Dr Bloch explained: ‘Horses started out small, about the size of a small dog like a miniature schnauzer.

‘What’s surprising is that after they first appeared, they then became even smaller and then dramatically increased in size, and that exactly corresponds to the global warming event, followed by cooling.

‘It had been known that mammals were small during that time and that it was warm, but we hadn’t understood that temperature specifically was driving the evolution of body size.’