Diseases started jumping from animals to humans at least 6,500 years ago, researchers found in a new study of ancient DNA.
After analyzing ancient DNA from 1,313 prehistoric humans from Europe and Asia, researchers charted a map and timeline of human infectious disease that spans 37,000 years. Within that long history, they uncovered the earliest-known evidence of zoonotic disease — in which pathogens in animals transfer to humans — dated to 6,500 years ago.
The researchers described their findings in a study published Wednesday (July 9) in the journal Nature, noting that cases of zoonotic disease probably occurred before that point. But they said the risk and extent of the transmission of such diseases probably increased as humans interacted with animals more frequently, namely through farming and animal husbandry.
Migration likely also played a role, as individuals may have carried zoonotic diseases to new populations that had not yet been exposed to them.
“Today, zoonoses account for more than 60% of newly emerging infectious diseases,” the researchers wrote.
The researchers found a peak in evidence of zoonosis in samples that are around 5,000 years old. They argue this coincides with the period when livestock domestication became more widespread. (Evidence suggests animal domestication began around 8,000 to 10,000 years ago and then likely took time to spread to various geographies.)
Related: 32 diseases you can catch from animals
Until now, questions remained about where and when known human pathogens first emerged and how they were distributed around the world. Thanks to new technology that can capture genomic evidence of such diseases in ancient DNA, some of these questions are beginning to be answered.
In total, 214 known human pathogens were detected in the study’s DNA samples, which were gathered from the bones and teeth of ancient human remains. The oldest case with a known pathogen uncovered in the study involved Corynebacterium diphtheriae, the bacterium behind diphtheria. The microbe’s DNA was discovered in a sample from the Mesolithic period and dated back as far as 11,400 years.
Twelve cases involved the Yersinia enterocolitica bacterium behind the zoonotic disease yersiniosis, which causes various symptoms including fever and diarrhea. The oldest remains showing evidence of this pathogen were found in Denmark and are about 6,500 years old.
The researchers also found evidence of some more well-known pathogens — including 42 suspected cases of the plague-causing bacterium Yersinia pestis — in about 3% of their samples. However, they did not detect the pathogen responsible for tuberculosis: Mycobacterium tuberculosis.
The team suspects that they didn’t detect M. tuberculosis because it is typically a low-load bloodstream infection. Due to the dataset they used, they were most likely to detect bugs that accumulate in high concentrations in the blood during an infection.
The samples from the human remains consisted of a mixture of human, germ and other DNA. After excluding any human DNA, the team then identified which DNA belonged to human pathogens and which came from other sources, such as bacteria involved in the decomposition process, the soil or from the human microbiome.
One limitation of the study is that the technology used does not detect RNA, a cousin of DNA that forms the basis of many germs. Flu viruses contain RNA, for instance, so analyzing RNA could have provided evidence of different influenza pandemics throughout history.
“There are many epidemic-type pathogens that are RNA viruses that we would like to study from the past. But the problem with those is that RNA is not as stable a molecule as DNA,” study lead author Martin Sikora, an associate professor who studies human and pathogen evolution at the University of Copenhagen, told Live Science. “So far, we haven’t really been successful at extracting this type of information from archaeological remains.”
This is “the largest study to date on the history of infectious diseases,” the researchers said in a statement, adding that it could potentially have implications for the future of medicine, including the development of vaccines.
Sikora said that while reconstructing the genomes of these ancient pathogens, sometimes they got enough data to recover the whole genome sequence of a particular germ. In theory, new vaccines could be developed based on this information and would be available to protect humans against viruses that are not around now but could emerge again in the future, he suggested.
