Evolution of Human Growth

Despite the importance of these adaptations, our understanding of precisely how and why human ontogeny differs from ontogeny in other primate species remains qualitative and rather speculative.1–6 Consequently, this study builds on previous analyses in fields such as allometry, heterochrony, human and primate growth studies, and life-history theory7–10 to address several issues regarding human growth. First it develops an interspecific allometric perspective to determine how human growth compares to trends established by other anthropoid primates. Because humans are comparatively large primates, we may simply follow a growth pathway that is consistent with expectations based on body size. Second, this allometric foundation enables evaluation of two competing models that seek to account for the pattern of human growth prolongation. Prolongation could represent extension or “retardation” of all phases of growth. Alternatively, the overall prolongation of human growth could represent extreme lengthening of a single growth phase. Obviously, a blend of these possibilities, such as prolongation of a few growth periods coupled with abbreviation of others, may exist. Third, this study reviews alternative life-history models that seek causal explanations for prolonged human growth. Allometric results are applied to qualitative evaluations of these models with the objectives of clarifying the focus of these models and directing theoretical attention to new areas. For example, life-history models often emphasize certain variables, such as age at maturation, while looking past others, such as neonatal growth rates. Exploring human growth in light of alternative life-history models has the potential to refine and redefine these models with the ultimate goal of providing an integrated explanation for the evolution of human ontogeny.