Humans versus nature

The story of biodiversity loss is a tale of decline spanning thousands of years. This month, the world will get a chance to change the narrative at Cop15 in Montreal

- Phoebe Weston

The story of the biodiversity crisis starts with a cold-case murder mystery that is tens of thousands of years old. When humans started spreading across the globe they discovered a world full of huge, mythical-sounding mammals called "megafauna", but by the end of the Pleistocene, one by one, these large animals had disappeared. There is no smoking gun and evidence from ancient crime scenes is - unsurprisingly patchy. But what investigators have learned suggests a prime suspect: humans.

Take the case of Genyornis, one of the world's heaviest birds, which was more than 2 metres tall and weighed in excess of 200kg. It lived in Australia until, along with many other megafauna, it went extinct 50,000 years ago. In North America, giant beavers weighing the same as a fridge and an armadillo-like creature called a glyptodon, which was the size of a small car, existed until about 12,000 years ago, when they, too, went extinct. In all, more than 178 species of the world's largest mammals are estimated to have been driven to extinction between 52,000 and 9,000BC.

These extinctions happened over hundreds or thousands of years, so it is unlikely people back then recognised it as a crisis. But, in some cases, the loss of these animals would have had landscape-scale impacts.

For a long time, the extinctions were thought to be linked to natural changes in the environment until palaeontologist Paul S Martin put forward his controversial "overkill hypothesis" in 1966. This stated that humans were responsible for the extinctions of megafauna, destroying the romantic vision of early humans living in harmony with nature.

By 10,000 years ago, when humans had spread across every continent except Antarctica, there were at most 10 million of them-so a lot of change had already been dished out by few people.