The limbs of several organisms are shown in the illustrations below. scientists sometimes compare the limbs of these organisms to look for evidence of common ancestry.

These limbs (see attached image) provide evidence of common ancestry because they have the same basic structure. In evolution we have convergent evolution and divergent evolution; In convergent evolution we focus on the analogous structure; which have different ancestry but perform similar functions while in divergent evolution, we have homologous structures which have a common ancestry and similar structure; but performs different functions, like the limbs described above. 

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