The question of when hominoids began to walk on two limbs rather than four is being defined by new fossil discoveries by anthropologists, but the question of why humans became bipedal may be more difficult to answer.
The prevailing theory has been that early ancestors of humans—those who lived in trees—were pushed onto the open savannah by climate and environmental changes that forced them out of the trees and onto the ground. Less vegetation meant traveling farther for food and relying more heavily on their wits. A bipedal gait was more efficient and better suited to this life out in the open than was walking on all fours. This theory was supported by environmental evidence indicating that other species common to harsh, open land environments were found in multitudes with the fossils of early bipedal hominids and that evidence of forest-loving animals was largely missing. Then, more evidence, this time from plants and animals, indicated that these early hominids lived near plants that flourished in a forest and near animals with extremities meant for forest life.
Other theories about the advent of bipedalism have included the theory that being upright made it easier for early humans to see across a plain and theories that humans needed free hands for gathering food, for holding children, and/or for carrying food back to a base camp. These theories feature either men as hunters, women as gatherers, or man as a “provisioner” who was improving his chances for his clan’s persistent survival by supplying a mate with food who would in turn produce offspring more frequently. Another theory proposed that shifts in the availability of food forced wider foraging, making bipedalism attractive. A chimpanzee is only about half as efficient walking on all fours (what anthropologists call “knuckle walking”) as he would be walking bipedally. So walking on all fours did not have an energy advantage.
Whether the human line became bipedal to free the hands, to hold children or provide provisions for them, or to gather food or avoid becoming food for predators, it is important to remember that humans are the only species that walks with a functional bipedal gait, despite the many similarities between the human knee and those of many animal systems.
That is partly why watching children learn to walk is such an excruciating but deeply cheering experience. Walking is at once one of the most fundamental aspects of being human yet one of the most difficult skills to master in very early childhood. It is as though in gaining the ability to walk, the child obtains much more than the ability to put one foot in front of another.
Dr. Jack E. Jensen, M.D.