Imagine a world in which horses of all colors, shapes, and sizes roamed the world, some barely larger than a small dog. That world no longer exists—but once it was real.
The earliest known horses evolved 55 million years ago. For more than half their history, most horses remained small forest browsers. But a changing climate allowed grasslands to expand, and by about 20 million years ago, many new species rapidly evolved.
Some horses—but not all—became larger and had the familiar hooves and grazing diets that we associate with horses today. Only these species survived to the present, but in the past, small and large species lived at the same time, often side by side.
The majority of horse species evolved in North America. From there, they occasionally walked to other continents. But about 10,000 years ago, horses became extinct in North and South America.
What ended their 55-million-year run? The prime suspects are changes in the environment, disease, and overhunting by humans, who likely killed them for food. Equus, the ancestor of all horses today, survived only in Eurasia and Africa. It’s the only surviving twig on an immense family tree that spans millions of years.