The ultimate horse - Works in Progress Magazine

In June 1973, chestnut colt Secretariat ran the Belmont Stakes so fast that no horse has matched him since. In less than two and a half minutes, Secretariat flew along the track, finally pulling away to win by 31 lengths, nearly three quarters of a football field. The other horses were so far behind they weren’t even in the television frame. Fifty years later, billions of dollars have been poured into breeding and training, yet Secretariat’s record still stands. 9th June 1973: Secretariat leaves the field behind at Belmont Park. Image Getty Images. This is the paradox of the thoroughbred: breeders have spent centuries trying to create a faster horse, but in doing so, they may have bred away the very genetic diversity that would allow the horse to keep improving, or even to remain robust. The Works in Progress Newsletter Get new articles from Works in Progress delivered to your inbox. The paradox is not confined to the racetrack. The horse began as a terrier-sized, multi-toed forest dweller and, over 55 million years of evolution and a few thousand years of human intervention, became everything from the compact Icelandic pony to the streamlined racehorse. For most of that history, natural selection shaped horses slowly. Only recently has human ambition taken over, breeding animals for speed, endurance, and docility. The result is extraordinary diversity in form and performance, but also a dramatic narrowing of genetic diversity.The first horsesThe story begins 55 million years ago, deep in the warm, humid forests that covered what is now North America. You’d be hard pressed to recognize the earliest equid. Known as Hyracotherium, it stood only 14 inches tall, closer to a terrier than a modern horse. With four toes on the front feet and three on the back, it was ideally suited to moving gingerly over soft, marshy ground and nibbling fruits and leaves.When global temperatures rose, between 18 and 15 million years ago, the equid family diversified into more than twenty genera (the subdivision above species living in woodlands and savannas). Most of these lineages have long since died out: today, only the genus Equus survives, which includes horses, zebras, asses, and donkeys.As grasslands spread, over millions of years, equids developed into the horses we see today. Few evolutionary stories are as well documented. In the nineteenth century, when paleontologist OC Marsh unearthed a rich collection of fossil horses in the American West, their significance was immediately recognized. Here, laid out almost step by step, was evolution in action: horses transitioning from small, many-toed forest-dwellers into large, single-toed animals that grazed on grass rather than eating fruit and leaves.That single toe, of which the hoof is the toenail, is the evolutionary innovation key to horses’ success. Life on the plains favored animals able to run efficiently over long distances. Rigid, lightweight hooves help with this: they minimize sideways wobble while storing and releasing energy like a spring, much like the shoes a human sprinter might wear, or even the prosthetic running blades sometimes used by amputees.Over millions of years, the number of toes fell gradually from several to just one. Between 43 and 33 million years ago, Mesohippus still sported three toes of roughly equal size, with each touching the ground. By the time of Hipparion, about 23 million years ago, the middle toe carried most of the animal’s weight: the outer two toes had shrunk into vestiges, and the middle toe had become dominant. Finally, Pliohippus took a pivotal step toward the modern form, with a single weight-bearing toe ending in a solid hoof. This evolutionary streamlining produced the feet of today’s horses: a single, sturdy keratin hoof perfectly suited for galloping over open ground. The route to the modern horse involved several detours. Hyracotherium, the progenitor of the horse family, appears in the fossil record in North America from around 55 million years ago, disappearing again 8 million years later. Around 12 million years ago, Hipparion, which, though three-toed, otherwise resembled modern horses, colonized Eurasia. Hippidion, which had short limbs and distinctively shaped facial bones, roamed the plains and mountains of South America from around 3 million years ago. Then, around 2 million years ago, the most recent common ancestor of present-day asses and zebras crossed the Bering land bridge formerly joining Alaska and northeastern Siberia.Within the next half a million years, their descendants rapidly expanded across Eurasia and entered Africa at least two separate times. The descendants of the first migration diversified into various zebras, while the second led to modern