Bipedalism

Eduzendium article

Bipedalism is the condition of having or using only two feet for locomotion. This form of movement is found in a few groups of animals on Earth. Throughout the course of evolutionary history, the use of bipedal movement came forward several times as an adaptation, including at what many consider to be an influential point in human evolution. The particular selective pressures that acted to bring bipedalism to the forefront time and time again are most likely diverse and unique to each group for which this type of locomotion was expressed. However, the prevalence of bipedal movement in animals both extant and extinct shows that this form of locomotion is sometimes advantageous.

The use of bipedal movement is thought to be a key element in human evolutionary history, as it is part of what separates humans and their ancestors from the other Great Apes. By virtue of similar musculoskeletal organization of human and ape bodies it is possible to trace common ancestry. Humans today bear many anatomical resemblances to their closest relatives. Residual anatomical traits that include but are not limitied to dorsally-placed scapulae and a wide range of motion in the shoulder, arm, and wrist point to an origin of bipedal locomotion that is very recent in evolutionary history of the Genus Homo. These residual traits suggest suspensory adaptations in the forelimbs of a primarily terrestrial group of animals that usually bear the bulk of its mass on their hindlimbs.

Species Displaying Bipedal Movement

There are many species on Earth that use bipedal movement as their primary means of terrestrial locomotion, and also some that can use bipedal movement when pressed to escape danger.

Birds

In nearly every species of bird, a form of terrestrial bipedal movement shows itself. Because of their wings many birds do not use bipedalism as their primary means of locomotion in life, but when they land they hop or walk on two feet. Birds whose lives are spent in a mostly terrestrial capacity such as chickens, kiwi birds, and larger forms like ostriches and emus have reduced wings and larger, stronger legs. These birds are often highly-efficient bipeds, able to run at high speeds and relatively agile for their size. Ostriches have been known to run at speeds in excess of 40 mph (65 km/h)when chased by predators <ref>Doherty, James G. (1974) Natural History Magazine, March 1974. The American Museum of Natural History; The Wildlife Conservation Society. Herons and similarly adapted marsh-living birds have an elegant, slow bipedal gait that they use for hunting.

Birds are thought to be descendants of theropod dinosaurs, and are thought to have split from that lineage before the event that cause the extinction of much of land vertebrates 65 million years ago at the end of the Cretaceous period.

Lizards

Several extant species of lizard walk or run on their hind legs when trying to escape predators. Lophognathus longirostris is an example that is closely related to the frill-necked lizards, and runs on its back legs to gain speed and agility over short distances. Physignathus lesueurii, another relative, is capable of running for short distances on the surface of water.

Anatomical Correlates of Bipedalism in Humans and Recent Human Ancestors

Through looking at the osteology of humans and their close relatives in the fossil record, a few key anatomical traits for the particular bipedalism associated with humans have come forward. A combination of these traits makes it possible for humans to walk upright and allows scientists to diagnose whether a fossil likely used bipedal movement or not.

Medially-Angled Femur

Also, larger medial condyle on the distal end of the femur.