Geologic Evolution of Horses: Common ancestry with tapirs and rhinoceroses

With many horse enthusiasts in Valencia County and rodeo season here, let us review the geologic record on horse evolution.

Only 10 million years after the demise of the dinosaurs, some rodent-size mammals had already evolved into a dog-size forest-dwelling primitive “horse” 55 million years ago.

Horses then evolved in North America with a common ancestry with tapirs and rhinoceroses, 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 common tooth structure.

The horse further evolved into the genus Equus about four million years ago, adapted to steppes and spread to Asia two to three million years ago, reaching Africa as zebras about 1.9 million years ago.

Horse fossils dated at about 13,000 years B.C. have been found near Carlsbad and at the White Mesa mine near San Ysidro along with fossil camels and bisons. Before 10,000 years ago, Native Americans would have known horses only as wild game for food; however, horses disappeared from America 10,000 years ago along with saber tooth tigers, camels, mammoths and many other large animals. (The cause of the disappearance of the megafauna is not clearly understood).

In Central Asia, around 3,500 B.C., horses started to get domesticated, and that practice then spread widely across the Old World around 2,000 B.C.

Although horses are often associated with having a wild spirit, the only truly wild horse alive today is the Mongolian Horse (also called Przewalski’s Horse). Other horses perceived to be “wild,” such as the North American Mustang, Australian Brumby and the Namib Desert horse are actually just feral horses who descend from once-domesticated breeds.

Native to central Asia’s steppes, the Mongolian horse was driven to extinction in the wild, with the last horse spotted in 1969. After conservation efforts in zoos and reserves, these horses have now found their way back into the wild, with about 500 horses in a National Park in Mongolia. These horse also occasionally are found at horse-trading facilities.

In particular, a Przewalski horse was sold in a horse-trading center in the western United States, where an unaware buyer bought the horse assuming it to be a mule, later to find out that this “mule” was untrainable. The horse was then resold until it was finally recognized as a rare Przewalksi’s horse.

On the American continent, where no horses were known to the Native Americans before the arrival of the Europeans, horses were reintroduced when Hernan Cortez, in 1519, landed with horses from Spain. Native populations were at first very frightened at the sight of them.

The advantages of the horse for hauling cargo and in battles were quickly realized by the Native populations, who rapidly proved excellent at using horses. Although the common interpretation is that Native Americans started using horses after the 1680 Pueblo Revolt when the Spaniards were sent back to Mexico, evidence reported in the Smithsonian Magazine suggests that some horses were taken from Mexico very early on, and Native Americans had started using horses as soon as they could get them in early 1600s.

In Central New Mexico, the Pueblo populations became more vulnerable to horse-enabled raids from Comanche, Apache and Navajo tribes than before the arrival of the horse.

Some areas of present New Mexico, such as Placitas and Ruidoso, have an overpopulation of wild horses, creating overgrazing and dangerous situations to livestock, people and the horses themselves. A proposed law that would have resulted in the capture and relocation of wild horses was pocket-vetoed by the governor in early 2025.