PART. 2
Joining forces
When paired with sequences from modern populations, ancient DNA can help to refine the calculations made by population geneticists and test the claims made by archaeologists. In 2008, Brian Kemp, now at Washington State University in Pullman, extracted mitochondrial DNA from a 10,300-year-old tooth found in On Your Knees Cave in Alaska. When he compared the DNA sequences with those from modern Native Americans, he found that the mutation rate was faster than previously thought8. The results, he says, effectively rule out the possibility that humans came to North America as early as 40,000 years ago — a date based on equivocal evidence from archaeological sites in the eastern United States. The finding also argues against the idea that people used boats before the thaw to go around the glaciers and come down the coast. Instead, the DNA evidence supports the consensus that people didn't migrate into the Americas — whether by boat or over land — until the end of the last glacial maximum, 16,500 years ago at most.
The DNA told researchers a few more things. The ancient man who died in that Alaskan cave had mitochondrial DNA most closely related to Native American groups living today along the west coast of North America. “Most of the people who descended from that type are still living near the coast,” Kemp says. So the first wave of migrants probably came down the coast and then spread east from there, developing tiny variations in their DNA as they went, Kemp says.
Dennis O'Rourke, a geneticist at the University of Utah in Salt Lake City, is using similar comparisons to fill in the map of ancient migrations in the New World. In the past ten years, dozens of similar studies have established a clear trend — comparisons of DNA from modern people with ancient DNA have shown that the geographic distribution of genetic groups in the Americas has been stable for millennia. “The patterns must have been established more than 4,000 years ago,” he says. That helps to constrain the timing of when people spread across the continent and when they stopped migrating, he says.
In Point Barrow, Alaska, O'Rourke recently began studying human remains from a cliff-top cemetery threatened by coastal erosion, where people have been buried for the past 1,000 years. By comparing the samples from ancient Alaskans to populations from Greenland, eastern Canada and elsewhere, O'Rourke hopes to learn more about the colonization of the Arctic, an environment similar to what the first Americans would have encountered towards the end of the last ice age.
O'Rourke's collaborators are also collecting DNA samples from Inupiat people in northern Alaska. By matching up the modern and ancient DNA sequences from that region, they hope to refine the genetic clock and improve estimates for when people arrived in the Americas. Similar work is going on at a cemetery on Prince Rupert Island off northern British Columbia, where local Tsimshian people are working with archaeologists to gather ancient and modern DNA evidence.
While geneticists open up intellectual frontiers, archaeologists are searching for ways to test the migration theories in the field. Direct evidence for coastal migration will be hard to come by, because a rise in the sea level since the end of the last ice age has flooded the ancient coastlines. But researchers are turning up indirect evidence in many locations. Last year, for example, Erlandson demonstrated that humans lived on California's Channel Islands as far back as 12,200 years ago9, which shows that they must have mastered the use of boats before that time.
And at the Monte Verde site in Chile, researchers have found evidence that the ancient occupants were fans of seafood10. “Monte Verde has ten different species of seaweed at the site,” Dillehay says. “Somebody was intimately familiar with seaweeds and the microhabitats where they could be found.” That lends support to the idea that the earliest Americans were seafarers, he says.
Dillehay's recent findings, which came 30 years after the first excavations at Monte Verde, show that previously studied sites can become potential gold mines, says Waters. Because so many sites were either dismissed or forgotten during the 'Clovis-first' era, Waters says that “the field can really be pushed forward by going back and taking a look at sites that were put up on a shelf”. He is already planning to reopen sites in Tennessee and Florida, where evidence of pre-Clovis mammoth hunting was uncovered in the 1980s and 1990s.
Geneticists and archaeologists agree that the death of the Clovis theory has injected the field with excitement and suspense. “There's a sense that there was something before Clovis,” says Jenkins, whose coprolite study shook the field four years ago. “But what it was and how it led to the patterns that we see in North and South America — that's a whole new ball game.”
Nature 485, 30–32 (03 May 2012) doi:10.1038/485030a
References
- Gustafson, C. E., Gilbow, D. & Daugherty, R. Can. J. Archaeol. 3, 157–164 (1979).
- Dillehay, T. D. Monte Verde: A Late Pleistocene Settlement in Chile Vol. 1 (Smithsonian Institution Press, 1989).
- Gilbert, M. T. P. et al. Science 320, 786–789 (2008).
- Waters, M. R. Science 334, 351–353 (2011).
- Waters, M. R. Science 331, 1599–1603 (2011).
- Tamm, E. et al. PLoS ONE 2, e829 (2007).
- Rasmussen, M. et al. Nature 463, 757–762 (2010).
- Kemp, B. M. et al. Am. J. Phys. Anthropol. 132, 605–621 (2007).
- Erlandson, J. M. et al. Science 331, 1181–1185 (2011).
- Dillehay, T. D. et al. Science 320, 784–786 (2008).