Backpedaling in human evolution – adaptive values of bipedalism in reverse.

J.K. McKee, A. Kolatorowicz, L. Reitsema, A. Ruth, S. Schlecht, O. Ulvi, T. Weston. Department of Anthropology, The Ohio State University.

Bipedalism and associated orthograde posture offer a number of selective advantages. One adaptive benefit that has eluded the literature is the unique way in which bipealism allows hominins to move in reverse. Whereas most mammals backpedal, few if any can do so with comparable quickness and agility. We hypothesize that rapid reverse motion may have provided a small but significant advantage for Homo erectus and later hominins.

Backward running today is relevant in many sports, particularly soccer, as one can move quickly across the field while keeping the eyes focused on play action. It is used in training, as it uses most of the same muscles as forward running, yet has less impact on the skeletal system. Vastus lateralis and vastus medialis are used more in backward running. We hypothesize that fossil evidence will show these muscles to have evolved their full biomechanical advantage with the attainment of modern body size in Homo erectus. Moreover, it is at that stage that the vestibular system allowed for backpedaling with adequate agility.

In hunting and scavenging niches, the advantage of backpedaling is that one can retreat rapidly while still facing a predator or dangerous prey, before turning one’s back and allowing attack. It has also been observed that running backward confuses animals, and thus would be significant in allowing greater distance for retreat or repositioning.

Thus, rapid backpedaling would not provide a selective advantage at the origins of bipedalism, but the morphological exaptations that allow such behavior would have had adaptive value.

American Journal of Physical Anthroplogy Supp. 46, pp. 152-153 (2008).

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