The first known horses were the size of house cats, weighing only 8 pounds but before they could evolve to the stately creatures they are today horses shrank even smaller because of global warming.

According to a new study the effect of global warming during the Eocene era shrank the earliest known horse, Sifrhippus to adapt to the changing climate. Only when the Earth cooled down did the horse get big. The study links the affects of temperature to body size. Could the Earth's rising temperature lead to animals shrinking again?

Animals' shrinking because of the effects of temperature isn't an unfamiliar concept. The phenomenon has been explained in what is known as Bergmann's rule that holds that animals are smaller in hot climates and bigger in cold climates. Although the rule refers to differences in locations it could also account for differences over animal sizes over a period of time. Ross Secord of the University of Nebraska, Jonathan Bloch of the Florida Museum of Natural History and a team of researchers set out to answer whether Bergmann's rule was the result of the temperature change or because of other factors. To do this the team studied current horses and their counterparts in the past.

56 million years ago, the earliest known horse, Sifrihippus first appeared in North America during the period called the Paleocene-Eocene Thermal Maximum. During that period the Earth experienced a spike in carbon dioxide in the atmosphere and oceans. Global temperatures rose by about 5.5 Celsius over 10,000 to 20,000 years. It was also during this period that horses body sizes shrunk by about 30 percent from 5.4kg to 3.8kg on average. Then during a period of cooling that occurred in the next 45,000 years the tiny horse shot up in size to about 6.8kg.

"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," said Jonathan Bloch. "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."

The researchers considered other factors that could have led to the horse's shrinking including the dryness of the environment, atmospheric pressure and even nutritional content of the food the animal ate. They ultimately concluded that the temperature was the primary driver of the size evolution of Sifrhippus.

The findings could have grave implications for mammals today. Paul L. Koch the head of the department of earth and planetary sciences at the University of California, Santa Cruz said the paper gives us a glimpse on the effects of global warming on mammals. Another researcher, Philip Gingerich at the University of Michigan noted that global warming could decrease body size in human beings.

"We're going to be walking around 3 feet tall if we keep going the way we're going," Gingerich said. "Maybe that's not all bad and if that's the worst it gets, it will be fine. You can either adapt, or you go extinct, or you can move, and there's not a lot of place to move anymore, so I think it's a matter of adaptation and becoming smaller."