CONSEQUENCE TO HUMAN EVOLUTION Growing Up Slowly

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.

1978).

Based on these historical data and data from traditional cultures

(Hill 6r

^Hurtado,

^1996;

Kaplan et al.,

2000),

it is likely that our ancient ancestors were closer to

18-20

years of age before being fully reproductive.

The tendency toward retardation in Homo supiens can be viewed as an extension of a trend observed in primates in general relative to other mammals. For example, primates live longer and mature more slowly than most other mammals of comparable body size. The gestation period is ex- tended, with primates having substantially longer gestation periods for their birth weights than most other mammals. Similarly, puberty is typically attained in farm mammals when an animal has reached about 30% of its final adult weight. In contrast, humans and chimpanzees usually do not attain puberty until they are about

60%

of their final adult weight (see S. J.

Gould,

1977).

Thus, humans’ tendencies toward retardation, in at least some aspects of their development, seem an extension of a phenomenon apparent in primates rather than a phylogenetic innovation.

Humans’ prolonged immaturity is all the more impressive when one considers the likely life expectancy of our hominid ancestors. Modem women can expect to survive childbirth, and both men and women, once they reach their teenage years, can anticipate another

60

years plus on this planet.

This was not true, however, for our ancestors. Even today, the life expectancy of men and women in many developing countries barely reaches

40

years (and actually does not reach this modest level for men in some of the most impoverished nations). In the United States, life expectancy as little as

150

years ago was only

38

years for White men and

40

years for White women.

High

rates of infant mortality contribute significantly to these low values, so that the actual life expectancy for someone who survived past childhood in our not-too-distant past was likely beyond

50

years, similar to the age attained by some chimpanzees. Moreover, it seems likely that there have always been some “old” people in human groups, although they would have been the exception. The

3

score and

10

years promised in the Bible forecasts a rosy future, not the typical life expectancy an ancient human could realistically expect.

Thus, the

15-

to 20-year wait that our hominid and hunter-gatherer forebears had before reaching reproductive maturity must have come at great expense. Many must have died of disease or fallen prey to predators before ever having a chance to pass their DNA on to the next generation and, given the risks of childbirth, many females must have died giving birth to their children, leaving their infants motherless and dependent on the kindness of strangers. Even in colonial America it has been estimated that 20% of the deaths of adult females were associated with childbirth, with one in

30

births resulting in the death of the mother. The rates were surely higher for our large-brain predecessors, dating back at least 1.5 million years ago to Homo erectus and possibly 2.5 million years ago to Homo

habilis.

In

hindsight, human’s delayed maturation had substantial perils. Given the risks involved in our slow growth, the selective pressures for this delayed maturation must have been derived from strong compensatory advantages of the immature state, most notably increased flexibility of learning.

Human development is not only prolonged relative to other primates, but reflects stages of life history that may be unique to Homo sapiens.

Anthropologist Barry Bogin

(1997, 1999)

argued that human development is comprised of five stages-infancy, childhood, juvenility, adolescence, and adulthood-two of which, childhood and adolescence, are not observed in any other species. In other mammals, infancy ends with the cessation of weaning and is followed by the juvenile period, in which the young animal is no longer dependent on its parents for survival but is not yet sexually mature (adulthood). In humans, weaning typically occurs in traditional cultures at about ages

3-4

years, but children are not able to fend for themselves until at least about age

7.

(For example, they still posses “baby teeth,” requiring special food preparation to receive adequate nutrition.) Human adolescence, with its characteristic growth spurt, usually begins early in the second decade and persists until reproductive maturity, which in humans is in the late teens or early

20s.

(As we mentioned earlier, although girls typically have their first menstrual period at about age

13

years, there is a period of about

4.5

years of low fertility; a similar period of low fertility is found in boys.)

No

other species displays this rapid growth spurt before adulthood, although chimpanzees and bonobos also apparently have a post- menarche period of infertility (see Bogin,

1999).

Based on fossil evidence such as bone size and dental development, Bogin

(1997, 1999)

has estimated that the life stages of our australopithecine ancestors were similar to those of chimpanzees (Pun troglodytes): a period of infancy lasting

5

or

6

years, followed by a juvenile period, with adulthood beginning about age

12

years. According to Bogin, it is only with the beginning of the Homo line that a period of childhood is seen, and only in modern Homo supiens is there evidence for a period of adolescence. Age of reproductive maturity apparently increased gradually in the Homo lineage, ranging from about

12-13

years for Homo habilis ^to

14-15

years for Homo erectus, to the late teens and early

20s

for modem Homo sapiens. In addition to the emergence of childhood and adolescence, the length of juvenility and adulthood is longer in humans than in other primates and almost certainly longer than in our hominid ancestors.

Although many reasons have been proposed for the extension of the developmental period in humans and for the additions of childhood and adolescence (see Bogin,

1999))

the fact of the extended prereproductive period in humans suggests that our ancestors were more successful at keeping their offspring alive than were other primates.

Child

mortality rates in hunter-gatherer societies are about

50%

compared with

67%

to nearly

90%

THE BENEFITS OF YOUTH

99

in other primates (Lancaster & Lancaster,

1983).

