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The evolution of the human brain has been a combination of reorganization of brain components and increases of brain size through both hyperplasia and hypertrophy during development, underlain by neurogenomic changes that have involved epigenetic changes largely effecting regulation of growth dynamics. While both genomics and comparative neuroanatomical studies are invaluable to understanding how brains and behavior correlate, it is paleoneurology, based on endocast studies ( chapter “Virtual Anthropology and Biomechanics,” Vol. 1), which are the direct evidence demonstrating volume changes through time. Some convolutional details of the underlying cerebral cortex do appear on the endocranial surface. These details allow one to recognize reorganizational changes that include (1) a reduction of primary visual cortex and relative enlargement of posterior association cortex, (2) expanded Broca’s regions, and (3) cerebral asymmetries. The size of the hominid brain increased from about 450 ml 3.5 Ma ago to our current average volume of 1,350 ml, with a slight reduction since Neolithic times. Many more data from additional fossils will be necessary to decide how and when these two changes through time occurred and whether these were gradual or punctuated.
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The Evolution of the Hominid Brain
Ralph L. Holloway
Contents
Introduction ..................................................................................... 1962
Lines of Evidence . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. 1963
Direct Evidence . . ........................................................................... 1963
Indirect Evidence . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 1969
Characteristics of the Human Brain . .......................................................... 1969
Brain Size, Absolute and Relative . . . . . .................................................... 1969
Encephalization (Encephalization Coefficient, EQ) . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . 1971
Brain Organization and Reorganization ................................................... 1973
Human Brain Asymmetry . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . 1978
Synthesis: Putting Together Size, Organization, and Asymmetry During
Human Evolution ............................................................................... 1980
And to the Future? .............................................................................. 1984
Conclusions . .................................................................................... 1984
References . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . 1985
Abstract
The evolution of the human brain has been a combination of reorganization of
brain components and increases of brain size through both hyperplasia and
hypertrophy during development, underlain by neurogenomic changes that
have involved epigenetic changes largely effecting regulation of growth dynam-
ics. While both genom ics and comparative neuroanatomical studies are invalu-
able to understanding how brains and behavior correlate, it is paleoneurology,
based on endocast studies (chapter Virtual Anthropology and Biomechanics,”
Vol. 1), which are the direct evidence demonstrating volume changes through
time. Some convolutional details of the underlying cerebral cortex do appear on
R.L. Holloway (*)
Department of Anthropology, Columbia University, New York, NY, USA
e-mail: rlh2@columbia.edu
#
Springer-Verlag Berlin Heidelberg 2015
W. Henke, I. Tattersall (eds.), Handbook of Paleoanthropology,
DOI 10.1007/978-3-642-39979-4_81
1961
the endocranial surface. These details allow one to recognize reorganizational
changes that include (1) a reduction of primary visual cortex and relative enlargement
of posterior association cortex, (2) expanded Broca’s regions, and (3) cerebral
asymmetries. The size of the hominid brain increased from about 450 ml 3.5 Ma
ago to our current average volume of 1,350 ml, with a slight reduction since Neolithic
times. Many more data from additional fossils will be necessary to decide how and
when these two changes through time occurred and whether these were gradual or
punctuated.
Introduction
The evolution of the human brain has largely been a matter of integrating
both increases in the size of the brain and the brain’s organization through the
past 3–4 myr mainly based on species of the genus Australopithecus and two of its
species, A. afarensis and A. africanus.
1
Earlier possible hominin forms such as
Sahelanthropus or Ardipithecus in the time range of 3–6 myr do not have sufficient
endocranial remai ns to do more than estimate volumes. Three lines of evidence are
used by paleoneurologists to ascertain how these events might have occurred:
(a) direct evidence from the brain endocasts of fossil hominids (paleoneurology)
and (b) the indirect evidence from comparative neuro science, where variations in
brain structures can be related to variations in behavior and be compared between
species. This latter evidence is “ind irect” because extant living animals are not
ancestral to humans and have undergone their own evolutionary changes. Indeed,
the last common ancestor for apes and the hominid line existed some 5–7 Ma ago.
(c) Newer neurogenomics evidence also promises to provide important clues to how
and when certain aspects of brain changes occurred during human evolution
(e.g., Preuss 2012; Zeng et al. 2012).
Our best paleoneur ological evidence suggests that the human brain evolved from
an early hominid 3–4MY, A. afarensis, having a size of roughly 400 ml to our
present average of 1,330 ml. These brain size increases, at different taxonomic
levels, were mostly allometric, i.e., related to body size, but not always. Integrated
with these changes in brain size was reorganization of the cerebral cortex, as well as
changes in subcortical structures such as the hippocampus, amygdala, etc., to
mention a few important structures that relate to aspects of social behavior but
that cannot be seen on endocasts. Reorganization simply refers to both qualitative
and quantitative changes through time of neural structures. Endocasts, of course,
cannot provide information regarding neural variables such as subcortical volumes,
cell densities, dendritic branching and connectivity, or any neurochemical or
neurophysiological information. Thus from the point of view of knowing what
1
This paper is adapted and expanded from an earlier chapter written for the Encyclopedia of
Human Biology, 3rd Ed. Elsevier, In press.
1962 R.L. Holloway
exactly the data indicate regarding human brain evolution, the direct evidence
of endocasts is critically important, however poor the data they contain actually
may be.
At least three areas of the reorganization of the cerebral cortex were affected at
different times: (a) a relative reduction of primary visual striate cortex (V1, PVC) and
an attending relative increase in posterior parietal association cortex; (b) a change in
Broca’s region, resulting in a more humanlike pattern; and (c) increasing degrees of
cortical asymmetry, as well as increases in overall brain size and number of neurons.
How exactly did the human brain evolve, and when did changes in it happen?
Obviously, to answer this question fully would require a time machine and thou-
sands of generations of observations to ascertain both the variability and direction of
selection pressures in the past. We can, however, flesh out an initial understanding of
how we got to be the animal par excellence that utilizes its brain for intelligent
rationalizations, based largely on the use of arbitrary symbol systems and on
behavioral adaptations involving a complementary social existence between males
and females permitting prolonged infant growth and nurturance (chapters Great
Ape Social Systems and Theory of Mind: A Primatological Perspective,”
Vol. 2). The evidence consists of two components: (a) the “direct” evidence from
the fossil record and (b) the “indirect” evidence of the comparative neuroscientific
record of extant living animals, particularly those most closely related to us such as
the chimpanzee and bonobo. There is also a third possibility: since the Human
Genome Project has sequenced almost all of the genetic code, the future study of
evolutionary neurogenomics might provide more data about the actual genetic
history of our genus through time, as well as that of the great apes mentioned
above (see, e.g., Hernando-Herraez et al. 2013; Gokcumen et al. 2013). As this
latter possibility is little more than a gleam in our eye at present, this article will
concentrate on the evidence provided by the first two components.
