today, leaving modern historians with glimpses of ancient culture and knowledge. Settlement The first inhabitants migrated into the Americas from Asia. Habitation sites are known in Alaska and Yukon from at least 20,000 years ago, with suggested ages of up to 40,000 years. Beyond that, the specifics of the Paleo-Indian migration to and throughout the Americas, including the dates and routes traveled, are subject to ongoing research and discussion. Widespread habitation of the Americas occurred during the late glacial maximum, from 16,000 to 13,000 years ago. The traditional theory has been that these early migrants moved into the Beringia land bridge between eastern Siberia and present-day Alaska around 40,000–17,000 years ago, when sea levels were significantly lowered during the Quaternary glaciation. These people are believed to have followed herds of now-extinct pleistocene megafauna along ice-free corridors that stretched between the Laurentide and Cordilleran ice sheets. Another route proposed is that, either on foot or using primitive boats, they migrated down the Pacific coast to South America. Evidence of the latter would since have been covered by a sea level rise of hundreds of meters following the last ice age. Both routes may have been taken, although the genetic evidences suggests a single founding population. The micro-satellite diversity and distributions specific to South American Indigenous people indicates that certain populations have been isolated since the initial colonization of the region. A second migration occurred after the initial peopling of the Americas; Na Dene speakers found predominantly in North American groups at varying genetic rates with the highest frequency found among the Athabaskans at 42% derive from this second wave. Linguists and biologists have reached a similar conclusion based on analysis of Amerindian language groups and ABO blood group system distributions. Then the people of the Arctic small tool tradition, a broad cultural entity that developed along the Alaska Peninsula, around Bristol Bay, and on the eastern shores of the Bering Strait moved into North America. The Arctic small tool tradition, a Paleo-Eskimo culture branched off into two cultural variants, including the Pre-Dorset, and the Independence traditions of Greenland. The descendants of the Pre-Dorset cultural group, the Dorset culture was displaced by the final migrants from the Bering sea coast line, the Thule people (the ancestors of modern Inuit), by 1000 Common Era (CE). Norse colonization Around the same time as the Inuit migrated into Greenland, Viking settlers began arriving in Greenland