African Apes and the Evolutionary History of Orthogrady and Bipedalism

Since the first discovery of human fossils in the mid-19th century, two subjects-our phylogenetic relationship to living and fossil apes and the ancestral locomotor behaviors preceding bipedalism-have driven the majority of discourse in the study of human origins. With few fossils and thus limited comparative evidence available to inform or constrain them, morphologists of the 19th and early mid-20th centuries posited a range of scenarios for the evolution of bipedalism. In contrast, there exists a rich hominin fossil record and the acceptance of Pan (chimpanzees and bonobos) as our closest living relatives is nearly universal, yet consensus about the ancestral condition from which hominins evolved remains elusive. Notably, while the earliest known hominins are generally congruent with parsimonious inferences of an African ape-like last common ancestor, our more distantly related Miocene ape cousins are frequently invoked as evidence in favor of more complex scenarios that require substantial homoplasy. Debate over these alternatives suggests that how we infer ancestral nodes and weigh evidence to test their relative likelihoods remains a stumbling block. Here we argue that a key contributor to this impasse includes the history of terminology associated with positional behavior, which has become confused over the last century. We aim to clarify positional behavior concepts and contextualize knuckle-walking and other forms of posture and locomotion chimpanzees and gorillas engage in, while arguing that the presence of homoplasy in ape evolution does not alter the weight of evidence in favor of an African ape-like evolutionary history of hominins.