Читать книгу Horse Genetics - Ernest Bailey - Страница 15

The earliest recognized ancestor of the modern horse belonged to a species in the clade hyracothere (an animal or fossil of the genus Hyracotherium). Hyracotheres were also referred to as Eohippus, for “dawn horse”, and made famous by Thomas Huxley’s 1876 humorous cartoon showing Eohomo (dawn man) riding Eohippus (Fig. 2.1).

Fig. 2.1. Thomas Huxley’s 1876 cartoon of Eohomo and Eohippus (courtesy of the Division of Vertebrate of Paleontology, Peabody Museum of Natural History, Yale University.)

Such an event as a man riding an Eohippus never occurred since Eohippus (aka Hyracothere) existed over 50 million years ago and our genus, Homo, only appeared in the evolutionary record within the last 2 million years. Hyracotheres were small, perhaps the size of a small dog, and browsed leafy vegetation across North America and Europe. The hyracotheres present in North America became isolated from the rest of the world when rising waters submerged the land bridges that existed between the American and other continents. The modern horse is descended from just one of the species of hyracothere present in North America 58 million years ago. Other hyracothere species gave rise to rhinoceros and tapir species. The evolutionary trajectory of hyracothere in North America led to the appearance of hundreds of descendant species that evolved and went extinct in the intervening 58 million years. Fig. 2.2 illustrates the current view of the evolutionary processes leading to the horse.

Fig. 2.2. The phylogeny, geographic distribution, diet, and body sizes of the family Equidae over the past 55 million years. Vertical lines represent the actual time ranges of equid genera. The first ~35 million years are characterized by browsing species of relatively small body size. The last 20 million years are characterized by genera that are primarily browsing/grazing or are mixed feeders, exhibiting wide diversity in body size. Horses became extinct in North America about 12,000 years ago but the family survived by migration across land bridges to Asia and Europe. (Figure reprinted with permission from MacFadden 2005.) Plio. = Pliocene; Quat. = Quaternary.