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Toes
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The Eohippus had three padded toes on each foot and a little hoof at the end of each toe. This allowed the early horse to pad its way through the lush and moist forest underbrush where it fed on various plants and small animals. As the climate changed, the forest habitat was reduced and the Eohippus was forced into grassland habitat. Over millions of years of evolution, the three toes developed into a single hoof, with remnants of the padded toes finding a place higher up on the horse's leg as vestigial bumps. Hooves were strong and durable for pounding on the baked earth of the grasslands.
Teeth
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The shift in habitat also meant a new diet of grasses. The "dawn horse" originally had teeth that were more like those sported by omnivores. During its shift to the prairies, the early horse became a herbivore. Its short molars grew much longer so that grasses could be cut and ground before digestion.
Facial Structure
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The facial structure of the Eohippus changed in response to the development of longer teeth for the herbivore diet. The facial structure elongated to house the teeth and grinding of the rough grasses. The digestive system of a horse is unique to other mammals in that it processes simple carbohydrates with enzymes in the fore gut and other sources of fiber are processed by bacteria in the hind gut. The reliance on plants for food means that the horse is constantly grazing. The development of the longer facial structure was required.
Leg Length
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As the length of the legs grew, there was a lengthening of the neck. This allowed the animal to continue to graze.
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Why Did Horses Evolve?
The Eohippus, or "dawn horse," was the original form of the modern horse. Its origins can be traced back to 50 million years ago. The evolution to the modern day horse was not a straight line. There were many branches of the species in various areas of the world. Despite the scattered origins, it is clear that the Eohippus evolved in certain ways to adapt to changes to its environment.