausa special focus in anthropology
tralopithecus. It has a projecting
and, more recently, genetics. Fossil
44mm 48
52
56
60
muzzle as in Sahelanthropus,
hominid skulls provide direct eviwhich gives it a decidedly ape-like
dence of skull evolution and inforgestalt. Yet the Ar. ramidus skull is
mation about diet, appearance, and
not particularly chimpanzee-like.
behavior. Skulls feature promi12mm 16
20
24
For example, the ridge above the
nently in the characterization of
species, in taxonomy, and in phy- (Right) Oblique and side views of a female chimpanzee (right) and the Ar. eye socket is unlike that of a chimlogenetic analyses of both extinct ramidus female reconstruction (left; the oblique view includes a separate panzee, and its lower face does
mandible). (Left) Comparison of brain and tooth sizes (arrows) of chimps (Pan;
and living primates.
blue), Ar. ramidus (red), and Australopithecus (green). Means are plotted not project forward as much as a
Unfortunately, hominid skulls except for individual Ar. ramidus and Au. afarensis cranial capacities. Canine chimpanzee’s face. Chimps priare relatively rare in the fossil unworn heights (bottom) are based on small samples, Ar. ramidus (females, n marily eat ripe fruits and have
record. A number of partial skulls = 1; males, n = 3), Au. afarensis (n = 2), Pan (females, n = 19; males, n = 11). large incisors set in a projecting
lower face. Ar. ramidus instead
and crania (skulls without a lower
jaw) of early Homo and its predecessor, Australopithecus (which lived was probably more omnivorous and fed both in trees and on the ground.
~1 to 4 million years ago), have been recovered, but relatively few are Additionally, in chimpanzees, forward placement of the entire lower
complete enough for extensive comparisons. One surprisingly com- face is exaggerated, perhaps linked with their large tusklike canines
plete but distorted cranium from 6 to 7 million years ago was discov- (especially in males) and elevated levels of aggression. This is not seen
ered in central Africa (Chad). This fossil, Sahelanthropus tchadensis in Ar. ramidus, implying that it was less socially aggressive.
Like Ar. ramidus, S. tchadensis had a brain that was less than
(a.k.a. “Toumaï ”), is thought by many to represent the earliest known
400 cm3 in size. It also resembled Ar. ramidus in having small nonhominid, although some have argued that it is a female ape.
The Ardipithecus ramidus skull is of particular interest because it sharpened canines. Details of the bottom of the skull show that both
predates known Australopithecus and thereby illuminates the early Ar. ramidus and Sahelanthropus had a short cranial base, a feature
evolution of the hominid skull, brain, and face. The Ar. ramidus skull also shared with Australopithecus. Furthermore, we infer that the rear
was badly crushed, and many of its bones were scattered over a wide of the Ar. ramidus skull was downturned like that suggested for
area. Because the bones were so fragile and damaged, we imaged Sahelanthropus. These similarities confirm that Sahelanthropus was
them with micro–computed tomography, making more than 5000 indeed a hominid, not an extinct ape.
These and an additional feature of the skull hint that, despite its
slices. We assembled the fragments into more than 60 key virtual
pieces of the braincase, face, and teeth, enough to allow us to digitally small size, the brain of Ar. ramidus may have already begun to develop
some aspects of later hominid-like form and function. The steep orienreconstruct a largely complete cranium.
The fossil skulls of Australopithecus indicate that its brain was tation of the bone on which the brain stem rests suggests that the base
~400 to 550 cm3 in size, slightly larger than the brains of modern apes of the Ar. ramidus brain might have been more flexed than in apes. In
of similar body size and about a third of those of typical Homo sapi- Australopithecus, a flexed cranial base occurs together with expansion
ens. Its specialized craniofacial architecture facilitated the production of the posterior parietal cortex, a part of the modern human brain
of strong chewing forces along the entire row of teeth located behind involved in aspects of visual and spatial perception.
its canines. These postcanine teeth were enlarged and had thick
enamel, consistent with a hard/tough and abrasive diet. Some species See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
exhibited extreme manifestations of this specialized chewing appara- When citing, please refer to the full paper, available at DOI 10.1126/science.1175825.
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Authors’Summaries
Paleobiological Implications of the
Ardipithecus ramidus Dentition
Gen Suwa, Reiko T. Kono, Scott W. Simpson, Berhane Asfaw, C. Owen Lovejoy, Tim D. White
T
eeth are highly resilient to degradation and
therefore are the most abundant specimens in
the primate fossil record. The size, shape,
enamel thickness, and isotopic composition of teeth
