Human civilization has come a long way, but it would not have been possible without the impact of farm animals. These animals have played a vital role in the development and sustenance of our societies. However, recent scientific discoveries show that the rise of animal-borne diseases coincided with the domestication and husbandry of livestock. In this article, we will explore the evidence that connects the emergence and spread of major human diseases to the interaction between humans and animals.
Thanks to advancements in ancient DNA analysis, researchers have been able to study the genomes of 1,313 ancient human remains from Eurasia. Led by geogeneticist Martin Sikora at the University of Copenhagen, an international team of experts identified various microbial genes within these DNA samples. Their findings shed light on a 12,500-year long timeline of disease emergence and spread.
The transition from nomadic hunter-gatherer societies to settled pastoral communities around 12,000 years ago brought about a significant increase in the risk of pathogens jumping from animals to humans. While many microbes that infect humans remained relatively consistent during this transition, zoonotic diseases – where pathogens spread between humans and animals – only became detectable around 6,500 years ago.
The discovery of the bacterium that causes the plague, Yersinia pestis, and the pathogen responsible for louse-borne relapsing fever (LBRF) coincided with the rise of agricultural societies. These zoonotic diseases were undetectable in human remains until approximately 6,000 years ago. As human communities grew denser and hygiene decreased, pests such as rodents, fleas, lice, and ticks proliferated, contributing to the spread of these diseases.
Zoonotic diseases have historically made up a significant portion of newly emerging infectious diseases, accounting for over 60 percent of cases today. However, these diseases were entirely new to human populations thousands of years ago. Pastoral communities in the Eurasian steppes, which were exposed to zoonotic pathogens earlier than others, may have had the advantage of developing some level of immunity and adjusting to these new pathogens.
Around 5,000 years ago, there was a notable increase in the detection rates of zoonotic microbial DNA in human remains throughout Eurasia. This suggests that Steppe pastoralist populations, who migrated to new regions with their agricultural knowledge, also brought their zoonotic diseases along. These migrations potentially facilitated the genetic upheaval in Europe as epidemic waves of zoonotic diseases swept through the continent.
As human communities became denser, zoonotic pathogens had the opportunity to cause endemic outbreaks that eventually 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. Genomic analysis reveals that Yersinia pestis was present at lower levels from 5,700 years ago to about 2,700 years ago. By medieval times, the plague had become a devastating mass killer.
Louse-borne relapsing fever (LBRF) peaked around 2,000 years ago when the plague was less active. Researchers suspect that crowding, poor hygiene, war, forced migrations, poverty, and famine all contributed to the spread of LBRF during this period. These factors created conditions that favored the transmission of the disease.
The relationship between human civilization and animal-borne diseases is a complex one. As humans domesticated and lived in closer proximity to livestock, the risk of zoonotic disease transmission increased. The advent of agriculture allowed for denser populations, which further facilitated the spread of these diseases. Understanding this historical context helps shed light on the challenges faced by early societies and the impact of these diseases on human populations. By studying ancient DNA, researchers continue to uncover valuable insights into the origins and spread of diseases throughout history.
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