Moreover, the lengthening of the developmental period, along with a threefold increase in brain volume since

A.

ufurensis, suggests a substantial increase in the complexity of homi- nid social systems and in the ability to exploit biological and physical resources (e.g., through tool use). An extended developmental period, along with increased social play and exploratory behavior (see chapter lo), would enable the refinement of increasingly sophisticated physical, social, and cognitive competencies.

Although Homo supiens have seemingly evolved many domain-specific

“programs” for dealing with specific problems and with other members of their species, humans, more than any other species, depend on learning and behavioral flexibility for their success. The complexities of human societies are enormous and highly variable, and it takes an extended childhood to acquire all that must be learned to succeed. Because brain growth continues well into adolescence, neuronal connections are created and modified long after they have become fixed in other species (Jacobson,

1969).

The result is a more “flexible” brain (in terms of what neural connections can be made), which means more flexible thinking and behavior. In addition, an extended youth provides the opportunity to practice complex adult roles which, because of their cultural variability and complexity, cannot be hard- wired into the brain.

Archeologist Steven Mithen (

1996)

has recently speculated that the slow brain growth of ancient Homo supiens was necessary to produce the cognitive architecture of modem humans. Consistent with contemporary assumptions of evolutionary psychology (Tooby & Cosmides,

1992),

he proposed that the hominid brain was modular, with separate components for social, technical (tool use), and natural history intelligence. ‘Cognitive fluidity,” which, Mithen claims, characterizes the modem mind, requires communication among these various modules (and general intelligence) and, he proposed, this requires an extended childhood to accomplish.

To

support his claim, Mithen pointed to evidence that brain development in Neanderthals, based on a discrepancy between the rate of dental and cranial development, was much faster than in modern humans (Akazawa, Muhesen, Dodo, Kondo, & Mizouguchi,

1995;

Stringer, Dean, & Martin,

1990;

Zolli- kofer, Ponce de Le6n, Martin, & Stucki,

1995;

but see Trinkaus & Tompkins,

1990

for an alternative interpretation). For example, based on dental devel- opment, a well-preserved skeleton of a Neanderthal infant was believed to be about

2

years old when it died. Yet the cranium size was equivalent to that of a modern 6-year-old

child

(Akazawa et al., 1995). Other fossil evidence supports the conclusion that Neanderthal brain development may have been completed substantially earlier than that of modem humans (Dean, Stringer, & Bromage,

1986;

Zollikofer et al., 1995). O n the basis of archeological evidence, Mithen proposed that Neanderthals demonstrated

minimal cognitive fluidity, and it was only through a prolongation of child- hood that the architecture of

the

modem brain could develop.

Humans are not the only slow-developing and big-brained primate.

As

we noted earlier, chimpanzees and orangutans in particular have extended juvenile periods relative to monkeys, and they also have larger brains. In fact, in primates, the size of the adult brain is related to the length of the juvenile period; species with longer prereproductive periods, on average, have larger brains (Bonner,

1980;

see Figure

4.3).

As

we have noted, human development is prolonged relative to other primates when considering body size. Chimpanzees and humans have about the same adult

body

weight, but humans take approximately

5-7

more years to reach reproductive maturity than

do

chimpanzees. The picture is a bit different if, rather than body size, length of the juvenile period is predicted as a function of brain size. For mammals in general, brain size predicts rate of maturation, including maximum life expectancy (see Allman,

1999).

1,300 1,200 1,100 1,000

-

900 ⁸⁰⁰ 700

*

⁶⁰⁰

m

e

500

-I

v a _N

C .-

.-

400 300 200 100 0

- -

-

-

- - - - - - - - -

0 2 4 6 8 10 12 14

Juvenile Period in Years

Figure 4.3. The brain sizes of various primates and humans as a function of the length of the juvenile period.

Note. From The evolution of culture in animals (p. 207), ^{by J. T.} Bonner, 1988, Princeton, N.J., Princeton University Press.

THE BENEFITS OF YOUTH

101

TABLE 4.1

Actual Maturation Stages and Predicted Maturational Stages From Brain Weight in Humans

Age (Years) Predicted From

Stage Brain Weight Actual

First molar 19.3 6.4

Second molar 29.2 11.1

Wisdom teeth 37.8 20.5

Sexual maturity 44.5 16.6

Maximum life span 101.5 105.0

Note. From Evolving Brains (p. 196). by J. M. Allman, 1999, New York:

Scientific American Library. Copyright 1999 by Henry Holt Co. Adapted with permission.

Among primates, brain weight predicts many aspects of physical maturation, for example, eruption of teeth and sexual maturity. Table 4.1 presents the actual and predicted age based on brain weight for several indices of matura- tion in humans.

