Theevolution of the horse, amammal of the familyEquidae, occurred over ageologic time scale of 50 million years, transforming the small, dog-sized,[1] forest-dwellingEohippus into the large, single-toed, modern-dayhorse.Paleozoologists have been able to piece together a more completeoutline of the evolutionarylineage of the modern horse than of any other animal. Much of this evolution took place in North America, where horses originated but became extinct about 10,000 years ago,[2] before being reintroduced in the 15th century.
The horse belongs to theorder Perissodactyla (odd-toed ungulates), the members of which sharehooved feet and an odd number of toes on each foot, as well as mobileupper lips and a similartooth structure. This means that horses share acommon ancestry withtapirs andrhinoceroses. The perissodactyls arose in the latePaleocene, less than 10 million years after theCretaceous–Paleogene extinction event. This group of animals appears to have been originally specialized for life intropical forests, but whereas tapirs and, to some extent, rhinoceroses, retained their jungle specializations, modern horses are adapted to life in the climatic conditions of thesteppes, which are drier and much harsher than forests or jungles. Other species ofEquus are adapted to a variety of intermediate conditions.
The early ancestors of the modern horse walked on several spread-out toes, an accommodation to life spent walking on the soft, moist ground of primeval forests. Asgrass species began to appear and flourish, theequids' diets shifted from foliage to silicate-rich grasses; the increased wear on teeth selected for increases in the size and durability of teeth. At the same time, as the steppes began to appear, selection favored increase in speed to outrun predators. This ability was attained by lengthening of limbs and the lifting of some toes from the ground in such a way that the weight of the body was gradually placed on one of the longest toes, the third.
Wild horses have been known since prehistory from central Asia to Europe, withdomestic horses and other equids being distributed more widely in the Old World, but no horses or equids of any type were found in theNew World when European explorers reached the Americas. When theSpanish colonists brought domestic horses from Europe, beginning in 1493,escaped horses quickly established large feral herds. In the 1760s, the early naturalistBuffon suggested this was an indication of inferiority of the New World fauna, but later reconsidered this idea.[3]William Clark's 1807 expedition toBig Bone Lick found "leg and foot bones of the Horses", which were included with other fossils sent toThomas Jefferson and evaluated by the anatomistCaspar Wistar, but neither commented on the significance of this find.[4]
The first Old World equid fossil was found in thegypsum quarries inMontmartre,Paris, in the 1820s. The tooth was sent to theParis Conservatory, where it was identified byGeorges Cuvier, who identified it as abrowsing equine related to thetapir.[5] His sketch of the entire animal matched later skeletons found at the site.[6]
During theBeagle survey expedition, the young naturalistCharles Darwin had remarkable success with fossil hunting inPatagonia. On 10 October 1833, atSanta Fe, Argentina, he was "filled with astonishment" when he found a horse's tooth in the samestratum as fossil giantarmadillos, and wondered if it might have been washed down from a later layer, but concluded this was "not very probable".[7] After the expedition returned in 1836, the anatomistRichard Owen confirmed the tooth was from an extinct species, which he subsequently namedEquus curvidens, and remarked, "This evidence of the former existence of a genus, which, as regards South America, had become extinct, and has a second time been introduced into that Continent, is not one of the least interesting fruits of Mr. Darwin's palæontological discoveries."[4][8]
In 1848, a studyOn the fossil horses of America byJoseph Leidy systematically examinedPleistocene horse fossils from various collections, including that of theAcademy of Natural Sciences, and concluded at least two ancient horse species had existed in North America:Equus curvidens and another, which he namedEquus americanus. A decade later, however, he found the latter name had already been taken and renamed itEquus complicatus.[3] In the same year, he visited Europe and was introduced by Owen to Darwin.[9]
The original sequence of species believed to have evolved into the horse was based on fossils discovered in North America in 1879 by paleontologistOthniel Charles Marsh. The sequence, fromEohippus to the modern horse (Equus), was popularized byThomas 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 oftransitional fossils was assembled by theAmerican Museum of Natural History into an exhibit that emphasized the gradual, "straight-line" evolution of the horse.
Since then, as the number of equid fossils has increased, the actual evolutionary progression fromEohippus toEquus has been discovered to be much more complex and multibranched than was initially supposed. The straight, direct progression from the former to the latter has been replaced by a more elaborate model with numerous branches in different directions, of which the modern horse is only one of many.George Gaylord Simpson in 1951[10] first recognized that the modern horse was not the "goal" of the entire lineage of equids,[11] but is simply the only genus of the many horse lineages to survive.