Lines of Evidence
Direct Evidence
The term paleoneurology is used to describe evidence relating to the size and
morphology of the casts made from the inside of actual fossil cranial remains.
Occasionally, the casts are “natural,” i.e., where fine sediments have filled the
inside of the cranial cavity, becomi ng infiltrated and compacted through time.
These casts sometimes retain some of the morphological details that were imprinted
on the internal table of bone of the cranium when the animal was alive. The famous
australopith (A. africanus) Taung child’s skull, describ ed by Dart (1925), is one of
the best-known examples, as are Sts 60 and SK 1585, the latter a fine example of
Australopithecus robustu s. Curiously, these “natural” endocasts are only found in
the South African australopiths (cha pter Analyzing Hominin Phylogeny:
Cladistic Approach,” Vol. 3) and date from about 3.0 myr to about 1.5 myr (see
Fig. 1). Traditionally, paleoneurologists have made casts of the insides of fossil
The Evolution of the Hominid Brain 1963
skulls using rubber latex, or silicone rubber, extracting these from the cranial
remains. The partial cast is then sometimes reco nstructed by adding plasticine
(modeling clay) to the missing regions. The whole is then measured by imme rsion
into water, and the amount of water displaced is regarded as the volume of the once-
living brain. Other measurements (linear chords and arcs) and observations (con-
volutions and asymmetries) may be made on the original cast. More recently,
“virtual” endocasts have been made from CT scans of intact or partial crania, an
approach that has the advantage of being noninvasive (chapter Virtual Anthro
pology and Biomechanics,” Vol. 1). As it is computer driven, there are various
algorithms for deriving the size of the endocast and other metrics (chapter
Virtual Anthropology and Biomecha nics,” Vol. 1; Weber et al. 2012; see also
Zollikofer and Ponce de Leo
´
n 2013). Of course, CT scans (medical and micro)
are not continuous, as is the case with actual casting materials such as silicone-
based materials that flow into all the cracks, crevices, and convolutional details
available.
During life, the brain is surrounded by three dural tissues (the dura mater, the
arachnoid tissue and its cerebrospinal fluid, and the pia mater) that interface
between the actual brain tissue (cerebral cortex, mostly) and the internal table of
bone of the skull. The gyri and sulci (convolutions) of the once-pulsating cerebral
cortex are thus imperfectly imprinted on the interior of the skul l, and the degree of
replication often varies in different regions, e.g., sometimes the frontal lobe
imprints more details than the parietal lobe, as well as by age. The degree of
replication also varies in different animals. Two extremely important consi der-
ations emerge from this: (a) the resulting imprints are never complete and are thus
Fig. 1 Casts of the Taung (left), Sts 60 (right), and SK 1585 (bottom) “natural” endocasts of
Australopithecines
1964 R.L. Holloway
Fig. 2 (continued)
The Evolution of the Hominid Brain 1965
in that sense “data poor,” never including subcortical structures, and (b) the con-
troversial interpretations of what the under lying brain once looked like are
guaranteed (see Fig. 2). Nevertheless, endocranial casts do provide extremely
important information regarding (1) overall size, (2) shape, (3) rough estimates of
Fig. 2 Dorsal (see previous page) and lateral (see above) views of a modern human brain and the
endocast to demonstrate the loss of detail on the endocast surface
1966 R.L. Holloway
the lobal dimensions of the brain, and (4) cortical asymmetries that have relation-
ships to hemispheric specializ ations and behavioral processes including handed-
ness. In addition (5), if the imprints of the underlying gyri and sulci are available, they
can provide important information regarding the organization of the cerebral cortex
and whether the patterns of these are the same or different as in known extant primate
brains. The infamous “lunate sulcus is a good example, as it is a demarcation
boundary between purely sensory primary visual striate cortex (PVC) and multi-
modal association cortex in both Old World monkeys and anthropoid apes. When the
lunate sulcus appears in an anterior position, it is most similar to the condition known
in modern apes. When it is found in a posterior position, it is in a more humanlike
condition. Ascertaining its correct position is thus essential in deciding whether or not
such a fossil hominid had a brain organized along human or ape lines. In modern
humans, the “lunate” is only partially homologous with that found in apes and is
usually fragmented (Allen et al. 2006). Hominins such as Homo erectus,
H. heidelbergensis, H. georgicus,andH. neanderthalensis unfortunately do not
have occipital lobes that allow clear-cut identification of the lunate sulcus if it were
a singular unfragmented sulcus. Figure 3 shows a comparison between a chimpanzee
brain with a lunate sulcus and that of the Taung child, A. africanus (See also
Holloway 1984, 2000). Finally (6), meningeal arteries and veins that nourished the
dura mater also imprint on the internal table of bone and sometimes show patterns
that are useful for deciding taxonomic issues; these have no known relationship to
behavioral functions of the brain. (See also Grimaud-Herve
´
in Holloway et al. 2004,
for further discussion and illustrations.) Figure 4 shows that a more recent
A. africanus specimen from Sterkfontein, S. Africa, Stw 505, shows a clear lunate
Fig. 3 Lateral view of a chimpanzee brain and the Taung A. africanus endocast. The lunate sulcus
separates PVC from association cortex and is in an anterior position in apes. The white dots on the
Taung endocast show where the average chimpanzee lunate sulcus would fall and that location
violates the sulcal morphology on Taung. Placing it more anteriorly would be a monkey-like
configuration. The Taung lunate sulcus would most probably be posterior, in a humanlike position,
which is near the lambdoid suture
The Evolution of the Hominid Brain 1967
sulcus in a relatively posterior position compared to chimpanzee brains. Falk (2014)
has made the bizarre suggestion that is perfectly crescentic lunate sulcus is the lateral
calcarine sulcus, which is not possible given the medially directed curvature of both
inferior and superior ends of the depicted lunate sulcus in Stw505. When in the course
of subsequent hominin evolution, the lunate sulcus changed into a more fragmented
partially homologous structure as found in modern humans is unknown, as
neither Homo erectus nor Neanderthals show detailed gyri in the occipital region
(chapters Later Middle Pleistocene Homo”and“ Neanderthals and Their
Contemporaries,” Vol. 3).
The frontal lobe is of course a major focus of examining endocasts in the hope of
understanding the evolutionary trajectories through time of the most crucial part of
neuroanatomy underlining our very humanness, intelligence, and social behavior.
