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The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling ''Eohippus'' into the modern horse. Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal. The horse belongs to the order Perissodactyla (odd-toed ungulates), the members of which all share hooved 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, an accommodation to life spent walking on the soft, moist grounds of primeval forests. As grass species began to appear and flourish, the equids' 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. This was attained through the 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. ==History of research== Wild horses were known since prehistory from central Asia to Europe, with domestic horses and other equids being distributed more widely in the Old World, but no horses or equids of any type were found in the New World when European explorers reached the Americas. When the Spanish colonists brought domestic horses from Europe, beginning in 1493, escaped horses quickly established large wild herds. In the 1760s, the early naturalist Buffon suggested this was an indication of inferiority of the New World 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 Wistar, but neither commented on the significance of this find.〔(Academy of Natural Sciences - Thomas Jefferson Fossil Collection - Ancient Horse Fossils )〕 The first Old World equid fossil was found in the gypsum quarries in Montmartre, Paris, in the 1820s. The tooth was sent to the Paris Conservatory, where it was identified by Georges Cuvier, who identified it as a browsing equine related to the tapir. 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. On 10 October 1833, at Santa Fe, Argentina, he was "filled with astonishment" when he found a horse's tooth in the same stratum as fossil giant armadillos, and wondered if it might have been washed down from a later layer, but concluded this was "not very probable".〔('Filled with astonishment': an introduction to the St. Fe Notebook ), Barlow, Nora (ed. 1945) ''Charles Darwin and the voyage of the Beagle''. London: Pilot Press, p. (210 )〕 After the expedition returned in 1836, the anatomist Richard Owen confirmed the tooth was from an extinct species, which he subsequently named ''Equus 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."〔〔Darwin, C. R. (ed. 1840). ''Fossil Mammalia Part 1 No. 4 of The zoology of the voyage of H.M.S. Beagle''. By Richard Owen. London: Smith Elder and Co. (p. 108–109 )〕 In 1848, a study ''On the fossil horses of America'' by Joseph Leidy 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 curvidens'' and another, which he named ''Equus americanus''. A decade later, however, he found the latter name had already been taken and renamed it ''Equus complicatus''.〔 In the same year, he visited Europe and was introduced by Owen to Darwin.〔(Academy of Natural Sciences - Joseph Leidy - Leidy and Darwin )〕 The original sequence of species believed to have evolved into the horse was based on fossils discovered in North America in the 1870s by paleontologist Othniel Charles Marsh. The sequence, from ''Eohippus'' 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 equid fossils has increased, the actual evolutionary progression from ''Eohippus'' to ''Equus'' 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〔Simpson, George Gaylord (1951): ''Horses''. Oxford University Press; New Impression edition. ISBN 0-19-500104-4 (1971 reprint)〕 first recognized that the modern horse was not the "goal" of the entire lineage of equids,〔The notion of a goal would contradict modern evolutionary synthesis,〕 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 of ''Dinohippus'' to ''Equus'', were indeed gradual progressions, a number of others, such as that of ''Epihippus'' to ''Mesohippus'', were relatively abrupt in geologic time, taking place over only a few million years. Both anagenesis (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 equids' traits was also not always a "straight line" from ''Eohippus'' to ''Equus'': some traits reversed themselves at various points in the evolution of new equid species, such as size and the presence of facial ''fossae'', and only in retrospect can certain evolutionary trends be recognized.〔 See also (downloadable pdf version )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evolution of the horse」の詳細全文を読む スポンサード リンク
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