A Quantitative Genetic Analysis of Limb Segment Morphology in Humans and Other Primates: Genetic Variance, Morphological Integration, and Linkage Analysis

Limb segment lengths (and, by extension, limb proportions) are widely studied postcranial features in biological anthropology due to the seemingly consistent phenotypic patterning among human and fossil hominin groups. This patterning, widely presumed to be the result of adaptation to thermoregulatory efficiency, has led to the assumption among biological anthropologists that limb proportions in humans are phenotypically stable unless long periods of extreme environmental conditions force adaptive change. Because these traits are considered stable, they have been used to inform multiple areas of anthropological inquiry, including investigations of phylogenetic relationships and fossil species identification, locomotor behavior and the evolution of bipedalism, and migration patterns.

The problem with this assumption is that phenotypic patterns may not accurately reflect evolutionary processes, and even if they do, there is no reason to expect phenotype to respond to natural selection solely. Investigations of phenotypic variation need to incorporate genetic variation and covariation to better understand the processes that produced observable patterns, including evolutionary processes. However, the incorporation of genetic parameters is often difficult given that knowledge of familial relationships are required. Therefore, the goal of this project is to use a quantitative genetics approach to estimate the genetic variance and covariance in limb segment lengths and then begin the task of identifying genes which may influence this normal variation. These tasks are accomplished using multiple large, pedigreed samples of primate species, including humans. Linkage analysis on a baboon sample, a well-accepted model organism for humans, is used to identify regions of the genome which may influence limb segment variation.

The results presented here suggest that 1) while patterns of genetic and phenotypic variance and covariance across limb segments are broadly similar, there are differences in the details, and 2) while patterns of genetic and phenotypic variance and covariance within and among limb segments generally adhere to expectations set forth by developmental and evolutionary-based hypotheses, there are exceptions. Additionally, several genomic regions are identified which influence limb segment variation. Thus, biological anthropologists must use caution in their assumptions and interpretations regarding limb segment lengths and limb proportions in humans and other primates.