The story of the horse

Eohippus
The development of the equine species can be traced back to the Eohippus,a small animal of approx. 35 cm (14 in) which existed 60 millions years ago on the american continent. It was equipped to survive in a secondary jungletype environment as a browsing animal. The Eohippus had 4 toes on the forefeet and 3 behind, all terminating in thick horn. Behind the toes was a pad like that on a dog's paw. The teeth of Eohippus were short-crowned. Experts believe that the coat would have been marked with light spots or blotches on a darker ground. This camouflage allowed the animal to blend in with forest surroundings and thus to conceal himself from predators.

Mesohippus
In the Oligocene period, 35-40 millions years ago, the Eohippus was superseded by the larger Mesohippus, which may have stood at about 45 cm (18 in). The legs were longer in proportion, and the toes on the forefeet had been reduced to 3, the central toe being the most prominent. There were the beginnings of pre-molar, incisor teeth taht were capable of a stronger, choping action.
The physical changes that occur in Mesohippus point to an environment in which the jungle conditions were gradually giving way to wooded scrub areas, probably supporting low brush and shrubs. There was also a changement in the defensive system, the emphasis shifting slowly away from concealment toward a mechanism baxed on detection and flight.Miohippus
In the upper Oligocene period 30 millions ago, Mesohippus was succeeded by Miohippus, a slightly more advanced form of Mesohippus in respect of feet and dentology. The outside toes still remain prominent and the incisor teeth are becoming more evident.

Merychippus
In the middle and upper Miocene periods, about 25-20 millions years ago, there is the Merychippus which was not yet the prototype for Equus, but a most important point in the progression . Merychippus was bigger than its predecessors: it measured up to and over 90 cm (36 in) at the shoulder and was in outline more recognizably a horse. The animal was still three-toed, but the weight was carried on the central toe. The incisor teeth, suited to grazing were now clearly defined. Moreover, the animal began to complete the defense mechanism by developing the heightened senses that remain characteristic of today's naturally high-strung horse.

Pliohippus
In the mid-Pleistocene period, about 6 millions years ago, Pliohippus emerged. This was the first single-hoofed "horse" and was the direct ancestor of Equus. It stood about 12 hh (48 in) at the shoulder. The development of the grazing teeth was complete. Pliohippus was also the source for the subgeneric group represented by zebras, domestic and wild asses, and the hemionids, the half-asses.

The primitive foundation
At the beginning of the equine evolution in the Eocene period, North America was connected by land bridges to the mainland of Europe and Asia. From North America, Equus caballus and related equidae spread outward over the existing land bridges to Asia, South America, Europe and finally Africa. These successive migrations persisted up to the end of the final Ice Age in about 9000 BC. However the receding of the ice packs removed the connecting land bridges, thus isolating the North American continent and leaving 4 related forms of Equus in the Old World, distributed as follows: horses in Europe and Western Asia; asses and zebras in the north and south of Africa respectively; and onagers in the Middle East.
The horse became extinct in America some 8000 years ago for reasons never established and was only reintroduced with the arrival of the Spanish conquistadores in the 16th century.