How the Snake Lost Its Legs

All vertebrates that live on land--from humans to alligators to birds--are collectively known as tetrapods, meaning four feet. The name sticks even though the legs to which two of these feet are attached have become arms in humans and wings in birds. In snakes, the change was even more radical: they lost all four of their limbs. One of the few remaining signs of their limbed heritage is the presence of vestigial hips imprisoned in the rib cage.