The genus Equus, encompassing all living equine species, traces its ancestry back to Dinohippus, evolving through an intermediate form known as Plesippus. Early species, like the zebra-like Equus simplicidens with its donkey-shaped head, date back approximately 3.5 million years, with fossils found in Idaho, USA. The genus rapidly expanded into the Old World, evidenced by the similarly aged Equus livenzovensis discovered in western Europe and Russia.
Molecular studies suggest the most recent common ancestor of all modern equids lived around 5.6 million years ago. More recent paleogenomic analysis of a 700,000-year-old horse bone from Canada points to a more recent common ancestor approximately 4.07 million years ago. The earliest divergences within the genus are observed in the Asian hemiones (subgenus E. (Asinus)), including the kulan, onager, and kiang, followed by the African zebras (subgenera E. (Dolichohippus) and E. (Hippotigris)). All other modern equines, including the domesticated horse, belong to the subgenus E. (Equus), which separated around 4.8 million years ago.
Pleistocene horse fossils have led to the description of numerous species, with over 50 equine species identified from North America alone. However, recent genetic research on fossilized remains has indicated only three genetically distinct equid lineages in Pleistocene North and South America. These findings suggest that North American fossils of caballine-type horses, alongside South American fossils traditionally classified under the subgenus E. (Amerhippus), all belong to a single species: E. ferus. Fossils previously attributed to various species and grouped as New World stilt-legged horses likely represent a second species endemic to North America, closely related to E. ferus. Surprisingly, a third species, endemic to South America and known as Hippidion, was found to be a third species within the genus Equus, closely related to the New World stilt-legged horse. The significant variations in body size and morphology within each lineage point to remarkable intraspecific adaptability, explaining the historical overestimation of taxonomic diversity based on morphoanatomical grounds.
The genus Equus is thought to have originated from a Dinohippus-like ancestor between 4 and 7 million years ago. It quickly spread to the Old World, diversifying into various species of asses and zebras. A North American lineage within the subgenus E. (Equus) evolved into the New World stilt-legged horse (NWSLH). Subsequently, populations of this species migrated to South America about 2.5 million years ago, evolving into the form now known as Hippidion. Hippidion is therefore unrelated to the morphologically similar Pliohippus, which likely became extinct during the Miocene. Both the NWSLH and Hippidion exhibit adaptations for dry, open terrain, with the shortened legs of Hippidion possibly being a response to sloped landscapes. The geographic origin of the closely related modern E. ferus remains unresolved. However, genetic data from extant and fossil Pleistocene material suggests two clades, potentially subspecies. One clade had a Holarctic distribution, spanning Europe, Asia, and North America, and became the foundational stock for the modern domesticated horse. The other population appears to have been confined to North America.
In June 2013, researchers announced the sequencing of DNA from a 560–780 thousand-year-old horse, extracted from a leg bone found in permafrost in Canada’s Yukon territory. This marked a significant advancement, as the oldest previously sequenced nuclear genome dated back only 110–130 thousand years. Comparative analysis with the genomes of a 43,000-year-old Pleistocene horse, a Przewalski’s horse, five modern horse breeds, and a donkey revealed that the last common ancestor of modern horses, donkeys, and zebras lived 4 to 4.5 million years ago. The study also indicated that Przewalski’s horse diverged from other modern horse types approximately 43,000 years ago and had never been domesticated throughout its evolutionary history.
Evidence from digs in western Canada indicates that horses existed in North America until around 12,000 years ago. However, all Equidae in North America eventually became extinct. The precise causes of this extinction, which coincided with the disappearance of various other American megafauna, have been a subject of considerable debate. The suddenness of the event, following millions of years of horse flourishing, suggests unusual circumstances. One primary hypothesis attributes the extinction to climate change; for instance, in Alaska, approximately 12,500 years ago, the characteristic steppe ecosystem grasses were replaced by shrub tundra, featuring unpalatable plants. Another hypothesis links the extinction to overhunting by newly arrived humans who were unfamiliar with the prey animals’ defensive behaviors. These extinctions occurred roughly concurrently with the end of the most recent glacial advance and the emergence of the Clovis culture, known for big-game hunting. Several studies suggest humans likely arrived in Alaska around the same time or shortly before the local extinction of horses. Furthermore, it has been proposed that the vegetation transition from steppe to tundra in Beringia may have been a consequence, rather than a cause, of the megafaunal grazer extinction.
In Eurasia, horse fossils reappeared frequently in archaeological sites in Kazakhstan and southern Ukraine approximately 6,000 years ago. From this point, domesticated horses, along with the knowledge of capturing, taming, and rearing them, likely spread relatively rapidly, with wild mares from various populations being incorporated along the way.
Horses did not return to the Americas until 1493 with Christopher Columbus. These were Iberian horses initially brought to Hispaniola and later transported to Panama, Mexico, Brazil, Peru, Argentina, and Florida in 1538. The first horses to reach the mainland continent were 16 specifically identified horses brought by Hernán Cortés. Subsequent explorers, such as Coronado and De Soto, introduced progressively larger numbers, some directly from Spain and others from breeding centers established by the Spanish in the Caribbean. Later, as Spanish missions were founded on the mainland, horses were occasionally lost or stolen, leading to the proliferation of large feral herds known as mustangs. The indigenous peoples of the Americas, lacking a specific word for horses, referred to them in various languages as a type of dog or deer, with terms like “elk-dog,” “big dog,” or “seven dogs” (referring to the animal’s pulling capacity).