provide a wealth of information about phylogeny, diet,
and social behavior. Ardipithecus ramidus was originally defined in 1994 primarily on the basis of recovered teeth, but the sample size was small, limiting
comparison to other primate fossils. We now have over
145 teeth, including canines from up to 21 individuals.
The expanded sample now provides new information
regarding Ar. ramidus and, using comparisons with
teeth of other hominids, extant apes, and monkeys,
new perspectives on early hominid evolution as well.
In apes and monkeys, the male’s upper canine tooth
usually bears a projecting, daggerlike crown that is
continuously sharpened (honed) by wear against a Dentitions from human (left), Ar. ramidus (middle), and chimpanzee (right), all males.
specialized lower premolar tooth (together these form Below are corresponding samples of the maxillary first molar in each. Red, thicker enamel
the C/P3 complex). The canine tooth is used as a slic- (~2 mm); blue, thinner enamel (~0.5 mm). Contour lines map the topography of the crown
ing weapon in intra- and intergroup social conflicts. and chewing surfaces.
Modern humans have small, stublike canines which
function more like incisors.
In modern monkeys and apes, the upper canine is important in
All known modern and fossil apes have (or had) a honing C/P3 com- male agonistic behavior, so its subdued shape in early hominids and
plex. In most species, this is more developed in males than females (in Ar. ramidus suggests that sexual selection played a primary role in
a few species, females have male-like large canines, either for territo- canine reduction. Thus, fundamental reproductive and social behavrial defense or for specialized feeding). The relatively large number of ioral changes probably occurred in hominids long before they had
Ar. ramidus teeth, in combination with Ethiopian Ar. kadabba, Kenyan enlarged brains and began to use stone tools.
Orrorin, and Chadian Sahelanthropus [currently the earliest known
Thick enamel suggests that an animal’s food intake was abrasive;
hominids at about 6 million years ago (Ma)], provide insight into the for example, from terrestrial feeding. Thin enamel is consistent with
ancestral ape C/P3 complex and its evolution in early hominids.
a diet of softer and less abrasive foods, such as arboreal ripe fruits. We
In basal dimensions, the canines of Ar. ramidus are roughly as measured the enamel properties of more than 30 Ar. ramidus teeth.
large as those of female chimpanzees and male bonobos, but their Its molar enamel is intermediate in thickness between that of chimcrown heights are shorter (see figure). The Ar. ramidus sample is now panzees and Australopithecus or Homo. Chimpanzees have thin
large enough to assure us that males are represented. This means that enamel at the chewing surface of their molars, whereas a broad conmale and female canines were not only similar in size, but that the cave basin flanked by spiky cusps facilitates crushing fruits and
male canine had been dramatically “feminized” in shape. The crown shredding leaves. Ar. ramidus does not share this pattern, implying a
of the upper canine in Ar. ramidus was altered from the pointed shape diet different from that of chimpanzees. Lack of thick enamel indiseen in apes to a less-threatening diamond shape in both males and cates that Ar. ramidus was not as adapted to heavy chewing and/or
females. There is no evidence of honing. The lower canines of Ar. eating abrasive foods as were later Australopithecus or even Homo.
ramidus are less modified from the inferred female ape condition The combined evidence from the isotopic content of the enamel, denthan the uppers. The hominid canines from about 6 Ma are similar in tal wear, and molar structure indicates that the earliest hominid diet
size to those of Ar. ramidus, but (especially) the older upper canines was one of generalized omnivory and frugivory and therefore difappear slightly more primitive. This suggests that male canine size fered from that of Australopithecus and living African apes.
and prominence were dramatically reduced by ~6 to 4.4 Ma from an
ancestral ape with a honing C/P3 complex and a moderate degree of See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
When citing, please refer to the full paper, available at DOI 10.1126/science.1175824.
male and female canine size difference.
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AUTHORS’SUMMARIES
Careful Climbing in the Miocene:
The Forelimbs of Ardipithecus ramidus
and Humans Are Primitive
C. Owen Lovejoy, Scott W. Simpson, Tim D. White, Berhane Asfaw, Gen Suwa
A
grasping hand and highly
skeleton yet found, had only two
mobile forelimb are defining
hand bones—far short of the number
characteristics of primates.
needed to interpret the structure and
The special ability to pick things up
evolution of the hand. The Ardipithand manipulate them has probably
ecus skeleton reported here changes
