How Did People Reach the Americas?
Ancient DNA sheds light on the prehistoric humans who colonized a
hemisphere
By ANDREW CURRY
Posted: July 24, 2008, U.S. News & Report
After years of spirited debate over how and when people first reached the Americas,
scientists finally seem poised to reach agreement. The emerging consensus: In
contrast to what was long held as conventional wisdom, it now seems likely that the
first Americans did not wait for ice sheets covering Canada to melt some 13,000
years ago, which would have allowed them to traipse south over solid ground.
Instead, early nomads might well have traveled by boat or at least along the coast
from Siberia to North America, perhaps navigating arctic waters near today's Bering
Strait. The telltale evidence: ancient DNA from those early people that's been
coaxed, by powerful analytical technology, into revealing its secret.
Rewriting the prehistory of the Americas is perhaps the most remarkable discovery—
but hardly the only one—so far achieved through the analysis of ancient DNA. Other
new insights about the past are being drawn from the same emerging scientific
discipline. In the past five years, the double helix has shed light, for example, on the
vanished woolly mammoth, the flightless dodo, and even humanity's long-lost kin,
the Neanderthals. Extracting and testing old DNA, once considered practically
impossible because too little of the stuff survives the eons intact, are now at the
cutting edge of archaeology, paleontology, and other fields, thanks to new
techniques and more powerful technology.
"Archaeologists are used to stone tools and bones," says Ted Goebel of Texas A&M.
"So for us to be presented with this kind of evidence is pretty intriguing." DNA, which
contains the blueprints for organisms, degrades over time, breaking down into tiny
pieces or disintegrating entirely. For years, the dearth of intact DNA in ancient
samples—a chunk of mammoth bone, for instance, or a human hair—stymied
researchers who were trying to analyze the material. But now, using a technique
called polymerase chain reaction, or PCR, researchers can "unzip" minute fragments
of surviving DNA and duplicate them millions of times over, until they have a sample
large enough to test. Then, by comparing differences between the ancient material
and modern samples of known provenance, they can analyze a long-extinct animal's
genome.
The resulting data, in some cases, can resolve a long-standing scientific deadlock.
For almost a century, most archaeologists believed that people arrived in the
Americas between 13,000 and 13,500 years ago. The date was based on flint tools
first found in Clovis, N.M., and later all over North America. From that evidence,
archaeologists sketched out a scenario in which fur-clad "Clovis" hunters chased
mammoths and other prey from Siberia to North America across a land bridge
exposed by low sea levels. Then, the theory goes, they hunted along a path from
Alaska down through Central America and all the way to Chile in just a few centuries.
Before Clovis. But the notion that Clovis came first has collapsed in the face of
recent evidence, including DNA that pushes the arrival of the first humans in the
Americas back at least 1,000 years, centuries before the ice that covered northern
Canada at the time had melted enough to allow migration. Instead, some argue, the
first Americans must have arrived by boat, skirting the coast from Siberia and sailing
south along the American coast.
University of Oregon archaeologist Dennis Jenkins discovered the critical new
evidence buried more than 4 feet below the floor of a dusty cave near Paisley, Ore.
The "artifacts" were 14,300-year-old fossilized pieces of excrement, or coprolites.
Jenkins, who has been digging in Oregon's high desert for decades, handed off bits
of coprolite to geneticist Eske Willerslev of the University of Copenhagen. In
Willerslev's laboratory, PCR pulled enough DNA from the ancient poop to prove it was
human and even genetically link it to modern American Indians.
Announced in April in the journal Science, the find backs up evidence previously
found at the other end of the Americas, at a site in Chile called Monte Verde. There,
a full-fledged campsite was radiocarbon dated to 14,500 years ago, putting people in
South America more than a millennium before those Clovis hunters supposedly
crossed the Bering Strait. But carbon dating is inexact, and the Monte Verde find had
not convinced some skeptics. With DNA analysis, says Jenkins, "we can directly date
the item and verify it's human." The Oregon find has largely silenced the last few
Clovis adherents. "It's pretty compelling stuff," says Goebel, a longtime Clovis
supporter.
The success of ancient DNA analysis is likely to open up valuable new sources of
archaeological information in the United States. Coprolites—stored in the thousands
in museums all over the country—may yield answers from the distant past without
infringing on American Indian beliefs about the sanctity of burial remains.
The technique has promise—and a quickly growing track record—in other arenas, as
well. In 2002, Penn State researcher Beth Shapiro, then at the University of Oxford,
successfully sequenced the DNA of the dodo bird, extinct for more than three
centuries—and discovered it was a close relative of the pigeon. In 2006, Hendrik
Poinar, a geneticist at McMaster University in Canada, sequenced most of a woolly
mammoth's genome from fragments of bone, proving—at least in theory—that
cloning one might be possible. And a lab in Germany has been researching the
genetics of Neanderthals to see how closely they were related to modern humans—
and if the two species interbred.
Willerslev, meanwhile, is probing other genetic remains for further discoveries. Last
summer, he announced the recovery of the oldest intact DNA ever found. It came
from a soup of plants and animals, now buried under a mile of Greenland glacier,
that made up a forest at least 450,000 years ago. The discovery showed that
Greenland was once covered in lush forest—and helps refine climate models of global
warming. He has also just announced another find from the frozen north: a human
hair sample from Greenland that yielded a complete human genome more than
3,400 years old. The DNA matches modern Siberians but not the Inuit who live in
Greenland today, suggesting that the wrinkled prehistory of the Americas has yet to
be fully ironed out. Now, Willerslev is planning a trip back to Greenland to hunt for
more ancient DNA. With traditional archaeology in one hand and cutting-edge
genetic techniques in the other, he may soon have more to tell us about the
Americas' earliest immigrants.