Horses, of the family Equidae, evolved over a geologic timescale of 50 million years. 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.

Horses, tapirs and rhinoceroses image from http://www.rhinoremedy.org/awesome-rhinos/meet-the-relatives

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.

Perissodactyla, as we know it today, is a small order of hoofed mammals, containing 17 Recent species in three families: Equidae (horses), Tapiridae (tapirs), and Rhinocerotidae (rhinoceroses). Despite being such a small order, the odd-toed ungulates have a wide distribution, being found across the Ethiopian, Palearctic, Oriental, and Neotropical zoogeographic regions. Two species have been domesticated – the horse (Equus caballus) and the donkey (Equus asinus); feral populations of these two species have been established across the globe, (re)expanding this order’s influence to the Nearctic and Australasian regions.

Perissodactyls were once much more diverse, including the enormous horned brontotheres, the bizarre browsing, clawed chalicotheres, and the largest land mammal of all time, the Eocene Indricotherium (formerly known as Baluchitherium). It stood five meters (over sixteen feet) tall at the shoulder.

Eocene Indricotherium

Image of horned brontotheres

Although Brontotheres resembled modern Rhinos, They were more closely related to Horses. Unlike the horns of Rhinos, their horns did not have keratin, and were side by side instead of front to back. When Brontotheres first appeared, They looked like early Horses (such as Hyracotherium).  

Another extinct Equus family  is the American Zebra, roaming the Idaho area around 3 million years ago.

Image of skeleton of American Zebra

In fact in their earlier stages of evolution, these two Families were often mistaken for each other. As each Family evolved, Horses lost their three toes whilst Brontotheres retained theirs.

Brontotheres roamed throughout North America. They encountered the volcanic Rocky Mountains. Many were killed by volcanic ash, and later became fossils. Millions of years later, their bones would emerge after heavy rains.

Domestication of Horses

Being sure where horses were first domesticated is still a mystery. But Ludovic Orlando along with 120 other researchers, molecular archaeologists from France’s CNRS research agency in Toulouse have made a major attempt to unravel the thread back through the centuries. Together they have amassed the world’s largest collection of horse DNA—some of it as old as 42,000 years. Now, after several years of intensive analysis, they still do not know when and where modern horses got their start. But he and his colleagues have a much clearer understanding of how humans shaped equine evolution, and they’ve uncovered two previously unknown lineages of horses: an ancient equine that roamed what is now Portugal and Spain some 4000 years ago, and another that lived in Siberia in Russia around the same time. Since then, both lineages have gone extinct, and there are no traces of them left in modern horse DNA.

Orlando also discovered that after the Arabs expanded into Europe in the seventh century, Arabian stallions outproduced males from other breeds, leading to their Y chromosome being present in all modern horses today. Thus, when the Arab stallion was introduced to the Iberian Peninsula by the Moors in 711 AD they brought with them the magnificent genetics of the Arab stallion. This has led to the famous Andalusian which has been recognized as an individual breed since the 15th century, and its conformation has changed very little over the centuries

The African Moors army, under their leader Tariq ibn-Ziyad, crossed the Strait of Gibraltar from northern Africa and invaded the Iberian peninsula ‘Andalus’ (Spain under the Visigoths). 800 years later, the Conquistadores would ride their Andalusian horses brought on their Spanish ships to the New World and wreak terror amongst the natives, who had never seen a horse before.

The horse has been integral to the battle victories around the world, to the building of the early industrial society and transportation in the most difficult of terrains. They have also been fed to people as horse meat when no longer thought useful to humans.

Intense breeding of horses has increased speed, strength and endurance but has led to genetic damage too. Orlando’s work, he says “really illustrates that horses some 1000 years ago and horses now are two different creatures”.