First horses (Eohippus)

The first horses lived about 55 million years ago in the forests of North America and Europe. Fossils named Hyracotherium by Richard Owen in 1841 and Eohippus (dawn horse) by Othniel Marsh in 1876 are now often grouped under names such as Sifrhippus and Eohippus. They were small, ranging from the size of a fox to a medium dog, with an arched back and four toes on the front feet and three on the hind feet, ending in small hoofs. They browsed on soft leaves and fruit.¹

They belong to the odd-toed hoofed mammals, a group that also includes rhinos and tapirs, which diversified after the Cretaceous extinction. Early Eocene forests were warm and wet, and small browsers prospered. A study by Philip Gingerich and colleagues showed that the earliest horse, Sifrhippus, shrank by about 30 percent during a short warm spell and then grew again as the climate cooled, a clear case of body size tracking temperature.¹²

Horses were then a modest group, living in woods. Over the next 50 million years they changed greatly, as grasslands spread and teeth and legs adapted. Fossil horses are some of the best-documented in the record, and Marsh's collections in the American West provided evidence for evolution at a time when Darwin's ideas were new. Thomas Huxley, visiting Marsh in 1876, used the sequence in lectures in New York.