The first known members of the genus "Mammuthus" are the African species "Mammuthus subplanifrons" from the Pliocene and "Mammuthus 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 known type has been named "M. rumanus", which spread across Europe and China. Only its molars are known, which show it had 8–10 enamel ridges. A population evolved 12–14 ridges and split off from and replaced the earlier type, becoming "M. meridionalis". In turn, this species was replaced by the steppe mammoth, "M. trogontherii", with 18–20 ridges, which evolved in East Asia ca. 1.8 million years ago. Steppe mammoth populations replaced "M. meridionalis" in Europe between 1 and 0.7 million years ago. The Columbian mammoth, "M. columbi", evolved from a population of "M. trogontherii" that had entered North America over 1 million years ago. Mammoths derived from "M. trogontherii" evolved molars with 26 ridges between 800,000 and 400,000 years ago in Siberia, becoming the woolly mammoth, "M. primigenius". The woolly mammoth would replace the steppe mammoth in Europe during the late Middle Pleistocene around 200,000 years ago. A 2011 genetic study showed that two examined specimens of the Columbian mammoth were grouped within a subclade of woolly mammoths. This suggests that the two populations interbred and produced fertile offspring. It also suggested that a North American form known as ""M. jeffersonii"" may be a hybrid between the two species.