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Co-evolution of humans and pathogens

Humans have lived through pandemics for centuries, and according to a reconstruction of ancient infectious diseases, pathogens and humans appear to have co-evolved with mutually favourable outcomes before the advent of modern medicine.

Evidence from ancient skeletons revealed that over the past 5000 years, prevalence of the major infectious diseases leprosy, tuberculosis and syphilis declined naturally, Australian researchers report in the journal PLOS ONE.

“Humans, over time, can adapt to tolerate infections,” says lead author Maciej Henneberg, from the University of Adelaide. “In most human populations are some people who do not get sick while exposed to an infection. This also applies to viral infections, including COVID.”

His team, in collaboration with Switzerland’s University of Zurich, re-analysed published data on the three historic diseases to explore how their prevalence changed over time, covering nearly 70,000 ancient human skeletons with records spanning more than 200 generations.

Diseases can leave their mark on hard tissue, or bone, in severe cases and the evidence is preserved indefinitely on the skeletal remains. This allows new historical insights into ancient infections through the growing discipline of paleopathology.

Results suggested that leprosy and syphilis (treponematoses) increased before dropping after the end of the Middle Ages in Europe and before Europeans invaded North America, respectively. Tuberculosis showed a steady decline, and the authors say it’s likely that this disease infected humans earlier and peaked before records became available.

As always, there are limitations to the data, the authors note. External circumstances would have also played a role – in major cities like London and New York, for instance, mortality from tuberculosis dropped after widespread initiatives such as improved sanitation and nutrition.

Overall, the findings do make sense according to what is known about pathogens. Like all organisms, they are subject to the laws of natural selection and seek hosts so they can replicate and spread. But it’s not in their best interest to kill off their targets.

“In the long run, it is advantageous for the pathogen to not cause the death of its host, thus ensuring the reproductive success of the pathogen,” write Henneberg and team. “At the same time, survival of the host can only be ensured through reproductive success.”

Eventually the host and pathogen adapt to each other, they explain, and the pathogen ends up becoming benign (commensal) – or even useful (symbiont). Tuberculosis, for example, only develops in 5–10% of people who are infected with Mycobacterium tuberculosis

This co-evolution is the most logical factor underlying the trajectory of the three diseases, the authors note, which have different pathologies and modes of transmission. It’s not clear which evolved though – whether it was humans, the pathogens or both.

But co-adaptation allows both organisms to survive. “Adaptation can occur only through the process of evolution,” they write. “It can be achieved by changes in pathogens, in hosts’ immune systems and in hosts’ bodies providing greater tolerance to infections.”