Much of this evolution took place in North America, where horses originated but became extinct about 10,000 years ago. The horse belongs to the order Perissodactyla (odd-toed ungulates), the members of which all share hooked 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. 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.
The early ancestors of the modern horse walked on several spread-out toes, accommodation to life spent walking on the soft, moist grounds of primeval forests. As grass species began to appear and flourish, the equips diets shifted from foliage to grasses, leading to larger and more durable teeth.
At the same time, as the steppes began to appear, the horse's predecessors needed to be capable of greater speeds to outrun predators. In the 1760s, the early naturalist Buffoon suggested this was an indication of inferiority of the SeaWorld fauna, but later reconsidered this idea.
William Clark's 1807 expedition to Big Bone Lick found “leg and foot bones of the Horses “, which were included with other fossils sent to Thomas Jefferson and evaluated by the anatomist Caspar Wis tar, but neither commented on the significance of this find. The first Old World equip fossil was found in the gypsum quarries in Montmartre, Paris, in the 1820s.
His sketch of the entire animal matched later skeletons found at the site. During the Beagle survey expedition, the young naturalist Charles Darwin had remarkable success with fossil hunting in Patagonia.
In 1848, a study On the fossil horses of America by Joseph Lady systematically examined Pleistocene horse fossils from various collections, including that of the Academy of Natural Sciences, and concluded at least two ancient horse species had existed in North America: Equus curves and another, which he named Equus Americans. The original sequence of species believed to have evolved into the horse was based on fossils discovered in North America in 1879 by paleontologist Thiel Charles Marsh.
The sequence, from Phipps to the modern horse (Equus), was popularized by Thomas Huxley and became one of the most widely known examples of a clear evolutionary progression. The horse's evolutionary lineage became a common feature of biology textbooks, and the sequence of transitional fossils was assembled by the American Museum of Natural History into an exhibit that emphasized the gradual, “straight-line” evolution of the horse.
Since then, as the number of equip fossils has increased, the actual evolutionary progression from Phipps to Equus has been discovered to be much more complex and multibranched than was initially supposed. George Gaylord Simpson in 1951 first recognized that the modern horse was not the “goal” of the entire lineage of equips, but is simply the only genus of the many horse lineages to survive.
Although some transitions, such as that of Dinohippus to Equus, were indeed gradual progressions, a number of others, such as that of Phipps to Mesohippus, were relatively abrupt in geologic time, taking place over only a few million years. Both an agenesis (gradual change in an entire population's gene frequency) and cladogenesis (a population “splitting” into two distinct evolutionary branches) occurred, and many species coexisted with “ancestor” species at various times.
The change in equips' traits was also not always a “straight line” from Phipps to Equus : some traits reversed themselves at various points in the evolution of new equip species, such as size and the presence of facial fossa, and only in retrospect can certain evolutionary trends be recognized. Phipps appeared in the Persian (early Eocene), about 52 MYA (million years ago).
It was an animal approximately the size of a fox (250–450 mm in height), with a relatively short head and neck and a springy, arched back. It had 44 low-crowned teeth, in the typical arrangement of an omnivorous, browsing mammal: three incisors, one canine, four premolars, and three molars on each side of the jaw.
Its molars were uneven, dull, and bumpy, and used primarily for grinding foliage. The cusps of the molars were slightly connected in low crests.
Phipps browsed on soft foliage and fruit, probably scampering between thickets in the mode of a modern mental. Its limbs were long relative to its body, already showing the beginnings of adaptations for running.
The forelimbs had developed five toes, of which four were equipped with small proto-hooves; the large fifth “toe-thumb” was off the ground. Its feet were padded, much like a dog's, but with the small hooves in place of claws.
For a span of about 20 million years, Phipps thrived with few significant evolutionary changes. The most significant change was in the teeth, which began to adapt to its changing diet, as these early Equine shifted from a mixed diet of fruits and foliage to one focused increasingly on browsing foods.
Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North American strata, mainly in the Wind River basin in Wyoming. Similar fossils have also been discovered in Europe, such as Propalaeotherium (which is not considered ancestral to the modern horse).
Approximately 50 million years ago, in the early-to-middle Eocene, Phipps smoothly transitioned into Orohippus through a gradual series of changes. It resembled Phipps in size, but had a slimmer body, an elongated head, slimmer forelimbs, and longer hind legs, all of which are characteristics of a good jumper.
In the mid-Eocene, about 47 million years ago, Phipps, a genus which continued the evolutionary trend of increasingly efficient grinding teeth, evolved from Orohippus. A late species of Phipps, sometimes referred to as Duchesnehippus intermedia, had teeth similar to Oligocene equips, although slightly less developed.
