The evolution of horses is a fascinating journey that spans millions of years. It all began during the Eocene Epoch, around 56 million to 33.9 million years ago. The first ancestral horse, Hyracotherium, also known as Eohippus or the “dawn horse,” emerged during this time. Fossils of Eohippus, found in both North America and Europe, show an animal quite different from the modern horse. It stood only about 4.2 to 5 hands (42.7 to 50.8 cm) high, had an arched back and raised hindquarters. Its legs ended in padded feet with four functional hooves on each forefoot and three on each hind foot, and its skull and teeth were adapted to a browsing diet.
As time went on, the horse lineage underwent significant changes. Orohippus and Epihippus in the Eocene showed changes in dentition, with the premolars becoming more molar-like. Mesohippus, in the Oligocene, was larger, had a more muzzle-like snout, longer legs, and a reduced fourth toe on the forefoot. Miohippus, evolving from Mesohippus, gave rise to different branches, one of which led to the modern horse. Parahippus, in the early Miocene, had teeth adapted to eating grass, which was becoming widespread. Merychippus, evolving from Parahippus, had a more modern pony-like appearance, with fused lower leg bones for swift running. Eventually, Equus, the genus to which all modern equines belong, evolved from Pliohippus around 4 million to 4.5 million years ago during the Pliocene.
The origin of horses has been a subject of much research and debate. While the evolutionary journey of the horse family started with Eohippus in North America and Europe, the domestication of horses has a more complex origin story.
Archaeological and genetic evidence suggests that modern domestic horses originated in the Western Eurasian steppes, especially the lower Volga-Don region. An international team of 162 scientists analyzed 273 genomes from ancient horses scattered across Eurasia. They found that a genetic profile, previously confined to the Pontic steppes (North Caucasus), began to spread beyond its native region around 2200 BCE. This genetic profile replaced all the wild horse populations from the Atlantic to Mongolia within a few centuries. Before this discovery, other regions such as Central Asia (Botai), Anatolia, and the Iberian Peninsula were considered as possible domestication centers, but DNA results from ancient horses in these areas did not match the ancestry of modern domestic horses.
Prehistoric horses were diverse in form and adapted to different environments. Eohippus, as mentioned earlier, was a small, browsing horse. As the climate and landscapes changed, horses evolved to adapt. For example, during the Miocene, as grasslands expanded, horses like Parahippus and Merychippus developed teeth and body structures suitable for grazing and running on open plains.
Przewalski’s horse, often considered the last remaining wild horse, is an important part of the prehistoric horse legacy. It has a distinct genetic makeup and is thought to be related to some of the early horse populations. However, it is not the direct ancestor of modern domestic horses. The tarpan, which became extinct in the early 20th century, was another prehistoric horse. Recent studies have shown that the tarpan came about through admixture between horses native to Europe and horses closely related to the DOM2 group, which is a major lineage of modern domestic horses.
The domestication of horses was a significant event in human history. It is estimated to have occurred around 6000 years ago, but the exact time and location have been difficult to determine. The Western Eurasian steppes are now identified as the homeland of modern domestic horses. Around 2200 BCE, horses with a specific genetic profile (DOM2) began to spread rapidly across Eurasia.
This expansion was synchronous with the development of equestrian material culture, such as the Sintashta spoke-wheeled chariots. Humans likely selected horses for certain traits during the domestication process. Genetic studies have found that early selection at genes like GSDMC and ZFPM1 in DOM2 horses suggests a shift towards more docile horses, more resilient to stress and suitable for new locomotor activities like endurance running, weight bearing, and warfare.
The history of horses is intertwined with human civilization. Once domesticated, horses played crucial roles in transportation, agriculture, and warfare. In ancient times, they were used to pull chariots, which were a significant military and transportation innovation. Horses also enabled humans to travel longer distances, facilitating trade and the spread of cultures.
In different parts of the world, horses were bred for specific purposes. For example, in Europe, heavy breeds like the Belgian, Clydesdale, and Shire were developed for farm work and pulling heavy loads. Light breeds such as the Arabian, Thoroughbred, and Akhal-Teke were bred for riding, racing, and light draft work. Ponies, like the Shetland and Welsh ponies, were used for various tasks and were popular mounts for children.
Eocene Epoch (56 - 33.9 million years ago): Hyracotherium (Eohippus) appears. It is a small, browsing horse with four-toed forefeet and three-toed hind feet.
Oligocene Epoch (33.9 - 23 million years ago): Mesohippus evolves. It is larger than Eohippus, has a more muzzle-like snout, and the fourth toe on the forefoot is reduced.
Miocene Epoch (23 - 5.3 million years ago): Miohippus gives rise to different branches. Parahippus, with teeth adapted to grass, appears, and Merychippus further develops characteristics for grazing and running.
Pliocene Epoch (5.3 - 2.6 million years ago): Equus evolves from Pliohippus. It shows greater development of the spring mechanism in the foot and straighter and longer cheek teeth.
Pleistocene Epoch (2.6 million - 11,700 years ago): Equus spreads to South America and the Old World. However, it disappears from North and South America around 10,000 - 8,000 years ago.
Around 6000 years ago: Horse domestication begins, likely in the Western Eurasian steppes.
Around 2200 BCE: DOM2 horses start to spread across Eurasia, replacing pre-existing horse populations. This is concurrent with the development of equestrian material culture like spoke-wheeled chariots.
In conclusion, the origin of horses is a complex and captivating story. From their humble beginnings as small, browsing animals to the diverse and important domesticated horses we know today, horses have had a profound impact on the world. Their evolution and domestication are a testament to the dynamic relationship between animals and the changing environments, as well as the influence of human intervention in shaping their development.