The Domestication and History of Modern Horses

The modern domesticated horse (Equus caballus) is today spread throughout the world and among the most diverse creatures on the planet. In North America, the horse was part of the megafaunal extinctions at the end of the Pleistocene. Two wild subspecies survived until recently, the Tarpan (Equus ferus ferus, died out ca 1919) and Przewalskis Horse (Equus ferus przewalskii, of which there are a few left). Horse history, especially the timing of the domestication of the horse, is still being debated, partly because the evidence for domestication itself is debatable. Unlike other animals, criteria such as changes in body morphology (horses are extremely diverse) or the location of a particular horse outside of its normal range (horses are very widespread) are not useful in helping resolve the question. Evidence for Horse Domestication The earliest possible hints for domestication would be the presence of what appears to be a set of postmolds with lots of animal dung within the area defined by the posts, which scholars interpret as representing a horse pen. That evidence has been found at Krasnyi Yar in Kazakhstan, in portions of the site dating to as early as 3600 BC. The horses may have been kept for food and milk, rather than riding or load-bearing. Accepted archaeological evidence of horseback riding includes bit wear on horse teeth—that has been found in the steppes east of the Ural mountains at Botai and Kozhai 1 in modern Kazakhstan, around 3500-3000 BC. The bit wear was only found on a few of the teeth in the archaeological assemblages, which might suggest that a few horses were ridden to hunt and collect wild horses for food and milk consumption. Finally, the earliest direct evidence of the use of horses as beasts of burden—in the form of drawings of horse-drawn chariots—is from Mesopotamia, about 2000 BC. The saddle was invented around 800 BC, and the stirrup (a matter of some debate among historians) was probably invented around 200-300 AD. Krasnyi Yar includes over 50 residential pithouses, adjacent to which have been found dozens of postmolds. The postmolds—archaeological remnants of where posts have been set in the past—are arranged in circles, and these are interpreted as evidence of horse corrals. Horse History and Genetics Genetic data, interestingly enough, has traced all extant domesticated horses to one founder stallion, or to closely related male horses with the same Y haplotype. At the same time, there is a high matrilineal diversity in both domestic and wild horses. At least 77 wild mares would be required to explain the diversity of the mitochondrial DNA (mtDNA) in current horse populations, which probably means quite a few more. A 2012 study (Warmuth and colleagues) combining archaeology, mitochondrial DNA, and Y-chromosomal DNA supports the domestication of horse as occurring once, in the western part of the Eurasian steppe, and that because of the horses wild natures, several repeated introgression events (restocking of horse populations by adding wild mares), must have occurred. As identified in earlier studies, that would explain the diversity of mtDNA. Three Strands of Evidence for Domesticated Horses In a paper published in Science in 2009, Alan K. Outram and colleagues looked at three strands of evidence supporting horse domestication at Botai culture sites: shin bones, milk consumption, and bitwear. These data support domestication of the horse between about 3500-3000 BC sites in what is today Kazakhstan. Horses skeletons at Botai Culture sites have gracile metacarpals. The horses metacarpals—the shins or cannon bones—are used as key indicators of domesticity. For whatever reason (and I wont speculate here), shins on domestic horses are thinner—more gracile—than those of wild horses. Outram et al. describe the shinbones from Botai as being closer in size and shape to those of Bronze age (fully domesticated) horses compared to wild horses. Fatty lipids of horse milk were found inside of pots. Although today it seems a bit weird to westerners, horses were kept for both their meat and milk in the past—and still are in the Kazakh region as you can see from the photograph above. Evidence of horse milk was found at Botai in the form of fatty lipid residues on the insides of ceramic vessels; further, evidence for consumption of horse meat has been identified at Botai culture horse and rider burials. Bit wear is in evidence on horse teeth. Researchers noted bitting wear on horses teeth—a vertical strip of wear on the outside of horses premolars, where the metal bit damages the enamel when it sits between the cheek and tooth. Recent studies (Bendrey) using scanning electron microscopy with energy dispersive X-ray microanalysis found microscopic-sized fragments of iron embedded on Iron Age horse teeth, resulting from metal bit use. White Horses and History White horses have had a special place in ancient history-according to Herodotus, they were held as sacred animals in the Achaemenid court of Xerxes the Great (ruled 485-465 BC). White horses are associated with the Pegasus myth, the unicorn in the Babylonian myth of Gilgamesh, Arabian horses, Lipizzaner stallions, Shetland ponies, and Icelandic pony populations. The Thoroughbred Gene A recent DNA study (Bower et al.) examined the DNA of Thoroughbred racing horses and identified the specific allele which drives their speed and precocity. Thoroughbreds are a specific breed of horse, all of whom today are descended from the children of one of three foundation stallions: Byerley Turk (imported to England in the 1680s), Darley Arabian (1704) and Godolphin Arabian (1729). These stallions are all of Arab, Barb and Turk origin; their descendants are from one of only 74 British and imported mares. Horse breeding histories for Thoroughbreds have been recorded in the General Stud Book since 1791, and the genetic data certainly supports that history. Horse races in the 17th and 18th centuries ran 3,200-6,400 meters (2-4 miles), and horses were usually five or six years old. By the early 1800s, the Thoroughbred was bred for traits that enabled speed and stamina over distances from 1,600-2,800 meters at three years of age; since the 1860s, the horses have been bred for shorter races (1,000-1400 meters) and younger maturity, at 2 years. The genetic study looked at the DNA from hundreds of horses and identified the gene as C type myostatin gene variant, and came to the conclusion that this gene originated from a single mare, bred to one of the three founder male horses about 300 years ago. See Bower et al for additional information. Thistle Creek DNA and Deep Evolution In 2013, researchers led by Ludovic Orlando and Eske Willerslev of the Centre for GeoGenetics, Natural History Museum of Denmark and University of Copenhagen (and reported in Orlando et al. 2013) reported on a metapodial horse fossil which had been found in permafrost within a Middle Pleistocene context in the Yukon territory of Canada and dated between 560,00-780,000 years ago. Amazingly, the researchers found that there were sufficiently intact molecules of collagen within the matrix of the bone to enable them to map the Thistle Creek horses genome. The researchers then compared the Thistle Creek specimen DNA to that of an Upper Paleolithic horse, a modern donkey, five modern domestic horse breeds, and one modern Przewalskis horse. Orlando and Willerslevs team found that over the past 500,000 years, horse populations have been enormously sensitive to climate change and that extremely low population sizes are associated with warming events. Further, using the Thistle Creek DNA as a baseline, they were able to determine that all modern existing equids (donkeys, horses, and zebras) originated from a common ancestor some 4-4.5 million years ago. In addition, Przewalskis horse diverged from the breeds which became domestic some 38,000-72,000 years ago, confirming the long-held belief that Przewalskis is the last remaining wild horse species. Sources Bendrey R. 2012. From wild horses to domestic horses: a European perspective. World Archaeology 44(1):135-157. Bendrey R. 2011. 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