As

can be seen, humans develop at a substantially faster rate than would be expected from brain size. (However, their maximum life expectancy is very similar to that predicted

by

their brain weight.) Brain Size, Maturation Rate, and Sociality

Large brains in primates are related not only to length of the juvenile period, but also to sociality. For example, Dunbar

(1992,

1995) has shown that, within primates, the relative size of the neocortex is significantly correlated with group size ( r =

.76).4

Several theorists have argued that primates, including humans, have evolved large brains to deal with conspe- cifics. That is, dealing with the challenges of cooperating and competing with other members of the social group was the driving force of intellectual evolution within the primate line leading to humans (Alexander,

1989;

Bjorklund & Harnishfeger,

1995;

Byrne & Whiten,

1988;

Geary & Flinn, 2001; Humphrey,

1976).

Large brains are associated with extended juvenile periods, resulting in the argument that a prolonged childhood and a large brain are necessary for primates to master the complexities of their societies.

+Dunbar (1992) noted that the correlation between the size of the neocortex and home range size was also significant, suggesting the possibility that foraging demands, and not sociability, were the driving force behind increasing brain size in primates. However, when controlling for body size, the statistical relation between group size and neocortex size was virtually unchanged, whereas the relation between size of the neocortex and home range size became nonsignificant. This pattern suggests that it was sociability, rather than foraging strategies, that was the primary selective pressure for an increase in brain size in primates.

Joffe

(1997)

presented direct evidence for this by comparing aspects of brain size and structure with length of the prereproductive period and aspects of social complexity for 27 primates, including humans. Joffe reported that the proportion of the lifespan spent as a juvenile was positively corre- lated with group size and the relative size of the nonvisual neocortex. This is the part of the primate brain that is associated with complex problem solving, including memory. Joffe argued that social complexity exerted selec- tion pressures for increased nonvisual neocortex in primates and an extension of the juvenile period.

We should be careful, however, in suggesting that big brains and an extended juvenile period were necessitated solely because of hominids’

increasing social complexity. Other nonsocial factors also may have provided selection pressure for these evolutionary effects. For example, Kaplan et al.

(2000)

proposed that a shift to a higher quality diet was primarily responsible for humans’ evolution of enhanced cognitive skills and an extended period of youth, which would be required to master the associated food-gathering skills. Kaplan and his colleagues noted that chimpanzees, for example, rely primarily on a diet of easily extracted fruit and plants with low nutrition density. Such foods, when available, can be obtained relatively easily even by juveniles. Although chimpanzees do hunt, they obtain only a small portion of their diet from vertebrate protein, which are foods of high nutrition density. Hunting, among both chimpanzees and humans, is engaged in mainly

by

adults (usually males) and takes considerable time to learn. Kaplan et al.

(2000)

examined food-gathering procedures in contemporary hunter- gatherer societies and noted that, similar to chimpanzees, young children often forage successfully for low-density, easily accessible foods, such as ripe fruit. In contrast, extracting foods of higher nutrition density, such as roots and tubers or vertebrate meat through hunting, are performed effectively only by older individuals and require many years to master.

Of

course, determining the direction of causality among these various characteristics is not possible. In fact, we argue that changes in multiple factors surely acted synergistically, with changes in one factor (e.g., rate of maturation or relative brain size) both influencing and being influenced by changes in related factors (e.g., complexity of social conditions). Although one cannot discern any simple causal relation among these variables in the evolution of human intelligence, it is their confluence and the positive feed- back among changes in these interacting variables that led to the evolution of Homo supiens. Alternative courses of evolution were possible, of course, and Homo supiens is not the inevitable product of a deterministic process involving increasingly large brains, social complexity, and delayed development. But these interacting factors were, seemingly, necessary for the evolution of Homo supiens and, from a developmental perspective, delayed ontogeny appears to

THE BENEFITS OF YOUTH

103

be the mechanism most susceptible to selection forces, and thus the linchpin in the evolutionary process that produced modem humans.

A

related argument for the importance of delayed development to human evolution centers on the foundation of the human family and social structures (Allman,

1999;

Crook, 1980;

S. J.

Gould,

1977;

Hattori,

1998;

Wesson,

1991).

The human infant is totally dependent at birth and will remain dependent on adults for well over a decade. Pair-bonding and some division of labor (both within and between families) may have been necessary adaptations to the pressures of slow-growing offspring to increase the likeli- hood that children would survive to sexual maturity. The long period of dependency also meant that a male’s genetic success could not be measured just

by

how many females he inseminated or

by

how many children he sired. His inclusive fitness would depend on how many of his offspring reached sexual maturity, assuring him of becoming a grandfather. To increase the odds of this happening, his help in the rearing of

his

children would be needed.

It has also been speculated that the juvenile features of human infants (babies are “cute”) invoke positive feelings toward the infant in both males and females, and such feelings in men foster attachment to and paternal care for the infant. In fact, Hrdy

(1986)

suggested that the father-child bond may have been the basis of the father-mother-child bonds in human families. Noting that male primates are unique in the mammal world in the attention and care they give to infants, Hrdy suggested that perhaps this capacity preadapted “members of this order for the sort of close, long- term relationships between males and females that, under some ecological circumstances, leads to monogamy” (p. 152).

BIG BRAINS, SLOW DEVELOPMENT, AND PLASTICITY