Here, we are plagued with by the fact that very few sulcal details are available on
the endocasts of early hominins, particularly the australopiths. A rece nt paper by
Carlson et al. (2011) describes the frontal portion of an endocast of MH1 (Malapa
Fig. 4 Oblique view of the Stw 505 A. africanus specimen, showing a prominent lunate sulcus in
a posterior position. This specimen makes it clear that at least some australopithecines had reduced
primary visual cortices and expanded posterior parietal lobes, evidence showing that reorganiza-
tion probably preceded brain size increases
1968 R.L. Holloway
Hominin 1) that they have named Australopithecus sediba and that shows some
possibility of prefrontal organization toward a more human condition.
Indirect Evidence
This line of evidence is “data rich,” providing comparative neurological informa-
tion on living species, such as brain size (both absolute and relative, i.e., related to
body size), the actual makeup of the brain from the gross to microscopic levels,
including neural nuclei, fiber systems and interconnections, and distribution of
neurotransmitters and neuroreceptors. Additionally, the brain can be studied onto-
genetically, and neuroscientists can actually study the relationships between how
the brain varies neurologically and how these variations relate to the behavioral
variation. Modern examinations including CT, MRI, fMRI, and tensor diffusion
techniques can be applied, yielding different kinds of data relevant to different
aspects of growth and development, genetic and epigenetic unfolding, and behav-
ioral consequences (chapter Virtual Anthropolog y and Biomechanics,” Vol. 1).
Neurogenomic information will also add considerable details as to how living
brains vary and operate, both within and between different species, and hopefully
inform us about selection events in the evolutionary past. This richness is simply
lost to the paleoneurologist. However, it is necessary to realize that the extant living
species often used as comparisons to humans, e.g., bonobo, chimpanzee, and
macaque (chapters Estimation of Basic Life History Data of Fossil Hominoids,”
Vol. 1, Evolution of the Primate Brain,” and The Hunting Behavior and
Carnivory of Wild Chimpanzees,” Vol. 2), are end points of their own evolutionary
lines of development and are not our ancestors, however closely related to us they
may be. It is thus the blending and complementation of these two approache s which
provide the best set of evidence for when and how our brains evolved. Another
aspect of the comparative evidence is the question of how well we can explain
species-specific behavior on the basis of what we know from comparative neurol-
ogy. Considering the behavioral differences betwee n chimpanzees and bonobos and
gorillas and orangutans, there are no current explanations to explain these in terms
of neuroanatomical detail.
Characteristics of the Human Brain
Brain Size, Absolute and Relative
The human animal is obsessed with size, and those who study the brain compara-
tively are perhaps more so than average. With a mean brain weight of 1,330 g and a
body weight of 65,000 g (Tobias 1971), the human species has the largest absolute
brain size within the primate order, but is actually dwarfed by elephants and some
of the whales, in which brain weight can exceed 7,500 g. Of course, body weights
are also very much higher in elephants and whales. But even for its body weight,
The Evolution of the Hominid Brain 1969
Homo sapiens does not have the largest relative brain weight (abou t 2 % of body
weight), being outdone by several monkeys, some rodents, and even some fish.
Normal modern human brain size varies between roughly 900 and 2,000 g, although
a very small number of exceptions do occur, with sizes in the 750–900 and
2,000–2,200 g range. Human populations vary, as do the sexes . In general, Arctic
peoples tend to have larger brains than those living in the tro pics, and the smallest
brains appear to be found among Ituri forest pygmies who also display small
stature. Males in all populations for which good autopsy or cranial data have
been gathered show brain sizes on the average of 100–150 g greater than females,
an amount roughly the same as the range of modern human racial variation. It
should be pointed out that these differences, and their possible relationship to
cognitive skills, are highly controversial, and simple correlations are deceptive
(Holloway 1996, 2008; Nyborg 2003). Table 1 provides a listing of the major fossil
hominid taxa and their respective brain sizes (See Neubauer et al. 2012 for
confirmation of my australopithecine volumes ). Notice that the range of values
Table 1 Fossil hominid brain volumes
Average
Group Number Location
Brain
volume Range
Dating
(myr)
A. afarensis 3 E. Africa 435 385–500+ 3–4
A. africanus 8 S. Africa 440 420–500+ 2–3
A. aethiopicus 1 E. Africa 410 na 2.5
A. garhi 1 E. Africa Ca. 450 na 2.5
A. sediba 1 S. Africa Ca. 420 na 2–3
A. robustus 6 E. and S. Africa 512 500–530 1.6–2.0
H. rudolfensis 2 E. Africa 775 752–800 1.8
H. habilis 6 E. Africa 612 510–687 1.7–2.0
H. georgicus 3 Georgia and
Europe
677 600–775 1.7
H. ergaster 2 E. Africa 826 804–848 1.6
H. erectus 2 E. Africa 980 900–1,067 1.0–1.6
H. erectus 8 Indonesia 925 780–1,059 1.0
H. erectus 8 China 1,029 850–1,225 0.6
Archaic
H. sapiens
6 Indonesia (Solo) 1,148 1,013–1,250 0.13
Archaic
H. sapiens
6 Africa 1,190 880–1,367 0.125
Archaic
H. sapiens
7 Europe 1,315 1,200–1,450 0.5–0.25
H. sapiens
(Neand.)
25 Europe and
M. East
1,415 1,125–1,740 0.09–0.03
H. sapiens
sapiens
11 World 1,506 1,250–1,600 0.025–0.01
Source: Holloway 1997
1970 R.L. Holloway
from the earliest australopithecine to modern Homo is roughly 1,000 ml or about the
same amount as the normal range of variation within our species.
