Feet and the GDF6 Gene

Feet. No one talks about them because honestly, they are kind of gross. For humans, having feet is an advantage, our big toe especially, because without them we wouldn't be able to walk normally. Bipedalism has thought to have occurred because of the formation of our big toe throughout the evolution of humans. So what may have caused the creation of the big toe in our earlier ancestors?

Bone morphogenic proteins are molecules that control the formation of bones and joints during development. These proteins have been suspected to be the cause of evolving new structures while preserving the function of old structures. By first looking at the drastic differences in the freshwater and salt-water species of stickleback fish using genetic mapping of skeletal traits, Indjeian and his colleagues were able to find that cis-acting increase of the Growth/Differentiation Factor 6 was involved in the difference in amor plates in these fish, meaning that the freshwater species shows smaller plate size as they evolved due to the increase of GDF6. Smaller plate size has benefitted the freshwater species by facilitating in faster bursts when swimming but what does this have to do with the evolution of bipedalism in humans?

As humans evolved, multiple changes were made to benefit the species. For example, as locomotion started for humans, the hindlimb toes started to shorten and the strengthening of the first digit, our big toe. It was shown that the GDF6 gene is responsible for normal growth of the digits and the expression decreases as it moves away from the first digit, that is why our other toes are so much smaller than our big toe. This gene is also responsible for skull sutures and eye development. Deletion of a region-specific enhancer allowed this gene to alter the structure of the hindlimbs over time to adapt to the ever-changing environment without negatively impacting forelimb development. This enhancer deletion is human-unique as it is still prevalent in primates and other mammals. 

For further reading: Indjeian, Vahan B., Garrett A. Kingman, Felicity C. Jones, Catherine A. Guenther, Jane Grimwood, Jeremy Schmutz, Richard M. Myers, and David M. Kingsley. "Evolving New Skeletal Traits by Cis-Regulatory Changes in Bone Morphogenetic Proteins." Cell 164.1-2 (2016): 45-56. Web.