A homologous structure is a structure or organ that can be found on many different animals that suggest that there is a common ancestor. It can be anywhere from a bone to a flap of skin to an organ. A common one to look at is the similarity between a human hand, bat’s wing, whale anterior flipper, and a cat’s paw. All of these have a common, 5-point structure to the bones which suggests that we all come from an animal that had 5 fingers.