Development and the Evolvability of Human and Primate Limbs

Human and ape limbs are unusual when compared to most other primates due to our disproportionately long legs or arms, respectively. Although one might predict that the upper and lower limb are independent of each other, their common genetic architecture predicts that they should actually respond in parallel to selection. How then can this pattern of independent evolvability be explained? Using a key insight into the relationship between correlations and population variation to correct for artifacts in integration estimates, my research shows that humans, apes, and by extension fossil hominins, escaped developmental constraints by significantly reducing phenotypic correlations between limbs, most likely by reducing genetic pleiotropy. This result indicates that an increase in the independent evolvability of ape limbs was a critical event that facilitated the later evolution of human adaptations like bipedalism. This result also has important macroevolutionary implications, as not only does it place the mosaic pattern of human limb evolution in a developmental context, but it also yields testable predictions about lineage-specific patterns. Specifically, we show that differences in evolvability are reflected in the disparity of limb proportions in both living and fossil primate taxa, providing evidence that links shared development to broader evolutionary patterns.

    Evolution of the Ape Postcranium

All living apes share a suite of postcranial adaptations for suspensory below-branch behaviors such as brachiation and hanging. This fact strongly suggests that their last common ancestor was a suspensory animal as well. However, most fossil apes exhibit a mix of characteristics indicating a much more complicated evolutionary history. I have active research in the reconstruction of ape postcranial evolution, which includes reassessments of interspecific and intraspecific postcranial variation using large comparative samples from museum collections and published sources. I have worked extensively on the growth, development, and variability of the primate scapula. I have 3D landmark data from 1300+ scapula representing ontogenetic series of 17+ anthropoid taxa. This work has resulted in several publications documenting variational patterns in this developmentally and functionally complex postrcranial bone. My analyses demonstrate that the scapula of apes and ateline monkeys exhibit weaker morphological integration and canalization of shape compared to the scapula of quadrupedal taxa. This suggests that differences in variability components are due to reduced stabilizing selection in suspensory apes compared to monkeys. In other words, the important selective factors on shoulder shape in an arboreal suspensory animal are less stringent compared to quadrupeds.

I have also worked on the paleobiology of the early fossil ape Morotopithecus to estimate the phylogenetic position of this taxon, which is an important datapoint for our understanding of the ancestral ape morphotype. This work is motivated by my interests in the practice of cladistics, how character choice affects hominoid phylogenetic reconstruction, and assessing how character choice, particularly with postcranial data, affects phylogenetic signal and tree reconstruction.