The Evolution of Horses
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. Today's horses represent just one tiny twig on an immense family tree that spans millions of years. All the other branches of the horse family, known as Equidae, are now extinct. The earliest known horses evolved 55 million years ago and for much of this time, multiple horse species lived at the same time, often side by side, as seen in this diorama.
Some 10 million years ago, up to a dozen species of horses roamed the Great Plains of North America. These relatives of the modern horse came in many shapes and sizes. Some lived in the forest, while others preferred open grassland.
Here, two large Dinohippus horses can be seen grazing on grass, much like horses today. But unlike modern horses, a three-toed Hypohippus tiptoes through the forest, nibbling on leaves. A small, three-toed Nannippus, shown here eating shrubs, ate both grass and leaves.
In the background are several other large mammals alive at that time, including Procamelus, a camel relative; a herd of Dinohippus horses; Gomphotherium, a distant relative of true elephants; and Teleoceras, a hornless rhinoceros.
A Brief History of Horses
By 55 million years ago, the first members of the horse family, the dog-sized Hyracotherium, were scampering through the forests that covered North America. For more than half their history, most horses remained small, forest browsers. But changing climate conditions allowed grasslands to expand, and about 20 million years ago, many new species rapidly evolved. Some--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 side by side.
Horses were once much smaller than they are today. But there was not a steady increase in size over time. Little Nannippus, shown in the diorama at full adult size, was actually smaller than its predecessors.
- Mesohippus 45 kg
- Merychippus 100 kg
- Nannippus 75 kg
- Equus 500 kg
- Single hoof
- Ate grass
- 11 to 4 million years ago
The Dinohippus shown grazing on the left is a close relative of horses today. Like modern-day Equus, Dinohippus had single-toed hooves and ate mostly grass. The other extinct species shown in the diorama had three toes and never developed single hooves.