An Evolutionary Journey to the Modern Brain of Homo Sapiens

An Evolutionary Journey to the Modern Brain of Homo Sapiens
By: Bethany McMillen
Bio 425 Evolution
December 2, 2014
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Table of Contents
Introduction
3
Early Mammals
4
Early Primates
8
Panin – Homin Divergence (MRCA)
Hominid Brain Evolution
Early Hominids
Australopithecus
Genus Homo
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13
13
14
19
Early Homo Species
19
Later Homo Species
22
Homo Sapiens
25
Cultural Selection on Brain evolution
26
Brain evolution now
30
Conclusion
31
Resources
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Introduction
Over millions of years the brain of H. sapiens has evolved to be the cognitive
powerhouse it is today. The brain increased in size multiple times coming from a brain weighing
less than five grams to a size of about 1400 cm3 (Kaas, 2008; Holloway et al., 2009) The first
mammals had a very small brain that was less than 5 grams (Kaas, 2008; Northcutt & Kaas,
1995; Striedter, 2005). The common ancestor of chimpanzees and humans had a brain 400 cm3
(Holloway et al., 2009). In the hominid lineage of Australopithecus and Homo the brain
increased from about 450 cm3 to 600 cm to 900 and finally about 1200 cm3 (Bownds, 1999;
Holloway et al., 2009; Neill, 2007; Striedter, 2005).
The organization of ancestral brains also changed to produce the modern human brain (de
Sousa & Cuhna, 2012; Holloway et al., 2009; Striedter, 2005). The formation of petalias or
asymmetries, particularly the left-occipital, right-frontal petalia, allowed H. sapiens to evolve
larger brains without any loss of cognitive function (Balzeau et al., 2012; de Sousa & Cuhna,
2012). The shift of the lunate sulcus to a more posterior position was another organizational
change to the brain. The shift allowed the enlargement of the inferior parietal and posterior
temporal lobes, which altered the behavior of early hominids, making them scavengers whose
diet could support a larger brain (Holloway et al., 2004; 2009). Also the enlargement of area
BA10 in the orbital frontal lobe, Wernickes area and others allowed the brain to evolve and the
emergence of language (de Sousa & Cuhna, 2012).
There were many factors influencing the evolution of the human brain including the
environment, behavior and culture (Chene et al., 2014; Cunnane & Crawford, 2014; Leonard et
al., 2003; Maslin et al., 2014; Striedter, 2005). The transition from tree-dwelling primates to
bipedalism indirectly allowed the expansion of the neonatal brain (Chene et al., 2014). The diet
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of hominids was also a strong factor by affecting the amount of nutrients available to the brain
and its expansion (de Sousa & Cuhna, 2012; Leonard et al., 2003). The environment affected
brain evolution by creating new habitats with food sources that would impact brain evolution by
providing nutrients important to brain functioning: DHA, iron, selenium and iodine (Cunnane &
Crawford, 2014; Leonard et al., 2003; Maslin et al., 2014).
Early Mammals
The earliest mammals evolved from cynodonts (mammal-like reptiles) about 100 million
years ago (Striedter, 2005) Cynodontia was a premammalian lineage that first appeared in the
late Permian age (260 million years ago). They possessed a small olfactory bulb and a narrow
cerebrum that is illustrated in figure 4 (Nomura et al., 2014).
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Figure 1. “Primitive mammals and evolution of the neocortex.”
Upper images: “Phylogenic trees of primitive mammals with their endocasts.” The endocasts
show they had a small brain with a large olfactory bulb.
Lower image: A representation of a Morganucodon, an early mammalian ancestor. (The
Morganucodon is eating an insect0. (Nomura et al., 2014)
The brains of early mammals were small, probably less than five grams, between the size
of a cat and mouse brain (Kaas, 2008; Northcutt & Kaas, 1995; Striedter, 2005). However, they
were still slightly larger than the brains of reptiles of a similar size (Striedter, 2005). There was
little change in brain size from the appearance of the first mammals until the splitting of
marsupial and placental lineages about sixty million years ago. Both groups contain mammals
with small, less-developed brains, suggesting that their common ancestor possessed a similar
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brain (Northcutt & Kaas, 1995). An evolutionary tree showing brains of the different mammalian
lineages is pictured in fig. 2 (Frugal to the point, 2013).
Figure 2. The brain size of different mammals.
The different mammalian lineages all possess an animal with a small, less-developed
brains, suggesting that the brain of our common mammalian ancestor was similar. An example of
the brain of the mammalian common ancestor is shown in the upper left and the a possible
example of the primate common ancestor is shown in the upper left. (Frugal to the point, 2013)
The brains were relatively smooth without encephalization and with little neocortex
(Northcutt & Kaas, 1995; Striedter, 2005). This is believed to be the case because the brains of
modern mammals, like opossums, with brains of a similar size, have little encephalization (the
ratio between their brain size and body size is low (Striedter, 2005). Encephalization is a
quantitative term that relates brain size to body size and is known as the encephalization quotient
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(Holloway, 1996). It is a “ratio of actual brain size to expected brain size that takes body size
into account” – Tobias (1971). The equation used to calculate encephalization is given below.
