Human Walking: The Origins of Bipedalism in Hominids

Evolutionary Context and the Emergence of Bipedalism

Bipedalism, or locomotion on two hind limbs, is a rare trait among mammals. Although most terrestrial mammals have been quadrupedal since the Triassic period (−251 to −199.6 million years ago), some groups have developed bipedal locomotion. Among hominids, it became dominant.

The first primates appeared about 70 million years ago. Orrorin tugenensis (≈6 Ma) has a femur suggesting an upright gait. Later, Australopithecus afarensis (≈3.6 Ma) exhibits bipedal characteristics confirmed by the fossil footprints of Laetoli (Tanzania). These footprints, attributed to A. afarensis, show a plantigrade gait similar to that of modern humans.

Bipedalism in Other Animals

Bipedalism is not unique to humans. Birds (descendants of theropods), some mammals (meerkats, kangaroos, bears), and many primates (chimpanzees, bonobos, etc.) can adopt it temporarily. Among hominids, it became permanent.

Major Theories on the Origin of Human Bipedalism

Several hypotheses seek to explain the origin of bipedalism in hominids:

• Savanna Theory: Walking upright would allow seeing above tall grasses and moving in open environments. However, paleoenvironmental data indicate that Australopithecus evolved in more wooded environments.
• Freeing of the Hands: Standing upright frees the arms to carry food. Experiments have shown a significant increase in bipedalism in chimpanzees when they carry objects.
• Tool Use: Similar to the previous theory, this suggests that free hands were used to manipulate tools, promoting a joint evolution of posture and dexterity.
• Squatting Feeding: Proposed by Kingdon, it involves arboreal or ground feeding favoring an upright posture.
• Aquatic Theory: Niemitz argues for a semi-aquatic origin, where bipedalism would have been advantageous for moving in shallow waters. Human thermoregulation supports this hypothesis.

Limitations of Current Hypotheses

All these theories remain speculative. Fossils are rare and partial, making it difficult to reconstruct the evolutionary scenario. It is likely that several selective pressures—ecological, behavioral, dietary—jointly favored bipedalism.

Towards a Synthesis: Arboreal Verticality

The upright posture is common among arboreal monkeys, useful for climbing or hanging. It is plausible that the common ancestor of hominids retained this verticality when moving to the ground. Human bipedalism would then be a terrestrial extension of an arboreal adaptation.

Bipedalism: A Multifactorial Process

Human bipedalism results from a slow evolutionary transition. It allowed the freeing of the hands, broadening of the diet, use of tools... but also imposes biomechanical constraints. This complex evolution illustrates the deep interaction between environment, behavior, and anatomy in human history.