50 million years ago caused the first horses, already tiny in stature compared with present-day animals, to shrink 30 percent to about 8.5 pounds, the size of a house cat today, a study suggests.

Later, as the climate cooled, the horse called Sifrhippus began to grow in size, according to research in the journal Science. Scientists the University of Michigan's Museum of Paleontology used fossilized teeth to make the size estimates.



The finding, which correlates to a 10 to 20-degree change in global temperature, follows Bergmann's rule, which says that smaller animals within the same species are usually found in hotter climes. The study also may suggest that creatures alive today may shrink if global warming continues, said Philip Gingerich, a study author and the museum's director.

"I joke about this all the time -- we're going to be walking around 3 feet tall if we keep going the way we're going," Gingerich said in a statement.

From 1906 to 2005, the world has been in an "unequivocal" warming trend, according to the Environmental Protection Agency's website. The earth warmed about 1 to 1.7 degrees Fahrenheit over that time, much less than the warming trend 56 million years ago.

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