The given picture depicts the transformation of the horse, especially emphasizing on the foot structure, throughout a 40-million-year period.
Overall, the evolution of the horse has four main stages with different names, through which the horse grows in size, whereas the foot structure gets simplified.
Examining Eohippus and Mesohippus, the two first stages of the transformation, the evolutionary changes include getting bigger and reduction in foot digits. 40 million years ago, the ancestor of the modern horse named Eohippus, which had a dog-like size, resembled common features of the horse. Its foot had four fully developed digit and was relatively small compared to the later type, Mesohippus, whose size was as big as a modern cheetah with three foot digits. The phalanges of the horse existed 30 million years ago were bigger and longer than Eohippus’, among the three digits, the middle one outgrew the other two in size, enabling the running ability of the species.
Regarding Merychippus and modern horse, there was a great change in the appearance and foot structure of the horse. Merychippus was as big as a modern deer with its foot digits grew bigger compared to the Mesohippus’. This third stage of transformation resembled bigger and shorter toe nails, yet it appeared to be tougher and more relevant to modern horse. Ultimately, horse in modern time is significantly bigger compared to its previous counterparts, with muscular appearance. Additionally, the phalanges of modern horse have reduced in the number of digits, yet have more joints.