In Nature this week, Harvard Medical School researchers published a study showing that there may have been three migratory waves from Siberia to the Americas thousands of years ago, instead of one as was originally thought — a large one before 13,000 years ago, followed by two smaller ones. Now, a study in Science suggests it may have actually been four waves, says New Scientist's Michael Marshall. While Harvard's David Reich used DNA samples from contemporary Native Americans from Canada, Central America, and South America, he wasn't able to get permission to use samples from Native Americans in the US, Marshall says. But the University of Copenhagen's Eske Willerslev was able to analyze stone tools and DNA found in ancient human feces from the Paisley Caves in Oregon, and that data shows that there may have been two migrations before 13,000 years ago, followed by the two smaller ones.
Willerslev originally found the samples in 2008 and found them to be 14,300 years old. But his findings were criticized as other researchers said the fossilized feces could have been contaminated by other humans, Marshall says. Willerslev has since re-analyzed the samples, confirmed their age, and found that other animal remains in the cave were not contaminated by human DNA, leading to the conclusion that the human feces were also untouched.