All four-limbed, land-based vertebrates came from a common ancestor with legs that ended in five toes. Over time, many animals lost some of their digits: Hippos, rhinos and camels have four, three and two toes on each leg. But only one living group of animals ended up with a single toe per foot: the group containing modern horses.

A comprehensive new study, published last week in Proceedings of the Royal Society B, lends support to existing hypotheses about the dramatic transformation in horses’ hooves through history. Namely, as horses evolved and got larger from their ancestral, dog-sized form, it was better to have one very robust toe than several smaller ones to support their increased body mass. Furthermore, having just one toe reduced the weight horses had to carry at the end of each leg, making it easier for them to run and maneuver.

The study is a careful examination of “a story everyone had taken for granted and hadn’t really tested thoroughly,” said Christine Janis, a professor emerita of evolutionary biology at Brown University who was not involved in the research.

As told in textbooks and evolutionary biology classes, the earliest horses were small, dwelled in forests and had four toes on their front legs and three on their back legs. Then, more than 20 million years ago, their habitat in North America started to shift from forest to grassland. In these new grasslands, ancient horses needed to move at faster speeds to evade predators and cover more ground for grazing. It made sense that a larger body and longer, more slender legs with fewer toes would help horses achieve that.