During its earliest evolution, bipedality is thought to have been a very costly form of locomotion. However, a reproductive advantage must have fallen to those in each generation that walked more frequently in a bipedal posture. Many theories have been developed to try and explain the evolutionary success of bipedality.
The main goals in any evolutionary game are to eat, stay alive, and reproduce. The edge was not speed because most four-legged animals move faster than humans. It was not efficiency because moving on two legs uses no more nor less energy than moving on four legs unless one is traveling a long distance. If this is the case, the number of calories that are expended for travel are lower in a biped than in a chimp walking on four legs or an antelope on four legs.
Even a rise of several degrees in body temperature, from 98.6° to 107° F, poses a special threat to the human brain, leading rapidly to convulsions, hallucinations, permanent neural damage, and sometimes death. Because of its large size, the brain itself generates lots of heat, which can accumulate to dangerous levels if not dissipated. Fortunately, like the engine of a car, the brain has a radiator to protect it from overheating - a network of tiny veins that originate in the scalp and face.
Unlike modern humans, apes lack a venous radiator. The evolution of bipedalism necessitated changes in blood circulation and may have laid the groundwork for its emergence. An examination of fossil skulls, however, reveals that a venous radiator did not arise immediately. It is absent in the earliest-known hominid (Australopithecus afarensis) and in some of its successors, the "robust" australopithecines. Brain size remained conservative in these species. But a prototype radiator is present in the "gracile" australopithecines. This is most evident in the greater number of holes in the skull, called emissary foramina, to accommodate emissary veins. I therefore conclude that the gracile australopithecines gave rise to our own genus.
Beginning about two million years ago, the number of emissary foramina in the genus Homo began to increase dramatically. Brain size also began to increase rapidly in this group, and these trends continued in tandem until the time of the Neanderthals (about 100,000 years ago). The evolving venous radiator apparently removed a major constraint on the increase of brain size.
Humans are not the only large-brained animals that evolved cranial radiators. Some fifty million years ago, the terrestrial ancestors of whales (including dolphins) began a major change in posture and locomotion that culminated in their becoming exclusively adapted to an aquatic habitat. Like humans, whales have evolved large, highly convoluted brains that generate potentially damaging heat.
Hominids, or humanlike primates, first appeared in Africa five to seven million years ago, when that continent's climate was becoming increasingly arid and large tracts of woodland and savanna were replacing the unbroken canopy of the equatorial forest. While the ancestors of chimpanzees and gorillas remained in the moist forests, the hominids started to exploit the more open, drier habitats. The intense sunshine in the new environment, combined with a scarcity of drinking water, must have severely challenged the ability of early hominids to regulate their body temperature.
Most savanna mammals possess special physiological mechanisms to cool the brain - notably the carotid rete, a network of fine arteries near the base of the brain, coupled with venous circulation through the muzzle. Humans, apes, and monkeys, however, lack these features. The first hominids could have prevented damaging elevations of brain temperature only by keeping their entire body cool. Walking on two feet - the unique mode of terrestrial locomotion that is widely recognized as the first key development in hominid evolution - conferred precisely these benefits.
Bipedalism dramatically reduces exposure to direct solar radiation during the middle of the equatorial day. Bipedalism also raises most of the body well above the ground, so that the skin contacts cooler and faster-moving air currents. This favors heat dissipation through convection.
Scientists have long reasoned that one of the most obvious and unusual human features, the loss of insulating body hair, is an adaptation to the hot savanna. Although follicles are still densely distributed over most of the human body, the hairs they produce are so short and fine that the underlying skin is exposed directly to the flow of air, promoting the shedding of excess heat by convection. The problem with this hypothesis has always been explaining why humans differ from other savanna mammals, which have retained dense coats of hair.
In environments where mammals are exposed to strong solar radiation, the coat acts as a shield, reflecting and reradiating heat before it reaches the skin. For most mammals, the loss of this insulation would create more problems that it would solve. For a biped, in contrast, a naked skin saves water because so little skin surface is exposed to the sun. Mainly the head and upper shoulders are exposed, and these can be protected by the retention of a relatively small amount of hair cover. Bipedalism and the strategy of cooling the whole body (rather than just the brain) probably explain why humans evolved a naked skin, while other savanna mammals of comparable size did not.