It is believed that the peopling of the Americas began when Paleolithic hunter-gatherers (Paleo-Indians) entered North America from the North Asian Mammoth steppe via the Beringia land bridge, which had formed between northeastern Siberia and western Alaska due to the lowering of sea level during the Last Glacial Maximum. These populations expanded south of the Laurentide Ice Sheet, either by sea or land, and spread rapidly southward, occupying both North and South America no later than 14,000 years ago, and possibly before 20,000 years ago. The earliest populations in the Americas, before roughly 10,000 years ago, are known as Paleo-Indians. Indigenous peoples of the Americas have been linked to Siberian populations by the distribution of blood types, and in genetic composition as reflected by molecular data, such as DNA.