Mammals, birds, reptiles, and amphibians all have two pairs of . They are all tetrapods, or animals. In the fossil record, tetrapods first appeared about million years ago (Ma). Where did they come from?

The closest relatives of tetrapods are . Fish and tetrapods are both vertebrates that have a backbone with a spinal cord. Fish predate (come before) tetrapods, because their appear in rocks that are more than million years old. This observation leads to the hypothesis that tetrapods evolved from .

If tetrapods evolved from fish, as Darwin predicted, you would expect to find fossils with features of both fish and tetrapods. In Darwin’s day, such fossils had not yet been found, but today, there are many such examples.

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