Final answer:
The common ancestor of modern horses and extinct equines dates back to about 55 million years ago with the genus Eohippus. Evolution progressed with changes in climate and habitats, resulting in anatomical adaptations for grazing and predator evasion. Today, Equus is the only genus remaining from this diverse lineage.
Step-by-step explanation:
Evolution of the Horse and its Common Ancestor
The common ancestor of the modern day horse and all the extinct, fossilized species mentioned in the diagram dates back to approximately 55 million years ago. This ancestor gave rise to the first horse-like species in the genus Eohippus. Over millions of years, these early equines evolved—an example being species of Mesohippus—as seen in the fossil record tracking anatomical changes like the transformation of the leg bones from four digits to the single hoof of today's horses and the changing shape of teeth. These adaptations were in response to a gradual drying climate and the shift from forest to prairie habitats, which selected for traits suitable for grazing and escaping predators. The oldest horse fossils depict a small animal with four toes adapted to a wooded marshland habitat. As environments changed, there was a diversification in equines, but now only one genus, Equus, remains, comprising a few species including the modern horse.