The Emergence of Human Limb Proportions

The current issue of PNAS carries an interesting paper on the evolution of human limb proportions. The authors, Young et al. (2010), propose that one key change in the evolution of humanlike limb adaptations is a reduction in the strength of the developmental links between fore- and hindlimbs, and moreover, that this change actually occurred in a non-hominin ancestor we shared with other great apes.

The quadrupedal primates, like most vertebrates, have strong serial homologies between their limbs. Each limb is composed of three units, specifically the thigh/arm, the leg/forearm and the foot/hand, which are, in most species, tightly coupled such that changes in the relative proportions of the parts of the hindlimb will bring about corresponding changes in the forelimb and vice versa (Young et al. 2010). Humans, in contrast, have differently proprtioned fore- and hindlimbs, with the patterns linked to their different functions in fine manipulation and bipedal locomotion respectively. In addition, the fossil record of human evolution suggests that the changes from the ancestral pattern occurred in an evolutionary mosaic, with fore- and hindlimbs changing independently and at different times, in response to separate selective pressures (Young et al. 2010).

For me, the most interesting part of this article is not the proposal that the move towards weaker coupling of fore- and hindlimbs was important to human evolution (as this seems fairly straightforward, although interesting), but Young et al.'s suggestion that the change actually happened in a human-ape common ancestor rather than within the hominin clade. Many great apes do have functionally differentiated and differently proportioned fore- and hindlimbs, likely as the result of a reduction in the number of pleiotropic genes. Pleiotropic genes are those which influence more than one anatomical structure. This, to me, suggests we are justified in spending more time developing our understanding of the locomotor anatomy of the great apes.



References

YOUNG, N., WAGNER, G., & HALLGRIMSSON, B. (2010). Development and the evolvability of human limbs Proceedings of the National Academy of Sciences, 107 (8), 3400-3405 DOI: 10.1073/pnas.0911856107