The skeletal adaptation to bipedalism is well documented in early hominids. What is less clear is what events led to this adaptation and its eventual success. Hypotheses about why bipedalism arose have been very common, but most lack the necessary evidence to test them. All apes can walk bipedally, so the behavior itself was within the capabilities of the common ancestors of hominids and chimpanzees. What is necessary is to explain how bipedalism became so essential that it provoked skeletal adaptations that made other forms of locomotion much more difficult.
One argument is efficiency. Human bipedalism is very efficient at normal walking speeds, because forward motion results from gravity swinging each leg forward like a pendulum. The walking biped recaptures this forward momentum by slowing the swinging leg before footfall. As a result, walking at normal speeds on level surfaces requires very little muscular activity, making bipedalism more efficient than knuckle-walking or quadrupedalism (McNeill Alexander 1985). Aside from its energetic efficiency, bipedalism also has the advantages of raising the head, and therefore allowing a wider range of vision in a grassland environment, and of freeing the hands for carrying items or for tool use.
Despite these advantages, bipedalism also has considerable disadvantages. The first is that it makes climbing considerably more difficult. Without the ability to grasp with the feet, hominids are less secure in an arboreal setting. There are many indications that climbing remained an important part of the behavior of early hominids, discussed below. The combination of features found in early hominids reflects a compromise adaptation to climbing, which is based on the presence of morphological adaptations to bipedalism in the pelvis and foot. Part of this compromise was structural, involving much more powerful arms and possibly human-proportioned hands for gripping branches rather than suspending from them. Another part of the compromise was behavioral. The loss of a grasping foot is also a serious problem for child-rearing. In chimpanzees and other primates, the young can use their hands and feet to grasp and cling to their mother's fur. For hominid infants, such clinging would have been much more difficult, if not entirely impossible. One of the adaptations to bipedalism must, then, have been a behavioral change toward carrying dependent offspring until they were old enough to walk.
Over time, scientists have devised many different theories to reconstruct the circumstances that led to the evolution of bipedalism. Charles Darwin himself correctly assumed that the African apes are the closest human relatives, and constructed a model for hominid origins that stressed the appearance of bipedalism from an ape-like ancestor. In Darwin's model, bipedalism is seen as the adaptation resulting when a quadrupedal ape is forced to assume a terrestrial adaptation. In Darwin's formulation, this adaptation was partly caused by the advent of a hunting subsistence pattern, where the hands needed to be free to carry weapons and meat. Additionally, Darwin thought that a change in habitat from woodland to savanna left early hominids without the refuge of trees, resulting in less importance of climbing and a greater need for efficient movement on the ground. Other later researchers picked up many of the themes of Darwin's model, stressing other important features of life on the savanna, such as the need to see over tall grass, and the need to adapt to intense solar radiation. Bipedalism has been suggested as an adaptation to both these factors, by placing the head high and upright, and decreasing the exposure of the trunk to direct light from overhead. This model came to be called the savanna model, or stressing the importance of hunting in the model, the killer-ape hypothesis.
Today, we have greater knowledge about the environments that early hominids occupied, and many aspects of the savanna model do not appear to describe the conditions under which bipedalism evolved. All of the sites before 3 million years ago seem to have been partially or fully wooded, and no early hominids are known from full savanna environments. Additionally, the fossil forelimb elements of early hominids demonstrate the continued importance of climbing. The importance of hunting has been questioned because chimpanzees hunt extensively without the adaptations of early hominids, and because no tools, weapons or adaptations to making tools are known from the earliest hominids. These observations imply that bipedalism was not the simple consequence of a single climatic change.
Lovejoy (1981) has suggested that social factors may have been principally responsible for the origin of bipedalism. In his hypothesis, food sharing was an important component of social behavior. Lovejoy speculates that males may have supplied food to females in order to gain mating access or to contribute to the parenting of their own offspring. This behavior would require the use of hands for provisioning. Such a hypothesis must be reconciled with the apparently high level of sexual dimorphism among early hominids, but may provide significant insights.
One reason for the proliferation of hypotheses to explain hominid origins is that we have almost no knowledge about the postcranial anatomy of the immediate ancestors of the hominids. Most hypotheses have assumed that the common ancestors of living African apes and hominids were essentially like chimpanzees, with suspensory locomotion in the trees and knuckle-walking on the ground. Whether hominids originally evolved from a knuckle-walking ape or not has been controversial. Some scientists, like Brian Richmond and David Strait (2000), argue that early hominids like Lucy bear anatomical features that indicate a knuckle-walking ancestry. In this formulation, the occasional bipedal locomotion of chimpanzees and gorillas is a model for how obligate bipedalism originated. The anatomical changes that characterize the known hominid fossils grow from a more intensive use of this basic hominoid behavior.
Other scientists point to the possibility that knuckle-walking evolved in parallel in chimpanzees and gorillas. The manner of arboreal locomotion in living and extinct apes seems to have been greatly influenced by body size. Known early hominids average slightly less than chimpanzees in body size, and it is possible that their common ancestor was small, rather than chimpanzee-sized. Wolpoff (1999) has suggested that the ancestors of hominids may have been small apes who often walked or ran bipedally above large branches, as well as on the ground. From this perspective, the knuckle-walking of chimpanzees and gorillas and the bipedalism of hominids represent different strategies for ground locomotion related to body size. Under this hypothesis, the large apes developed a suspensory adaptation in response to increases in body size, with locomotion on the ground occurring later than or secondary to this increase. In contrast, early hominids adapted more fully to the ground before their body size increased, resulting in an anatomical adaptation to bipedalism, with climbing secondary.
None of the factors here excludes any of the others, and probably the origin of hominid bipedalism involved a complex combination of these and possibly others. Until scientists have more knowledge about the anatomy of the first hominids and their ancestors, we will be unable to rigorously test these hypotheses. Nevertheless, even as the record of hominid evolution has been pushed back to six million years, bipedalism remains the hallmark adaptation of our lineage. For this reason, explanations for its origin remain one of the most important parts of paleoanthropology.
Lovejoy CO. 1981. The origin of man. Science 211:341-350. JSTOR
McNeill Alexander Ra. 1992. Human locomotion. In: Jones S, Martin R, Pilbeam D, editors, The Cambridge encyclopedia of human evolution. Cambridge: Cambridge University Press. p 80-85.
Richmond BG, Strait DS. 2000. Evidence that humans evolved from a knuckle-walking ancestor. Nature 404:382-385. PubMed
Wolpoff M. 1999. Paleoanthropology. Second edition. New York: McGraw-Hill.