December 8. Horse evolution

Modern mammals all evolved in the past 65 million years from animals that had survived as pretty low-key critters for tens of millions of years before the start of the Cenozoic Era.

The evolution of the horse is a classic example of mammalian development during the Cenozoic, from small animals with four-toed feet to the much larger modern horse with a single hoof. As I mentioned the other day, the diversification of many mammals is tied fairly closely with the Paleocene-Eocene Thermal Maximum, perhaps at least in part because the greenhouse conditions at that time favored plant growth that could have provided fodder for herbivores like the ancestors of horses.

Illustration by Mcy jerry, used under Creative Commons license. Do not visualize it as a straight-line descent; there were many branchings and changes on the way to the modern horse.

The hoofed animals, or ungulates, include cows, pigs, camels, deer, giraffes, horses, and, surprisingly, whales and dolphins. Obviously whales and dolphins don’t have hooves, but their ancestry puts them in this group. They probably branched from the land-dwelling ungulates early on, probably in very early Eocene time, around 54 million years ago. The cetacean branch, including whales, is probably more closely related to hippopotamuses, which are also ungulates.

Hyracotherium was a dog-sized mammal that lived in Eocene time, around 55 million years ago. It was not a horse in any sense, but it is considered to be ancestral to horses. It was a herbivore a couple feet long, and probably survived for some time as its cousins, true horses, evolved. It’s important to not see the evolution of the horse – or, probably, most anything – as a simple linear set of changes, even though that’s how it is often portrayed. Think of it more as multiple lineages branching off, with some branches surviving and others going extinct.

Eohippus, “dawn horse,” appeared during the Eocene, around 52 million years ago, and thrived for close to 20 million years. It ran on feet with four discrete toes with small proto-hooves on each, and a fifth toe that was off the ground. The leg bones were longer in proportion to the body than in Hyracotherium, but it was still only about a foot and a half high. Some of the first fossils of Eohippus are now referred to Hyracotherium, so the two were really quite closely related to each other.

Eohippus lived on the Eocene plains and forest floors of western North America, where most of the evolution of the horse took place. Orohippus, “mountain horse,” and Epihippus, “related horse,” were two later Eocene lineages whose teeth were different from Eohippus, reflecting some changes in diet – they needed grinders for the plants they browsed on. By late Eocene or early Oligocene time, 32 to 40 million years ago, Mesohippus appeared. It had only three toes for support, and the animal was larger, close to two feet high. A slightly later Oligocene horse, Miohippus, appeared about 32 million years ago, but was a contemporary with Mesohippus for four million years or so.

Mesohippus and Miohippus began to show changes that coincided with the change in climate in western North America to drier environments with fewer forests and more plains. These early horses were taller than their predecessors, and the foot was evolving into the hoof. One of the descendents of Miohippus, Parahippus, shows a clear adaptation in its teeth to grazing on grass, which was becoming abundant and dominant in the North American plains. Parahippus appeared during the early Miocene, around 20 to 22 million years ago, and it led to the Miocene pony-like Merychippus, which was more than three feet high. Its leg bones were fused together, like modern horses. Among its many descendents was Pliohippus, dating to the Pliocene, about 4 million years ago. Pliohippus had only one toe and was the direct ancestor to Equus, the modern horse. A separate branch of horse-related animals that descended from Miohippus was more suited to life in the forests. It reached Eurasia, but was extinct by the end of the Miocene.

The modern genus, Equus, arose about at the end of the Pliocene, about 2½ million years ago, and spread to Eurasia by way of the Bering Land Bridge in Alaska and Russia, which was exposed by falling sea levels related to the onset of the glacial period. Horses and their kin became extinct in North America only about 9,000 years ago. We’ll talk more about the extinction of the large mammals in North America later in the month.

—Richard I. Gibson

Horse evolution 

Illustration by Mcy jerry, used under Creative Commons license. Do not visualize it as a straight-line descent; there were many branchings and changes on the way to the modern horse.