How to identify (as opposed to define) a homoplasy: Examples n *

Transcription

Journal of Human Evolution 52 (2007) 559e572
How to identify (as opposed to define) a homoplasy: Examples
from fossil and living great apes
David R. Begun*
Department of Anthropology, University of Toronto, Toronto, ON M5S 3G3, Canada
Received 30 December 2005; accepted 21 November 2006
Abstract
There is much debate on the definitions of homoplasy and homology, and on how to spot them among character states used in a phylogenetic
analysis. Many advocate what I call a ‘‘processual approach,’’ in which information on genetics, development, function, or other criteria help
a priori in identifying two character states as homologous or homoplastic. I argue that the processes represented by these criteria are insufficiently known for most organisms and most characters to be reliably used to identify homoplasies and homologies. Instead, while not foolproof,
phylogeny should be the ultimate test for homology. Character states are assumed to be homologous a priori because this is falsifiable and
because their initial inclusion in the character-state analysis is based on the assumption that they may be phylogenetically informative. If
they fall out as symplesiomorphies or synapomorphies in a phylogenetic analysis, their status as homologies remains unfalsified. If they fall
out as homoplasies, having evolved independently in more than one clade, their status as homologous is falsified, and a homoplasy is identified.
The character-state transformation series, functional morphology, finer levels of morphological comparison, and the distribution and correlation
of characters all help to explain the presence of homoplasies in a given phylogeny. Explaining these homoplasies, and not ignoring them as
‘‘noise,’’ should be as much a goal of phylogenetic analysis as the production of a phylogeny. Examples from the fossil record of Miocene hominoids are given to illustrate the advantages of a process-informs-pattern-recognition-after-the-fact approach to understanding the evolution of
character states.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Miocene hominoids; Mastication; Suspension; Emergence; Homology; Cladistics
Introduction
Whatever homoplasy is, at least on one level it is a property
of our concept of a character or a character state. A character,
the most basic component of phylogenetic analysis, is also
very difficult to define, but a working definition of it includes
something about an organism that we take to be informative in
some domain. One aspect of the concept we have of a specific
character state in a particular group of taxa is its evolutionary
history; character-state similarities across taxa reflect either
commonality of descent (homology) or independent acquisition in multiple taxa (homoplasy). We know that characters
* Tel.: þ1 416 978 8850.
E-mail address: [email protected]
0047-2484/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jhevol.2006.11.017
can appear in separate lineages as a result of the same or of
different evolutionary events and developmental processes
(Hall, 1992; Goodwin, 1994; Wagner, 1994), which in itself
lends credibility to character recognition. Characters are
more easily accepted as ‘‘real’’ if they can be shown to be affected in understandable or predictable ways by natural processes, such as evolution, growth, or use. However, it is
a paradox that, while we can readily identify homoplasies if
we know the sequence of evolutionary events represented by
the characters we use in an evolutionary analysis, we cannot
define a homoplasy in any meaningful way. As with characters, a working definition of a homoplasy is similarities that
define different taxonomic groups (Cracraft, 1981), acquired
independently and only identifiable once the phylogeny of
a group is determined. Homoplasies are similarities that
were assumed to be homologous but turn out not to be, that
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D.R. Begun / Journal of Human Evolution 52 (2007) 559e572
is, similarities without common ancestry (Cracraft, 1981).
This is a little like defining naturally occurring crystals or minerals that look remarkably biological in origin as ‘‘coral without life.’’ We can identify the processes that produce coral or
gyserite, for example, with a detailed analysis of each, but we
cannot do the same with homoplasies. We need to know the
history of the character a priori.
It is common in the literature on great ape and human systematics to read about the relative likelihood of homoplasy in
different anatomical regions. These ideas are often based on
observations of plasticity in postcranial anatomy or knowledge
of the relatively tight genetic control over tooth morphology
(Szalay and Delson, 1979; Lockwood and Fleagle, 1999;
Collard and Wood, 2001; Pilbeam and Young, 2001). However,
cladistic analysis of hominoid phylogeny fails to support the
premise that specific regions of the body have predictable
levels of homoplasy (Begun et al., 1997), and the same is
true in primate and mammalian evolution more generally
(Sańchez-Villagra and Williams, 1998; Williams, 2007). It is
also common to read that characters that are proposed by
some to be synapomorphies in support of a specific phylogenetic hypothesis are thought likely to be homoplasies by others
simply because they are known to occur in parallel at other
levels of the taxonomic hierarchy. Recent examples in
hominoid systematics include (1) the idea that postcranial similarities among various late Miocene hominids could be homoplasies simply because postcranial homoplasies also occur in
atelines; (2) an absent (or filled) subarcuate fossa (a depression
on the endocranial surface of the petrous portion of the temporal bone) is not a reliable synapomorphy of the Hominidae because it occurs occasionally in large cercopithecoids; or (3)
that the fused os centrale is not a good synapomorphy of the
African ape/human clade because such fusion occurs in
some indrids and in the occasional orangutan and hylobatid
(Schultz, 1936; Spoor and Leakey, 1996; Schwartz and
Yamada, 1998; Pilbeam and Young, 2001; Kivell and Begun,
2007). These are fallacies of character analysis because homoplasy is a hierarchical phenomenon (Nelson, 1994). That is,
within different clades, different anatomical complexes have
different levels of homoplasy depending on emergent characteristics and ecologically mediated evolutionary events unique
to those clades. The absence of a subarcuate fossa in some cercopithecoids or the presence of a fused os centrale in some indrids is uninformative of the evolutionary history of these
characters in distantly related taxa. Similarly, at different
levels of the classification of anthropoids, anatomical complexes as diverse as phalangeal morphology and gnathic structures show high levels of homoplasy, while at other levels they
provide strong evidence of phyletic relations (homology). A
number of these examples are considered below.
Classic Hennigian theory states that homoplasies are identified a posteriori as emerging from a cladistic analysis (Hennig, 1966; Kluge, 1999; Farris et al., 2001). New approaches
have been developed to attempt to predict homoplasy, or to
distinguish homoplasy and homology a priori (Lovejoy
et al., 1999, 2002; O’Higgins and Cohn, 2000; Line, 2001).
