DNA from the bones and teeth of 1300 people who died up to 37,000 years ago has revealed what infectious diseases some of them had when they died 鈥� as well as how the incidence of some of these diseases changed over time. The findings show that animal diseases were much more likely to jump to humans after the advent of farming.
This is the first direct evidence that the domestication of animals led to humans acquiring more infectious diseases, according to at the University of Copenhagen in Denmark and his colleagues. This 鈥減rofoundly impacted global human health and history throughout the millennia and continues today鈥�, the team writes.
The study looked at a wide range of microbes in human remains from all over the world and many different times, making it the largest and most comprehensive of its kind so far. The oldest human remains were from around 37,000 years ago, but most were between several thousand and a few hundred years old.
Advertisement
The researchers exploited the fact that it is becoming common to sequence the genomes of ancient people, and that the DNA of bacteria or DNA viruses present in the bones or teeth gets sequenced, too.
These microbial sequences are filtered out when ancient human genomes are reassembled. To identify them, the team analysed raw data from the sequencing of more than 1300 ancient human genomes, including 130 that haven鈥檛 yet been published.
Most of the microbial DNA the researchers found was from soil bacteria, suggesting bacteria got into the bones after burial. In teeth, much of the microbial DNA came from bacteria known to live in people鈥檚 mouths.
However, the team was also able to identify many disease-causing bacteria and viruses that were in the blood of people before they died and may have caused or contributed to their death.
The most common was the bacterium that causes the plague, Yersinia pestis, found in 39 people, which is 3 per cent of the remains. This bacterium mainly infects rodents, but can be spread to people by fleas.
The oldest plague cases were in three people who lived in various parts of Asia around 5700 years ago. The bacterium was also found in a person entombed on Orkney in Scotland around 4800 years ago 鈥� about 800 years before the previous .
Overall, the team found lots of plague cases between 6000 and 3000 years ago. Then there was a gap until 2000 years ago, when there was a wave lasting for a few centuries, then another gap until a second wave corresponding with the medieval 鈥淏lack Death鈥� plague pandemic.
The researchers think that the gaps where no plague cases were detected 鈥渞epresent an actual reduction in the underlying incidence of the disease鈥�. Their findings fit with other studies suggesting that the early form of plague wasn鈥檛 very transmissible and died out, to be replaced later by more transmissible strains that caused pandemics.
The next most common microbe was Borrelia recurrentis, which causes a disease spread by body lice聽called louse-borne relapsing fever. This disease is now rare, but the team found it in 31 people, 2.3 per cent of the total, suggesting it was widespread in the past.
The first cases the researchers found were in Scandinavia around 4500 years ago, implying B. recurrentis first jumped from animals into people around this time, but it isn鈥檛 clear what the original animal source was.
Other diseases identified include malaria, hepatitis B, leprosy and leptospirosis, also known as Weil鈥檚 disease.
The researchers divided the kinds of microbes they found into five broad types, including zoonotic diseases, those that jumped from animals into humans. They found there was an increase in zoonotic diseases from around 6000 years ago, but not of any of the other four types.
鈥淭he risk and extent of zoonotic pathogen transmission likely increased with the adoption of more widespread husbandry practices and pastoralism,鈥� the study says.
at the Francis Crick Institute in London says the work is 鈥減romising鈥�. While it is possible to identify trends in the incidence of pathogens in the distant past, studies like this need to take account of potential biases, he says. For instance, people who died of disease may have been buried in different ways to the standard, or cremated instead.
Another issue with the study is that standard DNA sequencing misses RNA viruses, such as flu and coronaviruses, which may have caused major outbreaks in the past. Specific techniques are needed to detect these viruses.
Willerslev declined to discuss the findings prior to publication in a peer-reviewed journal.
bioRxiv
