“Pathogen breeding” in the course of cattle breeding

Livestock farming created complex interactions between animals, humans and bacteria. (Image: Annette Günzel)

Herding instead of hunting: The development of livestock farming made life easier for our ancestors – but close contact with the animals also had a catch, as a study now shows: Livestock farming encourages the development of new pathogens, according to studies of ancient bacterial genomes , They come from remains of the victims of Salmonella infections that are up to 6500 years old. According to the genetic characteristics, the early forms of these pathogens, unlike today’s versions, infected both humans and animals, which suggests their origin in the context of livestock farming.

Around 10,000 years ago there was a groundbreaking development that would forever shape the fate of mankind: the Neolithic Revolution. Instead of hunting and gathering, domesticated people grew livestock and planted crops – the first peasant cultures to feed on agriculture and animal husbandry developed. So sheep, pig and co found their way into the lives of our ancestors. There was close contact with animals to which there was previously a predator-prey distance. As a result, people were confronted with pathogens more intensely than before. It was therefore already assumed that neolithization was linked to the development of new human diseases. An international team of researchers is now supporting this previous assumption for the first time with research results.

Tracking down ancient Salmonella

As part of their study, the scientists examined teeth from 2,739 skeletons for the genetic traces of pathogens from the Salmonella group. The human remains came from different European regions and could be assigned to hunters and gatherers, nomadic cattle breeders or farmers from the period up to 6500 years ago. A newly developed computer-controlled method was used for the genetic analyzes. “This enabled us to examine a large number of archaeological samples for traces of Salmonella DNA,” says co-author Alexander Herbig from the Max Planck Institute for the History of Humanity in Jena.

This genetic search was successful: The researchers were finally able to reconstruct eight different genomes of the pathogen Salmonella enterica with an age of up to 6500 years. As you emphasize, these are the oldest known bacterial genomes to date. Since the genetic material was discovered in the area of ​​the dental nerve of the skeletons, it seems clear that the people had a salmonella infection in the body at the time of their death. The results thus reflect what is likely to be a serious health problem in the past. Above all, however, they show how the bacterial pathogen developed over the 6500 years, say the scientists.

The influence of animal husbandry is emerging

Basically, the genetic comparisons showed that all reconstructed genomes come from pathogen forms that are related to the diverse representatives of the Salmonella enterica pathogen group. It was particularly interesting that the Salmonella genomes from farmers and cattle breeders were precursors of the Salmonella Paratyphi C bacterial strain. Today’s version infects humans and triggers typhoid-like symptoms that can lead to death. On the other hand, the genetic characteristics of the historic Salmonella show that they were not yet fully adapted to humans and also infected animals.

As the researchers explain, this suggests that the development of these pathogens was linked to new cultural habits in the context of neolithization. In other words: the close contact with farm animals has facilitated the development of human-specific diseases. “The broad temporal, geographical and cultural spectrum of our findings made it possible for us for the first time to use molecular genetics to link the evolution of pathogens with the advent of a new human way of life,” sums up Herbig.

Through their study, the researchers also want to draw attention to the potential of their methodology: “Historical metagenomics can open up a new window for us in the past of human diseases,” says Felix Key, the first author of the study. He and his colleagues hope that other scientists are now using the method to link the link between human-cultural evolution and the development of diseases. “As in the case of Salmonella, we could now begin to understand the genetic changes necessary to adapt to the host. We can use this knowledge to understand the mechanisms that enable diseases to be transmitted from animals to humans, ”says Key.

Source: Max Planck Society, professional article: Nat Ecol Evol, doi: 10.1038 / s41559-020-1106-9

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