For most researchers interested in predicting homoplasy, it is
no longer sufficient to state a priori that homoplasy may account for observed similarities. Analysis of heterochronic
change in hominoid development is now used to describe differences in growth processes that suggest homoplasy a priori
(Dainton and Macho, 1999; Alba et al., 2001). However, in
my view, any explanation that proposes a process to uncover
homoplasy is more complicated and more difficult to test
than one that uses a pattern to identify homoplasy, and it is especially precarious when the processes in question are very
poorly understood. Analysis of growth processes may well uncover different growth trajectories that lead to similar anatomical outcomes. However, without a phylogenetic analysis, it is
not possible to know whether specific growth trajectories are
primitive or derived, making their phylogenetic significance
unclear. For example, the demonstration of heterochronic differences in chimp and gorilla hand growth does not mean,
a priori, that ‘‘it is parsimonious to suggest that knuckle walking has evolved in parallel in the two lineages’’ (Dainton and
Macho, 1999). By this same logic, we would be forced to conclude that bipedalism in australopiths is homoplastic with respect to modern human bipedalism. It is clear that, given
evidence of developmental differences between australopiths
and modern humans and from evidence of significant differences in morphology, limb proportions, etc., there are substantial differences in the developmental biology of skeletal
characters related to bipedalism among hominins. This does
not mean that these must have developed independently in
each group. Heterochronic and associated kinematic differences among African apes in their forelimbs, like the differences between australopith and modern human bipedalism,
probably evolved after divergence from a common ancestor
with the basic pattern of positional behavior already established (Richmond et al., 2001). Among African apes and humans, differences either represent autapomorphies within
each clade or they indicate that one of the growth patterns is
primitive for the clade. A phylogenetic analysis is needed to
address this issue.
The epistemological basis for the argument in this contribution boils down to this: in proceeding toward an awareness of
a hypothesis of phylogenetic relationships, pattern recognition
is simpler, requires fewer assumptions, and is less subjective
and more easily testable than process-driven explanations of
phylogeny. This is, however, a difficult approach to defend.
Character analysis clearly has a large subjective component.
Individual experience and sense of what is important essentially determines which characters are included in any analysis, even if these decisions are also informed from previous
work. This is an unavoidable limitation of phylogenetic analysis, and, I would argue, of science in general. We as scientists
choose to examine phenomena that we consider likely to reveal something significant about the natural world. However,
this difficulty exists in both the cladistic (or pattern recognition) and adaptationist (or processual) realms. Until we
know everything there is to know about the origin and transformation (in developmental and evolutionary time scales) of
a character, we will not be able to avoid a certain degree of
subjectivity in choosing characters. How these characters are
used is the difference between these approaches. A cladistic
approach generates patterns and clusters of taxa based on character-state similarity and difference. A processual approach
generates a phylogeny based on presumed mechanisms of
development, selection, adaptation, exaptation, stochastic evolution, or other processes. This extra step is what makes processual approaches more complex and less easily tested because
the hypothesized processes that account for a phylogeny are
difficult to test.
Earlier I equated ‘‘adaptationist’’ with ‘‘processual’’ approaches, which contrasts with the dichotomy of Lovejoy
et al. (2002), who equated ‘‘adaptationism’’ with cladistics.
These authors held that the use of ‘‘investigator-defined traits’’
(characters included in a character-state analysis) is ‘‘truly
adaptationist’’ because of a presumption of a direct genetic
connection to them (Lovejoy et al., 2002). To me, all characters in all analyses are ‘‘user defined’’ (to use another one of
their phrases). As evolutionary biologists, we all have drawers,
file cabinets, and hard drives filled with character data, but I
have yet to learn of any data choosing themselves to be
included in an analysis. We select things to examine from
whatever perspective we have. In phylogenetic systematics,
character states are not, in fact, presumed to have a direct
genetic basis; they are assumed to represent (independent)
evolutionary events (homology). This assumption is stated
and testable (contra Lovejoy et al., 2002) via a parsimony
analysis followed by functional, developmental, or other processual analyses, and indeed, many of these assumptions are
falsified (i.e., homoplasies are revealed).
This is the principal strength of pattern (cladistic) approaches over processual approaches, be they functional, historical, or ‘‘evo-devo.’’ Lovejoy et al. (2002) qualified
cladistic approaches as ‘‘atomistic’’ and ‘‘adaptationist’’ given
their sense of the significance that cladists attach to traits. It
could equally well be said that ‘‘evo-devo’’ is the new adaptationism. Adaptive scenarios (freeing the hands to use tools; bipedalism and the savanna; crosspoterygians crawling onto
land) are processual arguments that cannot truly be tested,
and they assume basically that most or all morphological traits
have an adaptive significance (the ‘‘adaptationist program’’;
Gould and Lewontin, 1979). Processual arguments from developmental biology have the same logical structuredassuming
a specific genetic origin and developmental trajectory to all
features of a character complex in a taxon. The sophisticated
developmental processes described by Lovejoy et al. (2002;
and citations therein) represent a tremendous leap forward in
our understanding of characters, and they have an advantage
over earlier processual arguments in being more testable, at
least in certain taxa, under certain circumstances. However, I
maintain that they are not yet sufficiently developed to allow
researchers to confidently assess various hypotheses of character evolution a priori. In other words (those of Lauder, 1994:
165): ‘‘If, however, through a detailed examination of two
characters by studying their genetics, development, and patterns of connection to other structural features, we could
make a determination of probability of homology, then a phylogenetic analysis would be unnecessary.’’ Rather than pigeon-
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holing and rejecting cladistic approaches as ‘‘adaptationist’’ or
‘‘atomistic’’ vs. ‘‘morphogenetically informed,’’ I expect that
the new paradigm of phylogenetic systematics will emerge
from a realization that both approaches inform one another,
and that, as individual or systemic character morphogenesis
becomes more understandable, morphogenesis will inform
our choice of the characters that we include in phylogenetic
analysis.
In this paper I attempt to illustrate the process-informspattern-recognition-a-posteriori approach with some examples
from the hominoid fossil record, though at this stage processual arguments are limited to functional morphology. Building
on earlier seminal works on function and phylogeny (e.g.,
Cracraft, 1981), I think we are now ready not just to seek to
explain homoplasy through processual arguments, but to evaluate competing phylogenies of similar parsimony level based
on the likelihood of specific morphological transformation series, given what we know of character function and integration. Where functional and developmental data are available,
we can and should choose to prefer less-parsimonious hypotheses if the transformations series they imply make more sense
in light of the processes we already understand (Begun and
Kordos, 1997). The phylogeny I choose as the basis for identifying homoplasies in fossil and living hominoids is based on
the most comprehensive data base currently published. While
it is likely that details of this cladogram will change in the
future, with new discoveries and refinements of previous analyses, the homoplasies discussed in this chapter are comparatively robust in the sense that they emerge regardless of the
position of many of the taxa whose phylogenetic position is
currently under debate.
