Download Tracing the Paths of the First Americans

NEWS&ANALYSIS
H U M A N E VO L U T I O N
1692
23 SEPTEMBER 2011
VOL 333
SCIENCE
Published by AAAS
experience similar climatic and environmental conditions, than along longitude
lines, which would take them into diverse
environments. Diamond argued that the
east-west orientation of Eurasia fostered
the spread of technology among peoples and helped Eurasians achieve
technological dominance over
Americans living along a northsouth continental axis.
Ramachandran and Rosenberg compared genetic variation
in 39 Eurasian populations and 29 Native
American populations to test that theory.
Such variation can serve as a proxy for
the speed of past population movements, Ramachandran explains:
Faster movements mean more genetic
exchanges among groups and
thus reduce variation, whereas
slower movements foster
isolation and more
genetic variation.
Ramachandran
and Rosenberg
found much
more genetic variation along the long
axis of North and
South America than
along the east-west axis of
Asia and Europe.
The findings “seem consistent with Diamond’s hypothesis,”
says anthropologist Brian Kemp of
Washington State University, Pullman.
Diamond agrees. “With this paper,
human gene flow joins crop diffusion
and state government spread as important
phenomena affected by the differing axis
orientations of Eurasia and the Americas,”
he says.
Despite this recent progress, researchers agree that more Native American samples—especially ancient DNA from very
early sites—are key to filling in the blanks
in the story of the Americas. That means
closer collaborations with sometimesreluctant Native American groups, researchers say (Science, 8 October 2010, p. 166).
But the collaboration can work, Malhi says:
Canada’s Heiltsuk people agreed to let his
lab sample 5000-year-old human remains
from Namu, in British Columbia, shortly
before the bones were ceremonially reburied earlier this month.
www.sciencemag.org
–MICHAEL BALTER
Downloaded from www.sciencemag.org on September 23, 2011
The last great peopling of the continents—
when humans finally spread into North and
South America—is among the most mysterious. This much is known: Perhaps about
20,000 years ago, hunter-gatherers in Asia
traveled east into the Bering Strait land
bridge, where they lingered for a while
in the now largely submerged region
known as Beringia. Then they spread
rapidly into North and South Ancient Americans.
America, reaching Chile by at Researchers have recovered
least 14,000 years ago.
mitochondrial DNA from
But many of the details remain more than 60 sites (circles)
unclear. Was there only one wave in the Americas.
of migration or many? How long
did people stay in Beringia, and how did pop- time. That diverse
ulations interact after they took up residence population eventufrom the Yukon to Tierra del Fuego?
ally headed east and
In a special issue of the American Journal south to give rise to
of Physical Anthropology published online all later Paleoindians and
last week, six research teams used the genet- Native Americans. A few years
ics of living and ancient people to probe this ago, University of Florida, Gainesmomentous migration. The findings support ville, anthropologist Connie Mulligan
earlier indications that the Paleoindians, the and her colleagues analyzed mtDNA
ancestors of today’s Native Americans, stem and archaeological evidence to confrom a single Asian source population. But clude that these Beringian populations
the data also suggest that this population may might have become genetically isolated
have become genetically quite diverse during from mainland Asians by 30,000 years
thousands of years of Beringian occupation. ago—and then stayed in Beringia for as long
Because of admixture with Europeans, it’s as 15,000 more years.
difficult to get a clear picture of Paleoindians
The ancient DNA findings “do suggest
from living Native Americans. So research- that there was a fair amount of diversity in
ers have sought ancient DNA from skeletons. Beringia just before the peopling” of the
More than 60 such studies are reviewed by Americas, says geneticist Ripan Malhi
a team led by anthropologists Jennifer Raff of the University of Illinois, Urbanaand Dennis O’Rourke of the University of Champaign. But although most researchUtah in Salt Lake City. Unlike the situa- ers now accept that there was a single
tion on the other continents, now including migration from Asia, O’Rourke cautions that
Australia (see p. 1689), researchers in the the evidence cannot entirely rule out the alterAmericas lack a full nuclear genome of a native scenario of multiple migrations.
native; most of the studies are of mitochonHowever the first Americans arrived,
drial DNA (mtDNA) from sites 6000 years the mtDNA evidence shows that by about
old or younger. But four sites feature five 5000 years ago, the basic geographic disindividuals older than 8000 years, including tribution of their descendants was set. With
the 14,000-year-old Paisley Cave in Oregon only a few exceptions, the pattern of distri(Science, 4 April 2008, p. 37).
bution of genetic markers in North and South
Surprisingly, each of these five ancient America has been stable for all that time.
individuals featured a different genetic
Indeed, once early Americans got settled,
marker, called a haplotype, out of the 15 they didn’t move around too much, accordcurrently recognized “founder haplotypes” ing to work by Sohini Ramachandran of
thought to represent the genetically diverse Brown University and Noah Rosenberg of
founder population. To some researchers, the University of Michigan, Ann Arbor. Their
this result supports the so-called Beringian paper supports a hypothesis by University of
Incubation Model, which suggests that Asian California, Los Angeles, biologist Jared
migrants occupied Beringia for thousands of Diamond that people—and their technology
years before moving on to the Americas, and and domestic animals—travel more quickly
shows that their genomes diversified in this and easily along lines of latitude, where they
CREDITS: ADAPTED FROM J. RAFF ET AL./AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY (AUGUST 2011)
Tracing the Paths of the First Americans