535 million years ago
Soon after animals evolved, evolution went through two major growth spurts. In the Cambrian Explosion, it seems almost every group of modern animals appeared within tens of millions of years. This apparent ‘explosion’ may be partly down to better fossilisation, as many animals now had hard shells. Then 489 million years ago, each animal group expanded in the Great Ordovician Biodiversification Event.
See other: History of Life