Quadrupedalism 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.
Quadrupeds and tetrapods
All four-limbed animals are not 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, some 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 Turkish siblings (ranging from 18 to 34 years old) walked on their palms, allowing them to preserve the dexterity of their fingers. Callouses found on their hands make the possibility of a hoax unlikely.
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 that the siblings' gait is an instinctive human behavior which has simply been abandoned over the course of evolution. Humphrey argues that the five siblings, who suffer from mental retardation related to cerebellar ataxia, have regressed to an earlier stage of human evolution, and have simply continued to walk "as infants" into adulthood.
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 which is responsible for human bipedalism. He speculates that the Turkish siblings lack this gene.
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