Researchers found that, particularly in the higher speeds or jumping, most of the fossil bones would have endured significantly higher bone stress than modern horses if the side toes didn’t carry some of the weight. | Photo: iStock

Our horses’ ancestors had four toes in front and three in back. Over the generations, they dropped to the modern-day single toe that ends in the hoof. We’ve known this for a while. But it’s only recently that scientists have finally understood why.

A single-toe format “outweighed the costs” of multiple-toe formats as horses gained body mass and grew longer legs, said Brianna K. McHorse, a PhD candidate in the Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology at Harvard University, in Cambridge, Massachusetts.

“There was some selective pressure, or set of pressures, that conspired to push (some species of) horses in the direction of having a single toe,” she said