DEVELOPMENTAL TIMING

In chapter

2,

we provided a brief sketch of human evolution, from Ardipithecw: rumidus and the various species of Australopithecines

,

through Homo habilis, Homo erectus, and the rest of the hominid family to modem humans. According to fossil and

DNA

evidence, we last shared a common ancestor with modern apes between

5

and

7

million years ago. Judging from fossil evidence, our common ancestor was likely similar in physical and social structure to modem chimpanzees.

By

what mechanisms did our genetic line evolve bipedality and, later, a big brain, among the other physical differences between Homo supiens and Pun troglodytes? The mechanisms involved in such changes are genetic, of course. But only a small percentage

87

of

DNA

actually “builds” things. Most

DNA

in the genome is inactive throughout the life of an organism. Some genes, however, regulate develop- ment

by

turning other genes on and off and, in this way, play a vital role both in ontogeny and in phylogeny.

In chapter

3,

we introduced the concept of heterochrony, genetic-based differences in developmental timing (de Beer,

1958; S. J.

Gould,

1977;

McKinney, 1998; Shea, 1989, 2000). de Beer

(1958)

originally proposed eight types of heterochrony, and more recently Shea (1989) and McKinney and McNamara

(1991)

suggested that this number be reduced to six. For our purposes, we can collapse the various forms of heterochrony into two general types: those in which development is in some way retarded relative to the development of one’s ancestors (sometimes referred to as paedomorphosis, neoteny

,

or underdevelopment) and those in which development is accelerated relative to one’s ancestors (sometimes referred to as peramorghosis, hyper- morphosis, or overdevelopment). We generally use the terms retardation and acceleration to refer to these two general forms of heterochrony.‘

Humans as a Neotenous Species

As discussed in chapter

3,

for most of this century, evolutionary biolo- gists and anthropologists have argued that human evolution is especially characterized

by

a retardation of development, particularly

by

the form of retardation termed neoteny (Bolk,

1926;

de Beer, 1958; Garstang,

1922; S.

J.

Gould,

1977;

Montagu, 1989; Schwartz,

1999).

Neoteny (literally “holding youth”) refers to the retardation of development or, more specifically, to the retention into adulthood of ancestral embryonic or youthful stages.

How can humans be described as a “retarded” or neotenous species?

Most claims made

by

advocates of “humans as a neotenous species” were based on the general juvenile characteristics of adult humans in comparison to adult apes. For example, humans retain into adulthood many embryonic or infantile characteristics of our distant ancestors (Bolk, 1926;

S. J.

Gould,

1977;

Montagu, 1989). These include the shape of the head and face, late eruption of teeth, the size and orientation of the pelvis, a delicate (or gracile) skeleton, and a nonopposable big toe, among others. Using the chimpanzee (Pun troglodytes) as a model for what our distant ancestors may have been like (see Wrangham & Pilbeam, in press), we find many features in adult humans that resemble those of infant chimps. For example, at

birth,

both

‘Following McKinney and McNamara (1991), the three types of paedomwhosis, or retardation, are progenesis, or earlier onset of some aspect of development; neoteny, or reduced rate of development;

and post-displacement, or delayed onset of development. The three forms of peramorphosis, or acceleration, are hypermmpghosis, or delayed offset of development; acceleration, or increased rate of development; and pe-displacement, or earlier onset of growth.

humans and apes (and many other species) have rounded heads that are large relative to their bodies with adult-sized eyes, round cheeks, a flat nose, short arms, and relatively little hair. Humans maintain into adulthood, to varying degrees, many of these characteristics more so than other primates.

Looked at another way, humans, compared to other primates, show little developmental change in form relative to the fetal stage. For example, the shape of the human skull goes through relatively minor changes in shape over infancy and childhood, whereas such changes are substantial for chimpanzees (see

S.

J. Gould,

1977).

Great apes and humans look startlingly similar in infancy compared to their appearance at adulthood. These infantile characteristics are endearing to adults; those helpless babies who keep adults awake all night are so “cute,” it is hard not to love them-a very adaptive characteristic, indeed. The Nobel prize-winning ethologist Konrad Lorenz proposed that caretaking behaviors in many animals are triggered by infants’

immature features. These features are found not only in infants, but also in loveable cartoon characters (Mickey Mouse), dolls (Cabbage Patch Kids), and make-believe movie creatures

(E.T.).