(Holloway, 1996)
Early mammals had a brain that was the size expected for their body size, there was no
increase in complexity that could allow for higher cognitive functions. That would be seen in
later mammals (Strieder, 2005). The forebrain was dominated by a large olfactory bulb (Kaas,
2008; Striedter, 2005). There was little forebrain devoted to the neocortex. How the neocortex
emerged is unknown; there is not enough archeological evidence available to determine when it
first appeared; though, it is known that it formed after the divergence of reptiles and mammals
about 300 mya as seen in fig. 2 (Kaas, 2008, Nomura et al., 2014). Due to the lack of fossil
evidence, the brains of small living mammals are often studied because they are probably similar
to the brains of the extinct early mammals (Striedter, 2005). The multi-layered lamination that is
illustrated in fig. 3, of the neocortex in mammalian brains must have occurred early in the
mammalian lineage because it is seen in monotremes. Monotremes diverged from eutherians, or
placental mammals, between 160 and 210 mya (fig. 2) (Nomura et al., 2014).
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Figure 3. Neocortex lamination in small mammals.
The lamination of the neocortex is seen in small modern mammals: the domestic rat, grey
squirrel and tree shrew. (Northcutt & Kaas, 1995).
Early Primates
The early primates that appeared were about the size of a squirrel, similar to the modern
tree shrew pictured in fig. 4 (“Large tree shrew pictures”, 2011). Like the early mammals, they
were nocturnal (Striedter, 2005). They were arboreal creatures with several features that adapted
them for life among the treetops: bones and muscles for swinging in trees, big toes that were
more similar to an opposable thumb for grasping branches, and shoulder girdle and pelvic girdle
that were looser than terrestrial mammals (Bownds, 1999).
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Figure 4. Tree shrew.
A drawing of a modern tree shrew; an animal believed to be similar to early mammals (“Large
tree shrew pictures”, 2011).
The coordination needed for living in trees (e.g. vision, gravity sensing, balance, motor
skills) and the large social groups that primates live in required primates to develop a more
complex brain with more neuronal connections (Bownds, 1999). The brain size increased in
primates. There was an increase in the neocortex size of the more social primates to deal with the
complexities of social interaction (Striedter, 2005). Social groups require the ability to interact
with others: the sharing of food, grooming, caste systems, and other social behaviors. A large
neocortex aided in the processing of the information associated with social groups. Individuals
with a neocortex able to process the information were favored in natural selection (Croney, &
Newberry, 2007). In primates there is a trend between an increase in neocortex and social group
size (van Schaik, 2012). Social group size, frugivory, longevity, home-range size, and metabolic
state are all associated with increased neocortex in primates (Striedter, 2005).
Primate prefrontal cortexes are divided into more regions than the prefrontal cortex of
nonprimate mammals; the primate prefrontal cortex evolved to have three, not two, regions.
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Nonprimate mammals only have two major regions, the anterior cingulate cortex, and the orbital
prefrontal region (Striedter, 2005). The division of the prefrontal cortexes of a human, macaque
monkey and rat is shown in fig. 5 (Wallis, 2012). The rat prefrontal cortex is only divided into
two regions and the human and macaque monkey into three (Wallis, 2012). The lateral or
granular prefrontal region is the region unique to primates. This region controls “rational”
aspects of decision making; when this area is disabled, “primates become less able to retrieve
and manipulate information about objects and to construct alternative scenarios of how to
interact with them” – Striedter (2005). The prefrontal cortex appears enlarged after ape
divergence about nine million years ago (mya). Particularly, the left medial frontal lobe, which is
important to foresight and planning, is absent in apes (Bownds, 1999).
Figure 5. Prefrontal cortex comparison of a) humans, b) macaque monkey and c)
rat.
The prefrontal cortex of humans, macaque monkey and rat are compared. The prefrontal
cortex of humans and macaque monkey is divided into three regions, while the rat prefrontal
cortex is only divided into 2 (Wallis, 2012)
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The early primates would eventually diverge into multiple primate lineages. Orangutans
diverged 12 mya and the apes about 9 mya. The hominid lineage would emerge 5-6 mya when
they split from chimpanzees (Bownds, 1999). Fig. 6 shows the divergence of these species
(Venema, 2014).
