Research

More than 50 million years ago, a small fox-sized animal crept through the forests of North America, browsing on fruit and leaves. Its arched-back body was only about a foot high at the shoulder, and a long tail and short-snouted head probably gave it a distinctly dog-like look. In fact, its feet sported pads like a dog’s, except each toe ended in a tiny hoof instead of a claw. Interestingly, in modern horses, one toe has become the hoof, and the others remain as vestigial bumps higher up the leg.

When fossil hunters first discovered the bones of this creature a century ago, they named it Eohippus — “the dawn horse” — and believed it was the first link in an evolutionary chain that led directly to today’s horse. Indeed, many museums and textbooks still have displays and pictures showing this neat, predictable progression, with horses gradually getting larger, shifting from many toes to modern hooves, and gaining longer teeth able to grind down tough prairie grasses.

These days, however, researchers have a far more complex picture of horse evolution — and they have given the dawn horse a much less colorful name. While they agree that today’s horse probably arose from that smaller ancestor, the path was by no means direct. Instead, paleontologists have uncovered fossils that show that horse ancestors varied in size: some large early horses gave way later to smaller ones. They also discovered that some lines of horse-like animals alternated between many and few toes over time. In addition, some proto-horses once thought to be direct forefathers of the modern animals were revealed to be distantly related cousins — just one dead-end branch on a bushy family tree.

Horses' anatomy enables them to make use of speed to escape predators and they have a well-developed sense of balance and a strong fight-or-flight response. Related to this need to flee from predators in the wild is an unusual trait: horses are able to sleep both standing up and lying down. Female horses, called mares, carry their young for approximately 11 months, and a young horse, called a foal, can stand and run shortly following birth. Most domesticated horses begin training under saddle or in harness between the ages of two and four. They reach full adult development by age five, and have an average lifespan of between 25 and 30 years.