Evolution of the Horse

Thursday, November 24, 2016

The Evolution of Horses, from Eohippus to the American Zebra

The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling Eohippus into the modern horse. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal.

The horse belongs to the order Perissodactyla (odd-toed ungulates), the members of which all share hooved feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that horses share a common ancestry with tapirs and rhinoceroses. The perissodactyls arose in the late Paleocene, less than 10 million years after the Cretaceous–Paleogene extinction event. This group of animals appears to have been originally specialized for life in tropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life on drier land, in the much harsher climatic conditions of the steppes. Other species of Equus are adapted to a variety of intermediate conditions.

This image shows a representative sequence, but should not be construed to represent a “straight-line” evolution of the horse. Reconstruction, left forefoot skeleton (third digit emphasized yellow) and longitudinal section of molars of selected prehistoric horses  The Earliest Horses – Hyracotherium and Mesohippus

The Earliest Horses – Hyracotherium and Mesohippus

Until an even earlier candidate is found, paleontologists agree that the ultimate ancestor of all modern horses was Eohippus, the “dawn horse,” a tiny (no more than 50 pounds), deer-like herbivore with four toes on its front feet and three toes on its back feet. (Eohippus was for many years known as Hyracotherium, a subtle paleontological difference of which the less you know, the better!) The giveaway to Eohippus’ status is its posture: this perissodactyl put most of its weight on a single toe of each foot, anticipating later equine developments. Eohippus was closely related to another early ungulate, Palaeotherium, which occupied a distant side branch of the horse evolutionary tree.

Toward True Horses – Epihippus, Parahippus and Merychippus

During the Miocene epoch, North America saw the evolution of “intermediate” horses, bigger than Hyracotherium and its ilk but smaller than the equines that followed. One of the most important of these was Epihippus (“marginal horse”), which was slightly heavier (possibly weighing a few hundred pounds) and equipped with more robust grinding teeth than its ancestors. As you might have guessed, Epihippus also continued the trend toward enlarged middle toes, and it seems to have been the first prehistoric horse to spend more time feeding in meadows than in forests.

Following Epihippus were two more “hippi,” Parahippus and Merychippus. Parahippus (“almost horse”) can be considered a next-model Miohippus, slightly bigger than its ancestor and (like Epihippus) sporting long legs, robust teeth, and enlarged middle toes. Merychippus (“ruminant horse”) was the largest of all these intermediate equines, about the size of a modern horse (1,000 pounds) and blessed with an especially fast gait.

This image shows a representative sequence, but should not be construed to represent a “straight-line” evolution of the horse. Reconstruction, left forefoot skeleton (third digit emphasized yellow) and longitudinal section of molars of selected prehistoric horses. Author: H. Zell


Next Step, Equus – Hipparion and Hippidion

Following the success of intermediate horses like Parahippus and Merychippus, the stage was set for the emergence of bigger, more robust, more “horsey” horses. Chief among these were the similarly named Hipparion (“like a horse”) and Hippidion (“like a pony”). Hipparion was the most successful horse of its day, radiating out from its North American habitat (by way of the Siberian land bridge) to Africa and Eurasia. Hipparion was about the size of a modern horse; only a trained eye would have noticed the two vestigial toes surrounding its single hooves.

Lesser known than Hipparion, but perhaps more interesting, was Hippidion, one of the few prehistoric horses to have colonized South America (where it persisted until historical times). The donkey-sized Hippidion was distinguished by its prominent nasal bones, a clue that it had a highly developed sense of smell. Hippidion may well turn out to have been a species of Equus, making it more closely related to modern horses than Hipparion was.

Some of the most notable prehistoric horses

American Zebra Also known as the Hagerman horse.

Anchitherium A long-lived “side branch” on the equine tree of life.

Dinohippus This prehistoric horse wasn’t quite as fearsome as its name.

Epihippus This tiny, prehistoric horse lived about 30 million years ago.

Eurohippus Scientists have discovered a pregnant specimen of this ancient horse.

Hipparion One of the most successful horses of the Miocene epoch.

Hippidion This donkey-sized horse had a prominent snout.

Hypohippus This Miocene horse had unusually short legs.

Hyracotherium The horse formerly known as Eohippus.

Merychippus An important intermediate step in equine evolution.

Mesohippus This “middle horse” was about the size of a deer.

Miohippus This “Miocene horse” actually lived much earlier.

Orohippus This prehistoric horse was a close relative of Hyracotherium.

Palaeotherium This tapir-like beast was remotely related to modern horses.

Parahippus This “almost horse” had noticeably enlarged middle toes.

Pliohippus This prehistoric horse was built for speed.

Quagga This South African zebra went extinct in 1883.

Tarpan The immediate predecessor of the modern horse.