The Cambrian explosion was an evolutionary event that began about 541 million years ago with the rapid diversification of major animal body plans.
It marks the start of the Cambrian Period, when many animal groups first appear clearly in the fossil record. Fossils from this interval show the early forms of arthropods, mollusks, chordates, and other lineages. The famous Burgess Shale in Canada preserves many Cambrian organisms, including soft-bodied animals that rarely fossilize.
The phrase does not mean that every animal suddenly appeared, or that evolution had never occurred before. Earlier organisms, including Ediacaran biota, lived before the Cambrian, and some Cambrian groups had simpler ancestors. Scientists investigate several possible causes, including rising oxygen levels, ecological interactions such as predation, developmental innovations, and improved fossil preservation.
The Cambrian explosion is therefore best understood as a comparatively rapid geological radiation of animal diversity, not a single day or a complete replacement of earlier life.