Materials and methods
The phylogeny used in this analysis is modified and updated from Begun (2002, 2007) and Begun et al. (1997)
(Fig. 1 and Table 1). It is important to note that the character
analysis on which this phylogeny is based is driven mainly by
the fossil evidence. It includes data on the best-known fossil
hominoid taxa, but it excludes many data known only from living hominoids (e.g., Shoshani et al., 1996; Gibbs et al., 2000).
It is equally important to note that this phylogeny, based
on fossil evidence, is consistent with molecular data bearing
on the question of extant hominoid relations (Stewart and
Disotell, 1998) despite the recent observation by some workers
that morphological data are unreliable guides to hominoid
phylogeny (Pilbeam, 1996, 1997; Collard and Wood, 2000,
2001; Pilbeam and Young, 2001). In particular, the relationships among hominins (African apes and humans) duplicate
those obtained from the majority of molecular analyses, and
they have for the most part done so since the first iteration
of this data base (Begun, 1992a, 1999). The relationships
among extant hominoids depicted in Fig. 1 are also consistent
with analyses that include soft- and hard-tissue data from extant hominoids (Shoshani et al., 1996; Gibbs et al., 2000), and
with estimates of divergence times based on paleontological
and molecular data (Page and Goodman, 2001; Young and
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Fig. 1. Phyletic relations among Miocene and living hominoids used in this
analysis. Modified from analyses in Begun (1992a,b, 1994, 2001), Begun
and Kordos (1997), and Begun et al. (1997).
Steiper, 2006). So, whether it is philosophical issues or issues
related to the usefulness of morphological data in analyses of
other groups of taxa that lead some to discard morphological
data, the fact remains that the conclusions of the analyses leading to the phylogeny produced in Fig. 1 should be considered
robust by any practical or conventional measure, whether that
is the consistency index (CI), the retention index (RI), or congruence with molecular or soft-tissue data.
Having said this, I consider certain aspects of this phylogeny more robust than others. More-robust hypotheses include
the relations among living hominoids, the distinction between
hominids (fossil and living) and hylobatids, and the distinction
between eohominoids and euhominoids (Table 1). Large numbers of characters from a diversity of anatomical regions need
to have evolved in parallel for these relations to be falsified
(Begun et al., 1997). The relationships among middle and
late Miocene hominoids are less robust but have little bearing
on the more theoretical issues discussed here. Thus, while the
phylogeny depicted in Fig. 1 is rightly considered controversial, or at least unresolved in its details, the clusters of homoplasies that are the focus of this essay persist whether or not,
for example, Dryopithecus is considered a member of the African or the Asian great ape clade.
In generating this phylogeny, character states were unordered to avoid a priori conclusions about the trajectory of morphological change. The debate regarding whether to order or
not to order characters hinges ultimately on the researcher’s
confidence, for a particular data set, in reconstructing how
characters transform (Begun and Kordos, 1997; Dayrat and
Tillier, 2000). There are no characters in this analysis for
which I feel confident to reconstruct the character-state transformation sequence independent of a phylogenetic hypothesis.
A good example is the premaxilla. It was noted long ago that
hominid premaxillae for the most part are elongated compared
to those of other mammals, and it was considered that premaxillary elongation proceeded along a continuum from short to
long. Given this apriority, a character-state analysis that imposes a sequence from short to long for the premaxilla makes
sense (Ward and Kimbel, 1983). However, premaxillary length
is now known not to have evolved in a linear fashion within
the Hominoidea. Some clearly primitive hominoids (e.g., Afropithecus) have comparatively long premaxillae, while others
have premaxillae that are quite short (Proconsul ). Similarly,
hominids vary in the relative length of their premaxillae
from very long (Pongo and Pan) to very short (Ouranopithecus, Dryopithecus, and Homo), and it is reasonably clear that
these character states are not homologous (Begun, 1994).
Similarly, it is assumed that premaxillary elongation and
reorientation are accompanied by, or coupled with (in a developmental sense), specific changes in the incisive-foramenfossa-canal complex (Conroy, 1994). In fact, premaxillary
elongation is known to be decoupled from this complex
(Begun and Gu¨leç, 1998). Sivapithecus and Ankarapithecus
both have elongated and relatively horizontally oriented premaxilla, as does Pongo, but Ankarapithecus lacks the extreme
restriction of incisive-canal caliber and the near occlusion of
the incisive fossa and incisive foramen found in Sivapithecus
and Pongo. It is natural to assume that a posteriorly elongated
premaxilla would tend to overlap the palatine process of the
maxilla, which would have the simple structural consequence
of reducing the anteroposterior dimension of all features that
intervene between the premaxilla and the rest of the palate.
A hypothesis of character-state transformation, such as growing from short to long or small to big, makes sense intuitively,
but it is incorrect in this specific instance. So, the character
states used in this analysis are unordered, and this is consistent
with the theoretical approach to homoplasy that I adopt. Experience suggests that it is unwise to order characters, at least in
the matrix of morphological characters used to reconstruct
hominoid phylogeny, no matter how clear-cut the transformation sequence may intuitively appear (Begun and Kordos,
1997; Kluge, 1999; Dayrat and Tillier, 2000). In the same
way, it strikes me as unwise to attempt to determine, a priori,
whether or not a shared character state is homoplastic or homologous. We simply do not know enough about the heritability and developmental biology of character states to determine
their phylogenetic information content a priori (Farris, 1983;
Lockwood and Fleagle, 1999). The justification for minimizing character-state ordering and homoplasy is the same: to
minimize the number of assumptions about the evolutionary
history of character states (Farris, 1983).
Results
Many homoplasies emerge from the analysis of character
states used to generate Fig. 1, only some of which can be
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Table 1
Systematics of hominoid taxa used in this analysis
Superfamily
Family
Hominoidea
Proconsulidae
Hylobatidae
Hominidae
Subfamily
Homininae
Ponginae
Tribe
Dryopithecini
Hominini
Gorillina
Hominina
Pongini
Ankarapithecina
Pongina
Ponginae incertae cedis1
Hominoidea incertae cedis2
Subtribe
Genus
Proconsul
Hylobates
Dryopithecus
Ouranopithecus
Gorilla
Ardipithecus
Praeanthropus
Paraustralopithecus
Australopithecus
Paranthropus
Homo
Pan
Ankarapithecus
Pongo
Sivapithecus
Lufengpithecus
Gigantopithecus
Afropithecus
Griphopithecus
Equatorius
Nacholapithecus
Kenyapithecus
1
Lufengpithecus is more clearly a pongine than is Gigantopithecus.
