Abstract
Bipedal locomotion is a hallmark of being human. Yet the body form from which bipedalism evolved remains unclear. Specifically, the positional behaviour (i.e. orthograde vs. pronograde) and the length of the lumbar spine (i.e. long and mobile vs. short and stiff) of the last common ancestor (LCA) of the African great apes and humans require further investigation. While fossil evidence would be the most conclusive, the paucity of hominid fossils from 5-10 million years ago makes this field of research challenging. In their absence, extant primate anatomy and behaviour may offer some insight into the ancestral body form from which bipedalism could most easily evolve. Here, we quantify the frequency of bipedalism in a large sample (N = 496) of zoo-housed hominoids and cercopithecines. Our results show that while each studied species of ape and monkey can move bipedally, hylobatids are significantly more bipedal and engage in bipedal locomotion more frequently and for greater distances than any other primate sampled. These data support hypotheses of an orthograde, long-backed and arboreal LCA, which is consistent with hominoid fossils from the middle-to-late Miocene. If true, knuckle-walking evolved in parallel in Pan and Gorilla, and the human body form, particularly the long lower back and orthograde posture, is conserved.
Highlights
We find that species significantly differ by the occurrence of bipedalism (χ2 = 51.99, d.f.(5), p < 0.001; Figure 2)
We interpret our findings as an indication that additional morphological features of the hylobatid body form, aside from their limb proportions, predispose them to walk bipedally more frequently than other primates – a long lumbar spine and orthograde positional behaviour
A limitation of this study is that we did not include any atelines, a family of primates that possesses this combination of long lumbar spine and frequent orthograde positional behaviour
Summary
Twentieth-century physical anthropologists and anatomists had difficulty identifying the ape body plan from which the earliest human ancestors evolved. Keith (1923) identified homologous characters in living hominoids to posit an upright, orthograde ancestor that passed through a large-bodied, ‘Troglodytian’ phase. Gregory (1928) envisioned an orthograde ancestor but hypothesised that bipedalism evolved from a gibbon-like brachiator as was suggested later by Avis (1962). Morton (1924) imagined a more generalised, small-bodied, quadrumanous ape capable of arboreal bipedal locomotion, he wrote of humans passing through a terrestrial ‘gorilloid’ phase. Straus (1949) diverged from this line of thinking and instead hypothesised that the earliest human ancestors descended from a primate that was more cercopithecoid-like – an above-branch, pronograde, arboreal quadruped (Figure 1).
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