Seals and sea lions probably brought tuberculosis to the Americas centuries before Christopher Columbus first set foot there, scientists said Wednesday.
A new study challenges the theory that Europeans introduced TB to the New World, where it killed millions of indigenous Americans along with other foreign diseases like whooping cough, chicken pox and flu.
As many as 20 million people lived in the Americas before Europeans arrived, and up to 95 percent of them were wiped out by new diseases to which they had no immunity.
But the latest data revealed that TB "may have had a hand in American Indian deaths prior to the influx of European diseases", according to the US National Science Foundation (NSF), which co-funded the study.
An international expert team analysed DNA of bacteria from three 1,000-year-old human skeletons found in Peru, and found a type of TB closely related to strains that infect seals and sea lions today.
The finding provides a "plausible, if unexpected, route of entry into the New World", the study's authors wrote.
It means that seals and sea lions probably contracted the disease from a host animal from Africa, where the disease likely originated, and swam across the Atlantic to South America.
The mammals were probably eaten by coastal people who were themselves then infected and spread the bacteria to others, said the study, published in the journal Nature.
"The source of tuberculosis in the New World has long been a question for researchers," Elizabeth Tran, the NSF's biological anthropology programme director said in a statement.
"This paper provides strong evidence that marine mammals may have been the likely culprits."
The analysis also revealed that tuberculosis in the form we know it was much younger than originally thought, and probably only evolved in Africa about 6,000 years ago.
- An extinct TB strain -
The study offers answers to a number of long-standing questions about TB in the ancient Americas.
Genetically, modern strains of New World TB are closely related to European ones, which led to the conclusion that Europeans introduced the disease after Genoese navigator Columbus's first contact with Amerindians in 1492.
Yet there is archaeological evidence in skeletons and mummies of tuberculosis in the Americas hundreds of years earlier.
Some have suggested the disease must have spread with early humans out of Africa, before the Bering land bridge between Siberia and Alaska was flooded at the end of the last ice age about 11,700 years ago.
But this fails to explain the European genetic likeness, or the fact that TB is probably a younger disease than that.
The latest study concluded that TB bacteria in the three ancient skeletons were different to strains found in humans in the Americas today.
Having been initially brought over by sea mammals, the disease seems to have been replaced by European strains.
"The connection with 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," said co-author Johannes Krause, of the University of Tuebingen in Germany.
"A marine introduction seems the most likely way that the disease could have reached humans in the Americas thousands of years after the inundation of the Bering land bridge, when terrestrial movements into the Americas was no longer possible."
More than 8.6 million people fell ill globally with tuberculosis in 2012, according to the World Health Organization, and 1.3 million died from it -- making it the biggest killer disease after HIV.
The strains that still exist in seals and sea lions today can still make people ill, though it happens extremely rarely, the researchers said.