Encephalization (Encephalization Coefficient, EQ)
Nevertheless, the human animal does come out on top of the evolutionary heap
when its absolute brain and body weights are considered together. When the log
(base 10) of brain weight is plotted against the log10 of body weight for a group
of relevant taxa, the result is usually a linear relationship, where (log10)
brain weight ¼ a + b (log10) body weight. For a large array of primate data
(e.g., Stephan et al. 1981), the slope of the line (b in the equation above) is about
0.76, and the correlation coefficient is 0.98, indicating that the relationship is almost
perfect (chapter Estimation of Basic Life History Data of Fossil Hominoids,”
Vol. 1). This relationship will naturally vary depending on the databases and the
transformations used. This is known as an allometric equation, and these are used
frequently in biology to assess the underlying relationships between the size of parts
of the body and the whole (see Fig. 5). The slope sometimes has an interpretation
suggesting functional relationships between the brain and other variables. For
example, in the above example, the slope is 0.76, extremely close to 0.75 or 3/4,
which often describes a metabolic relationship (Martin 1983). The slope of 0.66, or
3.2
3.0
Homo = 1
Prosimians = 21
Monkeys
(Old + New World)
Apes = 4
1.6
2.0 2.4 2.8
log10 body weight (mean)
3.2 3.6
4.0
4.4 4.8
Gibbon
Chimpanzee
Orang-utan
Homo
Gorilla
a
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
log10 brain weight (mean)
1.0
0.8
0.6
0.4
0.2
Fig. 5 Graph showing human deviation from a plot of log brain weight against log body weight
for primates, with Homo at the extreme upper right position
The Evolution of the Hominid Brain 1971
2/3, has been championed by some (e.g., Jerison 1973) as indicating an important
geometric relationship between volume and surface area. It is important to realize
that these slopes vary depending on the taxa examined. In general, as the taxa
become more similar, the slope decreases. Species within a genus generally have a
slope around 0.3; within a species, the slope is smaller yet, being about 0.2, and the
correlation coefficient is also reduced (see also Martin and Isler 2010).
Just as the human animal is curious, it is also vainglorious, always trying to find a
measure that places it at the top. Thus we can fabricate a device, the
Encephalization Coefficient or EQ, which shows that relative to any database, the
human animal is the most encephalized animal living. The point for Homo sapiens
shows a clear positive residual above the expected regression line, and in fact the
human value is about three times that expected for a primate with its body weight.
Table 2 provides a number of different equations based on differing databases,
which happily give H. sapiens the highest value. (Actually, young immature
dolphins will provide a higher number, but when compared to an immature
human, the value is higher in the latter.) Two additional points should be made:
(a) EQs are relative to the databases used, and thus there is an inherent “relativity”
to relative brain sizes; and (b) EQs do not evol ve, only brain weight/body weight
relationships do, and EQs are simply a heuristic device enabling comparisons
between taxa; they have no reality outside of the database chosen, or species within
a taxa, and are not designed to discuss within-species variation. For example,
female humans are “more” encephalized than males, given their smaller body
sizes, more body fat which is not innervated, and smaller brains, but the relationship
might be simply a statistical artifact with no known gross behavioral manifestation
given the sexes equal overall intelligence. It is more likely that small differences in
Table 2 Some examples of encephalization quotients
Species
Brain
wt. (g)
Body
wt. (g)
EQ
Homo
a
EQ Jerison
b
EQ
Primates
c
EQ
Stephan
d
Lemur 23.3 1,400 21 1.56 (22.6) 0.94 (32.7) 5.66 (19.6)
Baboon 201 25,000 28 1.97 (28.5) 0.90 (31.3) 7.94 (27.5)
Gorilla 465 165,000 23 1.56 (22.5) 0.61 (21.2) 6.67 (23.2)
Orang 370 55,000 31 2.15 (31.1) 0.91 (31.7) 8.90 (30.9)
Chimp 420 46,000 39 2.63 (28.1) 1.81 (41.1) 11.3 (39.3)
Human 1,330 65,000 100 6.91 (100) 2.87 (100) 28.8 (100)
Source: Holloway 1997. Note: each formula is based on a different set of data. The EQ Homo
equation simply uses the average brain and body weight for Homo sapiens and assumes an
intercept where both brain and body weights are zero. The value of whichever animal is calculated
is then given as a direct % of modern Homo sapiens . EQ Jerison is based on data for almost
200 mammals, while the EQ Primates is based on Martin’s (1983) data set for primates only. The
EQ Stephan equation is based on insectivores only. The numbers in the parentheses are the % of
the Homo sapiens value
a
Formulae: EQ Homo ¼ Brain wt/1.0 Body wt
0.64906
b
EQ Jerison ¼ Brain wt/0.12 Body wt
0.66
c
EQ Primates ¼ Brain wt/0.0991 Body wt
0.76237
d
EQ Stephan ¼ Brain wt/0.0429 Body wt
0.63
1972 R.L. Holloway
neural reorganization might be related to behavioral differences such as language
ability or math and spatio-visual manipulation rather than brain size or EQ.
I will discuss later how the processes of hypertrophy and hyperplasia have been
positively selected for in the course of the last 2–3 myr of hominid evolution.
(Hypertrophy refers to increases in size of the neural components, e.g., neurons,
dendritic branching, nuclei, and fiber tracts; hyperplasia refers to increased produc-
tion of cells through mitotic division.) It is most probably the case that these
processes are controlled by regulatory genes, and one of the major differences
between ourselves and our closest nonhuman primate relative, the chimpanzee
(brain size ¼ ca. 385 g), relates to the schedules by which hyperplasia and hyper-
trophy are turned on and off during ontogenetic development (Holloway 1980,
1995; Miller et al. 2012).
Brain Organization and Reorganization
It is well known that the brains of most animals are extremely similar to each other
in terms of their overall organization, by which are meant neural nuclei and fiber
systems. The human animal does not appear to show any different structures when
compared to Old World monkeys such as the macaque or the great apes, including
bonobo, chimpanzee, gorilla, and orangutan . Even the neural fiber tracts that are
involved in human language appear in these primates (Deacon 1997). One might
ask, then, given the obvious species-specific repertoires that exist in all animals,
how can these behaviors differ without differences in the underlying nervous
systems? This is one of the major challenges of studying brain evolution and in
particular understanding what neural organizations account for the specificity of,
say, human behavior, the ability to use language composed of arbitrary symbols. In
other words, all mammals have a cerebral cortex, a thalamus, cerebellum, hypo-
thalamus, etc., and basically these structures possess almost identical divisions of
nuclei and do the same neural tasks. Clearly, brain size alone will never explain
species-specific behavior, and the relationships between neural nuclei and fiber
tracts will only go so far in explaining behavioral differences.
Allometric equations showing the relationship between individual bodily com-
ponents and the whole are instructive here. If we were to plot the logs (base 10) of
primary visual cortex (PVC) against brain volume, we would find that the human
PVC is 121 % less than predicted, and similarly, the lateral geniculate nucleus of
the thalamus is about 144 % less than expected for a primate of our brain size (see
Fig. 6). In contrast, if one plots the amount of cerebral cortex against brain weight
the result is a strai ght line, and the human point lies almost exactly on the line. In
short, the human cerebral cortex is as large as would be expected for a primate of its
brain size. But do portions of the cerebral cortex vary in size between different
primates? In humans, the residua ls mentioned above suggest that compared to
chimpanzees, the amount of PVC is significantly smaller in humans, or alterna-
tively put, the posterior association cortex of the parietal and temporal lobes is
relatively larger in humans. Since there are no essential differences between
The Evolution of the Hominid Brain 1973
Fig. 6 Graph showing log striate cortex (area 17) versus log brain volume, where the value for
Homo sapiens (upper right) is 121% less than expected from the log-log regression (see also
Table 3, which shows other departures between actual and predicted values for different brain
structures)
Table 3 Human brain structure residuals
a
Dependent
variable
Independent
variable
Number
species
Correl.
coeff.
