Origin: A Genetic History of the Americas

constructions, and artwork tell us about the countless societies that
flourished, diminished, or continued into the present day across the
American continents. Each society developed sophisticated ways of living
in different environments, from the elaborate irrigation canals created by
the Hohokam to water their crops in the Sonoran Desert about 1,100–500
years ago to the igluvijait (snow houses) that allowed the Thule to thrive in
the long bitter winters above the Arctic Circle a thousand years ago.
Their histories are also told in their genomes. Although contemporary
Native Americans are genetically very diverse, with ancestries from all over
the globe, their forebears could trace biological ancestry to one (or a very
few) founder populations (7). Many contemporary Native Americans have
these ancient signatures in their genomes as well. In the last few decades,
these genetic legacies from their ancestors have become recognized as a
source of information about the past fully as important as artifacts and
structures. Researchers—unfortunately, almost all non-Native—have rushed
to sequence DNA from contemporary and ancient Native Americans in
order to understand the secrets those genes have to tell.
As we discussed in earlier chapters, anthropologists and historians once
thought the prehistory of the Americas constituted a single entry event, one
that we could use as a starting point to understand the more complex
population histories elsewhere in the world. This event started toward the
end of the Pleistocene Ice Age, when temperatures were so cold that much
of North America was covered by massive ice sheets. Sea levels were so
much lower than they are today that Asia was connected to North America
by the Bering Land Bridge. The Ice Age ended around 13,000 years ago;
during its waning years the Earth warmed enough that the ice sheets began
to melt and a thin corridor between them ran down western North America.
A small group of people migrated rapidly from Siberia across the Bering
Land Bridge and then down through this corridor into the ice-free regions
of central North America. They may have been following herds of
mammoths or bison, whose bones are sometimes found with finely made
13,000-year-old spear points—called Clovis points—embedded within
them. These Clovis peoples were initially few in number, but as they moved
throughout the previously unpopulated lands, they increased in numbers
and eventually gave rise to all Indigenous peoples in the Western
Hemisphere.