Bipedalism
Primates

Most bipedal animals move with their backs close to horizontal, using a long tail to balance the weight of their bodies. The primate version of bipedalism is unusual because the back is close to upright (completely upright in humans), and the tail may be absent entirely. Many primates can stand upright on their hind legs without any support. Chimpanzees, bonobos, gorillas, gibbons and baboons exhibit forms of bipedalism. On the ground sifakas move like all indrids with bipedal sideways hopping movements of the hind legs, holding their forelimbs up for balance. Geladas, although usually quadrupedal, will sometimes move between adjacent feeding patches with a squatting, shuffling bipedal form of locomotion. However, they can only do so for brief amounts, as their bodies are not adapted for constant bipedal locomotion.

Humans are the only primates who are normally biped, due to an extra curve in the spine which stabilizes the upright position, as well as shorter arms relative to the legs than is the case for the nonhuman great apes. The evolution of human bipedalism began in primates about four million years ago, or as early as seven million years ago with Sahelanthropus or about 12 million years ago with Danuvius guggenmosi. One hypothesis for human bipedalism is that it evolved as a result of differentially successful survival from carrying food to share with group members, although there are alternative hypotheses.

Injured chimpanzees and bonobos have been capable of sustained bipedalism.

Three captive primates, one macaque Natasha and two chimps, Oliver and Poko (chimpanzee), were found to move bipedally. Natasha switched to exclusive bipedalism after an illness, while Poko was discovered in captivity in a tall, narrow cage. Oliver reverted to knuckle-walking after developing arthritis. Non-human primates often use bipedal locomotion when carrying food, or while moving through shallow water.
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