(R)
Actual
value
(A)
Expected
value (E)
(A)/
(E) ratio
%Diff.
(A)/
(E) homo
Striate
cortex
Brain weight
(C)
37 0.971 22,866 50,598 0.45 121.30
19 0.977 38,097 0.60 66.60
Lateral
geniculate
Brain weight
(C)
37 0.978 416 1,026 0.41 146.60
19 0.982 857 0.49 106.00
Cerebellum Brain weight
(C)
44 0.990 137,421 128,932 1.07 6.20
26 0.994 150,535 0.91 9.50
Dienceph. Brain weight
(C)
44 0.995 33,319 51,512 0.65 54.60
26 0.998 47,899 0.70 43.70
Septum Brain weight
(C)
44 0.983 2,610 2,085 1.25 20.10
26 0.991 2,201 1.19 15.70
Amygdala Brain weight
(C)
16 0.990 3,015 4,633 0.65 53.70
7 0.985 3,753 0.80 24.50
Lateral
geniculate
Thalamus 21 0.979 416 731 0.57 75.72
10 0.988 416 636 0.65 52.88
a
Based on Stephan et al. (1981) data
1974 R.L. Holloway
chimpanzees and humans in their visual abilities and competencies, these differ-
ences most probably reflect selection for expanded functioning of the association
cortex in humans. This is precisely what is meant by “reorganization” (Table 3).
When used in a comparative or evolutionary context, reorganization means
changes in the sizes and proportions thereof of neural nuclei and their fiber tracts
(see Fig. 7). Given that chimpanzees and hominids had a last common ancestor
some 5–7 myr and that chimpanzees appear to have large PVC cortices, we infer
that one aspect of human brain evolution has been some reorganization of the
Fig. 7 Types of reorganization without necessary brain size increase. The dashed lines represent
boundaries between frontal, parietal, and occipital lobes, which if changed in relative positions
from T1 to T2 would suggest reorganization
The Evolution of the Hominid Brain 1975
cerebral cortex, namely, an increase in posterior association cortex (or, equally, a
reduction in PVC) involved in polymodal cognitive tasks, where visual, auditory,
and motor inf ormation are brought together in a synthetic whole. The trick, of
course, is to demonstrate objectively when, where, and why these changes took
place. This example of PVC has been purposefully chosen because one of the sulcal
landmarks of the cortex that defines the anterior border of PVC is the “lunate”
sulcus, named for its crescentic shape, and there is some hope of identifying its
position on some of the early hominid brain endocasts. In this regard, endocasts are
most often frustratingly mute on other convolutional details.
Neuroanatomists have been trying for many decades to demonstrate the major
differences between us and other primates, and aside from gross brain size, very
little else of significance has been shown as most of the differences can be explained
as allometric scaling. The frontal lobe, and particularly its prefrontal portion, has
been a favorite target, and indeed, Br odmann (1909) claimed it was proportionally
larger in humans, a view most recently championed by Deacon (1997). Unfortu-
nately, other work has shown that the human brain has just as much frontal lobe as
would be expected for a primate of its brain weight (von Bonin 1937, 1948;
Semendeferi et al. 1997; Uylings and van Eden 1990), although the picture regard-
ing prefrontal cortex has yet to be determined objectively using cytoarchitectonic
criteria, which is how prefrontal cortex is differentiated from the pure motor cortex
behind it (Schenker et al. 2010; Sherwood et al. 2003; Rilling et al. 2008). Hominid
brain endocasts do not, alas, provide any sulcal landmarks with enough reliability to
determine the boundaries of prefrontal cortex, which is so important to impulse
control, and higher cognitive functions such as planning and abstraction and
recognition of social actors and behavioral elements suggesting “theory of mind”
abilities. Thus, these regions cannot be accurately measured in a phylogenetic
sequence. However, given the apparent closeness between us and the great apes
in terms of percentage of prefrontal cortex, it strikes this writer as extremely
doubtful that there could be any major quantitative differences in prefrontal relative
volume among the various hominin taxa. The Neanderthals, living from about
300,000 to about 28,000 years ago, have frequently been described as having
smaller frontal lobes; this is not based on objective measurements, but rather a
perception that the large brow ridges on these humans were constraining frontal
lobe development. Studying the Neanderthal brain endocasts and comparing them
to modern humans, I have failed to see any significant difference between these two
groups, and Bookstein et al. (1999) showed that their prefrontal profiles were
practically indistinguishable. More recently, Pearce et al. (2013) have suggested
that Neanderthal orbital size meant they had larger visual cortices and thus less
parietotemporal association cortex and were thus less intelligent than modern
H. sapiens. Unfortunately, these authors did not cont rol for facial size which is
larger in Neanderthals, nor did they bother to take into account the large degree of
occipital lobe variation in those Neanderthal endocasts providing such details.
There is nothing in the external morphology of Neander thal endocasts that can
pinpoint any primitive characteristics in cortical morphology; and yes, their brains
were on average larger than ours today, but not necessarily than Upper Pleistocene
1976 R.L. Holloway
anatomical modern humans. Their bodies, being larger in terms of lean body mass,
might have required larger brains.
Similarly, regions such as “Broca’s and Wernicke’s areas,” anterior and poste-
rior association cortical regions involved in motor (Broc a’s) and receptive
(Wernicke’s) aspects of speech, are determinable on most fossil endocasts, and
Fig. 8 Dorsal view of
KNM-ER 1470, Homo
rudolfensis (1.8 myr),
showing a typical Homo
pattern of petalias, the left
occipital projecting more
posterior and being wider
than the right side (A) and the
right frontal being wider than
the left (B)
Fig. 9 Neanderthal cerebral asymmetries. Left is Monte Circeo and right is La Ferrassie. Both
show a larger width of the right-frontal lobe and a larger left-occipital region (as in Fig. 8)
The Evolution of the Hominid Brain 1977
we can determine, for example, that Broca’s region is more humanlike on one brain
cast of an early Homo, some 1.8 Ma. This is the famous KNM-ER 1470 endocast of
Homo rudol fensis from Kenya, which had a brain volume of 752 ml. It may not be a
direct ancestor to our o wn line o f Homo, but it does show cerebral asymmetries
similar to those found in mode rn Homo (Fig. 8, KNM-E R 1470). We know that
Broca’s regions in modern Homo are asymmetrical both in overall size and
cytoarchitectonic divisions between areas 44, 45, and 47 of Brodmann (Amunts
et al. 2010; Schenker et al. 2010). Interestingly, Neanderthal endocasts show
similar asymmetry to modern humans in Broca’s region (Fig. 9).