Whether Duchesnehippus was a subgenus of Phipps or a distinct genus is disputed. In the late Eocene and the early stages of the Oligocene epoch (32–24 MYA), the climate of North America became drier, and the earliest grasses began to evolve.
The forests were yielding to flatland, home to grasses and various kinds of brush. In a few areas, these plains were covered in sand, creating the type of environment resembling the present-day prairies.
In the late Eocene, they began developing tougher teeth and becoming slightly larger and leggier, allowing for faster running speeds in open areas, and thus for evading predators in nonwooded areas . In the early Oligocene, Mesohippus was one of the more widespread mammals in North America.
Judging by its longer and slimmer limbs, Mesohippus was an agile animal. Mesohippus was slightly larger than Phipps, about 610 mm (24 in) at the shoulder.
Its back was less arched, and its face, snout, and neck were somewhat longer. It had significantly larger cerebral hemispheres, and had a small, shallow depression on its skull called a fossa, which in modern horses is quite detailed.
Mesohippus also had the sharp tooth crests of Phipps, improving its ability to grind down tough vegetation. As with Mesohippus, the appearance of Miohippus was relatively abrupt, though a few transitional fossils linking the two genera have been found.
Miohippus was significantly larger than its predecessors, and its ankle joints had subtly changed. Its facial fossa was larger and deeper, and it also began to show a variable extra crest in its upper cheek teeth, a trait that became a characteristic feature of equine teeth.
Miohippus ushered in a major new period of diversification in Equine. The forest-suited form was Kalobatippus (or Miohippus intermedia, depending on whether it was a new genus or species), whose second and fourth front toes were long, well-suited to travel on the soft forest floors.
Kalobatippus probably gave rise to Anchitherium, which travelled to Asia via the Bering Strait land bridge, and from there to Europe. The Miohippus population that remained on the steppes is believed to be ancestral to Parahippus, a North American animal about the size of a small pony, with a prolonged skull and a facial structure resembling the horses of today.
Its third toe was stronger and larger, and carried the main weight of the body. Its four premolars resembled the molar teeth; the first were small and almost nonexistent.
In the middle of the Miocene epoch, the grazer Merychippus flourished. It had wider molars than its predecessors, which are believed to have been used for crunching the hard grasses of the steppes.
The hind legs, which were relatively short, had side toes equipped with small hooves, but they probably only touched the ground when running. Three lineages within Equine are believed to be descended from the numerous varieties of Merychippus : Riparian, Protohippus and Pliohippus.
The most different from Merychippus was Riparian, mainly in the structure of tooth enamel : in comparison with other Equine, the inside, or tongue side, had a completely isolated parapet. A complete and well-preserved skeleton of the North American Riparian shows an animal the size of a small pony.
They were very slim, rather like antelopes, and were adapted to life on dry prairies. In North America, Riparian and its relatives (Cormohipparion, Nannies, Neohipparion, and Pseudhipparion), proliferated into many kinds of equips, at least one of which managed to migrate to Asia and Europe during the Miocene epoch.
Pliohippus arose from Calliopes in the middle Miocene, around 12 MYA. It was very similar in appearance to Equus, though it had two long extra toes on both sides of the hoof, externally barely visible as callused stubs.
The long and slim limbs of Pliohippus reveal a quick-footed steppe animal. Until recently, Pliohippus was believed to be the ancestor of present-day horses because of its many anatomical similarities.
Dinohippus was the most common species of Equine in North America during the late Pliocene. It was originally thought to be monodactyl, but a 1981 fossil find in Nebraska shows some were pterodactyl.
Plesippus is often considered an intermediate stage between Dinohippus and the extant genus, Equus. The famous fossils found near German, Idaho were originally thought to be a part of the genus Plesippus.
German Fossil Beds (Idaho) is a Pliocene site, dating to about 3.5 MYA. The fossilized remains were originally called Plesippus Shoshones, but further study by paleontologists determined the fossils represented the oldest remains of the genus Equus.
Their estimated average weight was 425 kg, roughly the size of an Arabian horse. At the end of the Pliocene, the climate in North America began to cool significantly and most of the animals were forced to move south.
One population of Plesippus moved across the Bering land bridge into Eurasia around 2.5 MYA. Skull of a giant extinct horse, Equus eisenmannae The genus Equus, which includes all extant equines, is believed to have evolved from Dinohippus, via the intermediate form Plesippus.
