Human civilization as we know it would not exist were it not for farm animals. But neither, it seems, would many major illnesses in human history.
For the first time, scientists have found direct evidence that the domestication and husbandry of livestock coincided with the rise of animal-borne diseases, like the plague (Yersinia pestis) and louse-borne relapsing fever (LBRF).
Archaeologists have long suspected that as nomadic hunter-gatherers in Eurasia began to settle into large, pastoral communities around 12,000 years ago, the risk of pathogens jumping from animals to humans would have increased.
Recent advances in ancient DNA analysis have finally allowed experts to put that hypothesis to the test.
Sifting through 405 billion DNA sequences collected from 1,313 ancient human remains from across Eurasia, an international team of researchers led by geogeneticist Martin Sikora at the University of Copenhagen has identified numerous genes belonging to microbes.
Their sweeping search for pathogen DNA provided sufficient detail for a 12,500-year-long timeline on the emergence and spread of major human diseases.
The team's results are not yet peer-reviewed, but according to a pre-print paper, a substantial fraction of these microbes came from external, environmental sources, like animals.
While many microbes that infect humans remained steady throughout the sample period, zoonotic diseases – where pathogens spread to humans from animals or vice versa – were only detected from around 6,500 years ago.
In fact, the bacterium that causes the plague, which also lives in small rodents and fleas, and the pathogen that causes LBRF, which lives in lice, were both undetectable in human remains until around 6,000 years ago – a time that broadly coincides with the transition from hunter-gatherer societies to agricultural societies.
From that point on, zoonotic microbial DNA was consistently detected in the genomes of the ancient human remains studied.
The uptick in zoonotic disease is not solely the result of direct human-animal interactions. It could also stem from the fact that as human communities grew denser in population, hygiene decreased, and pests like rodents, fleas, lice, and ticks increased.
Outbreaks of LBRF, for instance, are historically associated with poor living conditions and hygiene.
"Our results thus provide the first direct evidence for an epidemiological transition of increased zoonotic infectious disease burden after the onset of agriculture, through historical times," conclude Sikora and colleagues.
Today, zoonotic diseases make up more than 60 percent of newly emerging infectious diseases, and yet all those millennia ago, such microbes were a new experience for humans.
Early societies in the Eurasian steppes that were exposed to zoonotic pathogens prior to others might have had a big advantage. Not only did these pastoral communities have access to regular sources of meat and dairy, their bodies had time to adjust to new animal pathogens.
Sikora and colleagues found a spike in the detection rates for zoonotic microbial DNA in human remains throughout Eurasia, occurring around 5,000 years ago.
This suggests that when Steppe pastoralist populations migrated to new regions around this time, bringing their knowledge of agriculture with them, they also brought their zoonotic diseases.
"It is possible that the Steppe pastoralists, through their long-term continuous exposure to animals, might have developed some immunity to certain zoonoses and that their dispersals carried these diseases westward and eastwards," the authors surmise.
"Consequently, the genetic upheaval in Europe could have been facilitated by epidemic waves of zoonotic diseases sweeping through the continent."
If this is true, many people in Europe likely died as a result of the human migration, mirroring what happened later to Indigenous people elsewhere in the world during European colonization. Over time, as human communities in Eurasia grew denser, zoonotic pathogens took off, turning endemic outbreaks into epidemics.
The bacterium responsible for the plague, which can live in horses, cattle, and sheep, caused its first epidemic in the Roman Empire around 540 CE. The recent genomic analysis also suggests Y. pestis was around at lower, relatively continuous levels from 5,700 years ago to about 2,700 years ago.
By medieval times, the plague was a mass killer. In just three medieval cemeteries in Denmark, researchers found 11 out of 39 individuals suffered from the illness at their time of death.
In comparison, LBRF peaked around 2,000 years ago, when there was barely any detectable plague activity. Researchers suspect it spread as a result of increased crowding and poor hygiene, war, forced migrations, poverty, or famine.
While more research needs to be done to understand why these outbreaks occurred, the team says their map provides "robust evidence" that a major shift in human lifestyle thousands of years ago ultimately led to an increase in zoonotic infectious diseases, "which has profoundly impacted global human health and history throughout the millennia and continues today."
The study was published in biorxiv.