Heterochronic perturbations in the craniofacial evolution of Homo (Neandertals and modern humans) and Pan (P. troglodytes and P. paniscus)
In this dissertation, I test whether modern human adults are paedomorphic (juvenilized) with respect to Neandertals, whether bonobo adults are paedomorphic with respect to chimpanzees, and whether paedomorphosis, if it exists, results from neoteny sensu Gould (1977). Various approaches are used to compare the ontogenetic transformation characterizing each taxon. Descriptive statistics and one-tailed t-tests of consecutive life cycle stages that evaluate the null hypothesis of no growth, are used to determine at what point trait growth ceases within each taxon (Chapter 4). Growth curves, derived from piecewise regression, are used to determine rates and durations of absolute growth (Chapter 4). These modeled growth curves also figure prominently in testing for heterochronic product and process using the software program Hetpad (Chapter 6). Shape change is explored in three regions of the skull (calotte, face and mandible). These regions are analyzed separately to increase the representation of fragmentary Neandertal fossils. Relative growth patterns are assessed using growth allometries; Euclidean distances and principal components analysis of craniofacial shapes are employed to examine the similarity of infants, juveniles, subadults and adults within and across taxa (Chapter 5). The results suggest that modern humans are only slightly paedomorphic with respect to Neandertals, and that this paedomorphosis is manifested mainly in the masticatory apparatus. Surprisingly, Neandertals can be characterized as paedomorphic in aspects of calotte shape, and the relation of the lower to the upper face, with respect to modern humans. Bonobos, when compared to chimpanzees, can only be poorly characterized as general craniofacial paedomorphs. The paedomorphosis of bonobos, with respect to chimpanzees, is only slight, and is manifested mainly in the calotte region.
Although the attribution of “weak” neoteny in bonobos vis-à-vis chimpanzees cannot be ruled out, the characterization of modern humans as neotenes with respect to Neandertals fails as an explanation to describe the craniofacial differences manifested in adults. Overall, the shape path differences exhibited by the four taxa, as well as the idiosyncracies of taxon-specific growth patterns, suggest that heterochrony explains only a portion of the craniofacial evolution of Homo and Pan. Many of the traits which characterize adults of the four taxa emerge early (before, during, or just after infancy). Only a few differences exhibited by adults arise later.