Figure 6. Timeline of Primate divergence. (Venema, 2014)
Panin – Hominin Divergence (MRCA)
Hominid’s and chimpanzee’s lineages diverged about five to six mya (Bownds, 1999;
Striedter, 2005; Venema, 2014). The brain of the most recent common ancestor of chimpanzees
and H. sapiens was small, only about 400 cc with little to no encephalization (Holloway et al.,
2009). Modern chimpanzees are believed to still have brains that are similar to our most recent
common ancestor and are often studied to determine what it was like (Bownds, 1999; Kaas,
2006). The comparison of our brains with chimpanzees helps us determine which features are
ancestral, present in our common ancestor, or if they appeared after the divergence of
chimpanzees and H. sapiens (Bownds, 1999). One particular similarity to note is that modern
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chimpanzees’ brains have homologues for the Broca’s and Wernickes’s areas in humans, shown
in fig. 6, these will be elaborated later on (Bownds, 1999; "FIGURE 2. Comparative
Neuroanatomy," 2003).). These homologues were most likely also found in our most recent
common ancestor (MRCA) (Bownds, 1999). It is also useful to observe chimpanzees’
capabilities, such as tool use, to determine what behaviors are unique to us (de Sousa & Cuhna,
2012). Modern chimpanzees use tools and in experiments have shown the capability to plan
ahead in certain situations by keeping a tool they find in anticipation of using it later (Dufour, &
Sterck, 2008). The observation of tool use in both chimpanzees and early hominids suggests that
tool use was a behavior of our MRCA (de Sousa & Cuhna, 2012).
Figure 7. Lateral view of cerebral hemispheres of a human and chimpanzee brain.
This imape shows the homologues for the Broca’s and Wernickes of humans in chimpanzees.
("FIGURE 2. Comparative Neuroanatomy," 2003).
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Hominin Brain Evolution
Early Hominids
The earliest hominids appeared about 6-8 mya (de Sousa & Cuhna, 2012; HarcourtSmith, 2010; Striedter, 2005). They consist of the following species: Sahelanthropus tchadensis
or Toumai, Orrorin tugenensis, and Ardipithecus kadabba and Ardipithecus ramidus (HarcourtSmith, 2010). A list of the earliest hominid species is given in Table 1 along with the time of
their existence and location of their habitat. Sahelanthropus tchadensis was the first hominid
species, appearing shortly after the divergence of humans and chimps about 6 mya. The brain of
this species was small, 320 – 380 cm3, similar to the size of modern chimpanzee brains (Brunet
et al., 2002). The skull of Sahelanthropus is pictured in fig. 8.
Figure 8. “Cranium of Sahelanthropus tchadensis gen. et sp. nov. holotype (TM 266-01060-1).” (Brunet, et al., 2002).
Orrorin tugenensis and Ardipithecus species also had brains similar in size to those of modern
chimpanzees, with a range of 300 – 350 cm3 (Harcourt-Smith, 2010). While there wasn’t much
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change in brain size or structure there were other physical changes that would alter their behavior
and the behavior of future hominids (Harcourt- Smith, 2010). The early hominids were the first
species in our lineage to start the transition from tree dwelling creatures to bipedal land-dwellers.
Ardipithecus had an opposable toe and lived in woodland habitats suggesting that it was treedwelling. Orrorin tugenensis lost this opposable toe indicating a transition from trees to land
(Harcourt-Smith, 2010). Also, studies of the femur of Orrorin fossil specimens have shown that
they were capable of both climbing trees and walking bipedally (Pickford et al., 2002).
Table 1 Summary
information
Taxon Age range (Ma) Type Specimen Type Locality
for early hominin species
Sahelanthropus tchadensis 6.8–7.2 TM 266-01-060-1 Toros-Menalla,
(Ma =
Chad
millions of years)
Orrorin tugenensis 5.7–6.0 BAR 1000′00 Tugen Hills, Kenya
Ardipithecus kadabba 5.2–5.8 ALA-VP-2/10 Middle Awash, Ethiopia
Ardipithecus ramidus 4.4 ARA-VP-6/1 Aramis, Ethiopia
Table one. A List of Early Hominid species.
The table summarizes the different species of early hominids, the years they lived and the
location fossil specimens of them were found. (Harcourt-Smith, 2010)
Australopithecus
Australopithecus afarensis lived in the savannahs and woodlands of Africa about 3 to 5
mya. They were bipedal, but their locomotion wasn’t as efficient as Homo sapiens because of
differences in the pelvic and femur bones, and muscles (Hunt, 1994). They lost their opposable
toe, which allowed them to walk more like modern humans (Striedter, 2005). This shift to
bipedalism is important in our evolutionary history because it indirectly led to an increase in
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brain size (Chene et al., 2014). The australopithecine pelvic bones were wider, or more open,
than their ancestors, allowing them to walk bipedally (Hunt, 1994; Marchal, 1998). The changes
to the pelvic bone created a wider birth canal, allowing infants with a larger brain to be born
(Chene et al., 2014). A comparison of the pelvic structure of Au afarensis, H. erectus and H.
sapiens is shown in fig. nine ("Forgotten Females / Gona," 2014).
Figure 9. Comparison of pelvic structure in Hominds.
