According to these results, it appears the genus "Equus" evolved from a "Dinohippus"-like ancestor ~4–7 mya. It rapidly spread into the Old World and there diversified into the various species of asses and zebras. A North American lineage of the subgenus "E. (Equus)" evolved into the New World stilt-legged horse (NWSLH). Subsequently, populations of this species entered South America as part of the Great American Interchange shortly after the formation of the Isthmus of Panama, and evolved into the form currently referred to as "Hippidion" ~2.5 million years ago. "Hippidion" is thus only distantly related to the morphologically similar "Pliohippus", which presumably became extinct during the Miocene. Both the NWSLH and "Hippidium" show adaptations to dry, barren ground, whereas the shortened legs of "Hippidion" may have been a response to sloped terrain. In contrast, the geographic origin of the closely related modern "E. ferus" is not resolved. However, genetic results on extant and fossil material of Pleistocene age indicate two clades, potentially subspecies, one of which had a holarctic distribution spanning from Europe through Asia and across North America and would become the founding stock of the modern domesticated horse. The other population appears to have been restricted to North America. However, one or more North American populations of "E. ferus" entered South America ~1.0–1.5 million years ago, leading to the forms currently known as "E. (Amerhippus)", which represent an extinct geographic variant or race of "E. ferus".