been a central selective force in makthat. Not only is it more than 1 milG
ing primates so unusually intelligent.
lion years older than Lucy (4.4 milIt’s something that porpoises can’t do
lion versus 3.2 million years old), its
B
E
at all and crows can’t do very well. It
hands are virtually complete and
F
may also be one reason why humans
intact. They show that Ardipithecus
A
alone eventually evolved cognition.
did not knuckle-walk like African
D
The hands of African apes are
apes and that it lacked virtually all of
specialized in a number of ways that
the specializations that protect great
C
make them dramatically different
ape hands from injury while they
H
from our own. Apes must support
climb and feed in trees.
their large body mass during climbArdipithecus hands were very
ing to feed and nest, especially in the Two views of the left hand of Ar. ramidus showing primitive features different from those of African apes.
middle and higher parts of the tree absent in specialized apes. (A) Short metacarpals; (B) lack of knuckle- Its wrist joints were not as stiff as
canopy. Their hands must therefore walking grooves; (C) extended joint surface on fifth digit; (D) thumb those of apes, and the joints between
withstand very high forces, and this more robust than in apes; (E) insertion gable for long flexor tendon their palms and fingers were much
is facilitated by their elongated palms (sometimes absent in apes); (F) hamate allows palm to flex; (G) sim- more flexible. Moreover, a large
and fingers. Our palms are much ple wrist joints; (H) capitate head promotes strong palm flexion. Inset: joint in the middle of the wrist (the
shorter and our wrists more mobile. lateral view of capitates of Pan, Ar. ramidus, and human (left to right). midcarpal joint) was especially
Dashed lines reflect a more palmar capitate head location for Ar.
This allows us to grasp objects and ramidus and humans, which allows a more flexible wrist in hominids. flexible, being even more mobile
compress them with great dexterity
than our own. This would have
and force—something often called a “power grip.” The differences allowed Ardipithecus to support nearly all of its body weight on its
between ape and human forelimbs become less pronounced going palms when moving along tree branches, so that it could move
from the hand to the shoulder. Ape and human elbow joints, for exam- well forward of a supporting forelimb without first releasing its
ple, diverge only moderately in their manner of load transmission.
grip on a branch.
The high loads that apes bear during locomotion have required
This discovery ends years of speculation about the course of
them to greatly stiffen the joints between their fingers and palms. human evolution. Our ancestors’ hands differed profoundly from
Because their thumb has not been elongated in the same way as their those of living great apes, and therefore the two must have substanpalms and fingers have, thumb-to-palm and thumb-to-finger opposi- tially differed in the ways they climbed, fed, and nested. It is African
tions are more awkward for them. We are therefore much more adept apes who have evolved so extensively since we shared our last comat making and using tools. All of these forelimb characteristics in apes mon ancestor, not humans or our immediate hominid ancestors.
have led them to adopt an unusual form of terrestrial quadrupedality, Hands of the earliest hominids were less ape-like than ours and quite
in which they support themselves on their knuckles rather than on different from those of any living form.
their palms. Only African apes exhibit this “knuckle-walking.” Other
Ardipithecus also shows that our ability to use and make tools did
primates, such as monkeys, still support themselves on their palms.
not require us to greatly modify our hands. Rather, human grasp and
It has long been assumed that our hands must have evolved from dexterity were long ago inherited almost directly from our last comhands like those of African apes. When they are knuckle-walking, mon ancestor with chimpanzees. We now know that our earliest
their long forelimbs angle their trunks upward. This posture has ancestors only had to slightly enlarge their thumbs and shorten their
therefore long been viewed by some as “preadapting” our ancestors fingers to greatly improve their dexterity for tool-using.
to holding their trunks upright.
Until now, this argument was unsettled, because we lacked an ade- See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
quate fossil record. Even Lucy, the most complete Australopithecus When citing, please refer to the full paper, available at DOI 10.1126/science.1175827.
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Authors’Summaries
The Pelvis and Femur
of Ardipithecus ramidus:
The Emergence of Upright Walking
C. Owen Lovejoy, Gen Suwa, Linda Spurlock, Berhane Asfaw, Tim D. White
V
irtually no other primate has a human-like
pelvic girdle—not even our closest living relatives, the chimpanzee and bonobo. Such
