Genetic evidence now backs up Spanish documents from the 16th century describing smallpox epidemics that decimated Native American populations.
Native American numbers briefly plummeted by about 50 percent around the time European explorers arrived, before rebounding within 200 to 300 years, say geneticist Brendan O’Fallon of ARUP Laboratories in Salt Lake City and anthropologist Lars Fehren-Schmitz of the University of Göttingen in Germany. Population declines occurred throughout North and South America around 500 years ago, the researchers report in a paper published online December 5 in the Proceedings of the National Academy of Sciences.
O’Fallon and Fehren-Schmitz analyzed chemical sequences in ancient and modern mitochondrial DNA, which is inherited from the mother, to calculate the number of breeding females in the Americas over time. Based on those results, O’Fallon estimates that a Native American population of several million fell to roughly half that size once European explorers entered the continent.
"If disease was the primary cause of mortality, surviving Native Americans would have been more resistant to infection after initial epidemics, helping them bounce back quickly,” O’Fallon says.
Researchers disagree about when people first reached the Americas. Whenever initial human settlers arrived, Native American numbers expanded rapidly between 15,000 and 10,000 years ago, several thousand years later than previous DNA-based estimates, the scientists say. Population size then stabilized until suddenly plummeting as the era of European contact dawned, they find.
Several earlier genetic investigations uncovered no signs of mass deaths among Native Americans around the time they first encountered Europeans.
"These new results confirm what’s known from historic sources, but the quality of ancient DNA data raises potential concerns,” remarks geneticist Phillip Endicott of Musée de l’Homme in Paris. An unknown number of chemical sequence changes in mitochondrial DNA preserved in Native Americans’ bones may have resulted from contamination in the ground or after being handled by excavators, Endicott says. These sequence configurations, if intact, provide crucial clues to population trends.
O’Fallon and Fehren-Schmitz analyzed partial sequences of ancient Native American DNA ranging in age from 5,000 to 800 years old. The researchers also examined mitochondrial DNA of 137 people representing five major Native American sequence patterns found in different parts of North and South America.
In the new analysis, only one, relatively rare mitochondrial DNA group repeatedly branched into new genetic lineages over the past 10,000 years. The other four groups display genetic splits bunched within the past few hundred years.
Reasons for these population differences are unclear, O’Fallon says. A closer examination of each of the five Native American genetic groups is needed to confirm that the new estimate of contact-era population losses is accurate, comments anthropological geneticist Connie Mulligan of the University of Florida in Gainesville.