The evolution of bipedalism in hominids and reduced group size in chimpanzees: alternative responses to decreasing resource availability

Abstract

One hypothesis for the evolution of hominid bipedalism is that bipedalism was more efficient than quadrupedalism for long-distance terrestrial locomotion, and was favored when resources became scarcer and more widely separated during the drying of African forests in the Miocene. Here we extend this scenario for the evolution of bipedalism based on principles of behavioral ecology of extant primates. Daily travel distance is not only an increasing function of resource scarcity, but also of group size (because of intragroup scramble competition). When faced with Miocene drying, hominoids were forced to evolve either energetically more efficient locomotory abilities or smaller group sizes. The cost of the latter strategy is that smaller groups are less successful than larger groups in intergroup contest competition (smaller groups are supplanted from limiting resources). Among the earliest hominids, bipedalism may have been favored over small group size as an alternative energetic response to decreased resource availability. The alternative was to maintain quadrupedal locomotion but evolve fission–fusion grouping to reduce daily travel distance for individuals and hence, their energetic costs of travel. We suggest that this strategy represents the evolutionary pathway taken by chimpanzees. This divergence of strategies may have been a result of inherent differences in feeding ecology, but could also have been enhanced by the pre-empting of niche space by the two closely related and presumably competing hominoid ancestors of humans and chimpanzees. If so, it could have been a potential factor in the speciation process that led to modern humans and chimpanzees.