first Americans arrived in Alaska from Siberia less than 23,000
years ago over a land bridge, called Beringia, during the
last Ice Age when mammoths roamed widely. (Artwork by Sussi
The original Americans came from Siberia in a single wave no
more than 23,000 years ago, at the height of the last Ice Age, and
apparently hung out in the north perhaps for thousands of
years before spreading in two distinct populations throughout
North and South America, according to a new genomic analysis.
The findings, which will be reported in the July 24 issue of
Science, confirm the most popular theory of the peopling of the
Americas, but throws cold water on others, including the notion
of an earlier wave of people from East Asia prior to the last glacial
maximum, and the idea that multiple independent waves produced the
major subgroups of Native Americans we see today, as opposed to
diversification in the Americas.
This Ice Age migration over a land bridge between Siberia and
Alaska is distinct from the arrival of the Inuit and Eskimo, who
were latecomers, spreading throughout the Artic beginning about
5,500 years ago.
The findings also dispel the idea that Polynesians or Europeans
contributed to the genetic heritage of Native Americans.
The analysis, using the most comprehensive genetic data set
from Native Americans to date, was conducted using three different
statistical models, two of them created by UC Berkeley researchers.
The first, developed by the lab of Yun Song, a UC Berkeley associate
professor of statistics and of electrical engineering and computer
sciences, takes into account the full DNA information available
from the genomes in the study. A second method, developed by Rasmus
Nielsen, a UC Berkeley professor of integrative biology, and graduate
student Kelley Harris, requires much less computation, but relies
on a summary of the genome data. These and a third method developed
by researchers at the Wellcome Trust Sanger Institute, England,
all yielded consistent results. Song and Nielsen are two of three
corresponding authors of the paper.
migration route of Siberians into North America and the subsequent
split into northern and southern Amerindian populations. Analysis
of current and ancient genomes shows that there also was some
later interbreeding between East Asians and Inuit.
Modern and ancient genomes
The data consisted of the sequenced genomes of 31 living Native
Americans, Siberians and people from around the Pacific Ocean, and
the genomes of 23 ancient individuals from North and South America,
spanning a time between 200 and 6,000 years ago.
"There is some uncertainty in the dates of the migration and
the divergence between the norther and southern Amerindian populations,";
Song noted, "but as we get more ancient genomes sequenced, we will
be able to put more precise dates on the times of migration.";
The international team concluded that the northern and southern
Native American populations diverged between 11,500 and 14,500 years
ago, with the northern branch leading to the present day Athabascans
and Amerindians broadly distributed throughout North America. The
southern branch peopled Central and South America, as well as part
of northern North America.
"The diversification of modern Native Americans appears to have
started around 13,000 years ago when the first unique Native American
culture appears in the archeological record: the Clovis culture,";
said Nielsen. "We can date this split so precisely in part because
we previously have analyzed the 12,600-year-old remains of a boy
associated with the Clovis culture.";
One surprise in the genetic data is that both populations of
Native Americans have a small admixture of genes from East Asians
and Australo-Melanesians, including Papuans, Solomon Islanders and
Southeast Asian hunter gatherers.
"It's a surprising finding and it implies that New World populations
were not completely isolated from the Old World after their initial
migration,"; said Eske Willerslev from the Centre for GeoGenetics
at the Natural History Museum, University of Copenhagen, who headed
the study. "We cannot say exactly how and when this gene flow happened,
but one possibility is that it came through the Aleutian Islanders
living off the coast of Alaska.";
Song added that the state-of-the-art statistical methods that
his and Nielsen's labs developed "are being made publicly available
so that they can be used by others to study complex demographic
histories of other populations.";