How did humans acquire erect bipedal walking?

Abstract

Recent fossil records have suggested that human erect bipedal locomotion started in Africa probably more than 6 million years ago. However, debate continues regarding how locomotion was acquired by our prehuman ancestors. Fossils and the genetic traits of recent humans and animals cannot answer this question directly. Therefore, the present paper reviews acquisition models of human bipedalism and explanations regarding how humans acquired bipedalism based on a comparative kinesiology of contemporary mammal species. Nonhuman primates are adequate models of human bipedal acquisition because of not only the phylogenetically close relationship with humans, but also the trend toward hindlimb dominance and orthograde positional behavior in daily life. Although dissimilar to the erect bipedalism seen in humans, nonhuman primates adopt bipedal positional behavior in the wild. All nonhuman primates use the arboreal environment, but some groups of species utilize the ground predominantly. Compared with relatively terrestrial nonhuman primates, relatively arboreal primates show more similarities with humans in their bipedal locomotion. Comparisons among primate species and between nonhuman primates and nonprimate mammals indicate that human-like bipedal characteristics are strongly related to arboreal life. Our prehuman ancestors likely started and adapted to bipedal locomotion while living in trees; this process is referred to as the generalized arboreal activity model. When humans began terrestrial locomotion, they likely performed proficient bipedalism from the first step. The generalized arboreal activity model presented here does not contradict the fossil records.