While the concept of reorganization has a heur istic value in directing our
attention to changing quantitative relationships between different neural nuclei
and fiber tracts, we cannot yet ascribe behavioral differences between closely
related animals such as chimpanzee, gorilla, and orangutans or different species
of the genus Macaca or indeed different breeds of dogs or cats with their different
temperaments, aptitudes, and sociality to particular b rain conformatio ns. We
simply do not know what magic level of neural descript ion is necessary to de scribe
species-specific behavior. Recent r esearch on prairie and mountain voles suggests
that the difference in the females’ ability to retrieve pups back to the nest depends
on the distribution and number of neuroreceptors for the hormone oxytocin found
in several nuclei of the brain, particularly the thalamus. Otherwise, their brains
appear id entical (Insel and Shapiro 1992). In additi on, it is ne cessary to remember
that the bra in possesses aspects of plasticity that we did not appreciate except
within the past decade and that as the brain’s organization unfolds ontogenetically,
interactions with environmental stimuli are always occurring, and the brain builds
its org anization partly through its plasticity. It is difficu lt enough to study an d
understand such patterns in laboratory animals, let alone in our fossil ancestors!
While the above suggests a somewhat pessimistic tone, we should remember that
advances in noninvasive technology such as MRI, fMRI, PET, and tensor diffusion
scanning have enormously increased our understanding of how the brain works
and how neural sy stems integrate and dissect data from the environment, always
providing us with newer paradigms for further exploration about our brains
and behavior. In time, they will do the same for those of our closest relatives,
the apes, in particular the bonobo and chimpanzee (see in particular Semendeferi
et al. 2010).
Human Brain Asymmetry
The cerebral cortices of the human brain are usually asymmetrical and tend to grow
in a torqued manner, reflecting minor differences in maturation rates. The hemi-
spheres are seldom, if ever , equipotential in terms of functioning. Our left hemi-
sphere is often characterized as “analytic” and involved with language tasks, while
our right hem isphere appears most competent in visuospatial integration and is
often thought of as the “intuitive” or “gestalt” hemisphere. These characterizations,
1978 R.L. Holloway
while crude, hold up fairly accur ately for right-handers and many ambidextrals.
From radiographic studies, it was possible for LeMay (1976) to ascertain differ ent
petalia patterns for right- and left-handed humans with a high degree of precision.
These petalias are small extensions of cerebral cortex that extend farther in one part
of a hemisphere than on the other side. For example, we speak of a left-occipital
right-frontal torque pattern of petalias as occurring with high frequency in right-
handed indivi duals. This means that the left-occipital lobe bulges somewhat more
posteriorly on the left hemisphere, while the right hemisphere is somewhat broader
in width in the frontal lobe. In true left-handers, who make up about 8–10 % of
human populations, the pattern is reversed, meaning they exhibit a right-occipital
left-frontal pattern. Petalia patterns for a large collection of apes indicated that
while chimpanzees, gorillas, and orangutans sometimes demonstrated asymmetries,
they did not show the particular torque pattern described above as frequently. The
gorilla, incidentally, was the most asymmetrical of the apes (Holloway and de
LaCoste-Lareymondie 1982). On the other hand, brain asymmetries, particularly in
the planum temporale (temporal cortex) of the chimpanzee, show a strong left-
hemispheric size difference compared to the right (Gannon et al. 1998). This is
simply puzzling as we do not have any evidence that chimpanzees use this structure
in communication as do humans, and the fact that we share this difference with
chimpanzees suggests that brain organizational features relating to complex cogni-
tive functioning has been around for at least 5–7 myr. As our noninvasive scanning
techniques become more sophisticated, we can expect to learn how these
asymmetries function in animals other than ourselves. In fact, asymmetries appear
in many animals and are hardly unique to primates (Hopkins and his colleagues
have been in the forefront in demonstrating chimpanzee asymmetries and possible
handedness: Hopkins and Nir 2010, Gomez-Robles et al. 2013, and references). It is
probably the degree of asym metry which is important in distinguishing humans
from other primates (Balzeau and Gilissen 2010; Balzeau et al. 2012). Wey
et al. (2013) have recently shown that intrinsic connectivity networks are more
complex with regard to asymmetry of frontoparietal connectivity in humans com-
pared to nonhuman primates. These connections probably, in part at least, account
for the usual petalial asymmetries that appear more frequently in human brains.
Hominid brain endocasts, when complete for both sides (unfortunately, this is
very rare), allow the paleoneurologist to assess the cerebral asymmetries, and
indeed, even australopithecines appear to show beginnings of the right-handed
torque pattern found in humans, and, as one progresses through time, the petalia
patterns become more accentuated in the modern human direction. If we add to
these observations those of Toth’s (1985) studies on the early stone tools (chapters
Overview of Paleolithic Archaeology,” Vol. 3 and Modeling the Past:
Archaeology,” Vol. 1) of about 2 myr, which strongl y suggest right-handedness,
this underlines the fact that our early ancestors’ brains, despite their small sizes
(sometimes within extant apes ranges), were reorganized and that they probably had
some modes of cognition very similar to our own (chapters Overview of
Paleolithic Archaeology,” Vol. 3 and Modeling the Past: Archaeology,” Vol. 1).
The Evolution of the Hominid Brain 1979
Synthesis: Putting Together Size, Organization, and Asymmetry
During Human Evolution
As mentioned earlier, human brain evolution has clearly been a process of inte-
grating neurogenomic processes that led to increased size of the brain (hyperplasia
and hypertrophy), and thes e neurogenomic changes also played roles in the reor-
ganization (quantitative shifts) of neural nuclei, fiber tracts, and cortical cytoarch-
itectonics. In addition, it is proba ble that other changes occurred at the
neurochemical level, involving neurotransmitters and receptor sites, but these are
not well known from the comparative record, let alone the fossil one. This integra-
tion was sometimes gradual, sometimes “punctuated,” at least based on the fossil
hominid record currently available. The only reliable evidence from
paleoneurology suggests that Brodmann area 17 (PVC) was reduced early in
hominid evolution, signs of the reduction being clear in A. afarensis some
3–3.5 myr. While this would have meant a relative increase in posterior parietal
cortex (area 39) and peri- and parastriate cortex (areas 18 and 19, respectively), the
faithfulness of sulcal impressions does not allow for unambiguous definition of
these areas. Similarly, it is not possible at this time to measure and delineate
remaining areas of the temporal cortex and superior parietal lobule unambiguously.