Detailed fossil information on the distribution and rate of change of new equid species has also revealed that the progression between species was not as smooth and consistent as was once believed. Although some transitions, such as that ofDinohippus toEquus, were indeed gradual progressions, a number of others, such as that ofEpihippus toMesohippus, were relatively abrupt ingeologic time, taking place over only a few million years. Bothanagenesis (gradual change in an entire population's gene frequency) andcladogenesis (a population "splitting" into two distinct evolutionary branches) occurred, and many species coexisted with "ancestor" species at various times. The change in equids' traits was also not always a "straight line" fromEohippus toEquus: some traits reversed themselves at various points in the evolution of new equid species, such as size and the presence of facialfossae, and only in retrospect can certain evolutionary trends be recognized.[12]
Phenacodontidae is themost recent family in the orderCondylarthra believed to be ancestral to theodd-toed ungulates.[citation needed] It contains the generaAlmogaver,Copecion,Ectocion,Eodesmatodon,Meniscotherium,Ordathspidotherium,Phenacodus andPleuraspidotherium. The family lived from theEarly Paleocene to theMiddle Eocene in Europe and were about the size of asheep, withtails making slightly less than half of the length of their bodies and, unlike their ancestors, good running skills.
Eohippus appeared in theYpresian (earlyEocene), about 52mya (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, fourpremolars, 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.Eohippus browsed on soft foliage and fruit, probably scampering between thickets in the mode of a modernmuntjac. It had a small brain, and possessed especially smallfrontal lobes.[12]
Its limbs were long relative to its body, already showing the beginnings of adaptations for running. However, all of the major leg bones were unfused, leaving the legs flexible and rotatable. Its wrist and hock joints were low to the ground. The forelimbs had developed five toes, of which four were equipped with small proto-hooves; the large fifth "toe-thumb" was off the ground. The hind limbs had small hooves on three out of the five toes, whereas thevestigial first and fifth toes did not touch the ground. Its feet were padded, much like a dog's, but with the small hooves in place of claws.[13]
For a span of about 20 million years,Eohippus thrived with few significant evolutionary changes.[12] The most significant change was in the teeth, which began to adapt to its changing diet, as these earlyEquidae shifted from a mixed diet of fruits and foliage to one focused increasingly on browsing foods. During the Eocene, anEohippus species (most likelyEohippus angustidens) branched out into various new types of Equidae. Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers of North American strata, mainly in theWind River basin inWyoming. Similar fossils have also been discovered in Europe, such asPropalaeotherium (which is not considered ancestral to the modern horse).[14]
Approximately 50 million years ago, in the early-to-middleEocene,Eohippus smoothly transitioned intoOrohippus through a gradual series of changes.[14] Although its name means "mountain horse",Orohippus was not a true horse and did not live in the mountains. It resembledEohippus 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. AlthoughOrohippus was still pad-footed, the vestigial outer toes ofEohippus were not present inOrohippus; there were four toes on each fore leg, and three on each hind leg.
The most dramatic change betweenEohippus andOrohippus was in the teeth: the first of the premolar teeth was dwarfed, the last premolar shifted in shape and function into a molar, and the crests on the teeth became more pronounced. Both of these factors increased the grinding ability of the teeth ofOrohippus; the change suggest selection imposed by increased toughness ofOrohippus plant diet.
In the mid-Eocene, about 47 million years ago,Epihippus, a genus which continued the evolutionary trend of increasingly efficient grinding teeth, evolved fromOrohippus.Epihippus had five grinding, low-crowned cheek teeth with well-formed crests. A late species ofEpihippus, sometimes referred to asDuchesnehippus intermedius, had teeth similar toOligocene equids, although slightly less developed. WhetherDuchesnehippus was a subgenus ofEpihippus or a distinct genus is disputed.[15]Epihippus was only 2 feet tall.[15]
In the late Eocene and the early stages of theOligocene epoch (32–24 mya), the climate of North America became drier, and the earliestgrasses began to evolve. The forests were yielding to flatlands,[citation needed] home to grasses and various kinds of brush. In a few areas, these plains were covered insand,[citation needed] creating the type of environment resembling the present-dayprairies.