The pelvis of Au. afarensis is shown on the left, H. erectus in the middle and H. sapiens on the
right. The pelvic bones shifted to adjust to bipedal posture, and indirectly created a wider birth
canal ("Forgotten Females / Gona," 2014).
Au. afarensis were about the size of chimps and probably still behaved similarly (de
Sousa & Cuhna, 2012; Wilson, 1999). Au. africanus and Au. robustus were large than Au.
afarensis; about five feet tall. Au. africanus was the first toolmaker, while Au. afarensis had only
been a tool user, similar to chimps (de Sousa & Cuhna, 2012).
The brain of Australopithecus showed some increase in brain size, but nothing dramatic,
only an increase from 400cm3 to 450-500cm3 (Bownds, 1999; Leonard et al., 2003). Table 2 lists
the changes in brain size through hominid evolution (Neill, 2007).
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Table 1.
Brain size changes during hominid evolution
Species
Pan troglodytes
Australopithecus africanus
Australopithecus boisei
Homo habilis
Early Homo erectus
Late H. erectus
Homo heidelbergensis
Homo neanderthalensis
Homo sapiens
Date (Mya) Brain size (cm3)
400
3
450
2
500
2
600
1.8
800
0.5
950
0.5
1100
0.25 1200 - 1750
1200 - 1600
Table 2. Changes in brain size in Hominid evolution. (Neill, 2007)
The Australopithecus brain was only about 30 percent larger than that of non hominin apes, like
chimpanzees as seen in table 2 (Striedter, 2005). Also, a large encephalization quotient, a
distinctive feature of the brain of Homo sapiens, began in Au. afarensis and became more
evident in Au. africanus (de Sousa & Cuhna, 2012).
What is very notable between Australopithecine and H. sapiens brains is the changes to
structures in their brains from the early hominids. First, is the evidence of a left-occipital rightfrontal petalia (LORF), which is first seen faintly in Au. africanus. A petalia is protrusion of one
lobe relative to the other; it’s an asymmetry (de Sousa & Cuhna, 2012). A fully developed LORF
petalia of modern humans is shown in fig. 10. Asymmetries are important in the human brain
because as brain size increases, connection density decreases, which results in lost neuronal
connections, asymmetries prevent the loss of neuronal connections (Striedter, 2005).
Asymmetries prevent the loss of neuronal capacity when brain size increases by “allowing
parallel and separate processing in the hemispheres” (Balzeau et al., 2012). The LORF
asymmetry pattern is related to right-handedness in human populations; the left-occipital petalia
is seen in 78% of right-handed people; left-handed people will have a right-occipital pattern or a
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more symmetrical occipital lobe (Balzeau et al., 2012; de Sousa & Cuhna, 2012). The difference
between asymmetries in right and left handed people occurs because it causes changes in the
neuronal circuitry, which causes changes in the processing of the different hemispheres (Balzeau
et al., 2012).
Figure 10. Modern human brain.
The inferior view of an MRI scan of a modern human brain, which illustrates the leftoccipital petalia and right-frontal petalia characteristic of the human brain. Note: The petalias
were exaggerated to make them clearer. (“FIGURE 2 | Petalia and Yakovlevian torque,” 2003).
Another change that occurred in early hominid brains is the reduction and posterior shift
of the lunate sulcus in Australopithecus brains, particularly Au. africanus (de Sousa & Cuhna,
2012; Holloway et al., 2009). The lunate sulcus, or area BA17, is a well-defined sulcus that is the
anterior boundary of the primary visual striate cortex (Holloway et al., 2004). The position of the
lunate sulcus can be seen in fig. 11, a comparison of the modern chimpanzee and human brains
(Holloway, 1996). An increase in the inferior parietal and posterior temporal lobe occurred with
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this reduction of the lunate sulcus (Holloway et al., 2004). These two changes are believed to be
caused by the expansion of Australopithecus into a new ecological niche of scavenging and
foraging (Holloway et al., 2009). The early Australopithecus diets consisted of vegetation with
Au. africanus having teeth that appear best suited for vegetation (Wilson, 1999). However, over
time, the Australopithecus became hunter/ scavengers, stealing kills from other animals, or
hunting weak animals (Bownds, 1999; Strieder, 2005; Tobias, 1971). Au. africanus remains
found with other animal remains supports the idea of them beginning to be scavenging hunters
(Striedter, 2005).
Figure 11. Comparison of modern chimpanzee and human brains.
The comparison of the modern chimpanzee and human brain show a difference in the
position of the lunate sulcus. The lunate sulcus has shifter posteriorly since divergence from
chimpanzees (Holloway, 1996).
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Finally, area BA10 in the orbital frontal lobe shows signs of expansion in Au. africanus.