What is suggested, however, is that visuospati al abilities were most probably
cognitively enhanced early in hominid evolution. It is not until we come to
H. rudolfensis ca. 1.8 Ma that a case can be made for some frontal lobe reorgani-
zation in the third inferior frontal convolution, Broca’s area. Thus, it would appear
there was a gradient of cerebral reorganizationa l changes starting posteriorly and
progressing anteriorly. Table 4 outlines these changes.
More recently, Falk et al. (2012) have argued that the Taung A. africanus
specimen possessed an open metopic suture that allowed the prefrontal lobe to
expand and widen despite the pelvic constraints thought to exist for this species in
relation to bipedal locomotion. These authors then expanded this idea to several of
Table 4 Summary of reorganizational changes in the evolution of the human brain
Brain changes, reorganizational Taxon
1. Reduction of primary visual striate cortex, area 17, and
a relative increase in posterior parietal and temporal
cortex, Brodmann areas 37, 39, 40, as well as 5 and 7
Australopithecus afarensis and
Australopithecus africanus
2. Reorganization of frontal lobe (3rd inferior frontal
convolution, Broca’s areas 44,45, 47)
Homo rudolfensis and early Homo
3. Cerebral asymmetries, left-occipital right-frontal
petalias
Australopithecines and early Homo
4. Refinements in cortical organization to a modern Homo
sapiens pattern
Homo erectus to present
Source: Holloway 1997. Note: (4) is inferred, as brain endocasts cannot provide that level of detail
necessary to demonstrate the refinements in cortical organization from surface features alone.
Areas 18 and 19 are peri- and parastriate cortex just anterior to area 17 and are included in
posterior association cortex here
1980 R.L. Holloway
the specimens regarded as early Homo, without providing any detailed evidence.
Unfortunately, a newer study using micro-CT scanning (rather than medical CT
scans) failed to show any evidence of a metopic suture except for a possible small
portion just superior to nasion (Holloway et al. 2013), strongly suggesting that the
infant metopic suture had already fused from nasion to bregma.
Table 5 outlin es the major size changes in the human brain during its evolution-
ary odyssey. Paleoneurological data simply are not detailed enough to integrate the
two tables of size and reorganizational changes into one holistic sequence of events.
Basically, the paleontological record supports an early reorganizational change
resulting in an increase in posterior cortex associated with visuospatial processing,
perhaps accompanied by a relative small allometric increase in brain size from
A. afarensis to A. africanus. This would correlate well with geological and pale-
ontological evidence that shows that early hominids were expanding their ecolog-
ical niches (chapter The Paleoclimatic Record and Plio-Pleistocene
Paleoenvironments,” Vol. 1) and becoming more diverse in their subsistence
patterns in mixed habitats. We know this based on the fact that stone tool types
are becoming standardized in form, tool inventories grow larger, and right-
handedness is highly probable. With the advent of Homo, we find strong evidence
for a major increase in brain size, both allometric (related to body size) and
non-allometric, and a reorganized frontal lobe, broader and showing a more modern
humanlike Broca’s area. This suggests that there had indeed been some strong and
dramatic selection pressures for a somewhat different style of sociality, one perhaps
based on a primitive proto-language that had some arbitrary symboling elements, as
suggested by the standardization of stone tools (e.g., Acheulean hand axes) (chapter
Dispersals of Early Humans: Adaptations, Frontiers, and New Territories,”
Vol. 3) that suggest social cohesion and control mediated through symbolically
Table 5 Brain size changes in hominid evolution
Brain changes Taxon Time (myr) Evidence
1. Small increase,
allometric
a
A. afarensis to
A. africanus
3.5–2.5 Brain endocast increase
from ca. 400 to 450 + ml
2. Major increase, rapid,
both allometric and
non-allometric
A. africanus to
H. habilis,
H. rudolfensis
2.5–1.8 KNM–1470, 752 ml
(300 ml increase)
3. Modest allometric
increase in brain size to
800–1,000 ml
H. habilis to
H. erectus
1.8–0.5 H. erectus brain
endocasts and
postcranial bones
4. Gradual and modest size
increase to archaic
non-allometric FOXP2
H. erectus to
H. sapiens
neanderthalensis
0.5–0.075 Archaic H. sapiens,
Neanderthal endocasts
1,200–1,700 + ml
5. Small reduction in brain
size among modern
allometric
H. sapiens
H. sapiens
sapiens
0.015–present Modern endocranial
volumes
Source: Holloway 1997 and more recent endocast data Holloway et al. 2004
a
Related to increase in body size only
The Evolution of the Hominid Brain 1981
based communication (Holloway 1981). Needless to say, this is only one specula-
tive account of the evidence. But from about 1.8 to roughly 0.5 myr, we think there
were minor allometric brain size increases to the earliest Homo erectus hominids of
Indonesia and China, where brain sizes ranged from 750 to 1,250 ml in volume. We
have very little evidence for body sizes, but we believe, on the basis of the
KNM-WT 15,000 Nariokotome youth from Kenya at ca. 1.6 myr, that these did
not differ significantly from our own.
This is also a time during which cerebral asymmetries are becoming more
strongly pronounced. With the advent of Archaic H. sapiens, about 0.15–0.2 myr,
we find brain sizes well within modern human values and no evidence for further
allometric increases, except possibly for the Neanderthals, in which it can be argued
that larger brain and body sizes (lean body mass: bone and muscle) were adaptations
to colder conditions. If further changes took place in cerebral and/or subcortical
organization, they are simply not apparent from a paleoneurological perspective. Yet
the Upper Paleolithic is the time when cave art makes its appearance, and one cannot
help but wonder whether the explicit use of art involving symbolization might not
also have been the time for the emergence of full language (see, e.g., Klein 2009).
However, there is nothing in the direct fossil evidence, and in particular
paleoneurology, to provide any evidence for such views. Claims for a single
mutation are extremely speculative, and while some genes have been identified
(chapters Genetics and Paleoanthropology, Vol. 1 and Homo ergaster and
Its Contemporaries,” Vol. 3) such as the FOXP2 (also in Neanderthals), these also
involve more general aspects of cognition. It is more likely that stone tool making
and its underlying cognitive elements are very simi lar to language, if not partially
homologous (Holloway 1969, 1981, 2012; Stout 2006). Finally, it would appear
that there has actually been a small reduction in brain size, probably allometric in
nature, from about 0.015 myr to the present (Henneberg 1988; Hawks 2012).
