How horses—whose ancestors were dog-sized animals with three or four toes—ended up with a single hoof has long been a matter of debate among scientists. Now, a new study suggests that as horses became larger, one big toe provided more resistance to bone stress than many smaller toes. To trace the evolution of the horse toe, researchers first examined 13 fossilized horse leg bones, from those of the 50-million-year-old, dog-sized Hyracotherium (which had three toes on its hind feet and four on its forefeet) to those of modern horses. They measured features like bone length and area using 3D scanning, which revealed the bones’ resistance to stresses such as squeezing or bending. The team then estimated the body weight of each of the horses and calculated how much stress their leg bones would have been subjected to when trotting or jumping. As their body mass increased, horses’ center toes got bigger and more resistant to stress, whereas their side toes shrank and eventually disappeared, the researchers report today in the Proceedings of the Royal Society B. Living horses have only one toe, but if you look closely, you might be able to spot minuscule vestigial ones just above their hooves.