Here's something that will never let you look at horses the same way again: a horse hoof is one giant toe. Horses evolved from animals with multiple toes, but eventually lost the extras over millions of years of evolution. But why? Research using 3D fossil scanning may have the answer.
All The Pretty Little Horses
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 it 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 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.
This Little Piggy
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 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.
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.