After the extinction of the dinosaurs, the Earth was populated by other, astonishingly gigantic animals: a 13-metre snake weighing as much as a truck, a shark as big as a school bus, a 20-ton rhinoceros and a sloth 200 times the size of sloths today. Although they dominate their ecosystems, giant animals are often more vulnerable than others. They are susceptible to climate change, loss of habitat, shortages of food and competitors invading their territory. In fact, today’s last giants – the descendants of those behemoths – are all in decline. Some of them are even threatened with imminent extinction. The series looks back on the evolutionary history of eight giant prehistoric animals and their eight modern descendants. The first two episodes examine four particularly amazing animals, seeking to explain why they became so huge and why they later disappeared. We join four experts as they study those ancient species.