EXTENDED GROWTH OF THE HUMAN BRAIN

tors. Other accelerated features of human development include the early fusion of bones in the wrist and early descent of the testes (see Shea,

1989).

Although acknowledging that some aspects of human evolution may have been brought about via neoteny, anthropologist Brian Shea

(1989),

in perhaps the most complete evaluation of neoteny in human development, stated that “a hypothesis of general and pervasive human neoteny is clearly no longer viable.

A

careful analysis of human development, morphology, and life history patterns reveals little concordance with predictions of neo- teny based on accepted criteria” (p.

97).

However, Shea

(1989) did

concur with earlier neoteny theorists

(S. J.

Gould, 1977) about the role of neoteny in influencing the relative size of the human brain and skull (see discussion below). However, the adult human brain itself shows no immature, or neotenic, features, but rather has more synaptic connections and fissures (as opposed to the smooth surfaces characteristic of fetal brains) of any mammal (see Gibson, 1991; Preuss,

2001).

We must concur with the critics of neoteny; humans’ evolution cannot be characterized as one of general retardation. Rather, human evolution reflects a mosaic of changes in ontogenetic trends, some notable for their accelerated character and others for their retarded character (see Shea,

2000).

In fact, we see aspects of retarded and accelerated development as often being different sides of the same coin. For example, we argue that our extended period of immaturity, coupled with the retention of fetal rate of brain growth in postnatal life-both examples of retardation-were neces- sary components for the expansion of the brain and cognition beyond that of our ancient ancestors-examples of acceleration.

example, given the typical pattern of changes in brain and body weight in mammals, brain weight should increase at a certain rate relative to increases in body weight. When a particular species’ brain is larger than expected for its body weight, the encephalization quotient will be greater than

1.0.

When a species’ brain weight is less than expected for its body weight, the ratio will be less than

1.0.

Using this technique, modem chimpanzees have an encephalization quotient of about 2.3. The encephalization quotient for modern humans, however, is more than triple this, about

7.6

(Jerison, 1973;

Rilling & Insel, 1999).

Another way of viewing the brain-body weight ratio is to plot the relation between the two factors and compute a regression line that reflects what size brain an animal of a certain weight should have. (This line is essentially equivalent to an encephalization quotient of

1

.O.) Species above this line have “more brain” than expected. Figure

4.1

graphs the brain-body weight relation for a selection of species, including humans. The distance a species is from the regression line can be interpreted as the residual variance in brain weight after removing body weight (Allman,

1999).

Species above the regression line have “more brain” than expected for their body weight and, presumably, can devote more nerve cells to nonbodily functions. From this perspective, the residual above the regression line can be loosely thought of as the brain mass available for “intelligence” after subtracting the brain processes associated with maintaining basic

bodily

functions (Deacon,

1997).

As can be seen, humans and porpoises have the highest positive residuals of any animal.

This

pattern of enlarged brain relative to body size is also found in the fossil record for our hominid ancestors. Figure

4.2

presents average encephalization quotients for four species of hominids over the past

3.5

million years based on fossil skulls (of Austrulopithecus afurensis, Homo habilis, Homo erectus, and Homo supiens) and for modem chimpanzees. As can be seen, the encephalization quotient for Austrulopithecus afurensis was only slightly greater than that of modern chimpanzees. From this point on, brain weight relative to body weight increased at a rapid rate.

that an animal‘s brain size varies as a function of its body size raised to the power of 2/3. The 2/3 exponent implies a surface-to-volume relationship, such that brain size is regularly related to a body’s surface area and not to its actual size. The equation for computing the expected brain weight for vertebrates as a function of body weight is Y = W, where Y and X are brain and body weights, respectively; k is a constant; and a is the exponent for body weight. When dealing with vertebrates, a = 2/3, although this value may vary for more specific groups of animals (e.g., only primates).

Because the ranges of brain and body weights for different animals vary greatly, they are customarily transformed to a logarithmic scale so that log Y = a log X ⁺ log k. This yields a linear equation of the relationship between log Y and log X with a slope of a. Log k is the intercept and has the value of log Y when log X ^{= 0.} The constant k will vary as a function of which groups of animals are being examined. The encephalization quotient (EQ) is computed as a ratio of a species’ actual brain weight to its expected brain weight, or EQ = actual brain weight/expected brain weight.

