Origin of Bipedality

Theories on the origin of bipedalism itself are by their very nature important and speculative. They are important because they involve the first change that differentiated the hominid evolutionary lineage from the rest of the animal kingdom. They are speculative because they require reconstruction of so many unknowns, especially the morphology, behavior, and ecology of the last common ancestor and the first hominid. Darwin's view of the origin of bipedalism remains useful: bipedalism arose when our ancestors came ". . . to live somewhat less on trees and more on the ground," which was due to ". . . a change in its manner of procuring subsistence, or to a change in the conditions of its native country." More specific causes for the adoption of upright posture have been proposed such as carrying, display or warning, new feeding adaptations, tools, or a combination of these. A conservative view is that the hominid ancestor maintained the typical hominoid foraging regime in a Miocene habitat in which food was becoming more and more widely dispersed and required greater terrestrial travel to harvest. Bipedalism could easily have been the mode of terrestrial travel for this arboreally adapted hominoid, as it is in all of the modern species of lesser apes, since modern hominoids are equally efficient as bipeds or as quadrupeds at normal speeds. Given the added advantage of free forelimbs, bipedalism for a small hominoid seems likely. An objection to this model is that bipedalism could only arise under special circumstances because it is presumably an inefficient way for a mammal to get about. Such an objection misses two important points, however: 1. our Miocene hominoid ancestor was probably not an efficient quadruped since modern hominoids are inefficient quadrupeds, and 2. hominid bipedalism is as efficient as average mammalian quadrupedalism at normal speeds. The first point was proved by Taylor & Rowntree, who tested two chimpanzees for energetic efficiency during bipedalism and quadrupedalism and found the surprising result that the two patterns used about the same amount of energy. Both gaits used 50% more energy than the average quadrupedal mammal. Such a result makes sense because the hominoid body is adapted to a certain style of arboreal locomotion which makes terrestrial gait energetically costly. The second point is confirmed by recalculating the energy expenditure of humans and chimpanzees walking at normal speeds: the human energetic cost is about average for the mammals, but the chimpanzee still uses about 50% more energy per body weight. The adoption of bipedalism by a Miocene hominoid need not be taken as such an unlikely event, especially given the fact that all lesser apes today are habitual bipeds and bipedalism can easily be adopted by modern chimpanzees in the wild.

A far more elaborate view is presented by Lovejoy using information from a wide spectrum of sources including mammalian demography, ecology, and social behavior. At the core of the theory is the notion that bipedalism evolved in response to selection pressures on food carrying by monogamous males who provision their home-bound mates and their dependent infants. Unlike chimpanzees, so the theory goes, early hominids could produce more than one infant per 5.6 years per female as long as the faithful male would share in rearing responsibilities. Any complex hypothesis like this, especially one about hominid origins, will receive its share of petty criticisms of details which may add up to rejection of the whole edifice unfairly. On the other hand, complex hypotheses should be compatible with most relevant information available. The scenario is not likely to win over many anthropologists, ecologists, or primatologists familiar with mammalian and human mating systems; all available evidence indicates that early hominids were polygynous and not monogamous and that male provisioning of immobile females and offspring was unlikely. The living descendants of these early hominids are certainly not strictly monogamous; only about 20% of human societies today have laws prohibiting polygyny and these are usually recent and commonly broken in practice. Among all monogamous primates, sexual dimorphism is nearly absent, but in A. afarensis it is pronounced (males may be nearly twice the weight of females), as one might expect in a polygynous species. All monogamous primates are highly territorial, with the adult male and female and their offspring forming the exclusive territorial unit. A. afarensis apparently lived in much larger groups, as indicated by the 333 site of Hadar where apparently 13 individuals died together 3.5 m.y. ago. No monogamous primate species is known to have a male who provides food for the female and her offspring. Among modern hunters and gatherers the female does more than her share of the food gathering even while carrying an infant. Among the !Kung San (Bushmen) the female walks an average of almost 5000 miles in the first 4 years of a child's life while carrying the infant. The average female gathers an estimated 12,000 calories per day in food while the male averages only about 7,230 calories. Nor is paternal care of infants limited to monogamous primates. Decrease in birth spacing is hardy a sufficient cause for the origin of monogamy and paternal care. As Hrdy & Bennett point out, a polygynous male with two females giving birth every 6 years would produce 1.6 babies for every one that a single female would produce with 5 year birth intervals.

McHenry, H.M. (1982) The pattern of human evolution: Studies on bipedalism, mastication, and encephalization. Annual Review of Anthropology 11: 151-173.