The relations among Afropithecus, Griphopithecus, Equatorius, Nacholapithecus, and Kenyapithecus are poorly understood. Each taxon appears to be successively more closely related to hominids than Proconsul, but each is too poorly known to clearly distinguish them phyletically.
2
considered here. I focus on clusters of homoplasies among
hominoids that have inspired discussion in the past in the
hope that their emergence once again in this analysis may
shed some light on the evolutionary biology of these characters. These characters fall into two major categories: features
of the limbs related to forms of suspensory and terrestrial positional behavior, and features of the jaws and teeth related to
the power of mastication.
Suspension
Suspensory positional behavior has evolved numerous
times independently among mammals ranging in size from
bats to the largest great apes (Pongo and Gorilla). Aside
from bats, which are probably posturally suspensory due to
the elaborate reorganization of their forelimbs into wings,
most suspensory mammals are larger than their most closely
related nonsuspensory arboreal relatives. This is probably related to the fact that, above a certain body mass and limb
length, the amount of torque necessary to maintain a stable
grasp above smaller branches becomes anatomically infeasible (Grand, 1972; Grand, 1978; Cartmill, 1985; Larson,
1998). In order to maintain the ability to move throughout
much of the canopy, many large-bodied arboreal mammals
have become suspensory. However, this is not an indication
that the evolution of suspensory postures is so inevitable in
large-bodied arboreal mammals that its anatomical correlates
should be considered a priori to be especially subject to homoplasy, as has been suggested or implied (Tuttle, 1975;
Benefit and McCrossin, 1995; McCrossin and Benefit,
1997; Pilbeam, 1997; Pilbeam and Young, 2001). As an example, bipedalism is universally accepted as a synapomorphy
of living humans and fossil hominids more closely related to
living humans than to Pan. Yet, bipedalism has probably
evolved many more times than suspensory positional behaviors, e.g., in numerous lineages of Dinosauria and Aves, not
to mention the occasional appearance of this form of positional behavior in pre-theropod vertebrates and in a number
of lizard lineages (Benton, 1990; Berman et al., 2000; Middleton and Gatesy, 2000). Despite the evidence of extensive
homoplasy in the evolution of bipedalism among terrestrial
vertebrates, few would suggest that features related to bipedalism are a priori targets of homoplasy. At the same time, details of the characteristics related to bipedalism in hominins
and theropods are vastly different and easily distinguished.
Among more closely related organisms, homoplastic similarities are more difficult to distinguish but often become apparent at greater levels of detailed examination, sometimes
beyond that used to define the characters for the phylogenetic
analysis. In sum, the frequency with which specific homoplasies occur is still no justification to assume prior to a phylogenetic analysis that those character states are likely to be
homoplastic in every case.
Figure 1 suggests that the capacity for frequent suspensory
positional behaviors evolved once in the Hominoidea (i.e.,
before the divergence of hylobatids and hominids), while moreterrestrial forms of positional behavior may have evolved several times in the pongines, hominines, Kenyapithecus, and/or
Griphopithecus. At the same time, Fig. 1 also suggests that,
while the capacity for suspensory positional behavior evolved
564
only once in the Hominoidea, suspensory locomotion became
elaborated or more specialized independently in Hylobates,
Pongo, and possibly Oreopithecus and Dryopithecus (Larson,
1998). Below-branch arboreality is probably a synapomorphy
of the Euhominoidea, while more-specialized suspensory adaptations emerged on several occasions as separately evolving,
relatively minor changes on an already established developmental program, leading to even more elongated forelimbs,
more strongly curved phalanges, etc. Furthermore, it is clear
that suspension has evolved independently in other clades of
anthropoids, most notably atelines and pliopithecoids, and to
a lesser extent in large colobines (Zapfe, 1960; Tuttle, 1975;
Fleagle, 1976, 1983; Scho¨n Ybarra and Scho¨n, 1987; Begun,
1993; Ford, 1994; Rose, 1996; Lockwood, 1999). These trends
illustrate the hierarchical nature of homoplasy and the fact that
it is not specific to, or an emergent feature of, one anatomical
region as opposed to another. Suspensory positional behavior
is homoplastic within the Anthropoidea, homologous for the
Hominoidea, and, in a modified form, homoplastic among euhominoids and within the Hominidae.
All euhominoids, including humans, share characters indicative of a suspensory adaptation in their ancestry. Classic features documented in living hominoids include features of the
vertebral skeleton, thorax, shoulder, arm, elbow, wrist, and
hand (Schultz, 1930, 1936, 1950, 1963a; Washburn, 1971; Tuttle,
1975; Stern and Larson, 2001). Even modern humans, who do
not engage routinely in suspensory positional behavior as a
species, retain many characters associated with suspension in
other hominoids. Thus, characters thought to be related to suspension are difficult to evaluate in the fossil record because
they can be found in nonsuspensory hominoids. Nevertheless,
most authorities recognize that curved phalanges with well-developed secondary shaft features, elongated forelimbs, certain
aspects of elbow joint morphology, vertebral morphology, and
some characters of the carpal bones found in many late Miocene hominids are indicative of some form of suspensory positional behavior (Preuschoft, 1973; Susman, 1979; Morbeck,
1983; Rose, 1983, 1988, 1992, 1994, 1997; Begun, 1992b,
1993; Larson, 1996; Moya`-Sola` and Ko¨hler, 1996; Jungers
et al., 1997; Ko¨hler and Moya`-Sola`, 1997; Ko¨hler et al.,
2001; Richmond and Whalen, 2001).
Gibbons, orangutans, and to a lesser extent smaller African
apes are suspensory, and they share a number of features not
seen in all fossil members of these different clades. These include especially well-marked phalangeal secondary shaft characters; large, broad metacarpal heads; elongated metacarpal
shafts; and proximally oriented proximal phalangeal articular
surfaces (Tuttle, 1969; Susman, 1979). Furthermore, living
great apes share features (such as transversely broad phalanges
and large, blocky carpals with extensive articular surfaces) that
are generally not present in Miocene hominids. Additionally,
some Miocene hominids lack other features that are characteristic of suspensory hominoids or hominids [i.e., a typically retroflexed and generally gracile humeral shaft (Sivapithecus)
and torsion of the proximal humeral shaft (Sivapithecus, debated in Dryopithecus); Pilbeam et al., 1990; Begun, 1992b,
2007; Larson, 1996; Rose, 1997; Richmond and Whalen,
2001; Madar, et al., 2002]. Some have interpreted the absence
in fossil hominoids of all characters potentially related to suspensory positional behavior in living hominoids to indicate
that no Miocene hominoid is closely related to any living hominoid (Benefit and McCrossin, 1995; Pilbeam, 1996, 1997;
McCrossin and Benefit, 1997; Pilbeam and Young, 2001). In
my view, this is a good example of throwing the baby out
with the bathwater. Once again, homoplasy is a hierarchical
phenomenon. Homoplasies occurring at one level in a phylogeny do not inform the interpretation of similarities occurring at
another level. Additionally, it is unjustified to assume that we
understand the relationships among characters sufficiently to
be sure that they are functionally coupled to the extent that
the absence of one suggests the absence of an entire complex
of characters.
