The Radiation of the First Animals

by Dr. Jerry Lipps
Department of Integrative Biology and The Museum of Paleontology
University of California, Berkeley

Narrative Index

Animals first appear in the fossil record around 580 million years ago as frond-like forms, jellyfish-like imprints, and trace fossils. These fossils appear simultaneously on all continents, except Antarctica, and each assemblage contains roughly the same kinds. This appearance is still about 700-300 million years later than the molecular data suggest that animals originated, leaving an enormous period of time without a fossil record. Perhaps, the first animals were small, unskeletonized, or destroyed by geologic processes.
At the base of the Cambrian period about 545 million years ago, all modern phyla of animals and many algae and protists appear or radiated--including phytoplankton, forminifera, archeocyaphids, trilobites. Most explanations for this radiation involve properties of animals themselves (intrinsic causes), but the coincident events in algae and protozoans suggest perhaps a more ubiquitous, ecologic trigger (extrinsic causes). One possibility is oceanographic changes that increased nutrient supplies to the shallow waters, hence a radiation of trophic links in the food chains of that time.