The Cambrian explosion (~541 million years ago) saw the rapid diversification of most major animal body plans in a geologically brief window of ~20 million years.
The causes are debated: rising oxygen levels, predator-prey arms races, the evolution of eyes, or the emptiness of ecological niches after the Ediacaran fauna disappeared.
What's certain: most animal phyla that exist today were present by the end of the Cambrian. The basic architectural plans (bilateral symmetry, segmentation, appendages) were established early and have been modified since but not replaced.
Related: succession-theory, convergent-evolution