9.2: Evolution of Bipedalism
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There are a variety of theories as to how bipedalism evolved and why it proved to be so successful for early hominins. One early idea suggested that by standing up, our ancestors would have been able to see above the grass and thus avoid predation. Baboons and patas monkeys provided living models for hypothesizing the environmental stresses early hominins might have faced on the open plains of Africa. While they likely traveled through open areas, we now know that the earliest hominins were exploiting forest resources, as evidenced by their thinner molar enamel, relative to later hominins.
There were also theories that involved the freeing up of the hands to make and use tools and for carrying resources to a safe place or home base. C. Owen Lovejoy believes that bipedalism allowed males to provision mates with resources (see Lovejoy, 1981). Those males with the most advanced bipedal capabilities would have had an increased chance of mating and possibly offspring survival, and thus bipedalism would have spread throughout the population. While Lovejoy makes a good case for how a trait could be favored in a population, it is not clear why females would have needed to be provisioned unless their offspring had already lost their ability to hang on with their feet, and hence became a burden to foraging. However, if resources had become extremely scarce, bipedal males may have ventured out onto dangerous ground for resources with which to provision their mates.
Another theory that sees males as being the impetus for bipedalism suggests that males may have been more terrestrial and females more arboreal, i.e., a case of niche partitioning, like gorillas and the mandrill and drill monkeys, where males forage on the ground and females and young spend more time in the trees. Other theories also suggest that bipedalism was a response to the changing nature of the resource base. For example, Meave Leakey and Kevin Hunt (a theory known as Hunt’s Postural Feeding Hypothesis, see Hunt 1996) believe that the ability to stand on two legs for long periods of time would have facilitated picking fruit from the terminal branches of low, scrubby trees in the increasingly open habitats of East Africa.
While the aforementioned theories are not mutually exclusive and there was likely a synergistic effect that resulted from our ancestors’ changing locomotor capabilities, a plausible model suggests that it was our ability to break out of the “ape habitat” that facilitated our evolutionary success. Most apes went extinct as their habitats dwindled and they competed for limited resources. However, with an efficient means of locomotion to move between forest patches when resources became depleted, hominins could continue to exploit those resources to which they were adapted. They also likely evolved new capabilities for exploiting newly encountered food items as they moved through and between ecozones. Loss of habitat and resources often leads to local extinctions. By expanding their home ranges and dietary niches, hominins survived while the majority of their close relatives did not.
Other contributing factors could have been it kept the body cooler by exposing less of it to the sun and that bipedalism is a very efficient method for long-distance travel. Referring to the previous chapter, the reduction of tooth size and increase in brain size also correlates with bipedalism. It makes sense that small hominids/hominins with small, weak teeth, would have used some type of rudimentary tool for protection. This also connects to the carrying hypothesis above (Sarah Etheridge-Criswell, 2018).