On that 2005 trip, the Canadian doctors found no yaws cases in or around Bartica. But they flew deep into the interior, over waterfalls and rapids, then sailed to the villages of the Akwio people. There, in the Jawalla area, they found the only two cases of yaws, in girls, aged eight and 13, both with lesions on their knees.
Since Ms. Harper had called after he packed, Dr. Silverman had no way to properly collect a bacterial sample for storage. "We ended up using tongue depressors to scrape the wounds," he said. "It wasn't as perfect as we would have liked, but they're probably the last cases we'll ever see."
The Guyana strain
Although the samples were degraded, Ms. Harper, after comparing the DNA from more than 20 Treponema subtypes, said it was possible to identify the Guyana strain as the closest ancestor of the sexually transmitted bug.
"It gave us an insight into an organism in transition," Ms. Harper said. She called Dr. Silverman immediately: "Guess what?" she said. "You found the missing link!"
Ms. Harper said it's likely that Treponema emerged first in Africa, became yaws and followed the route of modern humans off the continent and up through Asia. It was then likely carried into the Americas by the Asian ancestors who gave rise to America's native populations and thrived in South and Central America.
"It may be that it is so hot and wet in these tropics," Dr. Silverman said, "that [the bacteria]evolved the ability to form ulcers, more like syphilis."
But when Columbus and his men met the New World natives, he said, they would not have contracted the disease in the same way it had spread among the local children.
"It didn't develop sexual transmission in the Amerindians because of their different clothing. But Europeans wore long pants and long sleeves, and the bacteria evolved quickly" to find another mode of transfer - sexual contact.
The only skin-to-skin relations Columbus's men conceivably had with natives, Dr. Silverman said, was "when they dropped their pants."
Once Columbus and his crew ferried the bug back to the colder climates of Europe, researchers say it became an advantage for the bug to bury itself in the body's warmer, moist and less-exposed folds.
In a commentary also published in PLoS, some researchers warn that firm conclusions should not be drawn from the degraded Guyana samples. But Ms. Harper said the genetic similarity between the two strains "is a pretty high coincidence."
Michael Gardam, infectious disease specialist at Toronto's University Health Network, called the findings "fairly striking" and "convincing." He said the new work jibes with the epidemiology of syphilis as a new infection that struck Europe.
"Often when a new organism gets introduced into a new population that has no existing immunity ... you would expect that at first it would be quite virulent," he said. But over time, Dr. Gardam explained, "it's not in a pathogen's best interest to kill everybody."
Dr. Silverman has continued to look for yaws in South America over the past two years. "But we haven't seen a single case," he said.
As for syphilis, Ms. Harper doubts her project will halt centuries of debate. "I'm sure there will still be a lot of controversy," she said. "We've seen it with HIV and other infections, especially sexually transmitted diseases. People are always so tempted to blame someone else."