For decades and centuries, a disease repeatedly left a black patch in the lives of ancient Eurasia, quickly spreading far and wide. The plague’s most infamous form — the Black Death in the 14th century — was spread through bites of infected fleas that lived on rats. This is still the most common form of transmission to date.
However, during the Bronze Age, the plague bacterium (Yersinia pestis) lacked the genetic makeup to spread through fleas. Scientists have thus been baffled as to how the illness persisted at that time.
Now, an international team of scientists has discovered the first plague bacterium genome in a sheep that lived in modern-day Russia, during the Bronze Age, 4,000 years ago. This discovery provides scientists with new clues about the transmission and ecology of the disease and how it spread in the ancient past. Scientists now believe that livestock played a vital role in the spread of the disease throughout Eurasia.
Yersinia pestis is a zoonotic disease, which is transmitted from other animals to humans and vice versa. This pathogen has been present since prehistoric times, but its ancient genome has been studied from human remains. This left us with the question about how the pathogen infected humans. Finding the ancient bacterium in an animal can help researchers understand the evolutionary pathway taken by the microbe. This can also help us understand modern diseases.
The Late Neolithic Bronze Age lineage is the name given to the ancient bacterium strain that spread from Europe all the way to Mongolia, with evidence of the disease found across 6,000 kilometers (3,700 miles).
Recent proof suggests that the majority of modern human diseases came into existence within the last 10,000 years. The period coincided with the domestication of animals such as livestock and pets. Animals other than rodents were suspected by scientists to have caused the Bronze Age plague; however, scientists couldn’t pinpoint the animals that spread the disease.
Scientists have discovered plague infection in the tooth of a 4,000-year-old sheep found at an archaeological site in Arkaim, Russia. This same strain was also discovered in human remains from that area.
According to Dr. Taylor Hermes, a study coauthor and an assistant professor of anthropology at the University of Arkansas, domesticated sheep served as a link between humans and infected animals.
“We’re sort of unveiling this in real time and trying to get a sense for how Bronze Age nomadic herders out in the Eurasian Steppe were setting the stage for disease transmission that potentially led to impacts elsewhere,” Hermes said, “not only in later in time, but also in a much more distant, distant landscape.”
During this time within the Eurasian Steppe, one in five bodies were infected, and likely died from plague. Although livestock is seemingly a part of the plague puzzle, much remains to be discovered. The identification of the bacterial lineage in an animal opens up new paths into the disease’s evolution.
Humans and animals were likely passing the disease to and fro, but it isn’t clear how they did so. One hypothesis suggests that the sheep likely picked up the pathogen from contaminated food and water, and then it was consumed by humans, effectively transmitting the bacteria through contaminated meat.
Plague strains that prevailed during the Bronze Age are now extinct; however, modern strains still prevail in parts of Africa and Asia, as well as the western United States, Brazil, and Peru. Annually, 1,000 to 2,000 cases of plague occur worldwide. There is no need for alarm; however, safety measures must be taken while cooking meat or if bitten by an animal.
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