Horse Evolution May Not Be as Straightforward as You Thought

Posted by Allie Layos
Herd of horses

If you think you know the story of horse evolution, you may want to think again.

Almost every equine text contains a chapter about horse evolution. Horse lovers know the story well -- modern horses descended from Eohippus, which was dog-sized, and had toes rather than hooves.

They grew larger in response to a changing environment, eventually becoming the majestic creature we know today, retaining only a few vestiges of their former life, like chestnuts and ergots. But what if that's not how it happened?

New studies and discoveries seem to suggest that horse evolution may not be quite that straightforward, and that the evolution of horses is not nearly that linear.

The earliest known horses evolved 55 million years ago in North America. They dispersed around the globe before they went extinct there about 10,000 years ago, but were reintroduced to the Americas by Spanish explorers in the 1600s.

If that all sounds familiar, it should. But the next part is where the story diverges from the commonly-held beliefs.

Herd of horses

The first members of the horse family, Equidae, were described in 1841 by English paleontologist Richard Owen. Mistaking them for a hyrax ancestor, Owen called the dog-sized creature Hyracotherium, but in 1876 American paleontologist Othniel C. Marsh discovered a complete fossil skeleton and created a new genus for it, naming it Eohippus or "dawn horse."

When it was discovered that the two animals were actually one species, the two names became basically synonymous.

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For the majority of their history, most horses remained small forest browsers. But grasslands expanded due to changing climate conditions in the new world, and about 20 million years ago many new Equidae species began to rapidly evolve.

Species of Equidae
American Museum Of Natural History

Some of these species became larger, with the hooves and grazing diet of modern day horses. But -- and here is the interesting part -- some, like Nannippus species, actually became smaller in size.

This challenges the conception that horses grew progressively bigger throughout their history, and that one species morphed smoothly into the next, larger, one. In fact, for much of their history, these different-sized species actually lived side by side.

As Kathleen Hunt, a biologist at the University of Washington in Seattle, said in an interview with National Geographic, the modern-day horse is "merely one twig on a once flourishing bush of equine species. We only have the illusion of straight-line evolution because Equus is the only twig that survived."

What do you think of this new view on horse evolution? Tell us in the comments below!

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Horse Evolution May Not Be as Straightforward as You Thought