The similarity in bone structure between many different animals was first observed back in 1555 by Pierre Belon. He interpreted this as evidence of the “great chain of being”—order in nature as dictated by the Almighty. These homologies were later used as evidence of Darwin’s theory of evolution, illustrating that all animals have evolved from a common ancestor.
This diagram shows the homologies of the forelimb in six different vertebrates or—in simple terms—that the underlying bones of some very different animals are exactly the same in format. Here we can see the limbs of six different creatures—a turtle’s flipper, a dolphin’s fin, a horse’s leg, a human arm, a chicken’s wing, and a bat’s wing. These are completely different animals, with the limbs being used for completely different purposes like swimming, flying, and walking. But as this diagram shows, underneath the surface the arrangement of the bones is identical.