Patterns of craniofacial integration in extantHomo,Pan, andGorilla

Abstract

Brain size increased greatly during Pleistocene human evolution, while overall facial and dentognathic size decreased markedly. This mosaic pattern is due to either selective forces that acted uniquely on each functional unit in a modularized, developmentally uncoupled craniofacial complex, or alternatively, selection that acted primarily on one unit, with the other responding passively as part of a coevolved set of ontogenetically and evolutionarily integrated structures. Using conditional independence modeling on homologous linear measurements of the height, breadth, and depth of the cranium in Pan (n = 95), Gorilla (n = 102), and recent Homo (n = 120), we reject the null hypothesis of equal levels of overall cranial integration. While all three groups share the pattern of greater neurocranial integration with distinct separation between the face and neurocranium (modularization), family differences do exist. The apes are more integrated in their entire crania, but display a particularly strong pattern of integration within the facial complex related to prognathism. Modern humans display virtually no facial integration, a pattern which is likely related to their markedly decreased facial projection. Modern humans also differ from their great ape counterparts in being more integrated within the breadth dimension of the cranial vault, likely tied to the increase in brain size and eventual globularity seen in human evolution. That the modern human integration pattern differs from the ancestral African great ape pattern along the inverse neurocranial-facial trend seen in human evolution indicates that this shift in the pattern of integration is evolutionarily significant, and may help to clarify aspects of the current debate over defining modern humans.

The meaning of Neandertal skeletal morphology

“…Covariance patterns are similar across human populations (22), and although there are some important differences, African apes and humans have broadly similar covariance patterns (23)(24)(25). The sum suggests that it is reasonable to assume that Neandertals would have had similar covariance patterns to humans, at least for most features.…”

Evolutionary Lability of Integration in Cambrian Ptychoparioid Trilobites

“…Graphical modeling is another useful method for exploring the structure of integration (Magwene 2001;Young 2004;Young and Hallgrímsson 2005;Polanski and Franciscus 2006;Allen 2008;Lawler 2008;Zelditch et al 2009;Webster and Zelditch 2011; but see criticisms by Bookstein 2007, 2009; reply by Magwene 2009). The analysis is done by analyzing correlations among pairs of variables, conditional on all others.…”

Building Developmental Integration into Functional Systems: Function-Induced Integration of Mandibular Shape

“…This matrix can be obtained by inverting the observed correlation matrix and rescaling the values so that elements along the diagonal are all 1.0. Magwene (2001) showed that graphical modeling can be useful for studying morphological integration, clarifying both concepts of modularity and revealing the structure of integration, and several recent empirical studies have used this method to test theories of integration/modularity (e.g., Young 2004;Young and Hallgrímsson 2005;Polanski and Franciscus 2006;Allen 2008;Lawler 2008).…”