Cambrian explosion

The Cambrian explosion is the name for a striking event early in the Cambrian Period, which began roughly 539 million years ago. Within perhaps 20 to 25 million years, a short time in geological terms, the fossil record fills with animals showing most of the basic body plans seen today, including early arthropods, mollusks, echinoderms, worms and chordates, the group that includes vertebrates. Many had hard parts such as shells, spines and exoskeletons, which fossilize far better than soft tissue.

Animals did not begin in the Cambrian. Simpler, mostly soft-bodied organisms of the Ediacaran Period lived earlier, and molecular clock estimates suggest that animal lineages split even before that. What changed was the speed of diversification and the arrival of complex bodies, eyes, armor and active predators. Proposed causes, which may have acted together, include rising oxygen in the oceans and air, changes in seawater chemistry that made shell-building easier, the evolution of genes such as the Hox genes that pattern animal bodies, and an arms race between the first predators and their prey.

Exceptional fossil sites such as the Burgess Shale in Canada and Chengjiang in China preserve soft bodies too, revealing animals like the large predator Anomalocaris alongside many trilobites.