Modern mammals, including humans, could be at risk of shrinking as a result of global warming, just as teeny prehistoric horses shrank to an even smaller size when temperatures rose 56 million years ago.

Early horses were already quite tiny to begin with in comparison to modern beasts, which have been helped along by people breeding them for size, speed and strength.

However, during the Paleocene-Eocene Thermal Maximum (PETM), when temperatures on the planet rose by around 10 to 20 degrees Fahrenheit (about 5.5 to 11 degrees Centigrade) over 175,000 years, they dwindled to about the size of a cat, according to a study published in journal Science.

“Horses started out small, about the size of a small dog like a miniature schnauzer,” said author Jonathan Bloch from the Florida Museum of Natural History.

“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.”