The diagram demonstrates the evolutionary development of the horse from its earliest species over a 40-million-year period.
Overall, the development consists of four main stages characterized by an increase in body size, the gradual decline in the number of toes, and the growth of features such as tails.
40 million years ago, Eohippus appeared with a small physical appearance and short legs, indicating limited speed and function. Noticeably, its leg included four toes, with three touching the ground and one located higher. By thirty million years ago, Mesohippus had a more structured body accompanied by a longer tail. Its number of toes had reduced to only three. However, all the toes grew in size, with the center one being more dominant.
15 million years ago, Merychippus possessed a significantly developed body. To be more specific, its body and legs had become more muscular and contemporary horse-like. Turning to its legs, despite remaining the same number of toes, the middle one had evolved to be the most functional, whereas the two remaining ones reduced in size. Finally, the horse has now been completely developed, marked by body growth and the elongation of legs. It now has long, splendid legs, enabling it to fully function. The former toes have entirely disappeared, forming a single hoof, highlighting the culmination of the natural evolution process.