At the same time that the dinosaurs were spreading and diversifying, the first mammals evolved. Their ancestors were reptiles called cynodonts, whose faces looked a little like those of dogs and may have had fur or whiskers. Early mammals such as Morganucodon were small and shrew-like, and probably only active at night. This may have spurred them to evolve warm-bloodedness: the ability to keep their body temperature constant.
Just as the reptiles were flourishing, life on Earth faced perhaps its greatest challenge. The Permian extinction was the worst mass extinction in the planet’s history, obliterating up to 96% of marine species and similar numbers of land animals. We don’t know for sure what caused it, but massive volcanic eruptions – creating what is now the Siberian Traps – may have been to blame. In the aftermath, the first dinosaurs evolved.
For the last time, all Earth’s continents came together to form one giant supercontinent. Known as Pangaea, it was surrounded by a world-spanning ocean called Panthalassa. It lasted until 175 million years ago, when it began to tear itself apart over tens of millions of years. Its shattered remnants became the familiar modern continents.
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.
The Ordovician period was a time when life flourished. But towards its end, the world cooled dramatically and ice sheets spread from the poles. The ensuing ice age is called the Andean-Saharan, because the evidence of it comes from the Andes mountains and the Sahara desert. The deep freeze led to the second-worst mass extinction on record, the Ordovician-Silurian. Most life was still confined to the sea, and 85% of marine species were wiped out. In the aftermath, fish became much more common.
Some animals ventured onto land as far back as 500 million years ago, but they only visited briefly – perhaps to lay eggs in a place without predators. Plants were the first to take up permanent residence on land. The first land plants were relatives of green algae, but they rapidly diversified.