The totality of evidence shows that the brain has always been evolving during
our evolutionary journey, with myriad changes taking place at different tempos
during different times. As suggested recently (Holloway 1997, p. 200):
In sum, the major underlying selectional pressures for the evolution of the human brain
were mostly social. It was an extraordinary evolutionary ‘decision’ to go with an animal
that would take longer to mature, reach sexual maturity later, and be dependent for its food
and safety upon its caretakers (parents?) for a longer period of time. The benefits for the
animal were many, including a longer learning period, a more advanced, larger, and
longer-growing brain, and an increasing dependence on social cohesion and tool making
and tool using to cope with the environments that they encountered. Needless to say,
language abilities using arbitrary symbol systems were an important ingredient in this
evolution.
The fossil record shows us that there was a feedback between the complexity of stone
tools (which must be seen as a part of social behavior) and increasing brain size and the
expansion of ecological niches. The ‘initial kick,’ however, the process that got the ball
rolling, was a neuroendocrinological change affecting regulatory genes and target tissue-
hormonal interactions that caused delayed maturation of the brain and a longer growing
period, during which learning became one of our most important adaptations.
1982 R.L. Holloway
These ideas have been detailed elsewhere (Holloway 1967, 1969, 1980, 1996,
2010), where more details may be found.
Finally, Fig. 10 provides the often-seen relationship between time and
endocranial volume, and as should be apparent, t here is considerable overlap
between fossil groups and considerable variation within each taxon (e.g.,
H. erectus). Needless to say, such depictions cannot reveal the complex interac-
tions between phases of reorganization, size increases through hypertrophy and
hyperplasia, asymmetries in between left and right sides, differ ent distributions of
neuroreceptors and neurotransmitters, and the intricate interactions between nat-
ural selection, environmental challenges, mutation, drift, sensorimotor adapta-
tions (think of the challenges of becoming fully bipedal), social behavior,
communication skills, emotions, e tc., all of whi ch were operati ng during the
whole of hominid brain evolution, each having some necessary relationship to
neural reorganization, both cortical an d subcortical. I hope the point is obvious
thatwhilewehavelearnedmuchoverthelast century from the fossil, compara-
tive, and neurogenomic evidence, we remain almost totally ignorant of how it
really happened.
H. sapiens neand.
H. sapiens neand.
H. soloensis
H. antecessor
H. antecessor
H. heidelbergensi
s
H. erectus
H. erectus
H. georgicus
H. georgicus
H. ergaster
H. ergaster
H. rudolphensis
H. habilis
H. habilis
MILLION YEARS AGO
0.0 .5 1.0 1.5 2.0 2.5 3.0 3.5
A. boisei
A. robustus
A. garhi
A. africanus
A. ethiopicus
A. afarensis
1800
1600
1400
ENDOCRANIAL VOLUME
1200
1000
800
600
400
200
Fig. 10 Endocranial volume plotted against time, showing an accelerated change in volume from
Homo erectus on to anatomically modern Homo in the late Pleistocene. This figure cannot include
times of reorganization events, changes in neurogenomic elements, or any of the finer-grained
differences in morphology of the endocasts. It is important to observe overlap of endocranial
volumes, as well as their variation within taxa
The Evolution of the Hominid Brain 1983
And to the Future?
There appear to be two common presumptions about our future brain evolution. One is
that our biological evolution has stopped. The second is that our brains will continue to
grow in size, with bulging frontal lobes, to handle our growing dependence on
technology. What we have witnessed from the past fossil record is that our brains
and bodies work largely in allometric fashion, and given the high metabolic cost of
operating bigger brains (about 20–25 % of our metabolic resources go to supporting our
brains, which constitute only 2 % of our total body weight), the second scenario seems
highly unlikely. To demonstrate the first scenario would require vast amounts of
information from each generation of many living populations: feasible perhaps, but
not currently being collected. Furthermore, it is quite controversial whether brain size
has any close relationship to intelligence; however, intelligence is actually defined and
measured. Recent research based on MRI determinations of brain volume and selected
batteries of cognitive tests have shown correlations between test scores and brain
volume ranging from 0.4 to 0.6 (Andreasen et al. 1993; Anderson 2003; Davies
et al. 2011). Most recently, Burgaleta et al. (2013) have found significant relationships
between Full Scale, Performance, and Verbal IQ scores and cortical thickness in their
study of cortical thickness development in children and adolescents. As more sophis-
ticated imaging and neurogenomic advances are made, it would appear that our genes
andepigenomicprocesseshavemuchtodowithbrainbiologyandfunction.Butif
protein resources were to nosedive throughout the world for a significant period of
time, selection would probably favor smaller body sizes in our species, and that could
result in smaller brains, given an allometric relationship of roughly 0.3 between
stature and brain size, at least in males (Holloway 1980). While genetic engineering
may well provide some respite from the correlation between the ever-increasing mass
of humanity and ecological and nutritive degradation, this too is likely to be nothing
more than short-term fending off of the unstoppable future. These degradations are
part and parcel of the human brain’s capacity to ignore warnings that should properly
curtail greed and stupidity. The paleontological record for most mammals suggests
that genera (such as Pan, Homo, Canis, Notocherus, etc.) typically span approxi-
mately 5–10 Ma. Our genus has thus far a duration of about 2 myr. We, as a genus,
despite our largish highly encephalized brains, have another 3 myr to go if we wish to
be as successful in the paleontological longevity game.
Conclusions
Minor controversies notwithstanding, the evolution of the human brain has been an
intermingled composite of allometric and non-allometric increases of brain volume
and reorganizational events such as the reduction of primary visual cortex and a
relative increase in both posterior association and (most probably) prefrontal cortex,
as well as increased cerebral asymmetries, including Broca’s and Wernicke’s
regions, with some of these changes already occurring in australopithecine times.
As outlined in Holloway (1967), positive feedback (amplification deviation) has
1984 R.L. Holloway
been a major mechanism in size increases. Exactly how this me
´
lange of organs
evolved will require many more paleontological discoveries with relatively intact
crania, an unraveling of the genetic bases for both brain structures and their
relationship to behaviors, and a far more complete picture of how the brain varies
between male and female and among different populations throughout the world.
After all, the human brain is still evolving, but for how long is quite uncertain.
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