The ancestors of horses–small, multi-toed quadrupeds–emerged around 50 million years ago, but horses as we know them (and their wild cousins) evolved from a common ancestor around 6 million years ago. Horses in those days were concentrated in North America, but spread via the Bering land bridge to Eurasia and Africa, where they differentiated into zebras, asses, and “wild” horses.
When humans first encountered horses, we ate them. American horses became extinct around 14,000-10,000 years ago, first in Beringia and then in the rest of the continent–coincidentally about the time humans arrived here. The first known transition from hunting horses to herding and ranching them occurred around 6,000 years ago among the Botai of ancient Kazakhstan, not far from the proto-Indo European homeland (though the Botai themselves do not appear to have been pIEs). These herds were still managed for meat, of which the Botai ate tons, until some idiot teenager decided to impress his friends by riding one of the gol-dang things. Soon after, the proto-Indo-Europeans got the idea and went on a rampage, conquering Europe, Iran, and the Indian subcontinent, (and then a little later North and South America, Africa, Australia, and India again). Those horses were useful.
Oddly, though, it appears that those Botai horses are not the ancestors of the modern horses people ride today–but instead are the ancestors of the Przewalski “wild” horse. The Przewalski was though to be a truly wild, undomesticated species, but it appears to have been a kind of domesticated horse* that went feral, much like the mustangs of the wild west. Unlike the mustang, though, the Przewalski is a truly separate species, with 66 chromosomes. Domesticated horses have 64, so the two species cannot produce fertile hybrids. When exactly the Przewalski obtained their extra chromosomes, I don’t know.
*This, of course, depends on the assumption that the Botai horses were “domesticated” in the first place.
Instead, modern, domesticated horses are believed to have descended from the wild Tarpan, though as far as I know, genetic studies proving this have not yet been done. The Tarpan is extinct, but survived up to the cusp of the twentieth century. (Personally, I’m not putting odds on any major tarpan herds in the past couple thousand years having had 100% wild DNA, but I wouldn’t classify them as “feral” just because of a few escaped domestics.)
Thus the horse was domesticated multiple times–especially if we include that other useful member of the equus family, the ass (or donkey, if you’d prefer). The hardworking little donkey does not enjoy its cousin’s glamorous reputation, and Wikipedia reports,
Throughout the world, working donkeys are associated with the very poor, with those living at or below subsistence level. Few receive adequate food, and in general donkeys throughout the Third World are under-nourished and over-worked.
The donkey is believed to have been domesticated from the wild African ass, probably in ancient Nubia (southern Egypt/northern Sudan). From there it spread up the river to the rest of Egypt, where it became an important work animal, and from there to Mesopotamia and the rest of the world.
Wild African asses still exist, but they are critically endangered.
All of the different species within the equus genus, except the domesticated donkey and its wild African cousins, have different numbers of chromosomes:
Onager (Persian wild ass): 56
Kulan: 54/55 (??)
Kiang (Asian wild ass): 51/52 (??)
Grevy’s zebra: 46
common zebra: 44
mountain zebra: 42
As far as I know, they can all cross and make mules, but all of these hybrids are infertile except domestic donkey/wild African donkey crosses. (Your only limit is African wild donkeys being endangered.)
I have no idea while equines have so much chromosomal diversity; dogs have been domesticated for much longer than horses, but are still interfertile with wolves and even coyotes (tbf, maybe horses could breed with tarpans.)
Interestingly, domestication causes a suit of changes to a species’ appearance that are not obviously useful. Recently-domesticated foxes exhibit pelt colors and patterns similar to those of domesticated dogs, not wild foxes. We humans have long hair, unlike our chimp-like ancestors. Horses also have long manes, unlike wild zebras, asses, and tarpans. Horses have evolved, then, to look rather like humans.
Also like humans, horses have different male and female histories. Male horses were quite difficult to tame, and so early domesticators only obtained a few male horses. Females, by contrast, were relatively easy to gentle, so breeders often restocked their herds with wild females. As a result, domesticated horses show far more variation in their mitochondrial DNA than their Y chromosomes. The stocking of herds from different groups of wild horses most likely gave rise to 17 major genetic clusters:
From these sequences, a phylogenetic network was constructed that showed that most of the 93 different mitochondrial (mt)DNA types grouped into 17 distinct phylogenetic clusters. Several of the clusters correspond to breeds and/or geographic areas, notably cluster A2, which is specific to Przewalski’s horses, cluster C1, which is distinctive for northern European ponies, and cluster D1, which is well represented in Iberian and northwest African breeds. A consideration of the horse mtDNA mutation rate together with the archaeological timeframe for domestication requires at least 77 successfully breeding mares recruited from the wild. The extensive genetic diversity of these 77 ancestral mares leads us to conclude that several distinct horse populations were involved in the domestication of the horse.
The wild mustangs of North America might have even more interesting DNA:
The researchers said four family groups (13.8%) with 31 animals fell into haplogroup B, with distinct differences to the two haplogroup L lineages identified.
The closest mitochondrial DNA sequence was found in a Thoroughbred racing horse from China, but its sequence was still distinct in several areas.
The testing also revealed links to the mitochondrial DNA of an Italian horse of unspecific breed, the Yunnan horse from China, and the Yakutia horse from central Siberia, Russia.
Haplogroup B seems to be most frequent in North America (23.1%), with lower frequencies in South America (12.68%) and the Middle East (10.94%) and Europe (9.38%).
“Although the frequency of this lineage is low (1.7%) in the Asian sample of 587 horses, this lineage was found in the Bronze Age horses from China and South Siberia.”
Westhunter suggests that this haplogroup could have originated from some surviving remnant of American wild horses that hadn’t actually been completely killed off before the Spanish mustangs arrived and bred with them. I caution a more prosaic possibility that the Russians brought them while colonizing Alaska and the coast down to northern California. Either way, it’s an intriguing finding.
The horse has been man’s companion for thousands of years and helped him conquer most of the Earth, but the recent invention of internal and external combustion engines (eg, the Iron Horse) have put most horses out to pasture. In effect, they have become obsolete. Modern horses have much easier lives than their hard-working plow and wagon-pulling ancestors, but their populations have shrunk enormously. They’re not going to go extinct, because rich people still like them (and they are still useful in parts of the world where cars cannot easily go,) but they may suffer some of the problems of inbreeding found in genetically narrow dog breeds.
Maybe someday, significant herds of wild horses will roam free again.