Where horses came from: evolution and domestication

A tapir-footed beginning

The distant ancestors of horses looked nothing like a modern horse. Animals such as Hyracotherium, from the Eocene, had feet like a tapir's: four toes on each front foot and three on each hind foot, each with its own small hoof and a pad underneath. Researchers at the University of Bristol describe North America as the original home of horse evolution.

The spring foot

The real step forward, Bristol palaeobiologists argue, wasn't losing toes but evolving the "spring foot". Three-toed ancestors developed an enlarged central toe and a pogo-stick-like foot that stores elastic energy in the leg tendons with each stride. This happened as grasslands spread across North America around 20 million years ago.

Spring-footed horses were hugely successful, as varied in their day as antelopes are in Africa now. By around 11 million years ago they had spread into Eurasia and Africa, where some grew larger than a modern horse. Most kept three toes for most of their evolutionary history; only one lineage, the one leading to modern horses, showed any tendency to lose toes.

One toe, for trotting

Single-toed horses appeared in North America around 12 million years ago. For millions of years they stayed pony-sized, and were neither diverse nor numerous.

It is often assumed that a single toe helped horses outrun predators. The Bristol researchers propose a different explanation. As these horses lost their side toes, their backs became shorter and stiffer, which doesn't suit a back-flexing gallop. They probably relied instead on the energy-efficient, medium-speed trot, roaming widely for food as the northern climate grew cooler and drier. In the words of Professor Christine Janis, their foot anatomy mattered "for finding food, rather than for avoiding becoming food themselves."

By around five million years ago, as the cooling and drying intensified worldwide, early species of Equus, the genus of modern horses, appeared. Equus left North America for the Old World about 2.5 million years ago, and by a million years ago every three-toed horse was extinct. The Bristol team thinks climate change, rather than competition, explains why the single-toed horses survived.

What happened to the toes?

A 2018 paper suggested that modern horses still keep their side toes inside the hoof, as part of the frog. A 2023 study involving the University of Bristol disagreed. The upper parts of the side toes survive as the splint bones, fused to the central one, but the toes themselves have simply been lost. The frog evolved separately, as a shock absorber. Each modern foot is the original third toe. See Inside the hoof and Skeleton and movement.

Gone from the Americas

Single-toed horses became the dominant equines worldwide over the past couple of million years. But in the Americas, where their story began, they died out at the end of the Pleistocene, around 12,000 years ago.

Domestication

Domesticating the horse, as a 2021 study in Nature puts it, "fundamentally transformed long-range mobility and warfare". But where and when it happened was long unclear. The study, by a large international team including a University of Edinburgh archaeologist, analysed 273 ancient horse genomes to find out.

Not the first horses people kept. The earliest archaeological evidence of domestic horses, with signs of bridling, milking and corralling, comes from Botai, in Central Asia, around 3500 BC. Yet the study found that modern domestic horses don't descend from that lineage. Iberia and Anatolia, two other long-suggested candidates, have also been challenged.

A steppe homeland. The study traced modern domestic horses to the western Eurasian steppes, especially the lower Volga-Don region.

A rapid takeover. A lineage of domestic horses became widespread from about 2200 BC. From about 2000 BC it spread rapidly across Eurasia, replacing almost all other local horse populations. Its spread coincided with new horse-related technology, including the spoke-wheeled chariots of the Sintashta culture, though horseback riding probably drove its first expansion. The researchers suggest that breeders were producing large numbers of these horses to meet a growing demand for horse-powered travel.

Bred for riding. The genomes show strong selection at two genes, GSDMC and ZFPM1, which the authors link to changes in movement and behaviour that suited riding and driving.

A myth overturned. It was commonly thought that horseback riding powered the great migration of Yamnaya herders from the steppes into Europe around 3000 BC. The study rejects that idea: the domestic horse lineage didn't accompany them. In Asia, by contrast, horses, chariots and Indo-Iranian languages spread together.

From these steppe horses come all the domestic breeds we know today, from Shetland ponies to Shires. See Horse and pony breeds, and Horses in British history for what came next.