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.
Climate, environment and evolution
The climate in America is profoundly different today than 50 million years ago. Geological records indicate North America had a tropical environment, characterized by marshy ground and lush, woody vegetation. The five toes of hyracotheria allowed it to walk easily through marshlands, its prehensile lips were ideal to browse tender shoots, leaves, and branches. As the climate became drier and marshes gave way to grasslands, species adapted. Characteristics that favored browsers were replaced by characteristics that favored grazers. For example, teeth evolved in response to the challenges of eating grasses in sandy soil (Fig. 2.3). Inclusion of sand and dirt during grazing completely wears down the enamel crowns of teeth. Individuals with higher-crowned teeth, and especially those with teeth that grew throughout life, were better able to eat grasses. They were better fed and had more offspring than equids with their short, crowned teeth. Horse teeth have been well preserved because they are so hard and have been particularly useful in tracking the evolutionary history of Equidae. From the Eocene to the early Miocene periods (58–20 million years ago) horses had short crown teeth (brachyodonts) but were replaced by species adapted to grasslands with high-crowned teeth (hypsodonts) (MacFadden, 2005).
Fig. 2.3. Cheek-tooth crown height in extant and fossil equids. A and B illustrate the appearance low crown (brachydont) and high crown teeth (hypsodont), respectively. C shows their appearance in the fossil record from 60 million years ago to present (reprinted from Janis and Fortelius 1988.)
Climate change triggered ecological consequences which drove the migration of many of these ancestral equid species. A land bridge appeared between North America and Asia, at the location of the modern Bering Strait, between 15 and 20 million years ago and relatives of the horse (Anchiterium and Sinohippus) crossed into Asia. They thrived in Asia, Africa, and Europe for millions of years before they also went extinct everywhere.
Approximately 18 million years ago, extensive grasslands developed in North America. Grazing became advantageous for the equid species that evolved hypsodont teeth for grazing. Furthermore, in the drier, open climates, the ancestors of horses that could travel farther and faster were favored. Adaptations for travel led horses to be more successful in finding food than others. Consequently, those with a single toe, the hoof, and other adaptations for travel had more successful offspring than others. Speed may have also provided protection from fleet predators. Migrations occurred a second and a third time, 5–10 million years ago, as more relatives of the horse (hipparions) crossed the land bridge to Asia and thrived for millions of years before, once again, becoming extinct on the Eurasian continent.
Meanwhile, the ancestors of modern horses continued to evolve in North America with the genus Equus appearing approximately 4–5 million years ago. These early species of Equus also crossed the land bridge to Asia, populating Asia, Africa, and Europe. This time, however, after the bridge closed about 12,000 years ago, Equus became extinct in the Americas while thriving on the Eurasian and African continents. We do not know why all of representatives of Equus became extinct in North America but continued to thrive in Asia, Europe, and Africa. Both climate change and predation by people are suspected. Regardless of the extinction in North America, Equus continued its evolution in Asia, Africa, and Europe, differentiating into the diverse species of horses, asses, Asiatic asses and zebras we have today.
Domestication of horses
The horse did well in Europe. Cave paintings of horses in France, dated to over 30,000 years ago, show that prehistoric man knew horses well. When the last Ice Age occurred, approximately 20,000 years ago, glaciers descended on northern Europe, driving most surviving animals into southern Europe, Africa, and Asia. The numbers of horses dwindled. Consequently, European species of horses may have been on their way to extinction as well. Indeed, domestication may well have prevented extinction of the horse.
Relationship of modern domestic horses to the Przewalski horse and other wild horse populations
Today, two species of horse exist, the modern domestic horse (Equus caballus) and the Przewalski horse (Equus przewalskii). The two species are so similar that both are considered horses (Chapter 3 discusses all extant species of Equus), but they have so many phenotypic and genotypic differences that they are recognized as different species. Scientists have even been able to isolate DNA from equid bones preserved for thousands of years for comparison of ancient to modern horses (Orlando et al., 2013). Molecular genetic studies of DNA from modern horses and DNA isolated from bone fragments suggest that ancestors of domestic horses and Przewalski horses diverged 41,000–70,000 years ago (Schubert et al., 2014). That study reported evidence suggesting the existence of at least three different species of prehistoric horse, one identified as living in arctic regions (Siberia) and described as “ancient” (ANC), one leading to the modern domestic horse (DOM) and one leading to the modern Przewalski horse (PRZ). The ANC species went extinct sometime in the last 5000 years. The only horse species found today are the modern domestic horses and the Przewalski horse.
Archeological evidence for domestication of horses
Domestication of the horse is thought to have taken place during the last 5500 years, possibly in Eurasia. Eurasia was the natural territory of the horse during the last 10,000 years. Before domestication, people hunted horses for food. Hunting would have become difficult once people developed a lifestyle based on agriculture, growing crops, and living in permanent villages. Wild animals in the vicinity of villages would have been harvested to extinction or driven off. Later, people may have captured wild horses and tamed them for domestic uses. Breeding captured horses to assure a continued supply of horses would be the next step. We do not yet know where this occurred, but this activity probably prevented the extinction of the horse and caused it to become, in evolutionary terms, one of the most successful large animals on the planet (Budiansky, 1997).
The earliest archaeological evidence for domestication of horses comes from the Botai area in the region
