(See a map of human migration.)
This migration probably occurred at least 15,000 years ago—the oldest human remains discovered so far are 13,000 years old—but some scientists have proposed that the first Americans arrived up to 40,000 years ago.
The Alaskan tooth was discovered in a cavern called On Your Knees Cave, named by the explorer who first crawled inside it.
(See a National Geographic magazine feature on the search for the first Americans.)
Using material taken from the tooth, Kemp isolated fragments of mitochondrial DNA (mtDNA), which is passed down from mothers to their offspring, and Y chromosome DNA, which is passed from father to son.
From a genetic database of 3,500 Native Americans, Kemp found 47 individuals in North and South America who exhibited the same genetic markers as the caveman. Some of the samples were drawn from living people and others from ancient bones.
He then compared the tooth DNA with the matching, modern samples and tracked the mutations that had occurred in that DNA over time.
By measuring the rate of mutation, Kemp found that so-called molecular evolution—the process by which genetic material changes over time—had taken place two to four times faster than researchers believed mtDNA could evolve.
That, Kemp said, suggests people entered the Americas within the last 15,000 years, because the DNA has evolved too fast for the arrival to have occurred any earlier.
"I would say that humans were probably not here much before that date," said Kemp. "A 15,000-year-old entry is [also] much more consistent with the archaeological record."
All of the mtDNA lineages among Native Americans are associated with five founding lineages believed to have originated in Asia.
But the caveman DNA turned out to be an independent founding lineage.
Of the 47 samples that matched the tooth DNA, 4 were from descendants of Chumash Indians living along California's central coast.
"The distribution of people exhibiting this [genetic] type today are all distributed in the western Americas," Kemp said.
"More or less the individuals are smack down the coast. It's a very neat western distribution."
John Johnson, an archaeologist and ethnohistorian at the Santa Barbara Museum of Natural History in California, collected the Chumash DNA samples.
To Johnson, the matching of the Chumash samples to the On Your Knees Cave man is indirect evidence of an ancient coastal migration that may have occurred very rapidly.
"We're interested in who were those first people to arrive here at the Pacific coast," Johnson said.
"I believe the Chumash descended from a very early coastal migration that resulted in the distribution of people down to the tip of South America."
But where did these coastal migrants come from?
DNA samples of people living in Japan and northeast Asia show some of the genetic mutations found in the cave-tooth and Chumash samples.
"I think that's a clue that there could be a genetic connection," Johnson said.
He said the Chumash ancestors may have been skilled fishers before they arrived in the Americas.
"Your techniques for exploiting coastal resources are easily [transferable] and something that maybe can allow you to migrate more quickly than people who are hunters and gatherers, who must get used to new environments as they move into uncharted territory," Johnson said.
"I think that may have allowed a more rapid migration along the Pacific margins of the Americas."
Kemp, meanwhile, said rapidly advancing DNA technology will help scientists piece together the story of the first Americans.
"No expert in morphology could look at the bones and say this person resembles a Tierra del Fuego person. It was only the DNA that could seal the case," Kemp said.
"This really highlights the importance of adding a molecular component to the study of these really ancient remains."
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