One possible explanation for the absence of some characters related to suspensory positional behavior in fossil hominoids that otherwise appear to have been suspensory and to
be closely related to living hominids (e.g., Sivapithecus and
Pongo, Dryopithecus and African apes) may be revealed by
differences in lineage evolutionary life span. Hominoids that
show the greatest degree of suspensory adaptation (Oreopithecus, Hylobates, and Pongo) are all terminal taxa with long periods of phyletic separation from less specialized relatives.
The relationships of Oreopithecus are unclear (Begun et al.,
1997; Harrison and Rook, 1997), but its highly specialized
anatomy, insular ecology, and recent age all suggest some significant amount of time since its separation from its closest relative. Of all fossil hominids, Oreopithecus has the most
strongly developed adaptations for suspensory positional behavior (Straus, 1940, 1957; von Koenigswald, 1955; Langdon,
1986; Sarmiento, 1987, 1988; Godinot and Beard, 1991;
Harrison and Rook, 1997; Ko¨hler and Moya`-Sola`, 1997;
Moya`-Sola` and Ko¨hler, 1997; Rose, 1997; C.V. Ward, 1997;
Moya`-Sola` et al., 1999; Rook et al., 1999; Wunderlich et al.,
1999; Sarmiento and Marcus, 2000). This may in part be
explained by the fact that Oreopithecus is the best-preserved
Miocene fossil hominid. As it has become better known, Dryopithecus, for example, is also showing greater evidence of
specialized suspensory positional behavior than previously believed (Begun, 1992b, 1993; Moya`-Sola` and Ko¨hler, 1996).
Nevertheless, the degree to which Oreopithecus is specialized
for suspension is striking. Among living hominoids, hylobatids
also have a very lengthy period of separation from all other
known hominoids and may well have diverged more than 18
million years ago from their common ancestor with hominids,
based on recent molecular data (Page and Goodman, 2001).
The pongines (including the Pongo lineage) probably diverged
from the hominines over 13 million years ago, and possibly as
early as 18 million years ago, based on molecular and paleontological data (Kappelman et al., 1991; Page and Goodman,
2001; Young and Steiper, 2006), though it is not known how
much time has elapsed since the lineage leading to Pongo separated from its closest known relative (Sivapithecus). The oldest specimens that preserve synapomorphies of the pongine
clade are attributable to Sivapithecus sivalensis at about 9.5 Ma
and Ankarapithecus meteai at about 10 Ma (Kelley and
Pilbeam, 1986; Kappelman et al., 1991, 1996; Begun and Gu¨leç, 1998; Andrews and Alpagut, 2001; Kelley, 2002), but in
the case of Sivapithecus, specializations of the forelimbs (see
below) may indicate that this taxon was already separated
from the Pongo lineage by this time. Older specimens attributed to Sivapithecus from the Chinji Formation in the Siwaliks
(w12.5 Ma) lack unambiguous synapomorphies of the pongine clade (Begun and Gu¨leç, 1998). The Pongo clade is
thus likely to be at least 8e10 million years old.
I suggest that the independent development of extreme specializations of suspensory positional behaviors in Oreopithecus, Hylobates, and Pongo results from the combined effects
of time since divergence and emergence or canalization.
Once set in motion, the developmental program that led to anatomical features necessary for suspensory positional behavior
will change over time in two main ways. On the one hand,
some taxa will become increasingly specialized suspensory arborealists; on the other hand, developmental programs will
change and new adaptive directions will evolve, leading to
the emergence of new clades. Morphologies in the Pongo,
Oreopithecus, and Hylobates clades are extreme expressions
of the basic hominoid capacity for suspensory positional
behavior evolved independently over millions of years of isolation. The development of highly evolved forms of suspensory positional behavior in various lineages of hominoids is
a good example of the emergence of homoplastic similarities
arising in closely related taxa from similar anatomical plans
and/or developmental programs ( parallelism, after Simpson,
1945; see also Hall, 2007). It could be said that the scenario
described above violates the premise of this chapter, i.e.,
that the phylogenetic significance of characters cannot be assumed based on complex functional or developmental hypotheses. However, I am not saying that it is inappropriate to seek
processual explanations for observed similarities in different
clades; rather, these explanations can only support parsimony-based analyses and cannot aid in the initial character
analysis.
Terrestriality
Patterns of positional behavior with a strongly terrestrial
component are comparatively rare among primates, though
within hominoids they do characterize the most successful living clades (African apes and humans), and a similar case could
be made for cercopithecoids as well (e.g., Papio and Macaca).
Among fossil hominoids, the presence of terrestriality is less
clear. Equatorius and Kenyapithecus may have been more terrestrial than Proconsul (Le Gros Clark and Leakey, 1951;
Napier and Davis, 1959; Morbeck, 1975; Rose, 1983, 1988,
1992, 1994, 1996, 1997; Beard et al., 1986; Senut, 1986,
1989; Ward, 1991; Ward et al., 1991; Begun et al., 1994;
Rose et al., 1996; McCrossin, 1997; McCrossin and Benefit,
1997; S. Ward, 1997). Sivapithecus parvada, and possibly Sivapithecus sivalensis, may have been partly terrestrial as well
(Pilbeam et al., 1990; Rose, 1994; Madar, et al., 2002). Based
on size, Gigantopithecus is presumed by many to have been at
least partly terrestrial. Beyond that, there is no evidence for
565
terrestriality in any other Miocene hominoid, and there is
much evidence to suggest none was terrestrial to any significant degree (see above). However, unambiguous terrestriality
developed in the latest Miocene, as inferred indirectly from
the relationships among the living African apes and humans
and their approximate times of divergence, and directly from
the morphology of late Miocene/early Pliocene and extant
hominines (Lovejoy, 1974; Latimer et al., 1987; Latimer and
Lovejoy, 1989, 1990a,b; White et al., 1994; Leakey et al.,
1995; Begun et al., 1997; Asfaw et al., 1999; Richmond and
Strait, 2000; Haile-Selassie, 2001; Richmond et al., 2001;
Senut et al., 2001; Brunet et al., 2002; Pickford et al., 2002).
