Animals with hooves carry all their weight on the middle toe of the foot. This was not always the case, however. The evolution of horses shows how they changed from five-toed creatures to single-hoofed animals. Early relatives of the horse were about 30 cm tall and lightly built for fast running. There were four toes on the hind legs and three on the front legs; each toe had a small hoof. As horses grew larger, the number of toes gradually reduced until Merychippus appeared some 23 million years ago. It had several toes but carried all its weight on the large central toe, which had a hoof. Merychippus stood about 1 m high at the shoulder, and looked like a small horse. The development of the horse from its primitive ancestors is one of the best examples of evolution, because unlike many other animals, the development of the modern horse and its specialized hoofs can be seen in detail in the fossil record.
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