This area is involved in higher cognitive functions, such as planning future actions, and abstract
thinking. These changes in the brains of Australopithecus species were most likely
preadaptation’s or changes that were beginning to appear physically, but wouldn’t produce
modern functions until later Australopithecines and Homo species (de Sousa & Cuhna, 2012).
Genus Homo
Early Homo species
The early species in the Homo genus include H. rudolfensis, H. habilis and H. ergaster.
This was when the first major increase in brain size occurred; brains increased from 497 cm3 in
Australopithecines to 600 – 700 cm3 (Bownds, 1999). Brain asymmetries characteristic of
modern human brains are well defined for the first time in these species (Holloway et al., 2009).
These brain asymmetries include the frontal pole asymmetry, the LORF petalia, and left> right
asymmetry in the superior parietal region. These asymmetries remain unchanged in the transition
to Homo sapiens; which shows that early Homo species had structurally similar brains to modern
humans (de Sousa & Cuhna, 2012).
An important early Homo species that clearly displays the transition between what Phillip
Tobias calls the animal hominids, Australopithecines, and human-like hominids is H. habilis
(Tobias, 1987; Wilson, 1999). The frontal, parieto-temporal, and occipital lobe of H. habilis
show no significant differences to the brain of modern H. sapiens (Balzeau et al., 2012). The lack
of differences is important because these lobes contain the important features for language
processing (Tobias, 1987) The parietal lobe contains Brodmann’s areas 39 and 40, and
Wernicke’s area. An important feature in the frontal lobe is the Broca’s cap (Tobias, 1987). The
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Broca’s cap overlaps Broca’s language area (Ba45 and BA44) and is important in language
processing in modern humans (de Sousa & Cuhna, 2012). The appearance of Broca’s cap in the
early homo species was the “elaboration of a pre-existing system” and not the development of a
new system (Manger, 2005). The Broca’s cap appearing is very important in human evolution
because it is the first time that our ancestors could have been capable of rudimentary language
(de Sousa & Cuhna, 2012; Tobias, 1987). The Broca’s cap can be seen in fig. 12, an endocast of
a H. habilis fossil specimen (Holloway, 1996). Physically or structurally, most aspects of the
modern brain evolved in H. habilis and were passed on to the later Homo species unchanged;
later changes were mainly in size (de Sousa & Cuhna, 2012).
Figure12. Brain endocast of Homo habilis fossil specimen KNM-ER 1470.
An endocast of the brain of H. habilis, which shows a modern, human like Broca’s cap
(Holloway, 1996).
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One important aspect of brain evolution in early Homo species relied not on the physical
changes of the brain in early Homo, but the environment they lived in. Before delving into why,
some background on the metabolic needs of the human brain is required. The human brain has a
high metabolic cost. It uses 20-25% of the body’s resting energy (Leonard et al., 2003).
Therefore, in order to evolve, the early hominin brain had to overcome two obstacles:
“(i) an energetic constraint: increasing energy requirements as the brain size increased, and (ii) a
nutritional constraint: increasing requirements for nutrients that play a specific role in mammalian brain
structure, development and function.” – (Cunnane & Crawford, 2012)
In order to support a brain like the one of modern Homo sapiens early hominins would have
needed to consume more nutrient dense foods like meat, which first became part of the diet late
in Australopithecines (Bownds 1999; Leonard et al., 2003). The positive relationship between
dietary quality and brain size is shown in fig. 13; as dietary quality increases or improves brain
size increases, as well.
An increase in hunting and consequently meat consumption in both Australopithecines
and H. habilis allowed the brain to increase from the 492 cm3 to 640 cm3 in H. habilis (Bownds,
1999; de Sousa & Cuhna, 2012). H. habilis enabled later brain size increase by living in shorebased habitats. The shore-based habitats provided a diet rich in nutrients that are important to
brain development and function: DHA (an omega-3 fatty acid), iodine, iron, zinc, copper,
selenium, and vitamins A and D (Cunnane & Crawford, 2012; Leonard et al., 2003). About 2
million years ago there was a transition in the climate of East Africa that caused lakes and rivers
to form (Maslin et al., 2014). H. habilis members lived around these lakes and consumed nutrient
dense foods like fish and shellfish. When a diet is nutrient dense, more energy is allocated to the
brain, which would have enabled it to produce the greater of number of neurons seen in H.
habilis and later Homo species, which was shown earlier (Leonard et al., 2003). Changes in the
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body composition of some Homo species later on would also support the increased brain size and
consequential nutrient requirements, as will be discussed later.
Figure 13. “Plot of relative brain size vs. relative diet quality for 31 primate species.
Primates with higher quality diets for their size have relatively larger brain size (r=0.63;
P<0.001). Humans represent the positive extremes for both measures, having large brain:body
size and a substantially higher quality diet than expected for their size.” (Leonard et al., 2003)
Later Homo Species
The next changes in hominin brain evolution were in the later Homo species, which
includes our own species, Homo sapiens. There is some debate, but it appears that H. sapiens
descended linearly from H. erectus (Striedter, 2005; Wilson, 1999).
