Horses evolved in North America, vanished there, and returned with the conquistadors
Every living horse, donkey and zebra belongs to one genus, Equus, which arose in North America and spread across the Old World. Then, around 12,000 years ago, horses disappeared from the Americas entirely. They came back only in 1519, carried ashore by Spanish conquistadors, returning to the continent where their lineage began.
Carl Linnaeus named the genus in 1758; its Latin name simply means horse, a relative of the Greek hippos. Seven species survive, and they are the only living members of the family Equidae. Their ancestors, such as Eohippus some 54 million years ago, were dog-sized browsers with several toes and soft pads. Over millions of years the animals grew larger and the side toes shrank until a single hoof, evolved from the middle digit, carried the whole weight.
The oldest Equus fossils come from Idaho, and a close cousin of similar age turns up in western Europe and Russia, showing how quickly the genus crossed continents. DNA from a 700,000-year-old Canadian horse bone suggests the shared ancestor of modern equines lived about 4 million years ago. Asses probably branched off first, while zebras diversified in Africa. Why horses died out in the Americas is still argued over, with climate change and hunting by newly arrived people both suspected.
Today wild equines live only in Africa and Asia: plains zebras in grasslands, mountain zebras in the southwest of Africa, Grévy's zebra in East African scrub, wild asses in deserts and Przewalski's horse in Mongolia. Grévy's zebra is the biggest wild species at up to 405 kg, though heavy draft horses can top 1,000 kg and some miniature horses stand just 76 cm tall. Most wild species carry faint stripes on a dun coat; evidence from 2014 suggests zebras' bold stripes deter biting flies.
Equines ferment tough plants in the hindgut, which lets them survive on poorer forage than cattle and clear food in 30 to 45 hours rather than 70 to 100. They may spend most of the day eating, doze upright by day and lie down at night. Species can interbreed: a mule comes from a male donkey and a female horse, usually sterile, while zorses and zonkeys mix in zebra.
Source: Equus (genus)
