bipeds

Bipeds are vertebrates that locomote primarily on two legs. The term contrasts with quadrupeds, which move on four limbs, and with species that switch between stances or use all limbs for movement.

In living animals, bipedal locomotion is most familiar in humans and other hominins, many birds such as ostriches and chickens, and a number of extinct lineages, notably theropod dinosaurs like Tyrannosaurus and Velociraptor. Some mammals, including kangaroos, use two hind limbs as their main propulsive mechanism, though they may use other limbs or tails for balance.

Anatomical adaptations supporting bipedalism include upright posture and limb proportion changes that stabilize the body during single-leg support. In humans, the spine forms an S-shaped curve, the pelvis and lower limbs align to keep the center of gravity over the stance leg, and the foot acts as a lever for efficient push-off. Birds have different leg structures with fused bones and specialized feet that support bipedal locomotion on land, while their lightweight skeleton and wings enable flight. In many bipedal species, the forelimbs or wings are repurposed for tasks other than locomotion, such as grasping or balance.

Evolutionary significance varies by lineage. In the human lineage, habitual bipedalism arose roughly 4 to 6 million years ago and is linked to factors such as energy-efficient long-distance travel, enhanced field of view, and the freeing of hands for tool use. Bipedalism has evolved independently in several groups, and today they range from obligate bipeds to facultative bipeds that may revert to foraging or climbing on all fours.