Science fiction writers and

UFO

aficionados seem to be aware of this evolutionary trend because they typically describe futuristic humans and visiting space aliens as short, hairless creatures with large heads and big eyes. The implicit assumption here is that if human evolution continued (or if evolution occurred elsewhere in the universe to produce a more intellectually advanced creature), the result would be an even more infantile-looking adult.

Another neotenous characteristic of humans is the orientation of the vagina (see Montagu, 1989). In chimpanzee and human fetuses, the vagina slopes toward the front of the body.

As

chimpanzees grow, the vagina slopes more toward the back and, as a result, copulation is done most easily (and frequently) from the rear. In humans, the vagina retains its forward-sloping position, so that copulation can be done face to face. Such face-to-face copulation may have facilitated the formation of pair bonds, with both the male and female working together to help raise their offspring. This is particularly important because human children are dependent on their par- ents for so many years, making the establishment of stable “families” critical (see Crook, 1980;

S. J.

Gould, 1977; Montagu,

1989).

Groves

(1989)

provided evidence that neoteny may have played a role in human evolution in his careful analysis of the characteristics of hominid skeletons from Austrulopithecines through Homo sujiens. In comparison to their presumed ancestors, Austrulopithecines displayed relatively few juvenile traits. In contrast, Groves noted that Homo habilis, possibly the oldest species in the Homo line, possessed a number of juvenile features that differentiated it from its presumed ancestors, including: reduction in size of the molars and canines, smaller noses, more gracile cranial form, reduced facial height, and smaller jaws, among others. Many of these and other features went

THE BENEFITS OF YOUTH

89

through further juvenilization again in Homo erectus and Homo supiens.

Further changes were observed between more closely related species, with the extinct Homo neunderthaknsis displaying fewer juvenile features than fully modem humans (Homo supiens).

The viewpoint from theorists earlier in this century that differences in developmental timing (particularly neoteny) played an important role in phylogeny, was, in part, in reaction to the long-held dogma of evolutionary theory that new species arise by the addition of features to ancestral adults.

This was the central tenet of the biogenetic law as postulated by Emst Haeckel in the latter part of the 19th century (see discussion of biogenetic law in chapter 3 ) . To recapitulate, the theory held that the development of the individual goes through the same sequences as the evolutionary development of the species, with evolutionary additions being added to the adult stages of an organism. Neoteny is an obvious violation of the biogenetic law.

Does Neoteny Really Describe Human Evolution?

The idea of humans as a neotenous species has been a favored interpre- tation by developmental psychologists with an evolutionary perspective (Bjorklund,

1997b;

Cairns,

1976;

Mason, 1968a). They viewed important aspects of human morphology and behavior as the result of retarded develop- ment and saw Homo supiens, in general, as a neotenous species. Homo supiens retained their juvenile appearance and certain aspects of juvenile behavior (e.g., play) into adulthood, and required a much longer time than their primate cousins to reach reproductive maturity.

More recently, however, the general neotenous nature of humankind has been seriously questioned (Gibson,

1991;

McKinney, 1998,2000; McKin- ney

6r

McNamara,

1991;

Parker,

1996;

Parker

6r

Gibson, 1979; Parker &

McKinney, 1999; Shea,

1989, 2000).

For example, many aspects attributed to neoteny are due not to true neoteny, in which ancestral patterns are retarded so that the descendants do not develop as far as their ancestors

did,

but merely to an extension of the developmental period (Rice,

1997).

This is a form of retardation, but it is not true neoteny. More critically, some aspects that have been classified as neotenous have been proposed to be due to other processes. For instance, the facial similarities of skulls of adult humans and infant apes have been proposed to be due to specific remodeling of the human face (see Shea, 1989), or even as a structural consequence of a large brain (Deacon,

1997),

and not to an actual retardation of development.

In fact, some researchers have argued that acceleration is the predomi- nant characteristic of human evolution (McKinney, 1998,2000). For exam- ple, human brain and cognitive development clearly exceed that of other primates. This most characteristic of human qualities cannot be seen as anything other than acceleration of development beyond that of our ances-

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.