Humans may be much shorter if global warming continues, according to the latest study.

A new study finds that ancient horses shrunk even smaller than their ancestors, a trend scientists say is likely the result of global warming.

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

The proposal follows from a study of Sifrhippus, the first horse, 56 million years ago. Sifrhippus shrank from about 12 pounds average weight to about eight and a half pounds as the climate warmed over thousands of years, according to a report published by a team of researchers and reported in the journal Science on Thursday.

The study finds that early horses were much smaller than their modern day ancestors, which have since been bred for speed, size, and a number of additional attributes. The earliest-known horse, Sifrhippus, first appeared in North America during the Paleocene-Eocene Thermal Maximum, when increased concentrations of carbon dioxide in the atmosphere and oceans caused average global temperatures to begin to rise.

During the Paleocene-Eocene Thermal Maximum, a period when temperatures on the planet rose by around 10 to 20 degrees Fahrenheit, scientists say the small horses shrunk even smaller in size. The team of scientists say the resulting shrinkage was the result of natural selection. The team of scientists noted that the small horses likely evolved to be smaller during warming because smaller animals did better in that environment, perhaps because the smaller more easily shed excess heat.

The researchers, led by Ross Secord of the University of Nebraska-Lincoln and Jonathan Bloch of the Florida Museum of Natural History at the University of Florida, studied the geochemical composition of the horse’s teeth to document the decrease of the horses body size through the geologic timeframe of the PETM.

“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.”

One was the fossils themselves, recovered from the Cabin Fork area of the southern Bighorn Basin near Worland, Wyoming, Stephen Chester, an undergraduate student at Florida at the time, had the task of measuring the horses’ teeth. What he found when he plotted them through time caught Bloch and Secord by surprise.

“He pointed out that the first horses in the section were much larger than those later on,” Mr. Bloch recalled. “I thought something had to be wrong, but he was right — and the pattern became more robust as we collected more fossils.”

A postdoctoral researcher in Bloch’s lab for the first year of the project, Secord performed the geochemical analysis of the oxygen isotopes in the teeth. The results caught scientists by surprise.

“It was absolutely startling when Ross pulled up the first oxygen isotope data,” Bloch said. “We looked at the curve and we realized that it was exactly the same pattern that we were seeing with the horse body size.

“For the first time, going back into deep time — going back 10s of millions of years — we were able to show that indeed temperature was causing essentially a one-to-one shift in body size within this lineage of horse. Because it’s over a long enough time, you can argue very strongly that what you’re looking at is natural selection and evolution — that it’s actually corresponding to the shift in temperature and driving the evolution of these horses.”

The study is the first to suggest that mammals may become smaller as global temperatures rise. In response to the increase in greenhouse gas, one-third of mammal species decreased in size, the study found. The findings raise questions as to how plants and animals respond to changes in the climate, including the changes in today’s climate. A number of scientists, including researchers at NASA, predict that the Earth’s temperature could rise upwards of 7 degrees Fahrenheit over the next couple of centuries, due to a 40 percent increase of atmospheric carbon dioxide.

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