The diagram depicts the evolutionary process of the equine species over a 40-million-year geologic time span, showcasing the transformation from the diminutive Eohippus to the contemporary horse.
From the overall perspective, it is apparent that the lineage from Eohippus to the present-day horse demonstrates a gradual simplification in the structure of the equine foot.
Approximately 40 million years ago, Eohippus, characterized by a short head and neck, developed five toes, four of which bore small hooves, while the fifth remained elevated. Dating back to 30 million years ago, Mesohippus emerged as an intermediary species, exhibiting a larger and more slender physique, and ambulating on 3 toes.
By 15 million years ago, Mesohippus had transformed into Merychippus, marking a significant milestone in equine evolution. While still retaining the primitive trait of three toes, Merychippus resembled a modern horse, featuring an elongated face, a newly-formed mane, and a larger body. Ultimately, the contemporary horse, derived from Merychippus, now possesses a single hoof, slim legs, an elongated head, a relatively extended neck, and a lengthy tail.