The Evolution of a Horse Throughout history, humans have depended on the horse. Horses have contributed to the growth of humanity as transportation, agricultural workers, and battle steeds. They have been trained to support humans in many ways. When did this relationship begin? At what point did the horse become a vital part of human society? Exploring the evolution of the horse can help answer these questions. A mammal with hooves that have an odd number of toes on the hind feet is known as an odd-toed ungulate. The middle toe on each rear hoof is usually larger than adjacent ones. Horses are members of the strange-toed ungulates that include horses, tapirs, and rhinoceroses. They are relatively large grazing animals with only one stomach. They digest plant materials in their intestines rather than in the stomach as even-toed ungulates do. (Wikipedia, 1) By the beginning of the Eocene period, about fifty-five million years ago, these mammals had spread to occupy many different continents. Horses and tapirs evolved first in North America. However, the evolution of the horse did not follow a straight path. Many varieties of equids often flourished at about the same time. Different traits also evolved at different times and even reversed (Hunt, 2). “Paleontologist Michael Voorhies…characterizes horse evolution as more bush-like than tree-like, with starts, stops, and jumps in the development of genetic traits” (“Wild Horses: Equus Family Tree” 1). This statement is demonstrated in Chart 1 from “Wild Horses: Equus Family Tree.” Between thirty-four and fifty-five million years ago, horses began their history on the North American continent. This occurred in the early to mid-Eocene epoch of… middle of paper… Equus family tree”). As the land changed, so did the adaptations of horses. The horses' teeth changed as they transitioned to grazing only grass. They developed Hypsodont teeth that could continuously grow out of the gum as the crown wore away. The height of the dental crown also gradually increased, as did the hardness of the teeth. Other changes occurred at the same time. These horses became larger in body size, leg length, and face length. The leg bones fused together sacrificing the flexible rotations of the legs to make forward and backward movement efficient. During this period the horse also began to stand permanently on tiptoe. Instead of pads, they developed specialized ligaments that supported their weight and aided running. During this period several species developed. Among the species that developed were the kalobatippus, the parahippus and the merychippus..