Summer School: Evolution of the Horse

From their humble beginnings as small, forest-dwelling creatures to the pampered pets of today, horses have undergone a series of transformative changes over millions of years.

The evolutionary lineage of horses can be traced back to the Eocene epoch, approximately 55 million years ago. The first known ancestors of horses were small, dog-sized animals called Hyracotherium, more commonly known as Eohippus, or “dawn horse.”

These early equids had four-toed front feet and three-toed hind feet, reflecting their adaptation to forested environments. They stood about twelve inches tall at the shoulders, and their teeth were low-crowned and included a large canine tooth.

“Dawn Horse,” Houston Museum of Natural Science. Photo: Amanda Uechi Ronan.

Next came Mesohippus or the “middle horse” about 38 million years ago. Mesohippus still had three toes, with all three bearing weight, and retained the low crowned molars of Hyracotherium, though its premolars were transitioning to be more like molars. This may indicate a shift in diet that included more grasses.

“Mesohippus bairdi being hunted by a ‘Killer Warthog’,” Houston Museum of Natural Science. Photo: Amanda Uechi Ronan.

As the climate gradually changed, grasslands expanded and equids faced selective pressures that favored adaptations for grazing and running. Over time, their limbs elongated, and their toe structure changed, with the middle toe becoming the dominant weight-bearing structure.

About 28 million years ago, Merychippus was similar in size to a Shetland Pony. It had three toes similar to the Mesohippus, but the side toes did not touch the ground. It was also one of the first equids with high-crowned teeth that are more resistant to wear with a thicker coating of cement and more complex folds and ridges, making them more suitable for eating tough grass.

“Merychippus insignis, Miocene Horse,” Houston Museum of Natural Science. Photo: Amanda Uechi Ronan.

The Miocene was a time of great diversification, and there were several species coexisting, possibly a dozen or more. These included Hipparion, which still had three toes, though the outer two were vestigial like Merychippus.

Hipparion spread out of the American West, crossing the Bering Land Bridge into Asia, Europe, India and Africa. As a result of their worldwide distribution, Hipparion fossils have become important biological markers for stratigraphic correlations, which is the process of determining which sedimentary strata are the same age in different geographical locations.

“Darwin’s War-Horse, Hipparion,” Houston Museum of Natural Science. Photo: Amanda Uechi Ronan.

At almost the same time, Pliohippus, the first true one-toed horse with high-crowned teeth, gave rise to Dinohippus, which grazed in North America 13 to 5 million years ago. Dinohippus is the first horse that shows evidence of the passive “stay apparatus” that helped the horse stand for long periods of time, meaning this was probably the first horse that could sleep while standing.

Eventually, Equus hit the ground galloping. The genus had high-crowned teeth and one distinct toe, as the two side toes had fully developed into the side bones we now know as splints. Still, there was a massive variety of Equus species, including Equus scotti, Equus excelsus, Equus lambei, the “Yukon Horse,” and Equus occidentalis, whose fossils have been claimed to have been found in abundance at the La Brea Tar Pits.

Today, only seven species of Equus remain: the domestic horse and its cousin, the Przewalski’s horse; three species of asses; and three species of zebras.

Note from the author: Scientists seem to disagree on whether the domestic horse and Przewalski’s horse are the same species. Many say yes, because they can interbreed and create fertile offspring. But neigh-sayers say the domestic horse has only 64 chromosomes while Przewalski’s has 66.

By H. Zell (User:Llez) – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11633334

Go riding.

Amanda Uechi Ronan is an author, equestrian and wannabe race car driver. Follow her on Instagram @uechironan.