African apes and humans are strongly terrestrial even though
most, including probably early humans, retain well-developed
arboreal capabilities (Schultz, 1963b; Tuttle, 1967, 1969; Stern
and Susman, 1984; Susman et al., 1984; Gebo, 1992, 1996).
Evidence for terrestriality in Equatorius from Maboko and
Kenyapithecus from Fort Ternan comes mainly from aspects of
the distal humerus and proximal femur, including the morphology of the greater trochanter, the olecranon fossa, and
the medial epicondyle (Senut, 1989; Rose et al., 1996;
McCrossin, 1997; McCrossin and Benefit, 1997). Other features are consistent with terrestriality but also with more generalized pronograde (above-branch) arboreality (Rose, 1988,
1994, 1997; Begun, 1992b; Begun et al., 1994; Rose et al.,
1996; Ward and Brown, 1996; Nakatsukasa et al., 1998,
2000a,b; Ward et al., 1999).
Much of the postcranial anatomy of Sivapithecus suggests
arboreality as the principal component of the positional repertoire, with a mixture of above- and below-branch quadrupedal
and climbing signals (Rose, 1983, 1984, 1986, 1988, 1989,
1994, 1997; Langdon, 1986; Spoor et al., 1991; Larson,
1996; S. Ward, 1997; Madar, et al., 2002). However, the robust
morphology of the humeral deltopectoral region and the retroflexion of the humeral shaft are reminiscent of the humeri of
large baboons and patas monkeys, as well as those of other terrestrial mammals. In combination with the evidence of other
aspects of the postcranial skeleton, the proximal humeral morphology of Sivapithecus suggests an autapomorphic form of
positional behavior that may have included either some degree
of terrestriality or a unique form of arboreal positional behavior (Rose, 1994, 1997; Moya`-Sola` and Ko¨hler, 1996b; Begun
et al., 1997; C.V. Ward, 1997; S. Ward, 1997; Richmond and
Whalen, 2001; Begun, 2007).
In contrast to these ambiguous indications, terrestriality appears to have been an important component of the positional
behavior of the common ancestor of African apes and humans.
African apes and humans spend much of their time on the
ground, and as a clade, it is more parsimonious to suggest
that terrestriality is shared among the members of this group
than to posit that it evolved independently (Schultz, 1963b;
Begun, 1992b, 1994, 2004; Gebo, 1992, 1996; Richmond
and Strait, 2000; Richmond et al., 2001). Terrestriality in hominoids is thus much less frequent and probably less an effect of
emergent properties of existing hominoid anatomical ground
plans or developmental programs and more the result of convergence, i.e., the occurrence of similarity more related to
566
specific ecological conditions than to structural emergence
associated with phylogenetic closeness. There are few, if any,
anatomical similarities in the morphology related to terrestriality in Equatorius and Australopithecus or Homo, suggesting
that these are ecological convergences and not homologous
as implied elsewhere (McCrossin, 1997; McCrossin et al.,
1998).
Regardless of the cause, the distribution of characters
potentially related to terrestriality once again illustrates the
hierarchical nature of homoplasy. Terrestrial adaptations in
Equatorius, Kenyapithecus, and Sivapithecus, if they exist,
and in the African apes and humans are both homoplastic
and homologous. Terrestriality is almost certainly homologous
in African apes and humans, given our present understanding
of the relationships among these taxa and the functional morphology of their positional behavior (knuckle-walking and bipedalism) (Gebo, 1992, 1996; Begun et al., 1997; Richmond
and Strait, 2000; Richmond et al., 2001; Begun, 2004). Terrestriality, while less clearly demonstrable in other Miocene hominoids, may have originated in the middle Miocene taxa
Equatorius and Kenyapithecus from a pronograde arboreal ancestor (Proconsul ), with which it otherwise shares most postcranial attributes (Begun, 1992b; Begun et al., 1994; Rose
et al., 1996; Rose, 1997; S. Ward, 1997; Nakatsukasa et al.,
1998, 2000a,b), and in Sivapithecus from a more antipronograde ancestor, possibly in response to similar ecological conditions (Pilbeam et al., 1990; Rose, 1997; S. Ward, 1997;
Richmond and Whalen, 2001). The distal humeri and proximal
femora of Sivapithecus and Equatorius/Kenyapithecus differ
considerably. These bones are those upon which a reconstruction of terrestriality is based in each taxon but for different reasons. The morphological differences in each taxon also suggest
that a similar end may have been accomplished by different
means in each.
Mastication
The jaws and teeth of hominids have been the focus of an
enormous amount of attention with regard to the issue of homoplasy and phylogenetic reconstruction, and it is not my intention to review this vast literature here. I focus instead on
instances in hominoid evolution revealed by the pattern of relationships depicted in Fig. 1, in which homoplasy in the masticatory apparatus occurs, and how a consideration of the
structural morphology of hominoid jaws and teeth provides
a plausible explanation for these events.
Masticatory robusticity appears to have evolved independently at least four, and as many as six, times during the course
of hominoid evolution covered by the events depicted in Fig. 1.
Early (Afropithecus) and middle (Griphopithecus, Equatorius,
Kenyapithecus, Nacholapithecus) Miocene hominoids all
show indications of masticatory robusticity (thickly enameled
molars, large or robust canines and premolars, transversely thick
mandibles, possibly procumbent incisors, and strongly developed attachment sites of the masticatory muscles), but in their
details, the anatomy of mastication differs among them (Leakey
and Walker, 1997; McCrossin and Benefit, 1997; Gu¨leç and
Begun, 2003; Ward et al., 1999; Kunimatsu et al., 2004). The
morphologies of the teeth of middle Miocene hominoids and
Afropithecus differ considerably, the mandibles are easily distinguished from one another, and, although very poorly known, the
anterior palate of most middle Miocene hominoids was probably not as prognathic as in Afropithecus, with the possible exception of Nacholapithecus (Pickford, 1985, 1986; Leakey et al.,
1988, 1991; Brown, 1989, 1997; McCrossin and Benefit,
1993, 1997; Leakey and Walker, 1997; Ward et al., 1999; Kunimatsu et al., 2004; and personal observations). It is not clear if
masticatory robusticity evolved completely independently in
middle Miocene hominoids and Afropithecus or if Afropithecus
represents the primitive morphotype from which the middle
Miocene hominoid pattern evolved (Heizmann and Begun,
2001; Begun, 2002; Kelley, 2002). Middle Miocene hominoids
may have preserved certain aspects of masticatory robusticity
(transversely thick mandibles and thickly enameled molars)
and evolved new features (reduced dentine penetrance, more bunodont postcanine cusps, longer maxillary central incisors) as
refinements of the same basic adaptation, in a manner similar
to that suggested earlier with regard to suspensory positional
behaviors.
