Modern horses, like all known four-limbed, land-based vertebrates, descended from one common ancestor that had legs with five toes.

Some land-based vertebrates eventually dropped a few of those digits, but only one living group of animals evolved to have just a single toe (or hoof) per foot – the group that includes modern horses.

How and why horses dropped nearly all their toes has remained a mystery, until now.

A paper published in the Proceedings of the Royal Society B revealed that as horses began to increase in size, the animals’ center toes also grew to accommodate the extra mass, while the side toes shrank.

Earlier horses were small – about the size of domestic dogs – and originally lived in forested areas of North America. They had four toes on their front legs, and three on the back legs.

But about 20 million years ago, the horses’ habitat began to shift from the forest to grasslands. This environmental change required horses to move faster through the grasslands to avoid predators, as well as the necessary ability to cover more ground for grazing. 

To adapt, horses grew larger in size. At the same time, their outside toes began to shrink, until eventually they disappeared. Researchers believe it was more efficient for the animal to have one robust toe to support their growing mass and pressure, than several smaller ones. Downsizing to just one toe also allowed horses to move easier and run faster.

The researchers reached these conclusions by micro-CT scanning leg fossils from 12 different kinds of extinct horses, ranging from ones that lived 55 million years ago, to species in the same group as modern horses.

The CT scans provided detailed 3-D images that allowed the research team to measure features such as bone length and area of the leg fossils. This information showed the bones’ resistance to stresses and bending.

The team then used an engineered “beam bending” analysis to estimate how much stress each species’ leg bones would have experienced during both daily, regular movements, as well as high-speed running.

The stress data was compared to the actual fracture stress of each bone.

The researchers determined that side toes shrank as a result of increased body mass, and the middle digit compensated to allow the animal to better resist stress and bending.

Now, horses can reach speeds of up to 25 mph.

Horses likely fully evolved into monodactyls about 5 million years ago, but the process toward eliminating foot digits began much earlier. Therefore, the theory that human domestication may have altered horse evolution can be ruled out, according to the researchers.