Factoring the Proto-biped Infant into the Origin of Bipedalism

Abstract

This thesis applies a single test method to a high number of proposed explanations (models) of hominin bipedalism. This has not been undertaken previously, due largely to the very variable ways in which the models have been built, and the disparity of their core ideas. Taken as a whole, however, the modelling literature contains a gap, and the thesis' main claim is that it can be exploited to assess relative levels of tenability, using non-statistics-based hypothesis testing. Infants are defined here as individuals too young to survive without caregivers and older infants are defined as those who are not completely dependent on others for nutrition and transport. Most models have omitted infants, and others have included them only in minor or circumscribed ways. A species' survival, however, depends on sufficient numbers of young reaching breeding age, and species which provide postnatal care need to provide adequate care Any viable model of how hominin bipedalism evolved ought, therefore, to be able to accommodate appropriate levels of infant survival and adequate caregiving, and factoring infants and caregivers into the models can thus serve as a test method. The method cannot assess overall robusticity, because models which accommodate infants and caregivers might be untenable relative to other criteria, but it can narrow down the possible ways in which bipedalism might have evolved by identifying models which are untenable in one crucial way. Seventy-six models, written between 1871 and 2017, are identified. They are classified into 12 model types, three of which are treated as obsolete. Seventy-one models are tested, 76.2% of which omit infants or include them only trivially. The models are tested either in blocs, according to type or variant, or individually. Forty-three tests are conducted. In order to factor a proto-biped infant into a model, it is necessary to have a clear idea of its likely basic characters. After its Introduction, Methods chapter and Literature Review, this thesis is therefore divided into two distinct parts. The first part extensively reviews available literature bases for character reconstruction, and one of these bases, the literature on nonhuman anthropoid infancy, is supplemented by a short-term captive study of clinging behaviour in an orthograde ape, presented as an appendix. The likely size of older proto-biped infants, relative to conspecific adult females, is then estimated, based on data for extant anthropoids. All this information is then synthesized to reconstruct the infant and to guide the tests' expectations of infant/adult differences and adequate caregiving behaviours. The second part of the thesis contains the 43 tests. The results show that 40.8% of models are untenable, including two whole model types, 43.7% are highly questionable, and only 11.3% accommodate infant survival well (4.2% did not support the test method). These results indicate that the ways in which bipedalism might have evolved are heavily constrained by the needs of infants. They also suggest that future models which consider the full implications of their proposed behaviours for infants and caregivers will have greater explanatory value than the majority of existing models. Show more