Masticatory robusticity became even more pronounced in the
pongine clade, as exemplified by the massive jaws and teeth of
Ankarapithecus and Sivapithecus (Ward and Kimbel, 1983;
Ward and Pilbeam, 1983; Kelley and Pilbeam, 1986; Ward
and Brown, 1986; Kelley, 1988; Brown, 1989, 1997; S. Ward,
1997; Begun and Gu¨leç, 1998; Andrews and Alpagut, 2001).
Parsimony suggests that the bulk of these adaptations are homologies and symplesiomorphies, again with refinements such as
substantial malar enlargement and zygomatic flare, premaxillary robusticity, deflation of the maxillary alveolar process, I1
enlargement, and increases in the robusticity of the palatine perpendicular plate and pyramidal process. Some of these characters, and others shared among hominids, including increases
in the height of the zygomatic process, reduction or elimination
of molar cingula, similarity in size of the first and second molars,
and lengthening of the premaxilla, occur in taxa that do not share
masticatory robusticity with Sivapithecus and Ankarapithecus
(such as Dryopithecus, Pan, and Gorilla). It may be that these
characters appeared initially as parts of a suite of features related
to the trend to increase overall masticatory robusticity from Proconsul to Sivapithecus, and they are retained and co-opted for
other functions, while others (such as thick enamel) were lost
(see below). The phyletic relationships of Gigantopithecus are
also unclear, but most authorities believe this taxon to be part
of the pongine clade (e.g., Kelley, 2002). If this is the case,
then we see in Gigantopithecus a further development of the
trend established in the early Miocene (Afropithecus) to increase
masticatory robusticity. The cranium of Gigantopithecus is
completely unknown, a curiosity given the immense size of
this taxon. A taphonomic process may be the cause, but it may
also be that cranial fossils of Gigantopithecus have gone unrecognized due to the highly unusual morphology of this huge primate. At any rate, the teeth and mandibles of Gigantopithecus
indicate that this genus was unrivaled among primates in masticatory robusticity, outstripping even the largest and most robust
fossil hominins. The extreme morphology of Gigantopithecus is
best seen in the same light as that of the postcranium of Pongo,
Hylobates, and Oreopithecus (see above).
Aside from Pliocene hominines, which are beyond the
scope of this analysis, the Miocene hominine Ouranopithecus
also shows aspects of the suite of features related to masticatory robusticity seen in pongines. In fact, it was the co-occurrence of these characters in Ouranopithecus and the pongines
Sivapithecus and Ankarapithecus that originally led Andrews
and Cronin (1982), Andrews and Tekkaya (1980), and Martin
and Andrews (1984) to place all of these taxa in Sivapithecus.
Figure 1 suggests that most if not all of the features that appear
to link Ouranopithecus phyletically with Australopithecus and
other Pliocene Hominini (de Bonis and Koufos, 1993a, 1997;
Koufos, 1993, 1995) are homoplasies related to the independent development of heavy mastication (Begun and Kordos,
1997). Ouranopithecus shares with some but not all Hominini
such characters as large, flat molars with low, broad cusps;
rounded crests and shallow occlusal basins; hyperthick
enamel; comparatively small male canines, relatively homomorphic premolars; transversely thick mandibular corpora;
and a low origin of the root of the zygomatic process of the
maxilla. Many of these characters also occur in fossil pongines, and a few in Griphopithecus, Equatorius, Nacholapithecus, and Kenyapithecus. Ouranopithecus lacks all of the
derived characters that define the pongine clade, and it shares
characters with Dryopithecus and the African apes and humans
(de Bonis and Melentis, 1984, 1987; de Bonis et al., 1991;
Koufos, 1993, 1995; de Bonis and Koufos, 1993a,b, 1997;
Begun and Kordos, 1997). Many taxa in this clade lack most
or all of the features related to masticatory robusticity in
Ouranopithecus, including Dryopithecus, African apes, Ardipithecus, and Homo sapiens. Thus, it is not at all clear whether
these characters are symplesiomorphies of the hominid or
hominine clade or autapomorphies of Ouranopithecus. The
fact that Ouranopithecus tends to share characters with robust
australopiths (hyperthick enamel, low zygoma, relative large
postcanine dentition) while lacking more basic hominin
characters [very small male canines; elongated premaxilla;
elongated, narrow incisive canals; spatulate maxillary lateral
incisors; a projecting supraorbital torus; and a pronounced
supratoral sulcus (the last three also shared with Pan)] suggests that this suite of features evolved in parallel in Ouranopithecus (Begun and Kordos, 1997). The fact that functionally
similar (but structurally distinct) anatomical complexes are
comparatively widespread in hominoids means that the homoplasies revealed by the analysis represented in Fig. 1 are explicable as a common developmental option within the clade.
Conclusions
Hominoid evolutionary history provides good examples of
homoplasy arising from both structural emergence (parallelism) and functional convergence. Specialized suspensory positional behaviors and robust jaws and teeth are emergent in
hominoid biology, and their repeated occurrence in the hominoid fossil record should come as no surprise. Terrestriality
567
persisted as an option, but it is less common and probably requires a stronger external ecological impetus. It is possible to
more completely understand the origin and nature of morphological similarities within a superfamily (the Hominoidea)
given a careful consideration to anatomical detail and a relatively well-corroborated phylogeny. However, the mere fact
that certain adaptations occur frequently and others less so
does not allow for the prediction of homoplasy in the absence
of a phylogeny. It simply provides the anatomical and functional framework to explain the homoplasies that emerge
from an analysis of phylogeny. Many similarities among fossil
and living hominoids are in fact homologies (synapomorphies
and/or symplesiomorphies) at one level of the taxonomic hierarchy and homoplasies at another. Phylogenetic hypotheses
that tend to concentrate homoplasies in a few morphofunctional complexes, as opposed to those that randomly distribute
homoplasies, are reinforced by plausible functional and/or
developmental explanations, provided that these models of
character transformation are generated independently of the
phylogeny (e.g., that characters are not weighted with apriority
about the likelihood of homoplasy or the differential phyletic
importance of specific characters).
