Origin: A Genetic History of the Americas

The reason for that is because both mitochondrial and nuclear genomes
of Native Americans show us that they had been isolated from all other
populations for a prolonged period of time, during which they developed
the genetic traits found only in Native American populations. This finding,
initially based on classical genetic markers and mitochondrial evidence,
came to be known as the Beringian Incubation, the Beringian Pause, or the
Beringian Standstill hypothesis (9).
Most geneticists are skeptical that the peoples ancestral to Native
Americans could have been completely isolated for any length of time if
they were living anywhere near other populations during the LGM. Thus,
we look north, rather than east, for the location of the refugia that may have
allowed the ancestors of Native Americans—whom I will call the
Beringians for the rest of this book—to survive the Ice Age.
Except for a few small islands, central Beringia is mainly underwater
today; it was a substantial land connection between 50,000 and 11,000
years ago. Scientists are drilling into the sediments across this region in
order to take cores, whose layers—which contain pollen, plant fossils, and
insect remains—provide snapshots of both the geology and the environment
across time at each site they are taken. Paleoclimatologists stitch these
snapshots together to reconstruct the LGM climate across Beringia,
including the regions that are underwater today.
Layers dating to the LGM show us that the environment was patchy
across Beringia. There were vast regions of steppe-tundra: dry and cold
grasslands, sprinkled with herbs and small shrubs like dwarf willows. This
environment, lacking firewood, would have been difficult for humans,
although the presence of megafauna across steppe-tundra would provide not
only food but also dung and bones to burn. But there were some places in
and near Beringia that could have served as more attractive refugia for both
humans and animals.
One possible refuge for humans during the bitterly cold Ice Age was the
southern portion of central Beringia—a region that is presently under about
164 feet of ocean but would have been lowland coastline 50,000 to 11,000
years ago. Unlike the steppe-tundra regions, the southern coast of the land
bridge would have been much warmer and wetter because of its proximity
to the ocean.
Paleoenvironmental evidence shows that it actually contained wetlands,