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The evolution of the horse is always regarded as the best evidence of the evolution by a lot of evolution theorists, to explain how the evolution takes place.

During 1870s, O.C. Marsh drew the evolution diagram of horse for T.H. Huxley, who was a strong supporter of Charles Darwin. Marsh drew a ladder type evolution diagram for a series of horse fossils. The Eohippus appeared before 58 million years ago was evolved into the Oligohippus 36 million years ago, which was evolved into the Merychippus 25 million years ago, and was evolved into the Pliohippus 13 million years ago. Finally, the modern horse appeared 2 million years ago. This story presents the perfect evolution result. In order to survive, the horse was evolved into single toe, tall and big body, and the teeth which were apt to chew the weeds. Soon, this evolution story was adopted by the American Museum of Natural History, and also included in the contents of the textbook at the same time.

When more new fossils are excavated successively, this evolution story is considered too simple apparently. It misleads general people to think that the evolution of the horse is the most perfect evidence of the evolution, and the evolution route is purposeful, directional, and predicable.

Fig1：Evolution of the Horse

Bruce Macfadden redrew the evolution diagram of horse in 1988. The appearance of the Hyracotherium from 55 million years ago to the modern horse of Equus demonstrated the evolution tree. The modern horse is only one of the evolution branches. It is emphasized that the evolution of horse is very fast. The traditional statement for the evolution of horse with bigger body and single toe was also refuted. The modern horse is the only remaining offspring of the Hyracotherium at present, which belongs to the Equus. There are only eight kinds of horse at present: Three

kinds of zebras, four kinds of donkeys and modern horse. The evolution tree is specified as follows:

Eocene: 54 million years - 34 million years ago The Hyracotherium was appeared 55 million years ago in the American continent. Its body size looked like dog. The forelimb had 4 toes and the hind leg had 3 toes. The Orohippus was appeared 50 million years ago. The body size was similar to the Hyracotherium, but the degraded first toe and second toe were disappeared. The molar crown was protruding obviously, which was suitable for chewing and eating the hard grass. The Epihippus was appeared 47 million years ago. The body size was still similar to the Hyracotherium, and the molar crown continued evolving.

Oligocene: 34 million years - 24 million years ago The Mesohippus was appeared suddenly about 40 million years later than the Eocene. The body size was slightly bigger than the Epihippus. The head was stretched longer. The nose kiss was protruded. The neck was slightly longer. It had the cheek nest (a small concave on the skull). The forelimb and the hind leg had 3 toes, respectively. The fourth toe of forelimb was degraded into a small nub. The Miohippus was appeared 36 million years ago, which was the close relative of the Mesohippus. The body size was bigger than the Mesohippus significantly. The skull was longer. The cheek nest was deeper and wider. There was new protrusion on the molar crown obviously. The Mesohippus coexisted with the Miohippus for nearly 4 million years. The Mesohippus was disappeared at the middle of Oligocene gradually.

Miocene: 24 million years - 5.3 million years ago The Miohippus was evolved into 3 big categories quickly. The first one was the Anchitherium, Hypohippus and Megahippus with 3 toes. Their body structure was close to the Miohippus. They existed nearly about 10 million years. The second one was the Archeohippus. It did not exist long. The third one was the Kalobatippus, which features between the Miohippus and Parahippus. The evolution of its teeth and legs was different the aforesaid fossil horse significantly. The crown became taller apparently, which was suitable for chewing and eating the hard grasses. The body size became bigger, and the foot became longer. It stood by the tip of toe, which was suitable for running. The face became longer. The Parahippus was appeared 23 million years ago. The body size was bigger than the Miohippus slightly. It had 3 toes, the crown was relatively high, and the crown was protruded and relatively complicated obviously. The Merychippus was appeared 17 million years ago. Its

structure was slightly similar to the modern horse. It still had 3 toes, but the size of side toe was varied for different individual. Some ones had complete 3 toes, and some ones had smaller and suspended side toe. At the later period of Miocene, the Merychippus was evolved into 3 main categories. The first one was the Hipparion with 3 toes. This category was evolved into 4 kinds of genus. The common feature was the large cheek nest. The fossil evidence shows that genus moved to the Eurasia. The second one was small-scale Protohippus and Calippus. The third one was commonly called the real horse, which had smaller side toe.

The above-mentioned real horse included the Pliohippus, which was used to be considered as the direct ancestor of modern horse. It was appeared 15 million years ago and had deep cheek nest and bent tooth shape, which was different from the modern horse. The Astrochippus was appeared 10 million years ago, which had single toe and deep cheek nest. It is predicted to be evolved from the Pliohippus. The Dinohippus was appeared 12 million years ago. It had single toe. Its direct ancestor was not clear. The body size, tooth shape, and head shape looked like the modern horse very much, and the cheek nest shrank obviously.

Pliocene and Pleistocene: 5 million years - 10,000 years ago The Equus was appeared 4 million years ago. The height of mature horse was similar to newly born small horse. It had single toe. But it still had the gene of multiple toes. The degraded 2nd toe and 4th toe would be appeared sometimes. It did not have obvious cheek nest. Its body shape looked like the zebra, and the head shape looked like the donkey. It was evolved into 4 big categories quickly. These horses existed in the America with single-toe Astrochippus, small-scale Protohippus, and 3-toe Hipparion at the same time, until the first glacier period at the end of Pleistocene. Some of Equus migrated to the Africa, which was evolved into the zebra. The donkey was evolved in the Middle East and North African. The modern horse was evolved in the Europe and Asia. The other horse genus was disappeared from the geological history.

The evolution of Hyracotherium to Equus was a long process with big variance. Many branches were evolved from a single category. But finally only the Equus is remained till now.

Reviewed by：Zhang, Yong-Ta、Liu, Hsin professor