In response to the changing environment, the then-living species of Equidae also began to change. 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[citation needed]. About 40 mya,Mesohippus ("middle horse") suddenly developed in response to strong newselective pressures to adapt, beginning with the speciesMesohippus celer and soon followed byMesohippus westoni.
In the early Oligocene,Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes remained, but were small and not used in walking). The third toe was stronger than the outer ones, and thus more weighted; the fourth front toe was diminished to a vestigial nub. Judging by its longer and slimmer limbs,Mesohippus was an agile animal.
Mesohippus was slightly larger thanEpihippus, 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 largercerebral hemispheres, and had a small, shallow depression on its skull called afossa, which in modern horses is quite detailed. The fossa serves as a useful marker for identifying an equine fossil's species.Mesohippus had six grinding "cheek teeth", with a single premolar in front—a trait all descendant Equidae would retain.Mesohippus also had the sharp tooth crests ofEpihippus, improving its ability to grind down tough vegetation.
Around 36 million years ago, soon after the development ofMesohippus,Miohippus ("lesser horse") emerged, the earliest species beingMiohippus assiniboiensis. As withMesohippus, the appearance ofMiohippus was relatively abrupt, though a few transitional fossils linking the two genera have been found.Mesohippus was once believed to haveanagenetically evolved intoMiohippus by a gradual series of progressions, but new evidence has shown its evolution wascladogenetic: aMiohippus population split off from the main genusMesohippus, coexisted withMesohippus for around four million years, and then over time came to replaceMesohippus.[16]
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 Equidae.[17]
The forest-suited form wasKalobatippus (orMiohippus intermedius, 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 toAnchitherium, which travelled to Asia via theBering Straitland bridge, and from there to Europe.[18] In both North America and Eurasia, larger-bodied genera evolved fromAnchitherium:Sinohippus in Eurasia andHypohippus andMegahippus in North America.[19]Hypohippus became extinct by the lateMiocene.[20]
TheMiohippus population that remained on the steppes is believed to be ancestral toParahippus, a North American animal about the size of a smallpony, 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. The incisor teeth, like those of its predecessors, had a crown (like human incisors); however, the top incisors had a trace of a shallow crease marking the beginning of the core/cup.
In the middle of the Miocene epoch, the grazerMerychippus flourished.[21] 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.[17]Merychippus radiated into at least 19 additional grassland species.
Three lineages within Equidae are believed to be descended from the numerous varieties ofMerychippus:Hipparion,Protohippus andPliohippus. The most different fromMerychippus wasHipparion, mainly in the structure oftooth enamel: in comparison with other Equidae, the inside, ortongue side, had a completely isolatedparapet. A complete and well-preserved skeleton of the North AmericanHipparion shows an animal the size of a small pony. They were very slim, rather likeantelopes, and were adapted to life on dry prairies. On its slim legs,Hipparion had three toes equipped with small hooves, but the side toes did not touch the ground.
In North America,Hipparion and its relatives (Cormohipparion,Nannippus,Neohipparion, andPseudhipparion), proliferated into many kinds ofequids, at least one of which managed to migrate to Asia and Europe during the Miocene epoch.[22] (EuropeanHipparion differs from AmericanHipparion in its smaller body size – the best-known discovery of these fossils was nearAthens.)
Pliohippus arose fromCallippus in the middle Miocene, around 12 mya. It was very similar in appearance toEquus, though it had two long extra toes on both sides of the hoof, externally barely visible as callused stubs. The long and slim limbs ofPliohippus 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. However, thoughPliohippus was clearly a close relative ofEquus, its skull had deep facial fossae, whereasEquus had no fossae at all. Additionally, its teeth were strongly curved, unlike the very straight teeth of modern horses. Consequently, it is unlikely to be the ancestor of the modern horse; instead, it is a likely candidate for the ancestor ofAstrohippus.[23]
Dinohippus was the most common species of Equidae in North America during the latePliocene. It was originally thought to be monodactyl, but a 1981 fossil find in Nebraska shows some were tridactyl.
Plesippus is often considered an intermediate stage betweenDinohippus and the extant genus,Equus.
