Human civilization owes much of its existence to farm animals, but it turns out that the domestication and husbandry of livestock have also played a role in the spread of major illnesses throughout history. Recent advances in ancient DNA analysis have allowed scientists to investigate the relationship between animal-borne diseases and the emergence of agriculture. By examining 405 billion DNA sequences collected from ancient human remains, an international team of researchers has identified genes belonging to microbes that were likely transmitted from animals to humans. This finding provides direct evidence for the hypothesis that the risk of pathogen transmission increased as humans transitioned from nomadic hunter-gatherers to settled pastoral communities around 12,000 years ago.
Through their extensive DNA analysis, the researchers were able to establish a timeline of disease emergence and spread. While many human infecting microbes remained relatively constant over time, zoonotic diseases, which spread between animals and humans, were only detected around 6,500 years ago. This coincides with the transition from hunter-gatherer societies to agricultural societies. The bacteria responsible for the plague and louse-borne relapsing fever, for example, were undetectable in human remains until around 6,000 years ago. From that point on, zoonotic microbial DNA consistently appeared in the genomes of ancient human remains.
The increase in zoonotic diseases was not solely attributed to direct human-animal interactions but also to changing living conditions. As human communities grew denser, hygiene decreased, and pests like rodents, fleas, lice, and ticks became more prevalent. Historically, outbreaks of louse-borne relapsing fever were associated with poor living conditions. The team of researchers concludes that the transition to agriculture led to an epidemiological transition, resulting in an increased burden of zoonotic infectious diseases throughout history.
Early societies in the Eurasian steppes that were exposed to zoonotic pathogens before others may have had an advantage. These pastoral communities had regular access to meat and dairy, allowing their bodies to adjust to new animal pathogens over time. The detection rates for zoonotic microbial DNA in ancient human remains throughout Eurasia spiked around 5,000 years ago, suggesting that Steppe pastoralist populations migrating to new regions brought these diseases with them. It is possible that continuous exposure to animals conferred some immunity on the Steppe pastoralists, leading to the westward and eastward spread of zoonotic diseases through epidemic waves. This genetic upheaval may have played a role in shaping the population of Europe.
As human communities grew denser over time, zoonotic pathogens proliferated, leading to endemic outbreaks that turned 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 that this bacterium was present at lower, relatively continuous levels from 5,700 to 2,700 years ago. By medieval times, the plague had become a mass killer, as evidenced by the high number of individuals affected by the illness in medieval Danish cemeteries. In contrast, louse-borne relapsing fever peaked around 2,000 years ago when there was little discernible plague activity. It is speculated that the spread of this disease was a result of increased crowding, poor hygiene, war, forced migrations, poverty, or famine.
The domestication and husbandry of livestock have had a profound impact on the spread of diseases throughout human history. The transition from hunter-gatherer societies to agricultural societies coincided with the emergence of zoonotic diseases, and as human communities grew denser, these diseases became more prevalent. Early exposure to zoonotic pathogens may have provided certain populations with immunity, leading to the spread of these diseases to new regions. The rise of zoonotic diseases has shaped the genetic makeup of European populations and contributed to the devastating epidemics that have occurred throughout history. It is clear that the history of human civilization and the emergence of diseases are intricately linked, highlighting the complex relationship between humans and the animals they rely on.