uniqueness evolved via substantial modifications of a
pelvis more originally suited for life in trees. This
arboreal primate heritage has left us rather ungainly.
Our legs are massive because they continue to house
almost all of the muscles originally required for climbing. Our hamstrings, the large muscles in our posterior
thighs, must decelerate the swinging limb with each
step, and when we run, the limb’s inertia is sometimes
too great and these muscles fail (not something one
would want to happen on a savanna).
Furthermore, when each limb leaves the ground to
Homo sapiens
Ar. ramidus
Au. afarensis
P. troglodytes
be swung forward, it and the pelvis are unsupported
and would slump toward the ground were it not for The Ar. ramidus pelvis has a mosaic of characters for both bipedality and climbing. Left to right:
muscles acting on the opposite side of the body (the Human, Au. afarensis (“Lucy”), Ar. ramidus, Pan (chimpanzee). The ischial surface is angled
anterior gluteals). One early anthropologist described near its midpoint to face upward in Lucy and the human (blue double arrows), showing that
human locomotion as a process by which we alter- their hamstrings have undergone transformation for advanced bipedality, whereas they are
nately almost fall on our faces. Chimpanzees and primitive in the chimpanzee and Ar. ramidus (blue arrows). All three hominid ilia are vertically
other primates cannot prevent such slumping when short and horizontally broad, forming a greater sciatic notch (white arrows) that is absent in
Pan. A novel growth site [the anterior inferior iliac spine (yellow arrows)] is also lacking in Pan.
walking upright because they cannot reposition these
muscles effectively. Their spine is too inflexible and
their ilia—the large pelvic bones to which the gluteals attach—are almost entirely ape-like, presumably because it still had massive
positioned and shaped differently than ours. Modifying a typical hindlimb muscles for active climbing.
chimp or gorilla pelvis to facilitate upright walking would require
Changes made in the upper pelvis rendered Ar. ramidus an effecextensive structural changes.
tive upright walker. It could also run, but probably with less speed and
Until now, the fossil record has told us little about when and how efficiency than humans. Running would also have exposed it to
the early hominid pelvis evolved. Even 3 to 4 million years ago (when injury because it lacked advanced mechanisms such as those that
our brains were still only slightly larger than those of chimpanzees), it would allow it to decelerate its limbs or modulate collision forces at
had already undergone radical transformation. One of the oldest its heel. Australopithecus, which had given up its grasping foot and
hominid pelves, that of Australopithecus afarensis (A.L. 288-1; abandoned active climbing, had evolved a lower pelvis that allowed it
“Lucy”), shows that her species had already evolved virtually all of the to run and walk for considerable distances.
fundamental adaptations to bipedality. Even the kinetics of her hip
Ar. ramidus thus illuminates two critical adaptive transitions in
joint were similar to ours. Although the human pelvis was later further human evolution. In the first, from the human-chimp last common
reshaped, this was largely the result of our much enlarged birth canal. ancestor to Ardipithecus, modifications produced a mosaic pelvis
Ardipithecus ramidus now unveils how our skeleton became pro- that was useful for both climbing and upright walking. In the second,
gressively modified for bipedality. Although the foot anatomy of Ar. from Ardipithecus to Australopithecus, modifications produced a
ramidus shows that it was still climbing trees, on the ground it walked pelvis and lower limb that facilitated more effective upright walking
upright. Its pelvis is a mosaic that, although far from being chim- and running but that were no longer useful for climbing. Because
panzee-like, is still much more primitive than that of Australopithecus. climbing to feed, nest, and escape predators is vital to all nonhuman
The gluteal muscles had been repositioned so that Ar. ramidus primates, both of these transitions would likely have been a response
could walk without shifting its center of mass from side to side. This to intense natural selection.
is made clear not only by the shape of its ilium, but by the appearance
of a special growth site unique to hominids among all primates (the See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
anterior inferior iliac spine). However, its lower pelvis was still When citing, please refer to the full paper, available at DOI 10.1126/science.1175831.
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AUTHORS’SUMMARIES
Combining Prehension and Propulsion:
The Foot of Ardipithecus ramidus
C. Owen Lovejoy, Bruce Latimer, Gen Suwa, Berhane Asfaw, Tim D. White
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CREDIT: RECONSTRUCTION, COPYRIGHT J. H. MATTERNES; CHIMPANZEE CLIMBING, J. DESILVA; BONOBO AND HUMAN FEET, S. INGHAM CREDITS (TOP TO BOTTOM):
