caballus is estimated to have occurred 120,000– 240,000 years ago, long before domestication. The Mongolian horse is theorized to be the founding stock for many other horse breeds in Asia, including the Tuvinian, Akhaltekin, Yunan, Japanese and Cheju. A comparison of Mongol horses, Japanese horses, and Arab Anglo/Thoroughbred horses found that Mongol horses had the highest genetic diversity, with a heterozygosity ranging from 0.75 to 0.77. Compared to low heterozygosity values for Thoroughbreds (0.461), Arabians (0.478) and the bottlenecked Przewalski's horse (0.474), the genetic diversity of the Mongol horses is exceptional. A 1918 census of Mongolian animals found 1,500,000 horses. The origins of the Mongolian breed are hard to determine. Nomads of the central Asian steppes have been documented as riding horses since 2000 BC. Tests have shown, that among all horse breeds, Mongol horses feature the largest genetic variety, followed by the Tuvan horses. This indicates that it is a very archaic breed suffering little human-induced selection. The data also indicate that many other breeds descend from the Mongol horses. Crossbreeding Recently, breeders have begun importing expensive foreign racehorse breeds like Arabians and Thoroughbreds with the goal of breeding them to native stock to produce faster horses, but these relatively fragile breeds are unable to survive on the steppe like Mongolian horses can; if left unsheltered, such horses inevitably freeze to death or starve. So, breeders have focused on created crossbreeds between foreign horses and native Mongolian stock. The ultimate goal is to produce a race horse that has one-quarter foreign blood and three-quarters Mongolian blood; this proportion is believed to create a horse hardy enough to survive in Mongolia and combine the Mongolian horse's stamina and endurance with foreign speed to produce a new breed with the best qualities of both. One of the drawbacks to breeding such crosses is that the foreign stallion is much larger than the smaller Mongolian mare. This results in large foals that can be difficult for the small mares to birth. Since Mongolian mares typically give birth on their own without human supervision—and seldom have problems doing so—breeders have little experience on how to deal with the birthing problems that result due to the size of the crossed foals. To reduce birthing problems, a foreign mare could be bred to a native stallion to avoid the large foal problem, but in practice this reduces the numbers of crossbreed foals that can be