the continent. The most recent, but most irrefutable, evidence of domestication comes from sites where horse remains were buried with chariots in graves of the Sintashta and Petrovka cultures c. 2100 BCE. Studies of variation in genetic material shows that a very few wild stallions, possibly all from a single haplotype, contributed to the domestic horse, mating with many mares in early domesticated herds. Przewalski's horse has been conclusively shown not to be an ancestor of the domestic horse, though the two can hybridize and produce fertile offspring. The split between Przewalskii's horse and E. ferus caballus is estimated to have occurred 120,000–240,000 years ago, long before domestication. Of the caballine equines of E. ferus, E. f. ferus, also known as the European wild horse or "tarpan", shares ancestry with the modern domestic horse. In addition, tarpans that lived into modern times may have been hybridized with domestic horses. Archaeological, biogeographical, and linguistic evidence suggests that the donkey was first domesticated by nomadic pastoral people in North Africa over 5,000 years ago. The animals were used to help cope with the increased aridity of the Sahara and the Horn of Africa. Genetic evidence finds that the donkey was domesticated twice based on two distinct mitochondrial DNA haplogroups. It also points to a single ancestor, the Nubian wild ass. Attempts to domesticate zebras were largely unsuccessful, though Walter Rothschild trained some to draw a carriage in England. Conservation issues Humans have had a great impact on the populations of wild equines. Threats to wild equines include habitat destruction and conflicts with local people and livestock. Since the 20th century, wild equines have been decimated over many of their former ranges and their populations scattered. In recent centuries, two subspecies, the quagga and the tarpan, became extinct. The IUCN lists the African wild ass as critically endangered, Grévy's zebra, the mountain zebra, and Przewalski's horse as endangered, the onager as vulnerable, the plains zebra as near threatened, and the kiang as least concern. Przewalski's horse was considered to be extinct in the wild from the 1960s to 1996. However, following successful captive breeding, it has been reintroduced in Mongolia. Feral horses vary in degree of protection and generate considerable controversy. For example, in Australia, they are considered a non-native invasive species, often viewed as pests, though are also considered to have some cultural and economic value. In the United States, feral horses