Challenges to human uniqueness: bipedalism, birth and brains

Historically, paleoanthropology has focused on explaining human uniqueness. This review paper highlights several recent challenges to key features that have been considered to be exclusive to hominins, testing three long‐standing theories in evolutionary anthropology. The knuckle‐walking quadrupedalism model describes the evolution of modern gorillines, panins and hominins from a common, knuckle‐walking ancestor. But the homology of knuckle‐walking in African apes has been questioned. Although habitual bipedalism is unique to humans, it may have developed from occasional bipedalism in ancestors, without a quadrupedal stage. The obstetric dilemma seeks to explain the helplessness of human infants. The timing of human birth is seen as uniquely constrained by fetal head size and maternal pelvic width. An alternative hypothesis suggests that birth occurs when fetal demand for energy threatens to exceed maternal supply; this mechanism also appears to operate in other mammals. The expensive tissue hypothesis suggests that the expansion of energy‐hungry brain tissue in hominins was offset by a reduction in gut tissue. But although large brains are correlated with both good quality diets and relatively short guts in primates, the causes of this correlation are not clear. An alternative suggestion is that the large human brain is paid for by savings in other functions, such as locomotion and reproduction, and that a concurrent expansion of low‐cost adipose tissue in humans keeps metabolic rate low. In the past, paleoanthropology may have focused on defining a boundary between humans and animals, but recent research has seen a shift of focus to exploring humans as animals. Aspects of bipedalism, birth and brains have been considered to be exclusively human, but in the last few years even these have been eroded. It is the package of features that characterizes Homo sapiens that is unique.