The diagram shows the evolution of the horse with particular emphasis on the changing of foot structure

The diagram indicates the evolution of the horse, a mammal of the family Equidae, evolved from an Eohippus to the modern horse with reference to the changing of the foot structure. As can be seen from the diagram, changes in appearance and the internal bone structure were noticed in a 40-million-year period. Initially, the original version of the horse appeared 40 million years ago, called Eohippus, and can be recognized by its dog size and walked on four-toed feet. Eventually, in the next 10 million years, its size started to enlarge, the development of the mane was witnessed. During this stage, the fourth toe was completely disappeared, and the middle one became bigger. Finally, it evolved to Mesohippus. In the next 15-million-year period, the ancestor of modern Equus, Merychippus, continued to increase its size with the growth of thigh and the mane are more obvious; besides that, the reconstruction of bone shows that it had one more knuckle on each toe and the basic digital reduced while the middle became bigger, resembling into hooves. Over time, it had transformed into the horse we know today, with long finely mane, thick thighs, developed elbows, and ankle joints.