The Evolution of a Horse

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The Evolution of a Horse
Throughout history, humans have depended on the horse. Horses have contributed to the growth of humankind as transportation, farm 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 to answer these questions.
A mammal with hooves that have an odd number of toes on the rear feet are known as odd-toed ungulates. The middle toe on each rear hoof is usually larger than the ones next to them. Horses are members of the odd-toed ungulates which includes the horses, tapirs, and rhinoceroses. They are relatively large grazing animals with only one stomach. They digest plant materials in their intestines rather than in their stomachs as the even-toed ungulates do. (Wikipedia, 1) By the beginning of the Eocene period some fifty-five million years ago, these mammals had spread to occupy many different continents. Horses and Tapirs evolved first in North America. However, the horse’s evolution did not follow a straight path. Many varieties of equids often flourished at about the same time. Also different traits evolved at different times, and even reversed (Hunt, 2). “Paleontologist Michael Voorhies… characterizes the evolution of horses as more like a bush than a tree, with starts and stops and jumps in the development of genetic traits” (“Wild Horses: Equus Family Tree” 1). This statement is demonstrated in Chart 1 taken 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 the...

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... Equus Family Tree”). As the earth changed, so did horses’ adaptations. Horses' teeth changed as they switched grazing only grasses. They developed Hypsodont teeth which could grow out of the gum continuously as the crown was worn down. The height of the tooth crown also gradually increased, as did the hardness of the teeth. Simultaneously, other changes occurred. These horses became larger in body size, leg length, and length of the face. The bones of the legs fused together sacrificing flexible leg rotations to make efficient backward and forward motion. At this time the horse also began to stand permanently on tiptoe. Instead of pads they developed specialized ligaments that supported their weight and promoted running. Several species developed during this time.
Among the species that developed were the kalobatippus, parahippus, and the merychippus.
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