H. erectus appeared about 1.5 – 1.7 mya (Holloway et al., 2009; Wilson, 1999). They
were about a foot taller than Australopithecines and more lean muscled than H. sapiens
(Striedter, 2005). Their faces were more elongated with more complex facial muscles; the
importance of this will be explained later (Bownds, 1999; Striedter, 2005). Their brain had
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enlarged to between 900 and 1000 cm3 with both increases in absolute and relative brain size
(Striedter, 2005).
Important changes to neural connections in the brain were beginning in H. erectus and
would be fully developed in H. sapiens. Unlike chimpanzees or earlier Homo species, the facial
muscles of H. erectus were beginning to be controlled directly by the neocortex and were more
complex allowing them to show more facial expressions as a way of communication. The
neocortex was beginning to invade parts of the medulla, giving it direct connections to motor
neurons for the muscles in our face, jaw, tongue, and vocal chords. These connections would
give descendent H. sapiens “oral and vocal dexterity” (Striedter, 2005). Also, the vocalization
components had become more complex (Bownds, 1999). These two developments combined
would lead to the progression of a sophisticated language in modern humans (Striedter, 2005).
As with H. habilis, the physical changes to the brain of H. erectus aren’t the main factor
that would drive further brain evolution, but their behavior and environment. H. erectus were
hunter/ foragers who shared amongst themselves and had more sophisticated tools (Striedter,
2005). In Africa, the grasslands were steadily increasing, causing the amount of grazing animals,
like gazelles, to increase as well. This provided a steadier food supply and a better diet (Maslin et
al., 2014). The smaller molars of H. erectus are evidence that their diet had shifted from being
plant-based to consisting of larger amounts of animal foods. Also, H. erectus were the first
species to control fire, allowing them to cook the food, making it more digestible (Leonard et al.,
2003). These factors all contributed to H. erectus being able to support larger brains as they
evolved.
The body composition in H. erectus was also changing in order to support larger brains.
As previously mentioned brains always need a constant level of nutrients and have high
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metabolic requirements. They can’t reduce the amounts of nutrients they need in times of
starvation, like organs and muscles can. Therefore, mammals have evolved to store larger
amounts of fat, as encephalization increases. In the hominid lineage, this is first seen in H.
erectus with larger fat stores in females. Females were the first to increase their fat stores
because they have to support the brain of developing fetuses. This change in body composition
goes even further in H. sapiens with both males and females have more fat stores and less lean,
muscle (Figure 14) (Leonard et al., 2003). Figure 9 shows log muscle mass (kg) as a factor of log
body weight (kg) for 15 primates species including humans. The log muscle mass shows a
positive correlation with log body weight with a small deviation error (r=0.994). Humans fall
below the regression line, which means they have a smaller muscle mass than expected for a
primate of the same size (Leonard et al., 2003).
Figure 14. “Log–log plot of muscle weight (kg) vs. body weight (kg) of 15 primate
species (including humans).”
The graph shows log of muscle mass (kg) as a factor of log body weight (kg). “Across all
primates, skeletal muscle mass scales isometrically with body weight (SM=0.33Wt1.06;
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r=0.994). Humans fall below the primate regression line (standardize residual=−0.86),
indicating that they are ‘under-muscled’ relative to other primates.” (Leonard et al.,
2003).
Another physical change in H. erectus that would allow an increase in brain size was
changes to the pelvic bones, associated with adjustments for bipedalism, created a wider space
between their pelvic bones (Chene et al., 2014). As previously mentioned a change in the pelvic
morphology of early hominids caused by a transition to bipedalism created a larger birth canal,
and allowed a greater prenatal brain size (Abitbol, 1987). The last change in pelvic morphology
occurred in H. erectus; their pelvic bones were more open or spread wider than their ancestors.
The widening of H. erectus pelvic bones opened up the birth canal allowing an infant with a
larger brain to pass through or a larger neonatal brain size; as shown previously in fig. 9 (Chene
et al., 2014; ("Forgotten Females / Gona," 2014).
Homo Sapiens
H. sapiens evolved from H. erectus in Africa between 150,000 and 300,000 years ago
(Hershilwood et al., 2001). Physically, the first H. sapiens were identical to humans today. They
had a spherical skull and a smaller jaw and teeth. They were erect from birth, with a basin shaped
pelvis, mostly hairless, except for the head and genital areas, and had abnormally long thumbs
for a primate (Wilson, 1999). One physical difference in modern H. sapiens was the difference in
body composition. They had less muscle than primates of a similar size as shown in Figure 14.
This difference is important because the muscle was replaced with fat stores that would be used
to support the high metabolic demands of the brain (Leonard et al., 2003).
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The brain of modern H. sapiens reached a size between 1200 and 1400 cm3. Overall, the
first H. sapiens had the anatomically modern human brain that is pictured in fig. 15. The frontal,
parietal, and temporal lobes are expanded more than in any other species (Holloway et al., 2009).
