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 of Kazakhstan (Eurasia) approximately 5500 years ago. Before domestication, people routinely hunted and slaughtered horses for food (Levine, 1999). However, bit wear on teeth from the Kazakhstan sites indicated that horses may have been ridden, an activity that is unlikely without domestication (Brown and Anthony, 1998). Mares’ milk residue found associated with pottery also strongly suggested that these horses were accustomed to human contact, as wild animals are not easily milked (Outram et al., 2009). Archaeologists continue to look for physical evidence of horse domestication in other parts of the world, however, some of the most interesting contributions to this topic have come from molecular genetic studies of DNA recovered from skeletal remains. In the case of the Botai horses, the DNA results produced a surprising twist to the story of horse domestication.

      Ancient DNA

      Since the skeletal remains of equids are often incomplete and therefore difficult to assign to a specific species (horse, donkey, zebra, etc.) the study of DNA in skeletal remains has been a boon to archaeology. At the time of death, the organic material in the body begins to degrade. However, DNA found within bone is often protected from harsh environmental exposure and may be recovered and sequenced after thousands of years. These types of material are called ancient DNA (aDNA) and their survival depends upon favorable environmental conditions such as freezing in permafrost, mummification, desiccation of remains, or simply burial in soil that is low in acid and lacking other elements that degrade DNA. Successful study of aDNA in the laboratory requires a very clean environment to avoid contamination with modern DNA, complex corrections for naturally occurring chemical changes, as well as sophisticated computer analyses (bioinformatics) to factor out contaminating DNA sources such as bacteria or plant material that may have been present at the burial site. Horse remains are plentiful in the archaeological record. The quality of DNA recovery is correlated with age and aDNA is routinely recovered from samples that are hundreds of years old to tens of thousands of years old. The record for the oldest ancient mammalian DNA sequence ever reported is from a 700,000-year-old horse (Orlando et al., 2013).

      Which horse species was first domesticated?