Figure 1.

First peopling of the American continent. Settlement is thought to have occurred from Eastern Siberia through several waves of migration (arrows) across a land bridge connecting Siberia to Alaska, existing at the time. Crossing was impossible during the last glacial maximum (LGM) (∼20,000 years ago) because of glaciers covering a large part of North America. Most genetic studies of contemporary Native Americans point to a settlement of the continents soon after the LGM, subsequent to the retreat of the ice sheets. Although classical studies associated initial settlement with the Clovis archaeological complex of North America (∼13,000 years ago), older sites have been identified, including Monte Verde in South America (dated at ∼15,000 years ago). The Native American populations placed on this map are those included in the phylogenetic tree shown in Figure 3. Analysis of genetic data from these populations is consistent with the important role of the coast during the initial settlement of the continent (Reich et al. 2012).