Flexi Says: The oldest animal fossils are about 630 million years old. By 500 million years ago, most modern phyla of animals had evolved. Scientists think that cells of some protist colonies became specialized for different jobs. After a while, the specialized cells came to need each other for survival. Thus, the first multicellular animal evolved. One type of sponge cell, the choanocyte, looks a lot like the protist cell.