Early human dispersals within the Americas

INTRODUCTION Genetic studies of the Pleistocene peopling of the Americas have focused on the timing and number of migrations from Siberia into North America. They show that ancestral Native Americans (NAs) diverged from Siberians and East Asians ~23,000 years (~23 ka) ago and that a split within that ancestral lineage between later NAs and Ancient Beringians (ABs) occurred ~21 ka ago. Subsequently, NAs diverged into northern NA (NNA) and southern NA (SNA) branches ~15.5 ka ago, a split inferred to have taken place south of eastern Beringia (present-day Alaska and western Yukon Territory). RATIONALE Claims of migrations into the Americas by people related to Australasians or by bearers of a distinctive cranial morphology (“Paleoamericans”) before the divergence of NAs from Siberians and East Asians have created controversy. Likewise, the speed by which the Americas were populated; the number of basal divergences; and the degrees of isolation, admixture, and continuity in different regions are poorly understood. To address these matters, we sequenced 15 ancient human genomes recovered from sites spanning from Alaska to Patagonia; six are ≥10 ka old (up to ~18× coverage). RESULTS All genomes are most closely related to NAs, including those of two morphologically distinct Paleoamericans and an AB individual. However, we also found that the previous model is just a rough outline of the peopling process: NA dispersal gave rise to more complex serial splitting and early population structure—including that of a population that diverged before the NNA-SNA split—as well as admixture with an earlier unsampled population, which is neither AB nor NNA or SNA. Once in the Americas, SNAs spread widely and rapidly, as evidenced by genetic similarity, despite differences in material cultural, between >10-ka-old genomes from North and South America. Soon after arrival in South America, groups diverged along multiple geographic paths, and before 10.4 ka ago, these groups admixed with a population that harbored Australasian ancestry, which may have been widespread among early South Americans. Later, Mesoamerican-related population(s) expanded north and south, possibly marking the movement of relatively small groups that did not necessarily swamp local populations genetically or culturally. CONCLUSION NAs radiated rapidly and gave rise to multiple groups, some visible in the genetic record only as unsampled populations. At different times these groups expanded to different portions of the continent, though not as extensively as in the initial peopling. That the early population spread widely and rapidly suggests that their access to large portions of the hemisphere was essentially unrestricted, yet there are genomic and archaeological hints of an earlier human presence. How these early groups are related or structured, particularly those with Australasian ancestry, remains unknown. Rapid expansion, compounded by the attenuating effect of distance and, in places, by geographic and social barriers, gave rise to complex population histories. These include strong population structure in the Pacific Northwest; isolation in the North American Great Basin, followed by long-term genetic continuity and ultimately an episode of admixture predating ~0.7 ka ago; and multiple independent, geographically uneven migrations into South America. One such migration provides clues of Late Pleistocene Australasian ancestry in South America, whereas another represents a Mesoamerican-related expansion; both contributed to present-day South American ancestry.