All the asymmetries that appeared in earlier species, frontal pole asymmetry, the LORF petalia,
and left> right asymmetry in the superior parietal region (pictured in fig. 10) are all present and
clearly defined (de Sousa & Cuhna, 2012). The enlargement of the Broca’s region, Brodmann’s
areas and Wernickes area is what allowed humans to develop behaviors that are unique to us,
like language and culture that are unique to humans (Striedter, 2005). The human neocortex has
the most encephalization; greatest brain size in relation to body size than any other mammal
(Holloway, 1996). The connections between the neocortex and motor neurons are complete,
which allowed humans to have the “vocal” and “oral” dexterity needed to evolve sophisticated
language (Holloway et al., 2009; Striedter, 2005).
Cultural Selection on Brain Evolution
In the course of human evolution the brain of modern Homo sapiens wasn’t the only one
to evolve. Other hominid species with brains that differ from our own evolved from Homo
erectus as well (Ehrlich, 2000; Kaifu et al., 2011; White et al., 2014). However, out of all the
species listed in Table two, H. sapiens were the only ones to survive to modern times. So why
did our species survive to modern times when no other hominin species did? Cultural selection
played a part in H. sapiens survival and the extinction of other hominid species (Ehrlich, 2000;
Neill, 2007; Shea, 2008; Sorensen, 2011).
An excellent example of how cultural selection played a role in hominid evolution is the
extinction of H. neanderthalensis in Europe after the arrival of H. sapiens (Ehrlich, 2000; Neill,
26
2007; Shea, 2008). H. neanderthalensis or Neanderthals first evolved in Eurasia about 130,000
thousand years ago; like H. sapiens they evolved from H. erectus (Ehrlich, 2000). H.
neanderthalensis was larger than modern humans and had heavier, more compact bodies.
(Bownds, 1999) A skeletal comparison of H. neanderthalensis and H. sapiens is shown in fig. 15
(Restures, n.d.). Physically, they looked very similar to the modern caveman image that society
has portrayed. They had prominent brow ridges, a broad, flattened nasal aperture, and large teeth
(Holloway, 1985).
Figure 15. A physical comparison of H. neanderthalensis and H. sapiens.
H. neanderthalensis was more muscular and compact than H. sapiens; H. sapiens were
leaner with less muscle mass (Restures, n.d.).
27
The brain of H. neanderthalensis, according to Ralph Holloway (1985), “was fully
Homo, with no essential differences in its organization compared to our own.” They show
definitive, modern cerebral asymmetries, like the LORF petalia pattern that began to appear in
Australopithecines (de Sousa & Cuhna, 2012; Holloway, 1985). Homo sapiens and H.
neanderthalensis species have the largest amount of encephalization in the Homo genus (Balzeau
et al. 2012). In terms of size, on average H. neanderthalensis had a larger brain than H. sapiens
with a range of 1200 – 1750 cm3 (Neill, 2007). There is some variation in brain size of H.
neanderthalensis populations, which causes some overlap between their brain size and that of
modern humans (Neill, 2007; Striedter, 2005). This larger brain size may have been caused by
the higher growth rates in infancy that appears to have been present in H. neanderthalensis
development (de Sousa and Cunha, 2012).
Holloway (1985) believes their larger brain was due to the environment, body
composition, and metabolism (Holloway, 1985). As previously mentioned, the improved diet of
previous species, combined with the skilled hunting of H. neanderthalensis, proved a diet that
would be able to support a larger brain (de Sousa & Cuhna, 2012; Holloway, 1985; Striedter,
2005). Also, he believes it was larger because it was metabolically efficient in freezing habitats
that they lived in (tundra/ preglacial areas) and correlated to the greater amount of lean body
mass present in H. neanderthalensis than in humans (Holloway, 1985).
H. sapiens and H. neanderthalensis first came into contact when H. sapiens dispersed out
of Africa about 100,000 years ago (Ehrlich, 2000). H. sapiens arrived in Europe between 60,000
and 45,000 years ago (Ehrlich, 2000; Sorensen, 2011). H. neanderthalensis went extinct between
30,000 and 45,000 years ago, shortly after the arrival of humans (Shea, 2008; Striedter, 2005).
