Brief Primer on the Evolution of the Horse

Equine Evolution

Equine Evolution

Horses, or equids, began their evolution in North America roughly 52 million years ago. The earliest relative of what we know today as Equus feras, was Eohippus. Eohippus roamed the tropical forests of the Eocene era in North America. Most mammals of the Eocene era were 60% smaller than the mammals of the preceding epoch, and it is thought that this is due to the hotter climate of the Eocene era. Eohippus was a grazing omnivore, that, in addition to the normal grinding molars and incisors, but also had canines. It was no bigger than a fox, and had a proportionately smaller head and neck, and a springy, rounded back, more like a feline than a horse.

The most drastic difference between Eohippus and modern equines was in the legs. All of the major bones in the legs were unfused, providing ancient horse with the flexibility to maneuver quickly in and out of trees and low-laying brush. Both fore- and hind-limbs had five toes which were padded, much like a dogs, but where the nail would be, small hooves were instead. The forelimbs had four proto-hooves, while the hindlimbs only had three.

I won’t bore you with the step-by-step evolution of the horse (this is just a primer) but a few biological changes that accompanied this evolution are important to note. As forest gave way to grassland, ancient horse’s back became flatter and stiffer. The articulations between the vertebrae went from being flexible and loose, to stiff and rigid. This looseness was especially important throughout the lumbar, allowing the lower back and hind legs to twist and rotate, allowing ancient horse to dart around trees like a rabbit. The stiffness that horses gained through evolution, accompanied by larger withers with which to anchor the counter-balance of the head and neck provide action like a bow: loading and recoiling.

This action, along with the longer, leaner legs, gave the horse increased speed over the flat terrain of the Oligocene Epoch (roughly 32-24 million years ago). The first, second, fourth and fifth toes gradually retreated up the leg, to become nothing more than a chestnut and two splint bones. The splint bones, while not affecting the purpose of the canon bone, do provide necessary support to the bones of the knee and hock.

The teeth gradually morphed from those of an omnivore to those of a herbivore, designed to eat exclusively rough prairie grasses.

Horse’s extinction in North America

Fossil evidence proves that horses roamed North America until roughly 12,000 years ago. The extinction of the horse in North America coincides with the extinction of many of the great American megafauna such as wooly mammoths, saber toothed tigers and giant sloths. There are a number of theories regarding this sudden mass extinction of many species, but I believe it’s not as simple as simply choosing one. I believe a combination of human over hunting, disease, and a rapidly changing climate that caused a drastically different food supply to emerge, probably at a rate that the horse’s evolution could not keep pace with, caused the mass extinction of horse’s in North America.

Luckily for us, however, ancient horse had crossed the Bering Strait into Asia around 2.5 million years ago. Cave drawings of horses in the French Laseaux caves 10,000 years ago are a testament to ancient man’s encounter with ancient horse in Europe. The increase in horse fossils found in archaeological sites in countries such as Kazakhstan and Ukraine, lead us to believe that roughly 6,000 years ago, horses were domesticated and most likely kept like cattle.

Other archaeological sites that have dug up tack, have proven that ancient man advanced from domesticating horses as livestock to riding them about 3,500 B.C. or 5,500 years ago. It wasn’t until 600 years ago, that horses were reintroduced to the Americas by Spanish settlers. Christopher Columbus, and later, Hernan Cortes brought horses to the “new world” to aid in transportation, agriculture and trade. Subsequent Spanish conquistadores bought even more horses to the Americas, and a breeding operation was even established in the Caribbean by the Spanish. Eventually, horses broke free, were lost, or stolen, and bands of wild horses quickly procreated and spread throughout the entirety of North and South America.

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