Quadrupedalism (from Latin, meaning "four legs") is a form of land animal locomotion using four legs. The majority of walking animals are quadrupeds, including mammals such as cattle and cats, and reptiles, like lizards. Birds, humans, insects, crustaceans and snakes are not quadrupeds. There are some exceptions, for example among the insects the praying mantis is a quadruped. A few birds may use quadrupedal movement in some circumstances, for example the shoebill will sometimes use its wings to right itself after lunging at prey .
Quadrupeds and tetrapods
Not all four-limbed animals are quadrupeds. Although arms and wings are, in the evolutionary sense, modified legs, four-limbed animals are in fact classed as tetrapods – members of the taxonomic unit Tetrapoda. These include all vertebrates with quadrupedal ancestors, including mammals, reptiles, amphibians, and birds.
The distinction between quadrupeds and tetrapods is important in evolutionary biology, particularly in the context of bipeds, winged animals, and animals whose limbs have adapted to other roles (e.g. fins, in the case of cetaceans and pinnipeds). All of these animals are tetrapods, but none are quadrupeds. Even snakes, whose limbs have become entirely vestigial, are nevertheless tetrapods.
Quadrupedalism in humans
In July of 2005, in rural Turkey, scientists discovered five Kurdish siblings who had learned to walk naturally on their hands and feet. Unlike chimpanzees, who ambulate on their knuckles, the The Family That Walks On All Fours (ranging from 18 to 34 years old) walked on their palms, allowing them to preserve the dexterity of their fingers. Calluses found on their hands make the possibility of a hoax unlikely. Another similar case has been reported in Chile, but the case is still being investigated and reports are not released as of March 22, 2006.
The discovery of the family has provided scientists a unique view into human evolutionary history. Nicholas Humphrey, a researcher from the London School of Economics, has suggested and colleagues have argued that their gait is due to two rare phenomena coming together. First, instead of initially crawling as infants on their knees, they started off learning to move around with a “bear crawl” on their feet. Second, due to their congenital brain impairment, they found balancing on two legs difficult. Because this, their motor development was channelled into turning their bear crawl into a substitute for bipedality.
Other scientists, such as Stefan Mundlos of the Max Planck Institute, believe that the family's unusual gait may result from a genetic abnormality. Mundlos has isolated a gene on Chromosome 17 (human) which is responsible for human bipedalism. He speculates that the Turkish siblings lack this gene.