donkeys and African wild asses. Painting of a wild horse in the Lascaux cave, France (approximately 15,000–10,000 BC). Image Wikimedia Commons. Although the Americas were home to the earliest proto-horses, horses (including Hippidion and others) later became extinct in the Western Hemisphere around 10,000 years ago, alongside other megafauna like giant sloths, human-sized beavers, and armadillos as big as cars. All horses in the Americas today, including the free-roaming mustangs of the American West, come from European horses introduced after the Spanish conquest – descendants of the branches of the horse family that crossed into Eurasia and avoided extinction.The first horse tamersExactly when, where, and how horses were domesticated remained a mystery until recently, because fossils of wild and early domestic horses look very similar. For decades, archaeologists pointed to the Botai people of northern Kazakhstan, who lived around 5,500 years ago, as the original domesticators. Excavations revealed corrals, possible evidence of harnesses, and even pottery with residues of horse milk, a clear sign that the Botai people were not merely hunting and eating wild horses but had integrated them into many aspects of their lives.While the Botai horses seemed like the obvious ancestors of modern horses, DNA told a different story: when scientists sequenced the genomes of twenty horses from the Botai site, they were not genetically similar to modern domestic breeds. Instead, Botai horses grouped closely with the Przewalski’s horse (pronounced shuh-VAHL-ski), a species discovered roaming the Mongolian steppe in the late 1870s and long considered the only truly wild horse. Przewalski's horse, the closest living relative of the Botai horses. Image Wikimedia Commons. This means that, contrary to the long-held belief that Przewalski’s horses were the last surviving truly wild horses, they are instead the feral relatives of the earliest known domestic horses found at Botai. While the Botai likely did domesticate horses first, their culture died out and their horses returned to the wild.From steppe to stableThe horses we know and love came from a different source. They emerged more than a thousand years later, about 4,200 years ago, in the Pontic-Caspian steppes of what is now southern Russia, the same region that gave rise to famous horse-riding warriors like the Cossacks, the Golden Horde, and the Huns.Scientists have worked this out through genetic analysis of ancient horse remains, combined with carbon dating to establish their precise age. A foal inherits half of its DNA from each of its parents. This is passed on with almost incredible accuracy, with around 2.7 billion base pairs (the data units of DNA) being copied correctly, but even so a few dozen errors creep in each time. A scientist can take the DNA of an ancient ancestor from a fossil, compare it with the DNA of a modern descendant, and then estimate how many generations have passed by counting the number of mutations that have accumulated.In 2021, scientists did just this, sequencing genomes from 273 sets of bones ranging from 50,000 to 200 years old. By looking at the mutation rate and carbon dating the bones to confirm their exact age, they could calculate how quickly generations passed. About 4,200 years ago, the generational clock began to tick twice as fast, halving relative to wild horses, suggesting deliberate breeding.Although bones can’t tell us how ancient horses behaved, their genomes give us clues. The researchers found that certain genetic variants became unusually common in these steppe horses. Such a pattern signals strong selective pressure: something in the environment consistently favored animals carrying particular versions of genes. One region under heavy selection lies near a gene called ZFPM1, thought to play a role in the development of brain cells involved in mood, fear, and aggression. This suggests that early breeders might have been either deliberately or indirectly choosing animals that were more docile, allowing training, handling, and eventually riding. A second region under strong selection sits just upstream of GSDMC, a gene linked in humans to spinal problems such as chronic back pain and narrowing of the spinal canal. The signal of selection at this locus in horses suggests that breeders may have favored genetic variants associated with stronger, more resilient backs. As horseback riding spread, horses that could bear weight without developing spinal pathologies would have been more desirable.These steppe horses, known to geneticists as DOM2 (that is, domesticated lineage two), spread like wildfire. By 2200 to 2000 BC, the DOM2 genetic profile spread beyond its region of origin to Anatolia, the lower Danube region, and Bohemia.