The picture illustrates the evolution of the horse over a period of 40 million years.
Overall, the transformation process of the horse consists of three main stages, starting with the Eohippus and ending with the modern horse. Notably, the foot structure has undergone significant modifications, from spread-out toes to a single hoof.
Regarding changes to the body, 40 million years ago, the Eohippus had some primitive features such as a hairless back and an undeveloped small body. After 10 million years, it transformed into the Mesohippus, whose body had become bigger with a longer face, back, limbs, tails. It is also noticeable that it had somehow developed some form of a mane. These attributes continued to be improved further in the next 15 million years when the animal evolved into a new type of species, called Merychippus. Finally, the physical appearance of the modern horse has experienced some small changes when its body parts become more evolutionarily advanced. Specifically, both the mane and the hind legs are bigger, while the face, the back, and the tail become more defined.
Considering changes around the foot structure, the Eohippus, found 40 million years ago, had small and short limbs, which were separated into four smaller jointed toes. These toes later on, had been reduced to three in the middle evolution stages, which were called Mesohippus and Merychippus. In addition, the tip of the biggest toe bone in the middle started to show some growth in size, creating a symmetrical structure with the middle toe protruding to the front. The final form of a horse’s foot is when its toes being replaced by a single hoof.