Evolution of human growth prolongation

This investigation evaluates hypotheses that seek to explain temporal retardation or prolongation of human ontogeny. Current hypotheses that address this issue are poorly defined and conflate several distinct theoretical positions. A model that predicts homogeneity in the extension of human growth periods is evaluated. This model is contrasted with two alternatives. The first alternative predicts heterogeneity in the extension of human growth periods. The second anticipates that human growth prolongation is the result of the uniquely derived "insertion" of a human childhood period into an ancestral ontogenetic trajectory. Allometric analyses of body mass growth data from 21 species of anthropoid primates suggest that human female and male ontogenies often depart from patterns established by other primates, but these departures are not uniformly exceptional. Comparisons imply that derived changes in human growth are heterogeneous. Relative to interspecific expectations, early growth periods are much prolonged, but later growth periods are actually reduced. Moreover, the attributes of early growth periods, including growth rates, timing of growth events, and size-for-age, are highly variable across primates. Low correlations among growth periods suggest independence among growth phases. These analyses highlight minimal distinctions between competing models (heterogeneous extension and insertion hypotheses) that attempt to explain human growth prolongation. More important, the present study facilitates refinements of causal models that have been proposed to explain human growth prolongation. Specifically, human growth prolongation may be related to derived changes in patterns of brain development. Alternatively, metabolic factors may have exerted influences on human ontogeny. However, models that predict long growth periods as a byproduct of metabolic factors do not adequately explain temporal retardation of human ontogeny.