The given diagram illustrates the evolution of horses over 40 million years.
Overall, the early species of the horse evolved through four development stages: Eohippus, Mesohippus, Merychippus, and modern horse, along with a brief description of their feet structure and external appearance.
In terms of the order of species, the Eohippus is believed to have existed first, about 40 million years ago.Their appearance was not yet that of a modern horse. They had a relatively small body and lacked a mane, with a toe bone structure consisting of four small bones. Over the next 25 million years, they evolved into mesohippus, which had a larger body, a short mane, and leg bones that had reduced to three largest toe bones.In terms of the third stage of the evolutionary process, Merrychippus appeared about 15 million years ago. Notably, this species was quite similar to modern horses in terms of body size and external appearance. However, it was still a three-toed species with the middle toe larger than the other two. This lineage led to the modern horse species, which have characteristics known today along with leg bones that fully developed into solid structures fused with hooves.In conclusion, after 40 million years, horse species have undergone significant changes to adapt to transformations in their living environment.