Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor

Abstract

Despite decades of debate, it remains unclear whether human bipedalism evolved from a terrestrial knuckle-walking ancestor or from a more generalized, arboreal ape ancestor. Proponents of the knuckle-walking hypothesis focused on the wrist and hand to find morphological evidence of this behavior in the human fossil record. These studies, however, have not examined variation or development of purported knuckle-walking features in apes or other primates, data that are critical to resolution of this long-standing debate. Here we present novel data on the frequency and development of putative knuckle-walking features of the wrist in apes and monkeys. We use these data to test the hypothesis that all knuckle-walking apes share similar anatomical features and that these features can be used to reliably infer locomotor behavior in our extinct ancestors. Contrary to previous expectations, features long-assumed to indicate knuckle-walking behavior are not found in all African apes, show different developmental patterns across species, and are found in nonknuckle-walking primates as well. However, variation among African ape wrist morphology can be clearly explained if we accept the likely independent evolution of 2 fundamentally different biomechanical modes of knuckle-walking: an extended wrist posture in an arboreal environment (Pan) versus a neutral, columnar hand posture in a terrestrial environment (Gorilla). The presence of purported knuckle-walking features in the hominin wrist can thus be viewed as evidence of arboreality, not terrestriality, and provide evidence that human bipedalism evolved from a more arboreal ancestor occupying the ecological niche common to all living apes.ince Darwin first discussed pathways of human evolution in The Descent of Man, there has been an ongoing and often rancorous debate over the nature of locomotion in our prebipedal human ancestor. The debate can be summarized with 2 competing models. One model envisions the prehuman ancestor as a terrestrial knuckle-walker, a behavior frequently used by our closest living relatives, the African apes (1-6). In the alternative model, early human bipedalism is seen as having evolved from a more generalized arboreal, climbing-oriented ancestor, a mode of locomotion that is used by all living apes (7-10). Each scenario has important and profoundly different implications for understanding the evolution of ape and human locomotion.If early human bipedalism evolved from an arboreal ancestor, current ape-human phylogeny showing chimpanzees and bonobos as the sister taxa of humans (supporting information (SI) Fig. S1) logically implies that knuckle-walking evolved independently in both African ape lineages (Gorilla and Pan). In contrast, proponents of a terrestrial knuckle-walking hypothesis of human locomotor evolution hypothesize that African apes and humans share a common knuckle-walking ancestor. Advocates for this model support their claims by arguing that there are specific morphological features, particularly of the wrist and hand, that refle...

Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees

“…[16][17][18][19] supports the argument that many living ape similarities are not homologies (20); thus the ancestral morphotype for both apes and humans [i.e., the "Miocene ape ancestor" (MAA)] would reflect a more primitive, generalized postcranial morphotype and positional behavior [an "ape convergence" (AC) model]. For example, some have argued from differences in African ape wrist morphology that knuckle walking behaviors evolved in parallel (21,22), in which case the LCA must be more primitive. Thorpe et al (23,24) used similar reasoning to assert that hominins did not evolve from a knuckle-walking LCA, but rather one that practiced the hand-assisted, arboreal bipedality observed in living orangutans (Fig.…”

Morphology of the Distal Radius in Extant Hominoids and Fossil Hominins: Implications for the Evolution of Bipedalism

“…These authors also found that the shape of the carpals on the ulnar side to be significantly different in Gorilla and Pan, and that these differences could not be accounted for by differences in locomotor pattern during ontogeny in the two taxa. Kivell and Schmitt (2009) found that the development of supposed knuckle walking characteristics in the wrist followed significantly different ontogenetic trajectories in chimpanzees and gorillas and were also found in non-knuckle walking taxa.…”

Upright human gait did not provide a major mechanical challenge for our ancestors

“…Recent research suggests that humans did not evolve from a knuckle-walking ancestor 27 . Instead, the erection of locomotion likely took place in an arboreal environment, similar to the environment in which orangutans live even at present.…”