Modern horses are part of the family Equidae. The fossil history of Equidae is well documented, but new evidence about its evolutionary history—and new interpretations of it—continue to accumulate.
The earliest known genus of the Equidae family is Hyracotherium, which included several terrier-sized species that lived 55 to 45 million years ago, during the Eocene epoch. Since then, multiple lineages of horses have evolved, with much diversification occurring during the Miocene, about 25 to 8 million years ago.
Over time, the number of digits on the limbs tended to decrease in number: While Hyracotherium had four toes on the forefoot and three on the hindfoot, in the lineage that led to modern horses these were reduced to a single digit on each limb. By about one million years ago, members of the one-toed genus Equus (Latin for “horse”) were found across Africa, Asia, Europe, and the Americas, in enormous migrating herds.
All surviving species of the family Equidae are members of this single genus, Equus. These species are:
· Equus caballus, the common horse. All horse breeds, from Shetland ponies to Shire horses, belong to this species.
· Equus przewalskii, the wild Przewalski's horse, or takhi, as it is known in its native Mongolia. Some systematists and conservation biologists consider this to be a distinct species, whereas others believe it is a wild subspecies of Equus caballus.
· Equus asinus, the North African wild ass, domestic ass, burro, or donkey. The species’ native range is in Ethiopia and Somalia, yet domesticated and feral populations now exist in many parts of the world. Equus africanus is currently considered to be the name for the wild form of Equus asinus, and is ranked as Critically Endangered in the 2007 IUCN Red List of Threatened Species.
· Equus burchellii, the plains zebra, common zebra, or Burchell's zebra. This species lives in east and southern Africa, from southern Sudan, Ethiopia, and Somalia, south to southeast Congo, southern Angola, northern Namibia and Botswana, and South Africa.
· Equus grevyi, Grevy's zebra or Imperial zebra. It lives in Ethiopia, Somalia, Kenya and is ranked as Endangered in the 2007 Red List.
· Equus hemionus, the kulan or “half ass.” It lives in southwestern Mongolia and adjacent China and is ranked as Vulnerable in the 2007 Red List.
· Equus kiang, the Asiatic wild ass, or kiang. It resides in northern India, Turkmenia, and Iran.
· Equus onager, the onager. It lives in central Asia.
· Equus zebra, the mountain zebra. It resides in southern Angola, Namibia, and South Africa and is ranked as Endangered in the 2007 Red List.
One additional species, Equus quagga—the quagga—was formerly distributed in South Africa but is now extinct, according to the 2007 IUCN Red List of Threatened Species. The last individual died in captivity in 1883.
Where Did Modern Horses Come From?
Modern species of horses, such as the common horse, Equus caballus, evolved on the North American continent and migrated across the Bering land bridge into what is now Siberia. From there, horses spread across Asia into Europe and south to the Middle East and northern Africa. At the end of the Pleistocene epoch—about 10,000 years ago—a series of devastating extinctions took place in North and South America. Many large mammal species died out on the American continents, including mammoths, saber-tooth tigers, and all horse species. The cause of these extinctions is unknown. Possible factors may have included climate change, hunting pressure from early tribes that came across the Asia-Alaska land bridge, or perhaps some epidemic disease.
Better Luck in Eurasia
On other continents, however, Equus was very successful. Equus caballus lived on the lowlands north of the great Asiatic mountain ranges, probably in abundant herds. By around 4,000 B.C., several subspecies had developed on these grassy lowlands. The tarpan (Equus caballus gmelini), a small gray horse with a black tail and mane, lived around the Black and Caspian Seas. This was probably the first domesticated horse, and indeed, most contemporary domestic horses may derive from tarpan stock. The forest horse (Equus caballus sylvaticus), which once may have occupied parts of Central Europe as far west as the Rhine, became the ancestor of the heavy horses used by knights in the Middle Ages, as well as of later-day draft horses. The takhi (Equus przewalskii) was found in Mongolia and northern China.
Humans Squeeze Out the Wild Horse
Most scholars agree that the horse was probably first domesticated several thousand years ago on the treeless steppes of west-central Asia, north of the Caspian Sea. Early peoples first domesticated whatever species or subspecies of horse was found nearby. Then, altering the gene pool via trading or selective breeding, they engineered different types of horse suited for different activities. Today, there are at least 200 different domestic breeds of Equus caballus around the world, from tiny ponies to racehorses to huge draft horses.
A combination of factors—including hunting and competition with humans who wanted land and water for their livestock—reduced the range of the wild horse and drove it into ever more remote areas. Wild populations might have existed into the Middle Ages in Spain, the Alps, and the valley of the Danube, but some of these “wild” horses were probably actually feral: stock that had escaped captivity and returned to the wild. The forest horse persisted in the wild in eastern Poland until about 1800, but is now an extinct subspecies. Widely hunted as game, tarpans too died out; the last specimen died in captivity in the Ukraine in 1918. By the nineteenth century, most naturalists believed that no truly wild horses still existed, so great excitement greeted the 1879 rediscovery by Nikolai Przewalski of wild horses in southwestern Mongolia.
Takhi—The Only Wild Horse
The takhi is the only true wild horse left in the world. These dun-colored, black-maned equids have not been domesticated, and they remain genetically distinct from the common (domestic) horse. The takhi has 66 chromosomes instead of the common horse's 64. Because of this genetic distinction, some scientists recognize the takhi as a separate species from Equus caballus, rather than a closely-related subspecies. However, when the common horse and the takhi are cross-bred, the first-generation offspring have 65 chromosomes and are fertile. If parents can produce fertile offspring, according to systematic convention, it usually indicates that they do not belong to different species.
Another factor suggesting that the takhi might be a subspecies of Equus caballus is that if a first-generation horse-takhi hybrid is bred with a horse, the second-generation offspring have only 64 chromosomes and bear little resemblance to the takhi ancestor. Scientists who study equid systematics and conservation, however, are likely to debate the matter further. In fact, there are many approaches to how a species should be defined and the takhi provides a very good example of how difficult it can be to decide what “species concept” is the best.
If we accept the opinion that Przewalski's horse is a distinct species—Equus przewalskii—then we can say that the species became extinct in the wild in the 1960s. Now, populations have been successfully reestablished in several Asian locations.