Language and life history: A new perspective on the development ...

Locke & Bogin: <strong>Language</strong> <strong>and</strong> <strong>life</strong> <strong>history</strong>tools, <strong>and</strong> a greater diversity of stone tool types associatedwith H. habilis than with any earlier hominins (Klein1989). There is considerable evidence that some of thesetools were used to scavenge animal carcasses, especiallyto break open long bones <strong>and</strong> extract bone marrow(Plummer 2004; Potts 1988). This behavior may be interpretedas a strategy to feed children. Such scavenging mayhave been needed to provide the essential amino acids,some of the minerals, <strong>and</strong>, especially, the fat (densesource of energy) that children require for growth of thebrain <strong>and</strong> body (Leonard & Robertson 1992; Leonardet al. 2003).A further increase in brain size occurred during the timeof H. erectus, which began about 1.9 million years ago. Theearliest adult specimens have mean brain sizes of 826 cc,but many individual adults had brain sizes between 840<strong>and</strong> 1059 cc (Coqueugnoit et al. 2004). Insertion or expansionof childhood would have provided the time needed fora rapid, human-like, pattern of postnatal brain growth. 16It should be noted in Figure 3 that the model of humanevolution proposed here predicts that from Australopithecusto H. erectus infancy shrinks as childhood exp<strong>and</strong>s. Inthe time of early H. erectus the transition from infancy tochildhood took place before M1 eruption. If the <strong>new</strong> childhoodwas stolen from infancy, or reduced its length, thenH. erectus would have enjoyed a greater reproductiveadvantage than any previous hominin. This seems tohave been the case, since H. erectus populations increasedin size <strong>and</strong> began to spread throughout Africa <strong>and</strong> otherregions of the Old World (Antón 2003).Figure 3 also shows later H. erectus, with an averageadult brain size of 983 cc <strong>and</strong> a further expansion of thechildhood stage. In addition to larger brains (somemeasuring 1100 cc), the archaeological record for laterH. erectus shows increased complexity of technology(tools, fire, <strong>and</strong> shelter) <strong>and</strong> social organization (Antón2003; Klein 1989). These techno-social advances, <strong>and</strong> aparallel increase in learning, may well reflect extendedchildhood (Bogin & Smith 1996). The evolutionary transitionto archaic, <strong>and</strong> finally, modern H. sapiens exp<strong>and</strong>sthe childhood stage to its current length.As for how childhood abetted language, we speculatethat there was a process of coevolution, according towhich an initially short childhood period facilitated theemergence of a small amount of vocal-verbal behavior.Even if the original cause of childhood was nonlinguistic,as we propose, any <strong>new</strong> time between infancy <strong>and</strong> juvenilitywould have provided the young with additional opportunitiesfor vocal-verbal interaction during a time ofincreased independence, <strong>and</strong> would have given adults<strong>new</strong> reasons to honestly inform any dependents in theircare (Fitch 2004). The disposition to do so is suggestedby the fact that primate mothers appear to prevent theiryoung from eating alien substances, <strong>and</strong> warn off younganimals that approach toxic fruits (Caro & Hauser 1992).Young hominins also would have needed to know aboutplants as well as game, tools, shelter, <strong>and</strong> predators.Even a small amount of vocal-verbal behavior wouldhave facilitated warnings <strong>and</strong> instruction. Childhoodlengthened, enabling more language which, in turn,extended childhood, until the <strong>new</strong> ontogenetic stagereached its present length. Childhood would thus haveoffered up <strong>new</strong> behaviors, ones that would also provebeneficial in the run up to sexual maturity.9.2. Evolution of adolescenceThis brings us to the evolution of the other uniquelyhuman stage of development. Nonhuman primates lackchildhood, but they also lack a post-juvenile period of dramaticgrowth of the sort that defines human adolescence.Unfortunately, very little is known about the evolution ofadolescence. It is possible, however, to suggest somebenefits of this late stage of development in terms of reproductivebiology, social ecology, <strong>and</strong> reproductive success.Our claim, detailed elsewhere (Bogin 1999a; 1999b;2003), is that adolescence became part of human <strong>life</strong><strong>history</strong> because it conferred significant reproductiveadvantages on our evolutionary ancestors, in part by allowingadolescents to learn <strong>and</strong> practice adult economic,social, <strong>and</strong> sexual behaviors before reproducing. As indicatedin section 2.7, the basic argument for the evolution<strong>and</strong> benefits of human adolescence is that girls bestlearn their adult social roles while they are infertile butperceived by adults to be mature, whereas boys bestlearn their adult social roles while they are sexuallymature but not yet perceived to be such by adults. Theproposed patterns correspond exactly to the course ofgrowth <strong>and</strong> development taken by boys <strong>and</strong> girls. Onaverage, healthy girls begin to develop adult-like fatpatterns (e.g., breast, hips) at about age 10 to 11 years,which is two years before the peak of the adolescentspurt. Menarche occurs a year after the spurt <strong>and</strong> isfollowed by about three years of adolescent sterility,ovulation being rare or absent. The female pelvis alsogrows slowly during adolescence <strong>and</strong> does not reach fulladult size, a necessity for successful birth, until aboutage 18 years. Externally, however, these infertile girlslook like women <strong>and</strong> this perception prompts adults toinclude the girls in a suite of adult social, economic,sexual, <strong>and</strong> political activities. The girls learn from involvementin these activities <strong>and</strong> arrive at young adulthood, atabout age 19 years, with considerable experience thattranslates into reproductive success. This may be whyhumans have the highest rate of birth survival of anyspecies: 50 to 60% in traditional foraging societies comparedwith 35% in chimpanzees (Lancaster & Lancaster1983). 17In the reverse pattern that characterizes male adolescence,fertile sperm are produced at about 13.5 years ofage, <strong>and</strong> yet external features of male adults (body size,muscularity, body hair, <strong>and</strong> other sexual features) do notdevelop until about age 18 years. The hormones thatcause spermatogenesis may prime boys to be emotionally<strong>and</strong> cognitively attentive to the behavior of adult men<strong>and</strong> women. In most human societies, boys pass throughseveral rites of passage that help them learn <strong>and</strong> practiceimportant adult male economic, social, political, <strong>and</strong>reproductive activities. By age 20 or so, these boys graduateto manhood, <strong>and</strong> are expected to compete successfullywith other men <strong>and</strong> to assume the responsibilitiesassociated with marriage <strong>and</strong> paternity.As for when adolescence evolved, it is possible that thisstage first appeared in H. erectus, a hominin that originallyevolved in Africa more than one million years ago (Antón&Leigh 2003). The evidence for this is patterns of tooth formationthat are not directly linked to the presence of anadolescent growth spurt, but are nonetheless suggestive.Other research indicates that H. erectus, later hominins274 BEHAVIORAL AND BRAIN SCIENCES (2006) 29:3