Thefamous fossils found near Hagerman, Idaho, were originally thought to be a part of the genusPlesippus.Hagerman Fossil Beds (Idaho) is a Pliocene site, dating to about 3.5 mya. The fossilized remains were originally calledPlesippus shoshonensis, but further study by paleontologists determined the fossils represented the oldest remains of the genusEquus.[24] Their estimated average weight was 425 kg, roughly the size of anArabian 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 ofPlesippus moved across theBering land bridge into Eurasia around 2.5 mya.[25]
The genusEquus, which includes all extant equines, is believed to have evolved fromDinohippus, via the intermediate formPlesippus. One of the oldest species isEquus simplicidens, described as zebra-like with a donkey-shaped head. The oldest fossil to date is ~3.5 million years old, discovered inIdaho. The genus appears to have spread quickly into the Old World, with the similarly agedEquus livenzovensis documented from western Europe and Russia.[26]
Molecular phylogenies indicate the most recent common ancestor of all modern equids (members of the genusEquus) 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.[27] The oldest divergencies are the Asian hemiones (subgenusE. (Asinus), including thekulan,onager, andkiang), followed by the African zebras (subgeneraE. (Dolichohippus), andE. (Hippotigris)). All other modern forms including the domesticated horse (and many fossilPliocene andPleistocene forms) belong to the subgenusE. (Equus) which diverged ~4.8 (3.2–6.5) million years ago.[28]
Pleistocene horse fossils have been assigned to amultitude 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.[29] Recent genetic work on fossils has found evidence for only threegenetically divergent equid lineages in Pleistocene North and South America.[28] These results suggest all North American fossils of caballine-type horses (which also include thedomesticated horse andPrzewalski's horse of Europe and Asia), as well as South American fossils traditionally placed in the subgenusE. (Amerhippus)[30] belong to the same species:E. ferus. Remains attributed to a variety of species and lumped asNew World stilt-legged horses (includingHaringtonhippus,E. tau,E. quinni and potentially North American Pleistocene fossils previously attributed toE. cf. hemiones, andE. (Asinus) cf.kiang) probably all belong to a second speciesendemic to North America, which despite a superficial resemblance to species in the subgenusE. (Asinus) (and hence occasionally referred to as North American ass) is closely related toE. ferus.[28] Surprisingly, the third species, endemic to South America and traditionally referred to asHippidion, originally believed to be descended fromPliohippus, was shown to be a third species in the genusEquus, closely related to the New World stilt-legged horse.[28] The temporal and regional variation in body size and morphological features within each lineage indicates extraordinaryintraspecific plasticity. Such environment-driven adaptative changes would explain why the taxonomic diversity of Pleistocene equids has been overestimated on morphoanatomical grounds.[30]
According to these results, it appears the genusEquus evolved from aDinohippus-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 subgenusE. (Equus) evolved into the New World stilt-legged horse (NWSLH). Subsequently, populations of this species entered South America as part of theGreat American Interchange shortly after the formation of theIsthmus of Panama, and evolved into the form currently referred to asHippidion ~2.5 million years ago.Hippidion is thus only distantly related to the morphologically similarPliohippus, which presumably became extinct during theMiocene. Both the NWSLH andHippidium show adaptations to dry, barren ground, whereas the shortened legs ofHippidion may have been a response to sloped terrain.[30] In contrast, the geographic origin of the closely related modernE. ferus is not resolved. However, genetic results onextant and fossil material of Pleistocene age indicate two clades, potentially subspecies, one of which had aholarctic distribution spanning from Europe through Asia and across North America and would become the founding stock of the modern domesticated horse.[31][32] The other population appears to have been restricted to North America. However, one or more North American populations ofE. ferus entered South America ~1.0–1.5 million years ago, leading to the forms currently known asE. (Amerhippus), which represent an extinct geographic variant or race ofE. ferus.
