Backpedaling in human evolution Adaptive values of bipedalism in reverse

1. 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 It is our contention that the extensor muscles would have evolved their full biomechanical advantage with the attainment of modern body size and proportions in Homo erectus, as scavenging and hunting niches became a more critical in the hominin dietary niche. This is consistent with the Bramble & Lieberman (2004) hypothesis regarding the evolution of endurance running in Homo. Moreover, it is at that stage that the vestibular system allowed for backpedaling with adequate agility (Spoor et al. 1994). Bipedalism and orthograde posture afford a number of selective advantages that eventually allowed hominins to enter new niches. One adaptive benefit that has eluded the literature is the unique way in which bipedalism allows hominins to move adroitly 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. A A B B Backward running, also known as retro-running, is important in many sports, particularly soccer, as one can move quickly across the field while keeping the focused on play action (Figure 1.) Speeds reached in backward running can vary from 60-80% the speed of forward running (Arata, 2000.) Backward running is also used in training, as it uses most of the same muscles as forward running, yet has less impact on the skeletal system. Moreover, it has been found to facilitate balance an proprioception. We conclude that rapid backpedaling would not have provided a selective advantage at the origins of bipedalism, but the morphological exaptations that allow such behavior would have had adaptive value for those who employed and mastered appropriate technique. Figure 2. From left to right, KNM ER 1481 (Homo), Al 288 (Australopithecus), AL 288 outlined; Al 288 size-adjusted and superimposed on KNM ER 1481. Homo is more human-like in positioning of vastus lateralis and medialis. At point A, there is a distal extension of the greater trochanter interrupting the positioning of vastus lateralis. Point B is where minor lipping of linea aspera begins much more distally in Australopithecus. Vastus lateralis and vastus medialis are used differentially in backward than forward running (Flynn & Soutas-Little, 1993) due to increased effort for extension of the leg at the knee. Observations of fossil femora of Australopithecus afarensis, early Homo, and H. sapiens, reveal that the proximal attachments of vastus lateralis would necessarily differ in Australopithecus (Figure 2). The lack of a pronounced linea aspera on the proximal half of the femur also suggests that both muscles would have been less developed in the early hominins. Finally, as with Neandertals (Trinkaus, 1983), the shorter load arm (tibia) for the extensors in A. afaraensis would would decrease speed relative to Homo. References: Arata, A.W. 2000. Kinematic comparison of high speed backward and forward running. http://darkwing.uoregon.edu/~btbates/backward/alan2.htm Bramble, D.M., and D.E. Lieberman. 2004. Endurance running and the evolution of Homo. Nature 432: 345-352. Flynn, T.W., and R.W. Soutas-Little. 1993. Mechanical power and muscle action during forward and backward running. J. Orthop. Sports Phys. Ther. 17(2): 108-112. Spoor, F., B. Wood, and F. Zonneveld. 1994. Implications of early hominid labyrinthine morphology for evolution of human bipedal locomotion. Nature 369: 645-648. Trinkaus, E. 1983. The Shanidar Neanderthals. New York: Academic Press. Figure 1. Backward running is recognizable by the upright trunk and greater extension of the leg at the knee. The advantage of rapid backpedaling in hunting and scavenging niches 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. Thanks to Owen Lovejoy for access to fossil casts.