28
There was once debate about the possibility of H. sapiens/ H. neanderthalensis
interbreeding as mentioned by Bownds in 1999. However, recent studies of nuclear (nDNA) and
mitochondrial DNA (mDNA) have shown that interbreeding rarely occurred (White et al., 2014)
Suzanna White and her colleagues compared the genomic sequences of mitochondrial and
nuclear DNA of modern H. sapiens and H. neanderthalensis. When the mitochondrial DNA was
compared there was a lack of any evidence of introgressed genes; which means they didn’t
interbreed. When they compared the nDNA it appeared to support some introgression of
H. neanderthalensis genes with ancestral H. sapiens, there may have been some instance
of interbreeding, but very few (White et al., 2014).H. neanderthalensis went extinct during a
severe cold, the Weichselian glaciation period, which did pay a role in their extinction (Shea,
2008; Sorensen, 2011). However, they had gone through a similar glaciation (big freeze) before
during the Saalian glaciation period 135,000 years ago and survived (Sorensen, 2011). Both
glaciation periods were equally severe; the difference between H. neanderthalensis surviving the
Weichselian, but not the Saalian was the presence of H. sapiens. Both H. sapiens and H.
neanderthalensis suffered population decline during the severe climatic change in the east
Mediterranean Levant (Shea, 2008; Sorensen, 2011). However, H. sapiens populations were able
to survive because of their cultural and behavioral flexibility that included advanced technology
and clothing, and lower energy requirements (Ehrlich, 2000: Neill, 2007).
Culturally and behaviorally Homo sapiens were more advanced than H. neanderthalensis
(Ehrlich, 2000; Neill, 2007). H. neanderthalensis culture existed unchanged until the arrival of
humans. They buried their dead, but the lack of burial adornments suggests it was for disposal of
the body and not cultural (McBrearty & Brooks, 2000). Humans on the other hand buried their
dead with adornment suggesting ritualistic culture (Neill, 2007). H. sapiens culture also included
29
art such as cave paintings and body adornment, and their clothing was decorated with beads
(Neill, 2007; Wales, 2012).
H. sapiens survived because of their cultural advantage (Ehrlich, 2000; Neill, 2007;
Wales, 2012). The difference between H. neanderthalensis and H. sapiens clothing seems to
have played a role in the cultural advantage of H. sapiens. H. sapiens were able to produce
tighter fitting clothing, which was able to keep them warmer (Wales, 2012). Evidence has been
found that they weaved and sewed clothing including dyed flax (a plant product used to make
clothing), sewing needles and weaving battens (Wales, 2012). H. neanderthalensis lacked the
tools needed to make tailored or tight clothing that would efficiently contain their body heat.
Their clothing was looser and didn’t cover their entire body. Overall, “the simple clothing of the
Neanderthals did not lead to mass extinction via hypothermia. However, it can be considered to
be part of a larger cultural and behavioral package that gave Neanderthals an evolutionary
disadvantage and ultimately led to their demise” – Nathan Wales (2012). H. sapiens also lived in
shelters that they built and stored food while H. neanderthalensis lived in caves (Neill, 2007).
The differences in tools also gave H. sapiens an advantage. H. neanderthalensis used spears to
kill prey, but H. sapiens were able to produce a variety of sophisticated tools including longer
spears to throw longer distances and fish hooks (Ehrlich, 2000; Neill, 2007). These tools
increased H. sapiens adaptability. Neanderthals seem unable to have adapted the new behaviors
of H. sapiens (Neill, 2007).
Brain evolution now
The human brain stopped increasing in size about 100,000 years ago (Strieder, 2005).
One reason is the constraints of childbirth. As mentioned above the hominid brain was able to
enlarge after the early hominids became bipedal, and again with the widening of the pelvic
30
diameter in H. erectus (Chene et al., 2014). The modern human pelvis hasn’t enlarged and can
only conform to allow larger heads to pass through so much before they will no longer fit
without causing harm to the mother and being evolutionarily disadvantageous (Striedter,
2005).Phillip Tobias (1971) believes that brain evolution plateaued because once language and
culture came onto the scene, it increased changes in behavior and humans didn’t need larger
brains to develop new behaviors for survival (Tobias, 1971). Striedter (2005) also believes that
brain evolution stopped because of the evolution of language; “once we had speech, we did not
need perpetually larger brains; we just needed to improve our usage of the language faculty” –
Striedter, 2005. Brain enlargement also stopped because the brain reached a point where
increased brain size advantages didn’t outweigh its high metabolic costs (Leonard et al., 2003).
A larger brain would require more nutrients or energy than the diet could supply, making it more
of a hindrance to survival than a benefit (Leonard et al., 2003).
Conclusion
The evolutionary process took us from our primitive mammalian ancestors, to primates,
and then through the many tree branches of the hominids, to finally arrive at Homo sapiens. Now
the question to leave with is thinking about the next step in brain evolution. As shown in this
paper the environment, and behavior and culture of human ancestors have produced the modern
brain of H. sapiens. What effect will natural selection continue to have on the brain of H.
sapiens? Phillip Tobias, 1971 believes that humans have reached a point where brain size no
longer needs to increase, “We have used these very brains to create conditions in which mere
brain size is of negligible importance; we have used them to develop new mechanisms of
adaptation, tools, shelters, clothing, fire, social institutions – and central heating, airconditioning, refrigeration, disinfection, mink coats, and sunshades.”.
31
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