When we think of farm animals, we often associate them with providing us with food and resources. However, a recent study has shed light on a much darker aspect of human-animal interactions. Scientists have discovered that the domestication and husbandry of livestock coincided with the emergence and spread of animal-borne diseases that have plagued human history.

Archaeologists have long suspected that when nomadic hunter-gatherers in Eurasia settled into large, pastoral communities around 12,000 years ago, the risk of pathogens jumping from animals to humans would have increased. Thanks to recent advances in ancient DNA analysis, experts were able to put this hypothesis to the test. By sifting through billions of DNA sequences from ancient human remains across Eurasia, researchers identified numerous genes belonging to microbes.

In their comprehensive search for pathogen DNA, the research team found that zoonotic diseases, where pathogens spread between animals and humans, were only detected from around 6,500 years ago. Surprisingly, the bacterium that causes the plague and the pathogen that causes louse-borne relapsing fever were not detectable in human remains until around 6,000 years ago. This coincided with the transition from hunter-gatherer societies to agricultural societies.

The increase in zoonotic diseases was not solely due to direct human-animal interactions. As human communities grew denser in population, hygiene decreased, and pests like rodents, fleas, lice, and ticks increased. Poor living conditions and hygiene led to outbreaks of diseases like louse-borne relapsing fever.

Societies in the Eurasian steppes that were exposed to zoonotic pathogens prior to others may have had an advantage. These pastoral communities had regular access to meat and dairy and their bodies had time to adjust to new animal pathogens. The detection rates of zoonotic microbial DNA in human remains spiked around 5,000 years ago, suggesting that Steppe pastoralist populations brought their knowledge of agriculture and their zoonotic diseases when they migrated to new regions.

If the Steppe pastoralists did carry zoonotic diseases with them, it could explain the genetic upheaval in Europe. Epidemic waves of zoonotic diseases sweeping through the continent may have caused significant fatalities, similar to what later happened to Indigenous people during European colonization.

As human communities in Eurasia grew denser, zoonotic pathogens thrived. The bacterium responsible for the plague caused its first epidemic in the Roman Empire around 540 CE. The recent genomic analysis suggests that the plague had been present at lower levels from 5,700 years ago to about 2,700 years ago. By medieval times, the plague became a mass killer, as evidenced by the high prevalence of the disease in medieval Danish cemeteries. On the other hand, louse-borne relapsing fever peaked around 2,000 years ago, potentially due to increased crowding and poor hygiene, war, forced migrations, poverty, or famine.

The link between farm animals and major human diseases is a significant aspect of human history. By analyzing ancient DNA, scientists have uncovered the impact of the domestication of livestock on the emergence and spread of zoonotic diseases. This research highlights the importance of understanding our relationship with farm animals in order to better prevent and manage the spread of animal-borne diseases in the future.

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