Equus The genus
Equus, which includes all extant equines, is believed to have evolved from
Dinohippus, via the intermediate form
Plesippus. One of the oldest species is
Equus simplicidens, described as zebra-like with a donkey-shaped head. The oldest fossil to date is ~3.5 million years old, discovered in
Idaho. The genus appears to have spread quickly into the Old World, with the similarly aged
Equus livenzovensis documented from western Europe and Russia. Molecular phylogenies indicate the most recent common ancestor of all modern equids (members of the genus
Equus) lived ~5.6 (3.9–7.8) mya. Direct paleogenomic sequencing of a 700,000-year-old middle Pleistocene horse metapodial bone from Canada implies a more recent 4.07 Myr before present date for the most recent common ancestor (MRCA) within the range of 4.0 to 4.5 Myr BP. The oldest divergencies are 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 forms including the domesticated horse (and many fossil
Pliocene and
Pleistocene forms) belong to the subgenus
E. (Equus) which diverged ~4.8 (3.2–6.5) million years ago. Recent genetic work on fossils has found evidence for only three
genetically divergent equid lineages in Pleistocene North and South America. These results suggest all North American fossils of caballine-type horses (which also include the
domesticated horse and
Przewalski's horse of Europe and Asia), as well as South American fossils traditionally placed in the subgenus
E. (Amerhippus) belong to the same species:
E. ferus. Remains attributed to a variety of species and lumped as
New World stilt-legged horses (including
Haringtonhippus,
E. tau,
E. quinni and potentially North American Pleistocene fossils previously attributed to
E. cf. hemiones, and
E. (Asinus) cf.
kiang) probably all belong to a second species
endemic to North America, which despite a superficial resemblance to species in the subgenus
E. (Asinus) (and hence occasionally referred to as North American ass) is closely related to
E. ferus. The other population appears to have been restricted to North America. However, one or more North American populations of
E. ferus entered South America ~1.0–1.5 million years ago, leading to the forms currently known as
E. (Amerhippus), which represent an extinct geographic variant or race of
E. ferus.
Genome sequencing Early sequencing studies of
DNA revealed several genetic characteristics of Przewalski's horse that differ from what is seen in modern domestic horses, indicating neither is ancestor of the other, and supporting the status of Przewalski horses as a remnant wild population not derived from domestic horses. The evolutionary
divergence of the two populations was estimated to have occurred about 45,000
YBP, while the archaeological record places the first horse domestication about 5,500 YBP by the ancient central-Asian
Botai culture. The two lineages thus split well before domestication, probably due to climate, topography, or other environmental changes. Several subsequent DNA studies produced partially contradictory results. A 2009 molecular analysis using
ancient DNA recovered from archaeological sites placed Przewalski's horse in the middle of the domesticated horses, but a 2011
mitochondrial DNA analysis suggested that Przewalski's and modern domestic horses diverged some 160,000 years ago. An analysis based on whole genome sequencing and calibration with DNA from old horse bones gave a divergence date of 38–72 thousand years ago. In June 2013, a group of researchers announced that they had sequenced the
DNA of a 560–780 thousand year old horse, using material extracted from a leg bone found buried in
permafrost in Canada's
Yukon territory. Before this publication, the oldest nuclear genome that had been successfully sequenced was dated at 110–130 thousand years ago. For comparison, the researchers also
sequenced the genomes of a 43,000-year-old
Pleistocene horse, a
Przewalski's horse, five modern horse breeds, and a donkey. Analysis of differences between these
genomes indicated that the
last common ancestor of modern horses, donkeys, and zebras existed 4 to 4.5 million years ago. The Botai horses were found to have made only negligible genetic contribution to any of the other ancient or modern domestic horses studied, which must then have arisen from an independent domestication involving a different wild horse population. This gives Przewalski's horse the highest
diploid chromosome number among all equine species. They can interbreed with the domestic horse and produce fertile offspring (65 chromosomes).
Pleistocene extinctions Digs in western Canada have unearthed clear evidence horses existed in North America until about 12,000 years ago. However, all Equidae in North America ultimately became extinct in the
Late Pleistocene extinctions, simultaneous with the extinctions of a variety of other American
megafauna, while other species like
Bison have survived there. The causes of this extinction are a matter of debate. Given the suddenness of the event and because these mammals had been flourishing for millions of years previously, something quite unusual must have happened. The first main hypothesis attributes extinction to
climate change. For example, in
Alaska, beginning approximately 12,500 years ago, the grasses characteristic of a
steppe ecosystem gave way to shrub
tundra, which was covered with unpalatable plants. The other hypothesis suggests extinction was linked to
overexploitation by newly arrived humans of naive prey that were not habituated to their hunting methods. The extinctions were roughly simultaneous with the end of the most recent glacial advance and the appearance of the
big-game hunting Clovis culture. Several studies have indicated humans probably arrived in Alaska before or shortly before the local extinction of horses. However, it has been proposed that the steppe–tundra vegetation transition in
Beringia may have been a consequence, rather than a cause, of the extinction of megafaunal grazers. In Eurasia, horse fossils began occurring frequently again in archaeological sites in
Kazakhstan and the southern
Ukraine about 6,000 years ago.
Return to the Americas Horses only returned to the Americas with
Christopher Columbus in 1493. These were
Iberian horses first brought to
Hispaniola and later to
Panama,
Mexico,
Brazil,
Peru,
Argentina, and, in 1538,
Florida. The first horses to return to the main continent were 16 specifically identified horses brought by
Hernán Cortés. Subsequent explorers, such as
Coronado and
De Soto, brought ever-larger numbers, some from Spain and others from breeding establishments set up by the Spanish in the Caribbean. Later, as Spanish missions were founded on the mainland, horses would eventually be lost or stolen, and proliferated into large herds of
feral horses that became known as
mustangs. ==Details==