The diagram illustrates the evolutionary development of the horse over a period of 40 million years, with particular emphasis on changes in foot structure.
Initially, around 40 million years ago, the earliest ancestor of the horse, known as the Eohippus, was relatively small and possessed a foot with four distinct toes. As evolution progressed, the Mesohippus, which lived 10 million years later, displayed a more developed body and a foot structure reduced to three toes, with the central toe becoming more pronounced.
By 15 million years ago, the Merychippus emerged, exhibiting a significant increase in size and a further adaptation in the foot. Although it still retained three toes, the middle one had become more dominant and supported most of the animal’s weight. Eventually, the modern horse (Equus) evolved into a much larger and more robust animal with a single-toed hoof, ideally suited for running on hard ground.
Overall, the diagram clearly shows a gradual transformation in both the size and foot structure of the horse, evolving from a small, multi-toed creature into a large, single-hoofed animal—an adaptation that enhanced speed and mobility in open environments.