Grade school history lessons often have it that American Indians largely were wiped out by diseases such as whooping cough, chicken pox, influenza and tuberculosis brought to the New World by European explorers.

One report says, while estimates vary, about 20 million people lived in the Americas shortly before Europeans arrived, and roughly 95 percent of them were killed by European diseases.

But new research led by anthropological geneticists Anne Stone of Arizona State University and Johannes Krause of the University of Tubingen in Germany indicates the diagnosis of what devastated American Indian populations is a little more complicated, particularly when it comes to tuberculosis.

Their study of pre-Columbian Mycobacterial tuberculosis genomes published in the journal Nature reveals that tuberculosis may have had a hand in American Indian deaths prior to the influx of European diseases. The research concludes seals and sea lions likely brought the disease to South America and spread it to people there long before Christopher Columbus arrived in 1492.

"What we found was really surprising," said Stone, referring to her team's examination of tuberculosis DNA from roughly 1,000-year-old human skeletons found in Peru that produced the discovery.

The results, the researchers write, provide unequivocal evidence that a member of the M. tuberculosis complex caused human disease in the pre-contact New World.

"Skeletal evidence of tuberculosis is present in the archaeological records in both the Old World and New World," said Elizabeth Tran, Biological Anthropology program director in the National Science Foundation's Division of Behavioral and Cognitive Sciences.

"The source of tuberculosis in the New World long has been a question for researchers. This paper provides strong evidence that marine mammals may have been the likely culprits, bringing tuberculosis to South America long before Europeans arrived there."

The National Science Foundation's Directorate for Social, Behavioral and Economic Sciences partially funded the research as part of an effort to assess tuberculosis' evolutionary history, its effects on human history and to understand recent reemergence of the disease.

Study researchers also hypothesize that once European tuberculosis strains arrived in the Americas, they completely replaced the prior strains brought over by seafaring animals. This event confused diagnosis of the impacts on Indian populations as researchers struggled to identify which tuberculosis strain was involved in American Indian deaths.

"We are not sure what the timeframe was for the replacement of American strains by European strains after contact," said Stone, indicating the role these strains played on Indian populations is still unclear. "This is one question that we hope to address in the future."

However, she said, "It is likely that the new European strain, which is more virulent, was a culprit--particularly since tuberculosis is really good at spreading during times of social crowding and distress."

Africa has the most diversity of tuberculosis strains, suggesting the pathogen likely originated there and spread. These research results give rise to speculation that humans gave tuberculosis to animals and within the last 2,500 years, marine animals carried it from Africa to South America where they gave it back to humans.

Researchers collected genetic samples from throughout the world and tested them for tuberculosis DNA. Of 76 DNA samples determined to be from pre- and post-European contact sites in the New World, three dated to around 750 to 1350 AD from the Osmore valley in Peru--from archaeological sites at El Yaral, El Algodonal and Chiribaya Alta--and had tuberculosis DNA that could be used for further study.

Project scientists compared these to a larger dataset of modern genomes that included animal strains. Research results showed a clear relationship to animal lineages, specifically tuberculosis lineages from seals and sea lions.

"The connection to seals and sea lions is important to explain how a mammalian-adapted pathogen that evolved in Africa around 6,000 years ago could have reached Peru 5,000 years later," Krause said.

"Tuberculosis is a disease that is on the rise again worldwide," said Jane Buikstra, director of ASU's Center for Bioarchaeological Research, who identified tuberculosis from most of the project's research samples. "This study and further research will help us understand how the disease is transmitted and how the disease may evolve."