Ancient evidence of a familiar foe has emerged in a fossil tick infested with what appears to be spirochetes, a group of rotini-shaped bacteria responsible for many human diseases. The spirochetes in question closely resemble those of modern-day Borrelia, the genus responsible for Lyme disease. The finding, recently described in Historical Biology, could offer insight into the evolutionary history of the Lyme disease–causing pathogen that plagues people today, but is also notable for its novelty.

“This is the first evidence of spirochetes in a fossil tick prior to Homo,” says George Poinar, Jr., a paleoentomologist and parasitologist at Oregon State University, and author of the new paper. Although Lyme disease did not exist back then, the spirochetes in the fossil tick probably contributed to the genetic diversity of the 12 or more species of Borrelia that cause Lyme and similar diseases today, he says.

Parasites represent at least half of all modern animal species, and that distribution probably held true millions of years ago, too. “In a sense, this [finding] is not surprising since virtually every species on the planet is parasitized,” says Armand Kuris, a parasitologist at the University of California, Santa Barbara, who was not involved in the study. Evidence of those ancient parasite–host associations is difficult to come by, however. “In terms of finding any kind of physical documentation in the fossil record, that’s really rare—especially for a microbial pathogen,” Kuris says. “That’s what makes this paper just plain interesting.”

The spirochete-carrying tick—a juvenile—turned up in a 15-million- to 20-million-year-old piece of amber, along with three other young ticks that did not reveal any spirochetes. Poinar acquired the specimen nearly 25 years ago during a visit to the Dominican Republic’s amber mines. It was not until he recently took a closer look with a powerful compound microscope—magnifying the specimen up to 1,000 times—that he noticed the tiny ticks within.

Like modern ticks, the ancient ones likely picked up spirochetes when they took blood meals from infected vertebrates. Which of those hosts served as a natural reservoir for the spirochetes found in the juvenile tick, however, remains unknown. Ancestors of modern-day jaguars, ancient woodpeckers and shrewlike solenodons are a few potential spirochete-harboring hosts that lived in the same hot, balmy forest as the tick. Poinar, however, thinks it is more likely that the tick inherited its spirochetes from its mother (a process called vertical transmission), rather than from an animal reservoir. He found no evidence that the young arachnid had feasted on blood prior to its fatal encounter with the amber-forming tree resin.

Poinar placed the newly discovered bacterium within its own genus, naming it Paleoborrelia dominicana. He could not attempt to analyze the ancient DNA to confirm whether or not the bacterium is related to modern Borrelia because those tests would destroy the specimen. So it is impossible to know how closely—if at all—the ancient spirochetes are related to contemporary Borrelia. But the bacteria’s morphology and its location within the tick’s alimentary tract indicate that it probably has ties to those notorious pathogens. “Logic and all other evidence there says it could well be a related spirochete to Borrelia,” Kuris says. “But we cannot know for sure, because there are other spirochete parasites that use ticks as vectors.”

The oldest evidence of Lyme disease dates back 5,300 years, to an ice mummy discovered to contain Borrelia’s genetic material, but the condition’s evolutionary origins are unknown. Lyme disease is only one of a number of human afflictions caused by spirochetes, however, so even if that condition arose more recently, our problems with spirochetes likely date back much further. When Homo sapiens arrived on the scene around 200,000 years ago, Poinar thinks, the ticks—and their spirochetes—were probably waiting “As long as humans have been around,” he says, “I’m sure that they suffered from ailments caused by spirochetes carried by ticks.”