bipédie

Bipédie, or bipedalism, is the locomotion on two legs. The term combines Latin bi- “two” and pes/ped- “foot.” In biology it refers to the habitual use of two limbs for walking or running, as distinct from quadrupedalism or occasional bipedal postures. In humans, bipédie is a defining trait of the Homo lineage, while many other animals also move primarily on two legs—birds such as ostriches and emus are obligate bipeds, and several extinct theropod dinosaurs were bipedal.

Anatomically, bipedal locomotion is supported by features such as an upright pelvis, an S-shaped spine, a foramen magnum positioned under the skull, knee joints aligned for standing and walking, and arched feet with a non-opposable big toe. These traits help balance the body and absorb loads during two-leg gait.

Habitual bipeds include humans and some extinct hominins; birds and many dinosaurs are habitual bipeds; other mammals such as bears can stand and walk on two legs temporarily. Some primates and other animals adopt bipedal postures briefly, but do not rely on two legs for regular locomotion.

Evolution and timeline: evidence for bipedalism in hominins appears around 6–7 million years ago, with early examples such as Ardipithecus ramidus and Australopithecus afarensis. Homo erectus shows further refinements for efficient long-distance walking and running, accompanying changes in brain size and behavior.

Functions and trade-offs: advantages of bipédie include energy efficiency for long-distance travel, the ability to carry tools and offspring, and improved field of view in open environments. Disadvantages include increased stresses on the spine, hips, and knees, potential back pain, and reduced stability on uneven terrain or at high speeds.