One ancestor of the modern horse is Hyracotherium, a dog-sized animal that lived about 50 million years ago. Importantly, it had toes: four on each front foot and three on each hind foot. At the time Hyracotherium was around, the planet was warm, wet, and overgrown with trees, a lot like the rainforests of today. Hyracotherium probably spent its days foraging in the forest, nibbling on leaves and hiding in the brush when predators approached.

But about 35 million years ago, global temperatures dropped to create a climate pretty similar to the one we live in today. In North America, many of the forests disappeared, leaving dry grasslands in their place — and leaf-eating browsers like Hyracotherium began to disappear with them. Starting 24 million years ago, the three-toed, German-shepherd-sized Parahippus was one sign of this evolution from forest to plains: Its teeth probably allowed it to eat both leaves and grass.

That change to grasslands made running speed more important for survival. A larger body size did too, since grass isn't as energy dense as leaves are and larger animals can conserve energy (through heat) better than small animals can. Over time, all of these evolutionary pressures led to big horses that could run fast. But that doesn't explain why they lost their toes.

For a study published in August 2017, evolutionary biologists at Harvard University collected 13 fossilized horse leg bones that spanned 50 million years, reaching all the way back to our friend Hyracotherium and forward to the horses of today. They used 3D scanning to measure bone length and area, which told them how resistant to squeezing and bending each bone was. Next, they estimated how much each horse would have weighed, and thereby how much stress their leg bones would have been under when running, trotting, and jumping.

They found that as horses got bigger, their center toes also got bigger to make their leg bones more resistant to stress. This didn't all happen at once: They found that the bones of Parahippus, that German-shepherd sized horse with the diverse appetite, wouldn't have been able to withstand physical stresses without the help of its two side toes. But as horses' legs got longer and that center toe got bigger, the side toes became less useful, soon turning into more of a hindrance than a help.

And according to research published in April 2019, there may have been something else going on that helped horses leave their extra toes behind. Early horses also went from having a "pad foot," like what you might see on dogs and cats, to a "spring foot," which can flex up to 90 degrees and pull back a series of specialized tendons to store elastic energy like a bow and arrow. That style of footwork can save up to 40 percent of the horse's locomotor energy with each trot.

The researchers say this change in foot structure helped horses evolve for long-distance travel. "Early members of the single-toed horse lineage were not only losing their side toes, but the bones of the remaining central toe showed evidence of the boosting-up of the 'spring foot' apparatus, implying that these horses were becoming more reliant on energy-efficient locomotion," said lead author Christine Janis in a statement.

But the extra toes of modern horses haven't disappeared completely. You can still spot them on many horses as a rough growth on the back legs — something commonly called a "chestnut" — or hair-covered growths just above the back of the hoof, called "ergots." Evolution is everywhere! You just have to know where to look.

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For more wild stories about how modern animals evolved, check out "Hen's Teeth and Horse's Toes: Further Reflections in Natural History" by legendary science writer Stephen Jay Gould. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.