Eske Willerslev and his colleagues, at the University of Copenhagen, in Denmark, began studying DNA from bacteria on teeth that caused plague during the Bronze Age and Iron Age. This time was marked between 4,800 and 3,000 years ago. Willerslev concluded the early plague germ, Yersinia pestis, was spread from person to person among the herders migrating across Europe and Asia during the Bronze Age. Coughing could have spread the plague, and if the germ caused a lung infection, the illness was considered pneumonic plague. If the blood of an individual became infected, it was considered septicemic plague.
Willerslev and his team identified Y. pestis DNA on teeth from Bronze and Iron Age individuals. The DNA pieces showed up in seven of the 101 people. Two people with the strain were from Russia roughly 4,800 years ago. Another was from North Central Europe, 4,500 years ago. The third infected person examined was in West Asia from almost 4,200 years ago. Several more teeth collected were from Siberia and Poland and were 3,700 to 4,000 years old. The last sample was from an individual who lived in Armenia 3,000 years ago.
From the evidence Willerslev and his team gathered, it is clear the germ spread over a wide area and for a large amount of time. Further investigation was concluded of the entire genome in the Bronze Age DNA. The analysis showed the earlier germ lacked the gene, which was present in the Iron Age individual. This gene allowed bacterium to survive in a flea’s gut. The plague germ would have been able to spread via flea bites at some point during 3,700 and 3,000 years ago.
The biologists have compared plague DNA in one of the oldest Bronze Age individuals and the most recent Bronze Age individual and concluded both lacked a form of one plague gene that keeps an infected persons immune system from attacking the disease. This suggests that the earlier forms of the germ would have been vulnerable to attacks by the immune system.
One reason I found this article to be interesting was due to the time periods Willerslev and his team worked with. They collected and screened 89 billion pieces of DNA from teeth of individuals from the Bronze and Iron Age. That was almost 5,000 years ago! It is fascinating to know a scientist can pull ancient DNA from teeth of individuals that lived thousands of years ago. Another captivating point in the article mentioned this disease took over even when there really was no urbanization. But certainly this plague evolved and survived longer than expected.