The Cambrian explosion is the interval early in the Cambrian when the animal fossil record suddenly becomes much richer and many major body-plan lineages become recognizable. The current international boundary for the start of the Cambrian is 538.8 ± 0.6 million years ago.

Big idea: It was not the moment animals appeared from nothing. It was a geologically rapid expansion of animal diversity, ecological complexity, hard parts, and fossil visibility after a long Precambrian history of microbes and earlier multicellular organisms.

Why does it look so dramatic? Probably because several things reinforced one another:

• predator–prey arms races favored shells, skeletons, burrowing, eyes, and mobility;
• developmental toolkits could generate more body forms;
• environmental oxygenation may have made larger, more active animals easier to sustain;
• mineralized hard parts and exceptional fossil deposits greatly improved preservation.

The word explosion is geological, not instantaneous. The diversification unfolded over tens of millions of years, and several animal groups have roots before the formal Cambrian boundary.