10,000 3 5,000

=

Elephant Modern Man Porpoise

\ /

Blue / Mole

+

1,000 : 500

=

h b 100: Y E 50- 01 .-

2

10.0: E m 5.0- 1.0, 5- Figure 4.1. Selected brainlbody weight relations. Note. From Evolution of the Brain and Intelligence (p. 44), by H. J. Jerison, 1973, New York: Academic Press. Copyright 1973 by Academic Press. Adapted with permission. v3 bJ

7

t

2

P troglodytes A. afarenis H. habilis H. erectus H. sapiens

Figure 4.2. Encephalization quotients for chimpanzees (Pan troglodytes) and four hominid species.

Note. The data for chimpanzees are from Evolution of the Brain and Intelligence (p.

44), by H. J. Jerrison, 1973, New York: Academic Press. Copyright 1973 by Academic Press. Reprinted with permission. The data for hominids are from “The Brain of Homo habilis: A New Level of Organization in Cerebral Evolution,” by P. V. Tobias, 1987, Journal of Human Evolution, 16, pp. 741-761. Copyright 1987 by Academic Press.

Reprinted with permission.

Big Brains as a Product of Delayed Development

This enlarging of the brain was achieved, in large part, however, by maintaining the rapid rate of prenatal brain growth into postnatal life. The rate of prenatal brain development is remarkably similar for all primates, including humans (see Bonner,

1988).

The brain develops rapidly in compar- ison to the overall size of the body. Brain growth slows quickly after birth for chimpanzees, macaque monkeys, and other primates, but much less so for humans. The pace of human brain development begun prenatally continues through the second year of postnatal life (see

S. J.

Gould, 1977).

By 6

months the human brain weighs

50%

of what it will in adulthood; at

2

years about 75%; at

5

years, 90%; and at

10

years, 95% (Tanner, 1978).

In contrast, total body weight is about

20%

of eventual adult weight at

2

years and only

50%

at 10 years.

This

extended period of brain growth is afforded

by

a prolongation of the closure of the cranial sutures well into

the third decade of life. Thus the brain, which grows rapidly before birth, continues its rapid development postnatally.

Increasing the time the brain grows and the number of neurons that are produced results in a larger brain (Finlay & Darlington, 1995; Finlay et al., 2001). The delay in brain maturation also results in the extension of dendritic and synaptic growth, so that the human brain has more intercon- nections among neurons than the brains of other primates (Gibson,

1991).

However, different parts of the brain have not been equally affected in human evolution. The human neocortex has been estimated to be about 200% the size expected for an ape of comparable body size (Barton &

Harvey, 2000; Deacon,

1997;

Eccles, 1989; Rilling & Insel, 1999). The prefrontal cortex has been implicated in complex human cognition (Fuster,

1984;

Luria, 1973); has connections to other areas of the brain, including the limbic system (Fuster,

1984);

and has been hypothesized to be the locus of important inhibitory mechanisms (Harnishfeger,

1995;

see Dempster &

Brainerd, 1995). Most other areas of the human brain are also substantially larger than expected for an anthropoid ape, although none approach the size difference of the neocortex, and some areas actually are smaller (the olfactory cortex and some areas related to vision and motor control). Thus, although the extension of prenatal brain growth rates contributed to the overall size of the human brain (a form of retardation of development), it cannot account for the differential rate of change for different areas of the brain. Instead, different selection pressures (e.g., for language) must have played a role in shaping the size and structure of various areas of the Homo supiens brain, presumably after (or during) the period when the brain was increasing in overall size (see further discussion in chapter

5).

The retention of embryonic growth rates for the brain into the first two years of postnatal life was necessitated in part

by

some physical limitations of human females.

If

a species is going to have a big brain in relation to its body, it

will

also, of course, have a big skull. But the skull that houses a 2-year-old human brain is far too large to pass through the birth canal of a human female. The evolutionary pressures that resulted in an enlarged brain required that gestation be extended only to the point where the infant skull

will

fit through the birth canal.

