Moreover, Harper said because three out of the four genetic variants that support the relationship between the nonvenereal strains and the syphilis-causing strains were found in one small stretch of the genome, the bacterium must have mutated into the modern form of venereal syphilis in a relatively short period of time.
"It looks like there's a lot of changes all in one spot, so it seems to have changed rapidly," Harper explained, adding that although this latest study fails to provide a date for when the yaws strain mutated into the syphilis-causing strain, researchers know that the syphilis bacterium has been present in the New World for thousands of years.
Some experts not involved with the research agreed with the team's conclusions. Bruce Rothschild, a rheumatologist at the Northeastern University's College of Medicine in Youngstown, Ohio, said the work supports the findings of his research on skeletal remains from the New World, which show signs of the disease.
Rothschild said the earliest signs of syphilis were found in skeletal remains on the Colorado Plateau of North America, dating back to about 2,000 years ago. More recently, Rothschild found evidence of syphilis in 1,200-year-old remains from the Dominican Republic — and in an area known to have been a stop on the famous voyage to the New World by Columbus and his crew.
According to Rothschild, his research has revealed that there was no Treponemal disease in Europe before the 11th century, when the disease arrived on slave-carrying ships from Africa. However, the disease appeared to have disappeared with the Black Death, not to re-emerge until Columbus returned from his voyage to the New World in 1495.
"The fundamental issue is that I think the question of where syphilis came from is resolved. I thought our work resolved it," Rothschild said. "But when you add it with something new that has reproducible techniques, I think it becomes clear that anyone not subscribing to the Columbian theory is probably believing in mythology."
But many experts remained unconvinced of the Columbian theory on the origins of the disease.
Given the controversial nature of the study's findings, the research was published alongside expert commentary on the study by some of the leading scientists in the field. Connie Mulligan, primary author of the commentary and associate professor of anthropology and genetics at the University of Florida, said this study fails to conclusively answer the question of where syphilis originated from and how it traveled around the globe.
"Honestly, I don't think their data speaks to [the] question of geographic origin of syphilis at all," Mulligan said. "Harper's paper looks at the evolutions of different variants within each strain so they can infer the evolutionary history of it. But that's a different kettle of fish because we make all sorts of assumptions about how we think evolution occurs from a small sample of the genome.
According to Mulligan and commentary co-author Sheila Lukehart, research professor of medicine, pathobiology and microbiology at the University of Washington in Seattle, there simply hasn't been enough research on the genetics of the bacterium to determine which genetic variants distinguish one strain of Treponema from another.
"To truly understand it we need a whole genome sequences of all subspecies [of Treponemas]," Mulligan said.
Despite Mulligan and Lukehart's criticism of the study's conclusions, Rothschild maintains that the skeletal evidence coupled with these latest genetic findings are enough to put a nail in the coffin on the debate over the origins of the disease.
"Think about this as if it's a legal case and you're trying to prove whether or not someone was present at the crime scene. If you have fingerprint evidence to prove that they were there, do you also need DNA evidence? I don't think so," Rothschild said.