Have you ever wondered why humans have evolved bipedalism and the vast majority of the Earth's terrestrial organisms have not? No, it is not because we are special. Rather, it is because in almost every instance, quadrupedalism has proven a more efficient means of locomotion than bipedalism. So why did humans evolve this queer habit in the first place?

The early paleoanthropologists believed the large brain gave rise to tool use, so that it was necessary to stand up in order to free the hands to manufacture and implement stone tools.¹ History proved them wrong. The anthropologist Sherwood Washburn proposed that bipedalism arose to accommodate hunting behaviors. History proved him wrong as well.² (Washburn, 1960).

Then, in 1981, Owen Lovejoy associated bipedalism with a shift in reproductive strategies (Lovejoy, 1981), an idea that became recognized as the “Man the Provisioner Model”. To this day, Lovejoy’s hypothesis remains the most popular. However, it predicts very little sexual dimorphism in early hominids when in fact paleoanthropologists observe it to a significant degree.³

As a result, several new hypotheses have been suggested, among them, the Alternative Responses Hypothesis, developed by Lynne Isbell and Truman Young in 1996. Their paper, published in the Journal of Human Evolution, argued that bipedalism was one of the responses to the diminishing food sources caused by the shift in habitat. (Isbell & Young, 1996). As temperatures plummeted in the mid-late Miocene, the rainforests thinned and food became scarcer and more widely scattered.⁴ Based on the composition of stable carbon at a site in Kenya, paleoecologists and geochronologists have concluded that a mixture of C3 and C4 plants (those adapted to cold, wet environments) existed between 12 and 5 million years ago. (Kingston et al., 1994).

Footnotes
1      To elaborate, the earliest stone tools date back to 2.6 million years ago, while the first indisputably bipedal remains extend back to at least 4.2 myr ago. The evidence from Aramis and other Ardipithecus ramidus-bearing sites are excluded in this situation since their bipedality has not yet been confirmed.
2      Washburn is wrong for essentially the same reason as the above: there is no evidence of tool use earlier than 2.6 myr ago. While a recent study by Sponheimer and Lee-Thorp indicate that Australopithecus africanus (3.2-2.3 myr ago) enjoyed a more protein-dominated diet than had been previously suggested, A. africanus occurs fairly late in early hominid evolution and the results do not apply to earlier species. All other evidence indicate that, overall, australopithecines were adapted for processing non-nutrient dense foods.
3      Although there are other explanations for sexual dimorphism other than inter-male competition for mates (e.g., ecology), rivalry over mating opportunities accounts for the vast majority of instances. In addition, the Lovejoy Hypothesis predicts traits that are either not observed or questionable. For example, it asserts that concealed ovulation and the absence of estrus are apomorphic traits, while there is contrary evidence that they may in fact be plesiomorphic. The bottom line is Lovejoy's model fails, at least as a possible primary cause for the evolution of hominid bipedalism.
4      It must be stressed that this does not necessarily imply savanna dominance. It may also be caused by reduced resource density within forested regions or even increased scattering of food-bearing locations such as forest patches.

       

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