1. Evolutionary History
Scientists are still uncertain exactly how the first invertebrates evolved. Most experts agree that the
first invertebrates evolved from single-celled, food-eating microorganisms. Scientists think that these
microorganisms formed permanent symbiotic groups and in doing so, they were no longer single-
celled, they had become multicellular. When this happened, animals had evolved. The first known
invertebrates, and in turn the first known multicellular animals, are sponges. Sponges are in the phyla
Calcarea and Silicea. Sponges are basal animals. This means that sponges branch from the base of
both vertebrates and invertebrates. Next came the eumetazoans, or “true animals”. True tissues
evolved when the eumetazoans came to be. The first members of the eumetazoan clade include comb
jellies and Cnidaria. They are generally diploblastic and feature radial symmetry. The next clade is the
clade Bilateria. Bilaterians have bilateral symmetry and three germ layers. The majority of animal
phyla belong to Bilateria. After the Bilateria is where scientists disagree. Some believe the Bilateria
split into Deuterostomes and Protostomes, with chordates resulting from the Deuterostomes. Others
believe that Bilateria split into Deuterostomes, Lophotrochozoans, and Ecdysozoans. The truth has yet
to be determined.