Mammals, birds, reptiles and amphibians all derive from a prehistoric ancestor that had four limbs with five fingers and five toes. As these vertebrates adapted to different environments and lifestyles, the form and function of their hands and feet evolved to best serve them. While all extremities are derived from this basic osteological structure, the number, shape, and size of bones belonging to different animals reflects their specific environments and lifestyle. The form of a mammal's hands and feet depends on what the animal moves through (air, water, underground, on solid surfaces - such as the ground, rocks/cliffs, or trees), and how the animal moves.