The oldest fossil to date is ~3.5 million years old from Idaho, USA. The genus appears to have spread quickly into the Old World, with the similarly aged Equus livenzovensis documented from Western Europe and Russia.
Molecular phylogeny indicate the most recent common ancestor of all modern equips (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 metaphorical 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 divergences are the Asian heroines (subgenus E. (Sinus) , including the Klan, Onsager, and King), 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.
Pleistocene horse fossils have been assigned to a multitude of species, with over 50 species of equines described from the Pleistocene of North America alone, although the taxonomic validity of most of these has been called into question. Remains attributed to a variety of species and lumped as SeaWorld stilt-legged horses (including H. Francisco, E. tau, E. Quinn and potentially North American Pleistocene fossils previously attributed to E. cf.
Kiang) probably all belong to a second species endemic to North America, which despite a superficial resemblance to species in the subgenus E. (Sinus) (and hence occasionally referred to as North American ass) is closely related to E. ferns. Surprisingly, the third species, endemic to South America and traditionally referred to as Hippidion, originally believed to be descended from Pliohippus, was shown to be a third species in the genus Equus, closely related to the SeaWorld stilt-legged horse.
The temporal and regional variation in body size and morphological features within each lineage indicates extraordinary interspecific plasticity. Such environment-driven adaptation changes would explain why the taxonomic diversity of Pleistocene equips has been overestimated on morphoanatomical grounds.
According to these results, it appears the genus Equus evolved from a Dinohippus -like ancestor ~4–7 MYA. It rapidly spread into the Old World and there diversified into the various species of asses and zebras.
A North American lineage of the subgenus E. (Equus) evolved into the SeaWorld stilt-legged horse (Nash). Subsequently, populations of this species entered South America as part of the Great American Interchange shortly after the formation of the Isthmus of Panama, and evolved into the form currently referred to as Hippidion ~2.5 million years ago.
Hippidion is thus only distantly related to the morphologically similar Pliohippus, which presumably became extinct during the Miocene. Both the Nash and Iridium show adaptations to dry, barren ground, whereas the shortened legs of Hippidion may have been a response to sloped terrain.
In contrast, the geographic origin of the closely related modern E. ferns is not resolved. However, genetic results on extant and fossil material of Pleistocene age indicate two clades, potentially subspecies, one of which had a Arctic distribution spanning from Europe through Asia and across North America and would become the founding stock of the modern domesticated horse.
However, one or more North American populations of E. ferns 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. ferns. The evolutionary divergence of the two populations was estimated to have occurred about 45,000 GBP, while the archaeological record places the first horse domestication about 5,500 GBP by the ancient central-Asian Bowie 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.
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. A new analysis in 2018 involved genomic sequencing of ancient DNA from mid-fourth-millennium B.C.E.
The study revealed that Przewalski's horses not only belong to the same genetic lineage as those from the Bowie culture, but were the feral descendants of these ancient domestic animals, rather than representing a surviving population of never-domesticated horses. In comparison, the chromosomal differences between domestic horses and zebras include numerous translocation, fusions, inversions and centromere repositioning.
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).
Digs in western Canada have unearthed clear evidence horses existed in North America until about 12,000 years ago. 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 at the same time or shortly before the local extinction of horses. Additionally, it has been proposed that the steppe-tundra vegetation transition in Bering 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. From then on, domesticated horses, as well as the knowledge of capturing, taming, and rearing horses, probably spread relatively quickly, with wild mares from several wild populations being incorporated en route.
These were Iberian horses first brought to Hispaniola and later to Panama, Mexico, Brazil, Peru, Argentina, and, in 1538, Florida. 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.
Modern horses retain the splint bones; they are often believed to be useless attachments, but they in fact play an important role in supporting the carpal joints (front knees) and even the tarsal joints (hocks). Throughout the phylogenetic development, the teeth of the horse underwent significant changes.
The type of the original omnivorous teeth with short, “bumpy” molars, with which the prime members of the evolutionary line distinguished themselves, gradually changed into the teeth common to herbivorous mammals. They became long (as much as 100 mm), roughly cubical molars equipped with flat grinding surfaces.
In conjunction with the teeth, during the horse's evolution, the elongation of the facial part of the skull is apparent, and can also be observed in the backward-set eye holes. In addition, the relatively short neck of the equine ancestors became longer, with equal elongation of the legs.
Reconstruction of possible ancestral coat colors. The ancestral coat color of E. ferns was possibly a uniform dun, consistent with modern populations of Przewalski's horses. Pre-domestication variants including black and spotted have been inferred from cave wall paintings and confirmed by genomic analysis.
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