T
Pan
Homo
he special foot adaptations that
the great toe closed during grasping,
enable humans to walk upright
has been relocated more toward the
and run are central to underfront of the foot. This makes the tenstanding our evolution. Until the disdon run more parallel to other joints
covery of Ardipithecus ramidus, it was
that cross the midfoot, and allows
generally thought that our foot evolved
apes to grasp with great power withfrom one similar to that of modern
out stiffening these other, flexible
African apes. Apes have feet that are
joints. Apes can thus both powerfully
modified to support their large bodies
grasp and mold their feet around
and to facilitate vertical climbing, thus
objects at the same time. However,
allowing them to feed, nest, and seek
their feet have become less effective
safety in trees. Our foot differs from
as levers, making them far less useful
theirs in myriad ways, and its evoluin terrestrial propulsion.
tion from theirs would consequently
The foot of Ar. ramidus shows that
have required an extensive series of
none of these ape-like changes were
structural changes. Some mid–20thpresent in the last common ancestor
century comparative anatomists were
of African apes and humans. That
so impressed with the profound differancestor, which until now has been
ences between human and extant ape
thought to be chimpanzee-like, must
feet that they postulated a deep, prehave had a more monkey-like foot.
ape origin for hominids.
Not only did it still have an os perAr. ramidus brings a new perspeconeum, it must also have had all of the
tive to this old controversy. Its foot
other characteristics associated with
turns out to be unlike those of the Foot skeleton of Ar. ramidus (bottom; reconstruction based on it (subsequently abandoned in chimAfrican apes in many ways. The par- computed tomography rendering shown) lacked many features panzees and gorillas). We infer this
tial skeleton of Ar. ramidus preserves that have evolved for advanced vertical climbing and suspension because humans still have these charmost of the foot and includes a special in extant chimpanzees (Pan, top left). Chimpanzees have a highly acteristics, so we must have retained
bone called the os peroneum that is flexible midfoot and other adaptations that improve their ability them from our last common ancestor.
critical for understanding foot evolu- to grasp substrates. These are absent in Ar. ramidus.
The mid–20th-century anatomists
tion. This bone, which is embedded
were correct to worry about the human
within a tendon, facilitates the mechanical action of the fibularis foot as they did: Ours turns out to have evolved in one direction,
longus, the primary muscle that draws in the big toe when the foot is while those of African apes were evolving in quite another.
grasping. Until now, we knew little about this bone’s natural history,
One of the great advantages of our more rigid foot is that it works
except that it is present in Old World monkeys and gibbons but gen- much better as a lever during upright walking and running (as it also
erally not in our more recent ape relatives. Monkeys are very accom- does in monkeys). However, Ar. ramidus still had an opposable big
plished at leaping between trees. They must keep their feet fairly rigid toe, unlike any later hominid. Its ability to walk upright was thereduring takeoff when they hurl themselves across gaps in the tree fore comparatively primitive. Because it had substantially modified
canopy; otherwise, much of the torque from their foot muscles would the other four toes for upright walking, even while retaining its
be dissipated within the foot rather than being transferred to the tree. grasping big toe, the Ardipithecus foot was an odd mosaic that
The African apes are too large to do much leaping. They have worked for both upright walking and climbing in trees. If our last
therefore given up the features that maintain a rigid foot and have common ancestor with the chimpanzee had not retained such an
instead modified theirs for more effective grasping—almost to the unspecialized foot, perhaps upright walking might never have
point of making it difficult to distinguish their feet from their hands. evolved in the first place.
Indeed, very early anatomists argued that the “quadrumanus” apes
were not related to humans because of their hand-like feet. Extant See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
apes lack the os peroneum, and their fibularis tendon, which draws When citing, please refer to the full paper, available at DOI 10.1126/science.1175832.
AUTHORS’SUMMARIES
Authors’Summaries
The Great Divides: Ardipithecus ramidus
Reveals the Postcrania of Our Last Common
Ancestors with African Apes
C. Owen Lovejoy, Gen Suwa, Scott W. Simpson, Jay H. Matternes, Tim D. White
E
volutionary biologists have long recognized
Pan  
that the living primates most similar to humans
 

