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 t heoretical positions. A model that predicts homogeneity in the extensi on 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 anth ropoid primates suggest that human female and male ontogenies often de part 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 expectat ions, early growth periods are much prolonged, but later growth period s are actually reduced. Moreover the attributes of early growth period s, including growth rates, timing of growth events, and size-for-age, are highly variable across primates. Low correlations among growth per iods suggest independence among growth phases. These analyses highligh t minimal distinctions between competing models (heterogeneous extensi on and insertion hypotheses) that attempt to explain human growth prol ongation. More important, the present study facilitates refinements of causal models that have been proposed to explain human growth prolong ation. Specifically, human growth prolongation may be related to deriv ed 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 d o not adequately explain temporal retardation of human ontogeny. (C) 1 998 Wiley Liss, Inc.