Research reveals evolution of earliest horses was driven by climate change, global warming affected body size

February 23, 2012
This is an artist's reconstruction of Sifrhippus sandrae (right) touching noses with a modern Morgan horse (left) that stands about 5 feet high at the shoulders and weighs about 1,000 pounds. Sifrhippus was the size of a small house cat (about 8.5 pounds) at the beginning of the Eocene (approximately 55.8 million years ago) and is the earliest known horse. Credit: Danielle Byerley, Florida Museum of Natural History.

When Sifrhippus, the earliest known horse, first appeared in the forests of North America more than 50 million years ago, it would not have been mistaken for a Clydesdale. It weighed in at around 12 pounds -- and it was destined to get much smaller over the ensuing millennia.

Sifrhippus lived during the Paleocene-Eocene Thermal Maximum, a 175,000-year interval of time some 56 million years ago in which average rose by about 10 degrees Fahrenheit, caused by the release of vast amounts of carbon into the atmosphere and oceans.

About a third of responded with significant reduction in size during the PETM, some by as much as one-half. Sifrhippus shrank by about 30 percent to the size of a small house cat (about 8.5 pounds) in the PETM's first 130,000 years and then rebounded to about 15 pounds in the final 45,000 years of the PETM.

Scientists have assumed that rising temperatures or high concentrations of primarily caused the phenomenon in mammals during this period, and new research led by Ross Secord of the University of Nebraska-Lincoln and Jonathan Bloch of the at the University of Florida in Gainesville offers new evidence of the cause-and-effect relationship between temperature and body size. Their findings also offer clues to what might happen to animals in the near future from global warming.

In a paper to be published in the Feb. 24 issue of the international journal Science, Secord, Bloch and colleagues used measurements and geochemical composition of mammal teeth to document a progressive decrease in Sifrhippus' body size that correlates very closely to temperature change over a 130,000-year span.

Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History, said multiple trails led to the discovery.

This photograph compares upper teeth of the earliest known horse, Sifrhippus, at its larger size during lower temperatures, top, with teeth from the same horse after it shrank as a result of higher temperatures about 56 million years ago. University of Florida researcher Jonathan Bloch co-authored a study appearing in Science Feb. 24 documenting this phenomenon using fossils from the Cabin Fork area of the southern Bighorn Basin in Wyoming. University of Florida photo taken Feb. 1, 2012, by Kristen Grace

One was the fossils themselves, recovered from the Cabin Fork area of the southern Bighorn Basin near Worland, Wyo. Stephen Chester, then an undergraduate student at Florida, now an anthropology Ph.D. candidate at Yale and a co-author on the paper, 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," 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. What he found provided an even bigger 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 tens 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."

University of Florida researcher Jonathan Bloch compares the upper jaw of the earliest known horse species (foreground) to the upper jaw of a modern-day horse. Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History on the UF campus, co-authored a study appearing in Science Feb. 24 showing the body size of the earliest horse, Sifrhippus, decreased as temperatures rose during the Paleocene-Eocene Thermal Maximum about 56 million years ago. University of Florida photo taken Feb. 1, 2012, by Kristen Grace

Secord, who came to UNL in 2008 as an assistant professor of Earth and atmospheric sciences and curator of at the University of Nebraska State Museum, said the finding raises important questions about how plants and animals will respond to rapid change in the not-too-distant future.

"This has implications, potentially, for what we might expect to see over the next century or two, at least with some of the climate models that are predicting that we will see warming of as much as 4 degrees Centigrade (7 degrees Fahrenheit) over the next 100 years," he said.

Those predictions are based largely on the 40 percent increase of atmospheric carbon dioxide levels (from 280 to 392 parts per million) since the start of the Industrial Revolution in the mid-19th century.

Ornithologists, Secord said, have already started to notice that there may be a decrease in among birds.

"One of the issues here is that warming (during the PETM) happened much slower, over 10,000 to 20,000 years to get 10 degrees hotter, whereas now we're expecting it to happen over a century or two," Secord said. "So there's a big difference in scale and one of the questions is, 'Are we going to see the same kind of response?' Are animals going to be able to keep up and readjust their body sizes over the next couple of centuries?"