are the great apes, and comparative genomic

Ardipithecus
sequence analyses confirm that we are most closely
   
   

related to chimpanzees and bonobos (genus Pan).
   
  
    
Because of our great genomic similarity (sometimes

   



even cited as ~99%), the presumption that we evolved
  
 
    
from a chimpanzee-like ancestor has become increas  

  
 
ingly common wisdom. The widely held view that the
   
 
genomic and phyletic split between Pan and humans
  
 
   


 
was as recent as 5 to 6 million years ago also fuels the
 
 

often uncritical acceptance of a Pan-like last common

ancestor. Ardipithecus ramidus at 4.4 million years

 

ago provides the first substantial body of fossil evi 
   

dence that temporally and anatomically extends our
   
    
knowledge of what the last common ancestor we
    
      
shared with chimpanzees was like, and therefore
allows a test of such presumptions.
Until now, Australopithecus afarensis, which lived
3 to 4 million years ago, represented the most primi- Cladogram adding Ar. ramidus to images of gorilla, chimpanzee, and human, taken from the
tive well-known stage of human evolution. It had a frontispiece of Evidence as to Man’s Place in Nature, by Thomas H. Huxley (London, 1863)
brain only slightly larger than that of chimpanzees, (with the positions of Gorilla and Pan reversed to reflect current genetic data). Numerous
and a snout that projected more than in later details of the Ar. ramidus skeleton confirm that extant African apes do not much resemble our
hominids. Assuming some variant of a chimpanzee- last common ancestor(s) with them.
like ape ancestry, the bipedality of Au. afarensis has
been widely interpreted as being so primitive that it probably apes such as Proconsul (which lived more than 15 million years
could not have extended either its hip or knee joints and was a ago). Its lower back was mobile and probably had six lumbar verteclumsy upright walker. Some researchers have even postulated that brae rather than the three to four seen in the stiff backs of African
Au. afarensis could walk but not run, or vice versa. Still others have apes. Its hand was unpredictably unique: Not only was its thumb
suggested that Au. afarensis had a grasping ape-like foot. Similarly, musculature robust, unlike that of an ape, but its midcarpal joint (in
it has been suggested that Au. afarensis had forelimbs that were ape- the wrist) allowed the wrist to bend backward to a great degree,
like, including long, curved fingers used to forage daily in the arboreal enhancing its ability to move along tree branches on its palms. None
canopy, and that its immediate ancestors must have knuckle- of the changes that apes have evolved to stiffen their hands for suswalked. Australopithecus males were noticeably larger than females, pension and vertical climbing were present, so its locomotion did
and this has often been interpreted as signifying a single-male, not resemble that of any living ape.
polygynous, Gorilla-like mating system. Unlike gorillas, it has
The hominid descendant of the last common ancestor we shared
diminutive canines, but these were argued to be a consequence of its with chimpanzees (the CLCA), Ardipithecus, became a biped by
huge postcanine teeth. Early hominids have even been posited to modifying its upper pelvis without abandoning its grasping big toe.
have possibly interbred with chimpanzees until just before the It was therefore an unpredicted and odd mosaic. It appears, unlike
appearance of Australopithecus in the fossil record.
Au. afarensis, to have occupied the basal adaptive plateau of
The Ar. ramidus fossils and information on its habitat now reveal hominid natural history. It is so rife with anatomical surprises that no
that many of these earlier hypotheses about our last common ances- one could have imagined it without direct fossil evidence.
tor with chimpanzees are incorrect. The picture emerging from Ar.
ramidus is that this last common ancestor had limb proportions more
like those of monkeys than apes. Its feet functioned only partly like See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliations.
those of apes and much more like those of living monkeys and early When citing, please refer to the full paper, available at DOI 10.1126/science.1175833.
www.sciencemag.org
SCIENCE
VOL 326
2 OCTOBER 2009
73
25
Authors’Summaries
AUTHORS’SUMMARIES
Reexamining Human Origins in Light
of Ardipithecus ramidus
C. Owen Lovejoy
C
74
26
himpanzees, bonobos, and
presume a chimpanzee- or gorilla-like
BIPEDALITY
gorillas are our closest living
ancestor to explain habitual upright
relatives. The most popular
walking. Ar. ramidus was fully capable
reconstructions of human evolution
of bipedality and had evolved a subPan
during the past century rested on the
stantially modified pelvis and foot with
1 cm
presumption that the behaviors of the
which to walk upright. At the same
VESTED
earliest hominids were related to (or
time, it preserved the ability to maneuPROVISIONING
