After Charles Darwin published his theory of evolution, biologists began to understand why organisms fall into natural groupings. For example, species in the dog genus Canis look like one another because they all had a common ancestor. Foxes (genus Vulpes) and dogs (genus Canis) do not look as much alike because their common ancestor was farther back in time. The farther back a common ancestor lived, the longer its descendants have had to evolve and change.
It is almost impossible to prove that two species share a common ancestor. But by making an extensive list of characteristics, scientists can show how likely it is that two species are related. The more traits two species share, the more likely they are closely related and got those traits from a shared ancestor.
For example, both sparrows and bats have arms and hands that are wings, but sparrow wings and bat wings are much different. Sparrow wings and bat wings evolved separately, and not because of a common ancestor. This is called convergent evolution. On the other hand, the wings of sparrows, eagles, ostriches, and all other birds are alike. This shows that today's bird species are closely related and came from a common ancestor.