A sweeping new genomic study published in Nature has uncovered compelling evidence that many of the most notorious infectious diseases—such as plague and leprosy—made the leap to humans only after ancient communities began domesticating animals. The research highlights how the shift from hunter-gatherer lifestyles to settled livestock farming created the perfect storm for zoonotic diseases to take hold.
Around 6,500 years ago, human societies across Eurasia began settling in permanent dwellings, keeping livestock such as cows, goats, and pigs. This agricultural transition, while transformative for civilization, appears to have also invited a host of animal-borne diseases into human populations for the first time.
“Zoonosis really became a problem only with this major lifestyle change,” said study co-author Astrid Iversen, a virologist at the University of Oxford. “It’s not a new idea, but they’ve actually shown it with the data,” added Edward Holmes, virologist at the University of Sydney.
A Genomic Time Machine
The study—described as a “technical tour de force” by experts—analyzed DNA from 1,313 ancient human remains, spanning 37,000 years of Eurasian history. Led by researchers from the University of Oxford and the University of Copenhagen, the team extracted microbial DNA from ancient blood traces preserved in teeth and bones.
They identified 5,486 microbial sequences, including bacteria, viruses, and parasites. Crucially, zoonotic pathogens—those capable of jumping from animals to humans—only began to appear in the data set around 6,500 years ago, peaking 5,000 years ago during the rise of agrarian societies.
“The scale of the work is really pretty breathtaking,” said Holmes.
The Livestock Link
The timing is no coincidence. Roughly 5,000 years ago, Steppe pastoralists migrated into Europe with horses, ox-drawn wagons, and large herds of livestock. Their mobile, animal-rich lifestyles—and their animal-based diets—offered pathogens new opportunities to evolve and spread.
“Such lifestyle changes gave pathogens ample opportunities to jump from animals to people and to spread quickly between both groups,” said Iversen.
For example, the bacterium Yersinia pestis, which causes plague, first appeared in the ancient human DNA record around 5,700–5,300 years ago, just after livestock farming became widespread. The study detected recurring peaks in the bacterium’s presence, supporting the idea that it repeatedly emerged and spread via new waves of human-animal contact.
Similarly, the bacterium Mycobacterium leprae, responsible for leprosy, first appeared in the dataset in seven Scandinavian individuals from around 1,500 years ago, coinciding with the growth of the squirrel fur trade—a practice that likely helped the disease cross species.
Modern Parallels and Ongoing Risks
The study’s implications extend beyond history. Today, zoonotic diseases remain a critical public health issue. An estimated one billion people are infected with such diseases annually, and nearly two-thirds of emerging infections—including COVID-19 and avian influenza (H5N1)—originate in animals.
“The findings are highly relevant today, given how humans continue to interact with and encroach on animal habitats,” said Iversen.
What’s Next?
Lead researcher Martin Sikora of the University of Copenhagen said the team now hopes to expand the geographic and temporal scope of their work. The current study focused on pathogens found in high enough concentrations in blood to be detected, meaning others may have gone unnoticed.
“I’m sure there’s more in there,” said Sikora, emphasizing the potential for future discoveries as techniques improve.