even natural amplifications of) behavver in trees, because it maintained a
iors observed in these living great apes.
grasping big toe and a powerful hip and
One effect of chimpanzee-centric
thigh musculature. Because upright
models of human evolution has been a Ardipithecus
walking provided no energy advantage
Reduced Intra-sexual
tendency to view Australopithecus as
for Ar. ramidus (it lacked many of the
OVULATORY
LOSS OF
Agonism and Increased
CRYPSIS
HONING CANINE
transitional between an ape-like ancesadaptations evolved in later hominids
Social Adhesion
tor and early Homo.
such as Australopithecus), reproducArdipithecus ramidus nullifies these Breakthrough adaptations can transform life-history by deviating tive success must have been central to
presumptions, as it shows that the from typical reproductive strategy. Early hominids show feminized its evolution in early hominids.
anatomy of living African apes is not male canines [left] and primitive bipedality [right]. These suggest
Loss of the projecting canine raises
that females preferred nonaggressive males who gained reproprimitive but instead has evolved ductive success by obtaining copulation in exchange for valuable other vexing questions because this
specifically within extant ape lineages. foods (vested provisioning). Success would depend on copulatory tooth is so fundamental to reproducThe anatomy and behavior of early frequency with mates whose fertility remained cryptic (e.g., tive success in higher primates. What
hominids are therefore unlikely to rep- absence of cycling in mammary size). The result would be reduced could cause males to forfeit their abilresent simple amplifications of those agonism in unrelated females, and cooperative expansion of day ity to aggressively compete with other
shared with modern apes. Instead, Ar. ranges among equally cooperative males, eventually leading to males? What changes paved the way
ramidus preserves some of the ances- exploitation of new habitats.
for the later emergence of the energytral characteristics of the last common
thirsty brain of Homo? Such questions
ancestor with much greater fidelity than do living African apes. Two can no longer be addressed by simply comparing humans to extant
obvious exceptions are its ability to walk upright and the absence of apes, because no ape exhibits an even remotely similar evolutionary
the large projecting canine tooth in males, derived features that trajectory to that revealed by Ardipithecus.
Ardipithecus shares with all later hominids.
When the likely adaptations of early hominids are viewed generally
Ar. ramidus illuminates our own origins because it clarifies our rela- rather than with specific reference to living chimpanzees, answers to
tionship to Australopithecus. For example, the enlarged rear teeth of such questions arise naturally. Many odd hominid characteristics
Australopithecus have long been viewed as adaptations to a rough, become transformed from peculiar to commonplace. Combining our
abrasive diet. This has led to speculation that canine teeth might have knowledge of mammalian reproductive physiology and the hominid
become smaller simply to accommodate the emergence of these other fossil record suggests that a major shift in life-history strategy transenlarged teeth, or that the importance of canine teeth in displays of formed the social structure of early hominids. That shift probably
male-to-male aggression waned with the development of weapons. reduced male-to-male conflict and combined three previously unseen
Ar. ramidus negates such hypotheses because it demonstrates that small behaviors associated with their ability to exploit both trees and the land
canines occurred in hominids long before any of the dental modifica- surface: (i) regular food-carrying, (ii) pair-bonding, and (iii) reproductions of Australopithecus or the use of stone tools. The loss of large tive crypsis (in which females did not advertise ovulation, unlike the
canine teeth in males must have occurred within the context of a gener- case in chimpanzees). Together, these behaviors would have substanalized, nonspecialized diet. Comparisons of the Ar. ramidus dentition tially intensified male parental investment—a breakthrough adaptation
with those of all other higher primates indicate that the species retained with anatomical, behavioral, and physiological consequences for early
virtually no anatomical correlates of male-to-male conflict. Consistent hominids and for all of their descendants, including ourselves.
with a diminished role of such agonism, the body size of Ar. ramidus
males was only slightly larger than that of females.
See pages 62–63
5–6 forfor
authors’
affiliations.
authors’
affiliation.
The discovery of Ar. ramidus also requires rejection of theories that When citing, please refer to the full paper, available at DOI 10.1126/science.1175834.
2 OCTOBER 2009
VOL 326
SCIENCE
www.sciencemag.org
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Ardipithecus Unveiled

Transcription

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