For most of Earth's 3.5 billion-year history, life consisted of single-celled microorganisms and, more recently, simple soft-bodied animals. The Cambrian explosion changed this in a matter of tens of millions of years: hard shells, jointed limbs, complex eyes, and biting jaws — in short, the fundamental body plans of modern animals — appear suddenly in the fossil record.

The Burgess Shale in Canada (510 million years) and the Chengjiang biota in China (518 million years) are the main windows onto this era, owing to their remarkable preservation of soft tissue. Predators like Anomalocaris, with five compound eyes and hook-shaped head appendages, hint at the ecological sophistication of Cambrian seas.

The cause of the explosion remains debated. A combination of factors is usually proposed: rising oxygen levels, a thickening ozone layer and UV shield, an ecological arms race between predators and prey, and the evolution of gene-regulatory (Hox) systems. The explosion is neither a starting point from nothing nor a truly instantaneous event; its roots lie in the late Ediacaran period, and it unfolds over several tens of millions of years.