Becoming the Horse
The horse as we know it today evolved over millions of years to adapt to changes in the environment. Ancient horses differed in many ways, but it was how their toes, teeth, and tummies adapted that helped them survive through the centuries.
You might not recognize the fox-sized creature known as Hyracotherium as the horse's earliest ancestor. Sometimes called “dawn horse,” it developed nearly 60 million years ago. Its feet were like paws, with four toes on the front feet and three on the back. The toes were awkward for fast running, but good for traveling over the marshy land where dawn horse lived.
Dawn horse was a browser—it nibbled the berries and soft leaves it found in the forest. It didn't graze in the open fields like today's horses. Its teeth were too soft to chew tough grasses and plants, and its stomach couldn't digest such food anyway.
Mesohippus, or “middle horse,” developed about 35 million years ago. It was still a browser, like dawn horse, but it was larger—the size of a sheep. It had just three toes—the middle toe being larger than the other two.
Over the next 18 million years, the Earth's climate slowly changed. The lush forests became dry, grassy plains. And the horse changed, too. Merychippus had strong teeth to grind and chew wild grasses. And its legs were long, each ending in a strong single toe, which made it faster on the plain and able to outrun predators.
Equus, the modern horse, finally developed about 3 million years ago. It was tall and fast. Its single hoof enabled it to spring forward for speed. And it had the teeth and tummy to survive on a food that was everywhere—grass.
- evolve: To develop slowly from or into something else.
- How has the horse changed over time? Write a few sentences to explain your answer.
[anno: Over time, the horse's foot has gone from having multiple toes to having a single toe, or hoof. The horse's teeth have become stronger. Its stomach has changed so that it can digest grasses.]
- Look at the graphics for this article. Do you think the ancestors of the horse were vertebrates? Write a sentence explaining your answer.
[anno: The ancestors of horses were vertebrates because it looks like it has always had a backbone.]