A mass extinction rid the world of dinosaurs 65 million years ago. With the biggest players out of the picture, the world was up for grabs, and a new branch of vertebrate would eventually take over. Join David Attenborough as he charts the fascinating story of the mammals, from their origins as tiny nocturnal forest-dwellers to their explosion in diversity and domination. Through new fossil discoveries, we chart how and when our ancestors developed hair, placentas, and milk, and laid the foundations for the major mammal groups we see today.