The first known members of the genus "Mammuthus" are the African species "M. subplanifrons" from the Pliocene, and "M. africanavus" from the Pleistocene. The former is thought to be the ancestor of later forms. Mammoths entered Europe around 3 million years ago. The earliest European mammoth has been named "M. rumanus"; it spread across Europe and China. Only its molars are known, which show that it had 8–10 enamel ridges. A population evolved 12–14 ridges, splitting off from and replacing the earlier type, becoming "M. meridionalis" about 2.0–1.7 million years ago. In turn, this species was replaced by the steppe mammoth ("M. trogontherii") with 18–20 ridges, which evolved in eastern Asia around 2.0–1.5 million years ago. The Columbian mammoth evolved from a population of "M. trogontherii" that had crossed the Bering Strait and entered North America about 1.5 million years ago; it retained a similar number of molar ridges. Mammoths derived from "M. trogontherii" evolved molars with 26 ridges 400,000 years ago in Siberia and became the woolly mammoth ("M. primigenius"). Woolly mammoths entered North America about 100,000 years ago.