320 million years ago
When the first reptiles appeared, Earth was in the middle of a long cold snap called the Late Paleozoic Ice Age. Reptiles evolved from newt-like amphibians. Unlike their ancestors they had tough, scaly skin and laid eggs with hard shells that did not have to be left in water. Thanks to these advantages, they quickly became the dominant land animals. The reptile-like Dimetrodon reached 4.5m long – but despite what you may have heard, it was not a dinosaur.
See other: History of Life