If

humans were as well developed bodily at birth as their simian cousins, their heads would never fit through the birth canal, which is limited in

width

because of the constraints of bipedality. The result is a physically immature infant, motorically and percep- tually far behind other primate infants (see Antinucci,

1989;

Gibson,

1991).

However, human brain and cognitive development soon accelerate relative to their primate cousins (due, in large part, to the retention of fetal brain growth rate ), all while their physical development remains delayed (Langer, 1998; McKinney, 1998).

THE BENEFITS OF YOUTH

95

Big Brains and “Intelligence”

Although absolute differences in brain size between species may be informative, it should not serve as a simple index of how intelligent an animal is. That is, we are not arguing, as others have

(A. R.

Jensen, 1980), that animals can be arranged on a linear scale in terms of intelligence (as reflected

by

brain size in the current discussion, or

by

a general intelligence factor, g). Over evolutionary time, the nervous systems of different species have evolved to solve recurring problems specific to their niche. Many species have evolved highly specific behavioral responses, or have particular sensory apparatus that is foreign to humans or higher primates (echolocation in bats; certain aspects of migratory behavior in some birds and insects).

The ecological success of such animals argues for the position that each species is smart in its own way (Deacon,

1997; S. J.

Gould, 1996). Further- more, a basic tenet of evolutionary psychology is that humans possess a wide range of domain-specific abilities, which is at odds with the conception that “bigger is always better.”

We concur with this central assumption of evolutionary psychology, but only to a certain degree. We find it unlikely that big brains (larger than expected for an animal’s body size) contain only more domain-specific modules, although some of the neural additions of big brains are surely dedicated to domain-specific skills. We believe, rather, that large brains afford greater plasticity of learning and enhanced memory. These more general abilities can provide sufficient processing capacity in terms of greater working memory or faster speed of processing, for example, for the effective operation of more domain-specific mindbrain modules, such as those in- volved in the ability to detect cheaters or to perform theory-of-mind prob- lems. Thus, we argue, species comparisons as a function of brain size can indicate plasticity of learning and memory, which are important components to the functioning of many animals, including humans, but less so for others. (The issue of the evolution of domain-general versus domain-specific cognitive abilities is discussed in chapter

5.)

Big brains, with enhanced learning and memory abilities, would not be adaptive for all animals.

A

strong relationship exists between brain size and longevity. Animals with bigger brains (and bigger bodies as well, of course) tend to live longer than animals with smaller brains. Long-lived animals are much more in need of bigger brains than short-lived animals.

A n animal that survives many years is likely to encounter a wide range of environments, some dangerous and others filled with valuable resources. It benefits such animals to have a nervous system that can readily learn and retain new information. This takes time, and, within mammals, brain size is also correlated with length of the juvenile period (discussed further below).

In contrast, a short-lived animal will have too few experiences and live in

too narrow a range of environments for it to benefit from the learning and memory advantages a large brain affords. Its inclusive fitness is better served by having more preprogrammed behavior patterns that depend less on “leam- ing” (as conventionally understood; see Bogin,

1999;

Deacon,

1997).

Brain size is also correlated (negatively) with litter size. Big-brained animals tend to have smaller litters and to give birth to infants at longer intervals than small-brained animals. This is the distinction of r versus

K

~election.~ Small-brained species tend to produce many offspring rapidly but invest relatively little care in each one. Most insects are r-selection species, laying thousands of eggs but providing no post-hatching care. In contrast, big-brained species are more likely to have fewer offspring but to invest more care in each one

(K

selection). Mammals, as an order, are such animals, with primates in particular having mostly single births that are typically spaced several years apart. Following the argument we presented earlier, big-brained animals require a sustained juvenile period in order to make good use of the organ residing in their skulls, and parents must therefore invest substantially in the raising of their offspring so that such learning can occur in a relatively safe environment. And possibly, big brains are useful in choosing a mate who

will

also invest in the offspring. Humans are at the extreme of

K

selection among mammals, spacing their offspring about

4

years apart (the spacing is actually longer for orangutans) and providing both maternal and paternal support for their offspring well into the second decade of life.