Many vertebrates have a very similar bone structure despite their limbs looking very different on the outside. This structure is known as the pentadactyl (five fingered) limb. This suggests that many vertebrates descended from the same common ancestor.
Although the limbs of crocodiles, birds, whales, horses, bats and humans all look very different they share the same five fingered bone structure. This provides evidence for the theory of evolution.
The pentadactyl limb seen in the horse provides an excellent example of how evolution has happened. As the marshy ground became drier, horses with smaller feet were generally faster. They had an evolutionary advantage because they were able to avoid predators. Over many, many generations horse's feet have evolved to be smaller and the horses themselves taller and stronger.