Asked by: Jamie Horner, Rotherham
Mammals are animals whose females have mammary glands and produce milk, and that includes us. These glands don’t survive in fossils, so most of what we know about mammal evolution depends on the fact that mammals use two small bones for hearing, which other animals, like lizards and dinosaurs, used for eating.
Although there was no abrupt transition to ‘true mammals’, the general idea is that the tetrapods (vertebrates with four legs) divided into amphibians (that lay eggs in water) and amniotes (that lay eggs on land). Amniotes then split into sauropsids (including dinosaurs) and synapsids (including mammal-like reptiles), which eventually led to mammals. Once the dinosaurs were gone, early mammals could stop living nocturnally and flourish in the many forms we find today.