Genomic Signals of Pleistocene Beringian Refugia and Dispersals

Traditionally, indigenous American populations have been viewed as descendants of a small subset of the Eurasian population that migrated to the Western Hemisphere via the Bering Land Bridge. Mitochondrial genetic diversity suggests an isolated resident population on Beringia as the source population for Southward dispersal following the Last Glacial Maximum (LGM). This Beringian refugium model parallels other global human populations that retreated to refugia during the LGM, and is supported by paleoecological evidence of comparatively mild climates and a rich biota in south-central Beringia throughout the LGM. Recent estimates of the timing of Native American dispersal from Beringia based on modern genomic data have been consistent with an archaeological record supporting a late-LGM divergence and subsequent migration south into the continent. A reasonable question is how constrained are lineage/genomic diversification and prehistoric population size estimates as a result of restricted sampling? Ancient genomic research demonstrates increased sequence diversity in prehistoric populations. Modern genomic data from North America come from a small geographic area and few populations, suggesting we have yet to adequately characterize the full extent of genomic diversity in the Americas. Moreover, some modern genomes routinely used for comparative analyses are known to be from populations with reduced genetic diversity due to historical bottlenecks. All of these factors likely contribute to depressed estimates of divergence times as well as source population sizes. Recent archaeological reports of much earlier occupations of the Americas predate habitable migration corridors that appear ~17kya along the Pacific coast and 13kya into the interior of North America, raising questions about the correspondence of the archaeological and genetic records. If estimates of ancient genomic diversity continue to increase with denser sampling, we may expect to obtain earlier lineage origin and diversification estimates, making LGM archaeological dates from throughout the hemisphere less controversial. A similar effect on population size estimates also seems probable, leading to a re-evaluation of assumptions about founding population size. The merging of the increasingly rich and robust genomic, archaeological, and paleoecological records is proving to be challenging in elucidating the origin of the earliest Americans in both time and space.