Increased temperatures are not the only change animals will have to adapt to, Secord said. Greenhouse experiments show that increased atmospheric carbon dioxide lowers the nutritional content of plants, which he said could have been a secondary driver of dwarfism during the PETM.

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More information: "Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum," Science (2012).

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1.2 / 5 (19) Feb 23, 2012
Glieck got caught doing his fraud. Give the fraud a rest, you've been rumbled.
4.6 / 5 (10) Feb 23, 2012
So Glieck faked some documents about climate change, therefore all research into evolution is somehow tainted by the same paintbrush?

So what happened? Did you lose your Phd grant to study creationism?
2.9 / 5 (15) Feb 23, 2012
Useful thing about Science = "The discipline of the acquisition of knowledge" is the fact it has a discipline and anyone faking documents for their own aggrandisement or other reasons will be caught out by the process of peer review - provided that data is available to all. Its also helpful that a lot of those in science (and that includes engineering mostly) have the same foundation in fundamental physics and chemistry and able to manage mathematics well enough to ponder the numbers with intellectual rigour and educated skepticism.

The problem with creationism and any arbitrary belief such as any religion is it has no discipline and is subject primarily to a static type of self-referential emotional hypnosis at worst and a mere emotional attachment at least - in both respects this quells intellectual rigour, does not require any understanding of mathematics (especially probability and permutations) or questioning or imaginative speculation.

It must be obvious which is superior...
4.4 / 5 (13) Feb 23, 2012
Fortunately in Europe we don't know Glieck nor creationists... Leave them all for you who think to be the center of the world.
The creationists I know are in America or in muslim countries
5 / 5 (4) Feb 24, 2012
The problem with creationism and any arbitrary belief such as any religion is it has no discipline and is subject primarily to a static type of self-referential emotional hypnosis - Mike_Massen

Very true. I always have to chuckle when I hear someone say that they're engaged in bible 'study' (as if any possible test could divine truth from fiction or any other interpretation one cares to make up).
1.7 / 5 (6) Feb 24, 2012
I am curious. In conformation with this temperature-body size hypothesis, i can accept that tropical Asians are generally smaller in statue than their temperate counterparts and Europeans. But many of tropical Africans are giants. Considering they have been around the longest, the heat should have shrunk them down. Anyone cares to explain? Thanks.
1 / 5 (4) Feb 24, 2012
Interesting question there Skepticus,

I recall seeing early photos of Australian Aboriginees too, tall and skinny like the Masai of Africa, one wonders if instead of being squat the body mass is roughly the same but the dimensions offer another variation to achieve the same overall effect...

Having said that, the excess of carbs for Australian aboriginees does not do them well at all, the tens of thousands of years of negligible carbs but higher protein may have had consequences in affecting their psyche when carbs are suddenly abundant on a western diet.

In all cases it dose seem as a work in progress and a temporary best fit that lasts a mere few thousand years - a blink of the anthropological eye...
5 / 5 (1) Feb 25, 2012
I am curious. In conformation with this temperature-body size hypothesis, i can accept that tropical Asians are generally smaller in statue than their temperate counterparts and Europeans. But many of tropical Africans are giants - Skepticus

Differences in stature is largely down to (poor) nutrition. When Asian populations that traditionally ate a rice based diet switched to a more varied and nutritious diet (after WW2), they started getting taller with each generation. But that's not the whole picture because genetics (extreme height) and sexual selection also plays a role.
1 / 5 (5) Mar 13, 2012
I don't believe temperature affected the size of these horses. The change in size is consistent with the Gravity Theory of Mass Extinction. Go to and click on the link with the above title.
1 / 5 (5) Mar 13, 2012
Meqapixel offered a strange idea
Go to and click on the link with the above title.
Thermal mass is likely to be a significant factor in some relationship with average temperature & the relative niche opportunities for species to adapt to their environment with other adaptations as well so size or average temperature may not be critical.

Although Earth is putting on weight, estimate is ~4000 to ~50,000 tonnes/day, gradual change claimed on the web site is not likely to be a step change in survival as clearly observed with global Iridium distribution consistent with the mass extinction event, evidence is compelling, those amounts of Iridium integrated over the land mass is high and not available from terrestrial sources over such a short period. Deduction is that it came from an asteroid or similar body impact, the timing also favours the probability this even was causal in mass extinctions. Gradual changes in gravity has no evidenciary support.

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