horses were domesticated, people became adept at selecting them for the characteristics they valued. This is readily apparent when we consider the diversity of coat colors in horses. Since we have discovered DNA variants associated with coat color genes (Chapters 7–13), we can investigate the coat colors of prehistoric horses using aDNA. The results of these studies suggest that early horse breeders practiced selection for coat colors. Ludwig et al. (2009) observed that DNA tests from 14,000-year-old horse bones indicated that horses were uniformly bay in color. Around the time of domestication, 4000–5500 years ago, genotypes for both bay and black were found. However, over the next few thousand years there was an increase of horses with DNA sequences characteristic of the chestnut, tobiano, and sabino coat colors. Color variation among horses occurred coincidentally with domestication and was a probable basis for early selection by man of preferred breeding stock. One might suppose that the original color provided camouflage in the wild and protect horses from predators. However, people may have found flashy white markings or dilution variants attractive and selected those horses for their breeding herd.
Summary
• The evolution leading to the horse occurred almost entirely in North America, beginning 55 million years ago.
• Following the evolution of the genus Equus in North America the species migrated across the Bering Strait and populated Asia, Africa, and Europe.
• Equus went extinct in North America approximately 12,000 years ago but continued to thrive in Asia, Africa, and Europe
• Domestication of the horse first occurred in Eurasia. However, aDNA testing proved that those horses were not the ancestors of modern domestic horses, rather, they were ancestors of a different horse species, the modern Przewalski horse.
• The precise region, or regions, responsible for domestication of the modern horse is/are still unknown.
• However, once horses were domesticated, it appears that early horse breeders practiced selection for color and other characteristics found in modern horse populations.
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The horse belongs to the family Equidae in the order of perissodactyl. The evolutionary history of perissodactyls is shown in Fig. 3.1 (Steiner and Ryder, 2012). The perissodactyls are herbivorous, hoofed mammals with an odd number of toes. The other extant perissodactyls include four species of Rhinocerotidae (Fig. 3.2) and four species of Tapiridae (Fig. 3.3).
Fig. 3.1. Phylogenetic tree for the perissodactyl family based on combined molecular genetic data. Dog (Canis) and cattle (Bos) serve as outgroups; data was collected from four species of Rhinocerotidae, four species of Tapiridae and nine species of Equidae (figure reprinted with permission from the Zoological Journal of the Linnean Society).
Fig. 3.2. Indian rhinoceros.
Fig. 3.3. Central American tapir.
The common ancestor of extant perissodactyls existed approximately 54 million years ago (MYA). Divergence of the three families occurred shortly after that (51 MYA) with the emergence of the Equidae family occurring only about 4–4.5 MYA.
The only extant members of the Equidae family are members of the genus Equus. The Equidae are generally regarded as belonging to four major groups: the horses (E. caballus and przewalskii), the donkeys (E. asinus), the Asiatic wild asses (E. kiang, kulan and onager), and the zebras (E. quagga, E. zebra, E. grevyi) (Table 3.1). Each species has the same general morphology and size as the domestic horse, but only the horse and donkey were amenable to domestication. Each species evolved adaptations to the environment in their region of the world. Consequently, they exhibit unique behaviors, unique physical characteristics, and unique genetic attributes.
Table 3.1. Extant species belonging to the genus Equus.