Early sequencing studies ofDNA 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.[33] The evolutionarydivergence of the two populations was estimated to have occurred about 45,000YBP,[34][35] while the archaeological record places the first horse domestication about 5,500 YBP by the ancient central-AsianBotai culture.[34][36] The two lineages thus split well before domestication, probably due to climate, topography, or other environmental changes.[34]
Several subsequent DNA studies produced partially contradictory results. A 2009 molecular analysis usingancient DNA recovered from archaeological sites placed Przewalski's horse in the middle of the domesticated horses,[37] but a 2011mitochondrial DNA analysis suggested that Przewalski's and modern domestic horses diverged some 160,000 years ago.[38] 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.[39]
In June 2013, a group of researchers announced that they had sequenced theDNA of a 560–780 thousand year old horse, using material extracted from a leg bone found buried inpermafrost in Canada'sYukon territory.[40] Before this publication, the oldest nuclear genome that had been successfully sequenced was dated at 110–130 thousand years ago. For comparison, the researchers alsosequenced the genomes of a 43,000-year-oldPleistocene horse, aPrzewalski's horse, five modern horse breeds, and a donkey.[41] Analysis of differences between thesegenomes indicated that thelast common ancestor of modern horses, donkeys, and zebras existed 4 to 4.5 million years ago.[40] The results also indicated that Przewalski's horse diverged from other modern types of horse about 43,000 years ago, and had never in its evolutionary history been domesticated.[27]
A new analysis in 2018 involved genomic sequencing of ancient DNA from mid-fourth-millennium B.C.E. Botai domestic horses, as well as domestic horses from more recent archaeological sites, and comparison of these genomes with those of modern domestic and Przewalski's horses. The study revealed that Przewalski's horses not only belong to the same genetic lineage as those from the Botai culture, but were theferal descendants of these ancient domestic animals, rather than representing a surviving population of never-domesticated horses.[42] 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.[42]
Thekaryotype of Przewalski's horse differs from that of the domestic horse by an extra chromosome pair because of thefission of domestic horse chromosome 5 to produce the Przewalski's horse chromosomes 23 and 24. In comparison, the chromosomal differences between domestic horses andzebras include numeroustranslocations, fusions,inversions andcentromere repositioning.[43] This gives Przewalski's horse the highestdiploid chromosome number among all equine species. They can interbreed with the domestic horse and produce fertile offspring (65 chromosomes).[44]
Digs in western Canada have unearthed clear evidence horses existed in North America until about 12,000 years ago.[45]However, all Equidae in North America ultimately became extinct in theLate Pleistocene extinctions, simultaneous with the extinctions of a variety of other Americanmegafauna, while other species likeBison 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 toclimate change. For example, inAlaska, beginning approximately 12,500 years ago, the grasses characteristic of asteppe ecosystem gave way to shrubtundra, which was covered with unpalatable plants.[46][47] The other hypothesis suggests extinction was linked tooverexploitation 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 thebig-game huntingClovis culture.[48][49] Several studies have indicated humans probably arrived in Alaska before or shortly before the local extinction of horses.[49][50][51][52] However, it has been proposed that the steppe–tundra vegetation transition inBeringia may have been a consequence, rather than a cause, of the extinction of megafaunal grazers.[53]
In Eurasia, horse fossils began occurring frequently again in archaeological sites inKazakhstan and the southernUkraine about 6,000 years ago.[31] 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.[32][54]
Horses only returned to the Americas withChristopher Columbus in 1493. These wereIberian horses first brought toHispaniola and later toPanama,Mexico,Brazil,Peru,Argentina, and, in 1538,Florida.[55] The first horses to return to the main continent were 16 specifically identified[clarification needed] horses brought byHernán Cortés. Subsequent explorers, such asCoronado andDe 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 offeral horses that became known asmustangs.[56]
The ancestors of the horse came to walk only on the end of the third toe and both side (second and fourth) "toes". Skeletal remnants show obvious wear on the back of both sides ofmetacarpal andmetatarsal bones, commonly called the "splint bones". They are the remnants of the second and the fourth toes. 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).
A 2018 study has found remnants of the remaining digits in the horse's hoof, suggesting a retention of all five digits (albeit in an "hourglass" arrangement where metacarpals/tarsals are present proximally and phalanges distally).[57]
Throughout the phylogenetic development, the teeth of the horse underwent significant changes. The type of the originalomnivorous teeth with short, "bumpy" molars, with which the prime members of the evolutionary line distinguished themselves, gradually changed into the teeth common toherbivorous 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 eyeholes. In addition, the relatively short neck of the equine ancestors became longer, with equal elongation of the legs. Finally, the size of the body grew as well.[citation needed]
The ancestral coat color ofE. ferus was possibly a uniformdun, consistent with modern populations ofPrzewalski's horses. Pre-domestication variants including black and spotted have been inferred from cave wall paintings and confirmed by genomic analysis.[58]Domestication may have also led to more varieties of coat colors.[59]
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