Reconsidering Whether Knuckle-Walking Gave Rise to Bipedalism in Light of the Extended Evolutionary Synthesis.

Paleoanthropologists debate whether the common ancestor of chimpanzees and humans moved terrestrially by knuckle-walking or in some other manner preadapted for bipedalism. The answer to this question would shed light on the emergence of one of the most definitive features of early hominins-habitual terrestrial bipedalism. Comparative morphological studies of apes, australopithecines, and modern humans, and investigations of the few putative earliest hominins have failed to resolve this issue. Here, the knuckle-walking question is approached by comparing the ontogenetic development of locomotor milestones in extant humans and chimpanzees in view of the extended evolutionary synthesis (EES), which extends the traditional modern synthesis' relatively narrow focus on natural selection and genetics by considering the plurality of factors that influence evolutionary processes, including development, organism-environment interactions, and epigenetics. Although the neuromuscular development of locomotor milestones is similar in chimpanzees and humans, chimpanzees do not crawl on their hands and knees, and human babies do not knuckle-walk. Palmigrade quadrupedalism briefly precedes development of knuckle-walking in chimpanzees. Converging data suggest that hands-and-knees crawling in hominin infants derived from a developmentally driven evolutionary transformation of chimpanzee-like palmigrade quadrupedalism in response to the progressive elongation of legs that accompanied selection for bipedalism. Modern human behaviors such as knocking on doors and boxing reinforce the hypothesis that hominins are, indeed, descended from knuckle-walkers.