Despite the fact that their limbs appear to be very different on the outside, many vertebrates have very similar bone structures. The pentadactyl (five-fingered) limb is the name given to this structure. This indicates that many vertebrates descended from a single ancestor.
Crocodiles, birds, whales, horses, bats, and humans all have the same five-fingered bone structure, despite their appearance. This lends credence to Darwin's theory of evolution.