We have not yet achieved the level of understanding in developmental biology that we need in order to determine the
phylogenetic information content of characters from their developmental origins, although we are making progress (Lovejoy
et al., 1999, 2002; O’Higgins and Cohn, 2000; Line, 2001).
Until we can unambiguously reconstruct the developmental
and evolutionary biology of specific characters, it is unwise
to constrain ideas of potential patterns of evolutionary change
with suppositions of specific patterns of character-state transformations (Begun and Kordos, 1997; Dayrat and Tillier,
2000). We do not yet have the luxury to exclude certain characters from a phylogenetic analysis that we believe a priori
would provide misleading information on evolutionary history
(Hennig, 1966; Begun and Kordos, 1997; Kluge, 1999, 2001;
Dayrat and Tillier, 2000). Nor do we have the knowledge to
restrict phylogeny reconstruction to a small number of characters we believe to be particularly informative (Moya`-Sola` and
Ko¨hler, 1995; Lovejoy et al., 1999, 2002; Dayrat and Tillier,
2000; Alba et al., 2001; Grandcolas et al., 2001). Hennig’s assumption of homology remains the most robust, elegant,
straightforward method of identifying homoplasy (Hennig,
1966; Farris et al., 2001). Homoplasies are falsified homologies, uncovered by parsimony analysis (Kluge, 1999, 2001).
The irony of parsimony as a point of logic that contributes
to the choice of a preferred phylogenetic hypothesis is that it
reveals the degree to which the results of the process of evolution are unparsimonious.
We need to embrace homoplasies and stop thinking of them
as noise. Homoplasies are in fact among the most interesting
data that emerge from a phylogenetic analysis. As noted by
Kluge (1999), the relationship between homology and homoplasy is one of ‘‘reciprocal clarification.’’ In my view, and in
modest contrast to the views of Farris (1983) and Kluge
(1999), the reciprocal clarification that homologies and homoplasies provide each other (see also Hennig, 1966) extends
568
also to phylogeny itself. There can be no unified theory of homoplasy because homoplasies are independent, hierarchical
phenomena that are not dependent on a single theorem or
law of natural causality, as are homologies (the necessary
consequences of descent with modification). However, in combination with functional and structural analysis, developmental, epigenetic, and genetic approaches can all explain the
presence of specific homoplasies revealed by phylogenetic
parsimony.
Examples in hominid evolutionary history include possible
developmental explanations for the transformation of the periorbital region and premaxilla. Phylogenetic parsimony distinguishes Pan from Pongo cladistically, despite the shared presence
of a long premaxilla (Begun, 1994). However, maxillarypremaxillary fusion is known to differ in timing between Pan
and Pongo, a growth difference that may explain the sharing
of a superficially similar morphology via differing developmental mechanisms (Begun, 1994; Begun and Gu¨leç, 1998).
Similarly, Dryopithecus and Ouranopithecus on the one hand
and African apes on the other, though linked phyletically, are
distinguished by the development of their periorbital structures, including the supraorbital torus and the frontal sinus
(the latter is not known for Ouranopithecus). The difference
between the fossil and living members of this clade can also
be explained by developmental differences: in this case, the
living, putatively descendant taxa exhibiting evidence of
morphology developed beyond that seen in the fossils. The
Hominini have more strongly developed supraorbital tori and
more expansive frontal sinuses than do the Dryopithecini,
with no evidence of a change in the manner in which these
structures develop other than a change in rate or timing
(Begun, 1994). The expansion of the homologous periorbital
structures of the Hominini compared to those of the Dryopithecini could have developed according to a number of different
heterochronic processes (acceleration, hypermorphosis). We
cannot know which process caused these changes to develop,
which is why we cannot assume a priori that such a process
occurred, but we can deduce a posteriori that pattern differences correspond to known processes that suitably account
for the observed differences.
The strength of various explanations of morphological
change can justify a preference for one of several competing,
equally parsimonious phylogenies, or even a revision of a phylogenetic hypothesis to produce an alternative that is less parsimonious (in the strict sense) but makes more functional or
developmental sense. In hominoid evolutionary biology this is
more than a trivial consideration given (1) the level of homoplasy in the morphological data used to generate hypotheses
such as those depicted in Fig. 1 and (2) the number of competing
hypotheses within a few steps of each other (Begun et al., 1997).
The sharp, unforgiving edge of Occam’s razor is unappealing as
the ultimate explanatory tool to many paleobiologists, especially in light of the knowledge that the results of the evolutionary process rarely follow the intuitively simplest path (no
comprehensive phylogenetic analysis to my knowledge has produced a character analysis with a consistency index of 1, and
0.5e0.6 is considered pretty robust). Post hoc explanations
based on observed (not presumed) patterns of character-state
Fig. 2. The logical relation between the analysis of character states and the interpretation of the significance of character states. The analytical phase is informed
only by a careful character analysis in which character states are defined as precisely as possible with reference to morphological detail and developmental or
genetic data when they are available (rare today, but growing). This phase should not be informed by suppositions of character-state transformation sequences
based on possible genetic or developmental models, or by perceptions based on expectations of the pattern of morphological transformation from comparative
extant or fossil data. While both phases of analysis are iterative and subject to modification based on diverse data sources, the interpretive phase is more iterative
and may be informed more strongly by models from functional, epigenetic, developmental, or genetic lines of evidence. Judged by the criterion of phylogenetic
parsimony, character states are either homologous or homoplastic. Homologies are either synapomorphies or symplesiomorphies, depending on their distribution in
time, and homoplasies are either parallelisms or convergences, based essentially on the same criterion. Either way, all of these sources of similarity inform
interpretations of the paleobiology of taxa under analysis.
transformation and reasonably well-understood developmental
and functional principles can and should inform decisions
about the choice of a preferred phylogeny, but they should not be
involved in the initial character analysis and resulting matrix
of phylogenetic alternatives (Hennig, 1966; Begun and Kordos,
1997; Dayrat and Tillier, 2000; Grandcolas et al., 2001). I
believe that, in evolutionary biology, this phylogenetic-parsimony-first-then-structural/functional-morphology-second protocol should be the ultimate goal of the integration of
phylogenetic and functional (or structural) data (Fig. 2).
Acknowledgments
I am grateful to Charlie Lockwood and John Fleagle for inviting me to participate in the symposium that inspired this
volume. I learned much from the experience, and from the
comments of Bill Kimbel and two anonymous reviewers,
which led to substantial improvements in this paper. Research
on Miocene hominoid paleobiology described here was funded
by NSERC, The Wenner Gren Foundation, and the Alexander
von Humboldt Stiftung.
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How to identify (as opposed to define) a homoplasy: Examples n *

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