Figure 4: Multicellular animals arose from single-celled organisms related to choanoflagellates, a group that originated about one billion years ago. Animals probably appeared more than 565 million years ago. The most primitive of living animal phyla are the sponges, forms of which have been found in Neoproterozoic fossils dating back 565 million years. Following the sponges are animals such as comb-jellies (phylum Ctenophora) and jellyfish and sea anemones (phylum Cnidaria), fossils of which date to the late Neoproterozoic period. The next major branch produced the flatworms (phylum Platyhelminthes). After the flatworms came a division that gave rise to a wealth of more complex animals--the split between protostomes and deuterostomes. The protostomes include most of the familiar invertebrates, including crabs and insects (phylum Arthropoda), earthworms (phylum Annelida), and snails, clams and squids (phylum Mollusca), as well as others. Deuterostomes include starfish and sea urchins (phylum Echinodermata), and the group that encompasses fish to mammals (phylum Vertebrata). (Illustration by Linda Huff.)

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