The Evolution of the Horse and its Contact with Early Humans, up to the First Appearance of Riding Behavior
About 230 million years ago, the first mammal-like creature appeared (Owl, 2004). For nearly 200 million years, then, mammals evolved and spread around the world. One of those mammals was Eohippus, the earliest equid (member of the family Equidae), which appeared 55 million years ago. The modern horse, genus Equus, evolved in North America and spread to Europe, Asia, and Africa. It was then eliminated from North America by a mass extinction during the Pleistocene era (Moehlman, 2004). Horses belong the order Perissodactyla (the odd-toed ungulates) and are one of the few species in this order to have been domesticated. Other odd-toed ungulates include rhinoceroses and tapirs (Cerveny, 2004).
For more (or different) information about these evolutionary changes, see Alex’s description of donkey evolution.
This figure shows how horse evolution occurred, listing genera present between Eohippus and Equus.
Earlier than 10,000 years ago, horses were widespread, inhabiting grassland steppe in every continent in the northern hemisphere and South America. At this time, they were hunted by early humans for food, but there is no compelling evidence that they were ridden. By 10,000 years ago, they were extinct in the Americas and by 7,000 years ago, a much smaller group of horses was located only on the central Asian steppe, where they were originally domesticated (Clutton-Brock, 1992). It is unclear exactly why this sharp decrease in population occurred. It could have been in part due to hunting and harassment by early humans and in part due to warmer temperatures causing grasslands to become forests, which are much worse habitats for horses (Clutton-Brock, 1992).
Horses were certainly eaten before they were domesticated and likely were first domesticated as food. Like cattle, they are large, with much meat on their bones, but unlike cattle, they are extremely cold hardy and will actually paw snow off of grass in order to eat it. It is likely that horses were first ridden in order to hunt and kill others of their own species. It is difficult to know when horses were domesticated for food, because one cannot tell if horse bones found at dig sites are from domestic horses that were killed and eaten or wild horses that were hunted and eaten (Anthony, 2007).
The domestication of horses can most reliably be tracked by looking at bit wear patterns on teeth of horse skulls found at archaeological dig sites, which tracks use of domestic horses for riding, and not for food (Anthony, 2007). When horses are bitted, or carry a bit in their mouths, they grasp the bit in their back teeth to relieve the pressure of the bit on the corners of their mouths, then release the bit. They repeat this many times throughout the time during which they are wearing the bit. This creates distinctive wear patterns on their first premolars, even when they are bitted with a soft material like leather or hemp (Anthony, 2007). David Anthony determined that bit wear patterns are a very accurate way to determine whether or not a horse has been bitted. We can assume that a horse would not be bitted unless it was being ridden. This system assumes that the first horses ridden were ridden with some sort of bit. Riding horses with no bit, in a type of bridle known as a hackamore, is very common, even today, in many disciplines. It is likely that people rode horses with some type of hackamore before it occurred to them to put something in a horse’s mouth to control it. Thus, Anthony’s bit wear system only tells us when riding did occur, not when it did not occur.
A recent analysis of mitochondrial DNA (which traces maternal line), Y chromosome DNA (which traces paternal line), and archaeological evidence found that horses likely were first domesticated in a specific region in the western Eurasian steppe. Maternal lines were highly variable, which indicates that wild mares were incorporated into domestic herds regularly. Lower levels of variability in Y chromosome DNA suggests that stallions were fewer in number—which makes sense, given that stallions are harder to manage than mares are. These first domestic populations likely originated from wild populations that expanded from Eastern Eurasia beginning about 160,000 years ago (Warmuth et al., 2012).