The explosion of animal diversity really kicked off during the Cambrian period, about 540 million years ago.
The first animals that lived on Earth roamed the seas. The oceans were ruled by jawless, mostly spineless creatures that look alien compared with modern ocean life. The earliest animals with backbones, the vertebrates, were around in the Cambrian, but they did not diversify extensively until the Silurian period, just over 420 million years ago.
The oceans then became overrun with a vast array of bony fish, while arthropods and insects were making their way onto land. In the Devonian period, around 400 million years ago, four-legged fish crawled out of the sea to become the ancestors of all vertebrate life on land—including humans.
Once on land, animals expanded into an assortment of shapes designed for running, flying, hopping, and crawling. The rise of dinosaurs kicked off during the Triassic period, around 230 million years ago. Pterosaurs, flying reptiles, took to the skies around the same time and became the earliest known vertebrates capable of powered flight.
Even while dinosaurs dominated the landscape, small rat-like mammals were becoming more common. They survived the mass extinction 65 million years ago that ended the Age of Dinosaurs, along with some birds, amphibians, and reptiles. Most major animal groups that survived this extinction event still exist today.