"A schematic representation of the placement of the original populations and their subsequent movements and combinations.
The solid arrow labeled 1 represents the Middle Pleistocene movement across the northern edge of the Old World.
Arrow 3 shows the terminal Pleistocene initial entrance into the New World, and the arrows labeled 4 show post-Pleistocene expansions made possible by the utilization of new food resources related in part to the development of agriculture.
Arrows 1 and 2 represent single population expansions into unoccupied land.
Arrow 3 represents movement into unoccupied land but by a population with both European and Asian roots.
Arrow 4 represents a technology-based expansion of different populations into areas that, with the exception of Oceania, were already occupied. The consequence was a much greater rate of genetic exchange than had been true for any of the earlier movements."