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On Your Toes

Matt Shanley/AMNH

Grazing Dinohippus

Horses, humans, and all other mammals share a common ancestor--with five toes. So how did horses end up with single-toed hooves? Over millions of years, many horse species lost most of their side toes. The middle toe evolved into a single large hoof, while the other toes became smaller and ultimately functionless.

Only one species in this scene, the grazing Dinohippus, has a single hoof. What's the connection between hooves and grazing? Hooves and long legs help horses run farther and faster on the open prairie, helping them flee from predators and find fresh grass for grazing. In the forest, where the ground is softer, many horses retained three toes.

Stretch Your Legs

Comparing a human leg to a horse leg shows which bones give horses their great speed. Horses that moved onto grasslands have longer legs than their forest-dwelling ancestors. But their leg bones did not all lengthen equally. Mostly it was the bones of the foot that grew longer, with the ankle moving relatively higher up on the leg. Long, sturdy, light-weight legs help a horse run faster---a useful trait on the open prairie, where there's no place to hide.

Adapting to a Changing Climate

Two major changes in climate affected the evolution of early horses. First, about 55 million years ago, global temperatures abruptly rose by 9 to 18F (5 to 10C), turning much of North America into a warm, wet, subtropical forest--much like what you'd see today in the Brazilian Amazonian rain forest. Small, leaf-eating horses thrived.

Then, about 35 million years ago, global temperatures dropped, creating a climate more similar to today's. Thereafter, dry grasslands replaced much of the North American forest, leading to rapid evolution among horses. By about nine million years ago, most forest browsers had disappeared, leaving primarily grass-eating grazers like those alive today.

David Goldman/Search4Dinosaurs

Hypohippus

Hypohippus
  • Three toes
  • Ate soft leaves
  • 18 to 9 million years ago

Long after hoofed, grass-eating grazers evolved and adapted to the American plains, three-toed forest browsers like the Hypohippus still continued to thrive for millions of years. This three-toed lineage is now extinct, but in the past many diverse horses lived side by side.

On Tiptoe Through Time

The earliest horses had three or four functional toes. But over millions of years of evolution, many horses lost their side toes and developed a single hoof. Only horses with single-toed hooves survive today, but the remains of tiny vestigial toes can still be found on the bones above their hoofs.

Where Did Horses Come From?

The majority of horse species evolved in North America. From there, they occasionally walked to other continents. About 20 million years ago, three-toed horses called anchitheres crossed to Asia and continued to Europe and Africa. About 11 million years ago, three-toed horses called hipparions spread from North America around the globe. About three million years ago, hoofed Equus, the ancestor of living horses, spread to several continents including South America.

The End of an Era

About 10,000 years ago, horses became extinct in North America and South America. Equus, the ancestor of all horses today, survived only in Eurasia and Africa. What ended their 55-million-year run in North America? The prime suspects are changes in the environment, disease, and overhunting by humans who likely killed them for food.

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