Some researchers argue that, even without the asteroid, the reign of the dinosaurs may already have been ending. “I take a slightly unorthodox view that dinosaurs were doomed anyway because of cooling climates,” says Mike Benton, a palaeontologist at the University of Bristol in the UK. “They had just about held their own to the end of the Cretaceous, but we know that mammals were diversifying… [and] dinosaurs had already been declining for 40 million years.” Benton believes mammals would still have replaced the dinosaurs. He is an author of a 2016 paper suggesting dinosaurs were slower than mammals at replacing extinct species.

Other experts take a very different view. Carnivorous dinosaur researcher Tom Holtz at the University of Maryland in College Park, US, agrees there would have been some extinctions 66 million years ago anyway, due to eruptions and massive lava flows at the Deccan Traps in India – but he says “there’s nothing otherwise, once you’re into the Palaeocene and Eocene, that would have affected general dinosaur biology. It would be a world that Cretaceous dinosaurs would still be comfortable in.”

Stephen Brusatte of the University of Edinburgh adds that dinosaurs had survived well, doing a great diversity of things, through changing climates, for 160 million years. “Dinosaurs were still very adaptable at the end of the Cretaceous, that’s not the sign of a group that’s wasting away to extinction, just waiting for some asteroid to knock them off. It’s the sign of a group that still has a lot of evolutionary potential.”

Assuming dinosaurs had survived, what factors might have shaped their evolution? Climate change might have perhaps been the first big hurdle. An event known as the Palaeocene-Eocene Thermal Maximum, 55 million years ago, saw average global temperatures reach 8C hotter than today, and rainforests spanning much of the planet.