The first strains of TB in America came from seals and sea lions. (Ricardo Bastida)

The origin of tuberculosis (TB) in the Americas is no longer a mystery - the seals and sea lions gave it to us. The oldest strains have since given way to our current variety, which comes from Europe. But until the Americas made contact with Europe, a study published Wednesday in Nature reports, our ancestors suffered from a fishier ailment.

"For a long time everyone assumed that we got TB from cows, and that as we'd domesticated them and cozied up to them in the barn, the European strain of TB had jumped to us." Anne Stone, a professor in the School of Human Evolution and Social Change at the Arizona State University and co-author of the new Nature study, said. But in the past decade or so, Stone said, researchers realized that the strain of TB found in cows was actually newer than the one found in humans. "So actually, we probably gave it to them," Stone said.

Furthermore, researchers found evidence of a case of TB in the Americas before any known European contact. But the analysis wasn't very sophisticated - researchers could diagnose the TB, but couldn't tell much about it. Stone and her colleagues took recent advances in genetic analysis in hand to find out more.

Sea lions in Peru. (Sara Marsteller) Sea lions in Peru. (Sara Marsteller)

"The common assumption, which was definitely our hypothesis, was that migration over the Bering Strait had brought Asian TB to America," Stone said, "but wow, were we wrong!" Instead of showing a close relation to ancient strains of TB found in Asia, the earliest cases of human infection in the Americas were related to those found in pinnipeds (seals and sea lions).

The results may have surprised Stone and her colleagues, but the pieces do fit: Only pinnipeds in the southern hemisphere get TB, and the oldest evidence of the disease in humans comes from South America. Scientists always assumed that this was because the remains there were better preserved, but it looks as if that's where the disease started.

From there, the disease adapted quite well to humans and probably switched to human-to-human transmission (studies of inland populations, where marine life was hard to come by, will help confirm how it spread). Later, European strains of the disease out-competed America's own.

But humans can still catch good old-fashioned seal TB, too. "You mainly see that reported in zoo keepers," Stone said. "Luckily, these animal strains of TB are fairly easily treated with current drugs." But discoveries like this one should serve as a warning to those who come into contact with wild animals, and especially people who hunt. Zoonosis - the transmission of non-human disease to humans - is the phenomenon that exposed us to HIV, Ebola, and countless other dangerous ailments. "This is certainly a mode of transmission for new diseases that people should be aware of," Stone said, "and to me, it really brings home the message that hunters should take care."