Out of Beringia?

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Science  28 Feb 2014:
Vol. 343, Issue 6174, pp. 979-980
DOI: 10.1126/science.1250768

Based on the distribution of tundra plants around the Bering Strait region, Eric Hultén proposed in the 1930s that the now-submerged plain between Chukotka and Alaska—the Bering land bridge—became a refugium for shrub tundra vegetation during cold periods (1), which include the last glacial maximum (LGM) between ∼28,000 and 18,000 cal BP (calibrated radiocarbon years before the present). Adjoining areas to the west and east supported drier plant communities with a higher percentage of grasses during glacial periods. According to Hultén, when warmer and wetter conditions returned to these areas, the land bridge, which he named Beringia, became a center of dispersal for tundra plants. Now it appears that it also may have been a glacial refugium and postglacial center of dispersal for the people who first settled the Americas.

Since 1960, much evidence has accumulated to support the shrub tundra refugium thesis, including data collected from the former surface of the Bering land bridge. Pollen, plant macrofossils, and insect remains from dated sediments extracted from the floor of the Bering Sea indicate a mesic tundra habitat during the LGM (2, 3). Although pollen data from islands in the Bering Sea suggest more steppic vegetation (or “steppe-tundra”), these islands represent former upland areas on the now-submerged land bridge (see the figure). Several tree species, including spruce, birch, and alder, also probably survived locally during the LGM (3, 4). Fossil insect remains from both sides of the Bering Strait suggest surprisingly mild temperatures during the coldest phases of the LGM, despite the high latitude. All of these data presumably reflect the impact of the North Pacific circulation, which brought comparatively moist and warm air to southern Beringia during the LGM (4). In fact, the latest study of Beringian vegetation indicates that grasses were less dominant in areas outside the land bridge than previously thought (5).

The shrub tundra refugium in Beringia may also have played a pivotal role in the peopling of the Americas. Genetic evidence suggests that most Native Americans are descended from a population that was isolated somewhere between northeast Asia and Alaska during the LGM (6). According to the Beringian standstill hypothesis, this population occupied northeast Asia before the LGM, became genetically isolated from its Asian source during the latter, and—following a protracted sojourn in Beringia—dispersed throughout the Western Hemisphere, when retreating glaciers opened access to coastal and interior corridors in northwestern North America (6).

A possible human refugium.

Shrub tundra is likely to have covered much of the now-submerged plain that lies between Chukotka and Alaska during the last glacial maximum (LGM), reflecting the effect of moist air from the North Pacific Ocean. Shrub tundra communities also probably extended into parts of western Alaska and Chukotka, whereas drier steppe-tundra covered much of the unglaciated landscape that lay outside central Beringia. Pollen studies indicate that some trees also probably survived in parts of central Beringia during the LGM. These conclusions are based in part on the analysis of sediment cores extracted from the sites shown on this map (2, 3). The shrub tundra zone in central Beringia represents the most plausible home for the people who were genetically isolated from their Asian parent groups during the LGM and later dispersed throughout the Americas.

Hultén's mesic tundra refugium offers not only a credible home for the Beringians but also a mechanism for their genetic isolation, because other high-latitude regions were apparently abandoned by human populations during the LGM. Wood fuel was probably the key variable in determining which regions remained occupied. People in interior treeless settings relied heavily on fresh bone, but experimental studies have shown that at least a modest quantity of wood is necessary to render bone practical as a fuel (7). The woody shrubs and occasional trees of the Beringian shrub tundra zone may have been the only substantive source of wood fuel at higher latitudes during the LGM.

The Beringian standstill hypothesis was first fully articulated in 2007 by Tamm and colleagues, who worked with a large sample of mitochondrial DNA (mtDNA) from living Native Americans. They identified a set of mutations that accumulated after the divergence of the major haplogroups (A, B, C, D, and X) from their Asian parents but before the dispersal throughout the Western Hemisphere. Tamm et al. concluded that ancestral Native Americans were isolated genetically from other populations for a least a few thousand years before the dispersal, probably in Beringia. Applying a mutation rate of 3.5 × 10−8 per year per position, they estimated that Asian and American mtDNA haplogroups had diverged more than 25,000 years ago but that the latter dispersed in the New World less than 15,000 years ago (6).

Although the Beringian standstill hypothesis is based mainly on mtDNA, analyses of nuclear (including Y chromosome) DNA provide some support for it. Similarly, an allele at a microsatellite locus on chromosome 9 in the nuclear genome (the D9S1120 locus) is found at appreciable frequency in all Native American populations examined, as well as two Northeast Asian populations, but is absent in the rest of the world (8). These observations strongly suggest an origin from a single source population for most Native American populations, consistent with the Beringian standstill hypothesis. A large survey of nuclear genetic variation concluded that initial settlement of the Western Hemisphere was followed by at least two separate and later population movements from Asia (9). Recent studies of Y-DNA in both North and South American indigenous populations found greater diversity than previously assumed among the initial population, reduced diversity as a function of distance from Beringia, and differentiation of haplogroup Q in Beringia (10, 11).

To what extent is the Beringian stand-still hypothesis supported by archaeological data and dated human remains? The analysis of ancient DNA from human skeletal remains dating to 24,000 cal BP from Mal'ta in southern Siberia appears to confirm the pre-LGM divergence of Native Americans from their Asian parent haplogroups (12). At the same time, dated archaeological and human remains indicate that settlement of the Western Hemisphere probably took place after the LGM (13). At this point, the major outstanding questions are where the standstill population was located during the LGM, and how it was isolated genetically from its Asian source.

The shrub tundra zone in central Beringia represents the most plausible home for the isolated standstill population. Although high-latitude archaeological sites of LGM age are unknown, postglacial submergence of the Bering land bridge would explain the absence of traces of people concentrated in central Beringia. On the other hand, occupation of western Beringia before the LGM is well documented (14). The post-LGM archaeological record contains two sets of remains, one of which represents a movement of people from central Siberia into Beringia about 15,000 cal BP (and may be an archaeological proxy for mtDNA subclade D2a) (6, 15). The other lacks obvious antecedents in Asia and might represent the post-LGM standstill population, expanding out of central Beringia with the shrub tundra (15). To confirm the hypothesis, archaeological sites of LGM age must be documented in Beringia. Although most such sites presumably would lie underwater, some might be found in areas of the LGM shrub tundra refugium that remain above sea level, such as low-lying portions of southwestern Alaska and eastern Chukotka.

References and Notes

  1. Acknowledgments: We thank N. H. Bigelow, O. K. Mason, G. R. Scott, and two anonymous reviewers who commented on earlier drafts.

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