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tion 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).
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 tree-less 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 sam-
ple 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 West-
ern Hemisphere. Tamm et al. concluded
that ancestral Native Americans were iso-
lated 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 posi-
tion, they estimated that Asian and Ameri-
can mtDNA haplogroups had diverged more
than 25,000 years ago but that the latter dis-
persed 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 standstill 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. E. Hultén, Outline of the History of Arctic and Boreal
Biota During the Quaternary Period (Lehre J. Cramer,
New York, 1937).
2. S. A. Elias, B. Crocker, Quat. Sci. Rev. 27, 2473 (2008).
3. R. Westbrook et al., GSA Annual Meeting, 180-10
(Charlotte, NC, 4–7 November 2012).
4. L. B. Brubaker et al., J. Biogeogr. 32, 833 (2005).
5. E. Willerslev et al., Nature 506, 47 (2014).
6. E. Tamm et al., PLOS ONE 2, e829 (2007).
7. I. Théry-Parisot et al., in The Zooarchaeology of Milk and
Fats (Oxbow Books, Oxford, 2005), pp. 50–59.
8. K. B. Schroeder et al., Biol. Lett. 3, 218 (2007).
9. D. Reich et al., Nature 488, 370 (2012).
10. M. C. Dulik et al., Proc. Natl. Acad. Sci. U. S.A. 109, 8471
11. V. Battaglia et al., PLOS ONE 8, e71390 (2013).
12. M. Raghavan et al., Nature 505, 87 (2014).
13. T. Goebel et al., Science 319, 1497 (2008).
14. V. Pitulko et al., in Paleoamerican Odyssey, K. E. Grant
et al., Eds. ( Texas A&M University Press, College Station,
2013), pp. 13–44.
15. J. F. Hoffecker, S. A. Elias, Human Ecology of Beringia
(Columbia Univ. Press, New York, 2007).
Acknowledgments: We thank N. H. Bigelow, O. K. Mason,
G. R. Scott, and two anonymous reviewers who commented on
170°E 180° 170°W 160°W 150°W
Bering land bridge
Sites indicating steppe-tundra
Sites indicating shrub tundra
Sites indicating a mixture of
steppe-tundra and shrub tundra
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.