Home Nature preserves DNA preserved in lice glue reveals secrets of South American mummies | Science

DNA preserved in lice glue reveals secrets of South American mummies | Science


Mummified man around 2,000 years old from Ansilta culture, Andes in San Juan, Argentina, had lice eggs and cement in his hair that preserved his own DNA
National University of San Juan

Anyone who has ever looked through a magnifying glass and struggled to pick nits knows how effectively female head lice cement each of their eggs to a human hair. Once these pests gain a foothold, they are notoriously difficult to dislodge. But even a school nurse might be shocked at their true endurance; scientists have already found louse eggs still stubbornly stuck to old hair after 10,000 years.

And now researchers have discovered something even more remarkable about the use of glue lice to adhere eggs to hair. Invertebrate biologist Alejandra perotti and his team found that lice cement is exceptional at trapping and preserving everything in it, including high-quality ancient human DNA from lice hosts. Their study, published this week in Molecular biology and evolution, was a case of life imitating art. It played out much like the scene from jurassic park, in which dinosaur DNA was preserved by mosquitoes that had sucked dinosaur blood before being then sealed in amber.

In this case, the female lice had secreted cement from the glands of their reproductive organs to attach eggs, called nits, to the hair of ancient humans, which later became 1,500 to 2,000 year old mummies in the past. Argentine Andes. In doing so, the lice trapped the skin cells of the human scalp in their cement. Perotti and his colleagues sequenced the genomes of skin cells to discover that these ancient inhabitants originally came from the rainforests of southern Venezuela and Colombia. In addition, they found that the DNA in the glue was kept at a quality similar to that typically extracted from teeth and higher than other common sources like dense petrous bone in the skull. This means that examples of ancient hair, clothing, and other textiles across the world, with their ubiquitous lice, could end up producing invaluable DNA that identifies their human hosts even though their remains are gone.

“If you have hair or clothes you can find nits attached,” says Perotti, of the University of Reading. “We can study thousands of years of the natural and evolutionary history of hosts and lice just by looking at DNA trapped in cement.”

Importantly, the method of Perotti and his colleagues allows scientists to study DNA without invasive or destructive techniques, such as opening skulls, which often poses cultural problems when studying DNA in human remains. old.

Team members from five different universities study South American mummies to learn more about when and how the continent was populated. The two mummies producing lice for this research were buried about two thousand years ago in the caves of Calingasta and rock shelters in the high Andes mountains of present-day San Juan province in west-central l ‘Argentina. In this cold and arid region where even the valleys rise to nearly 3,000 meters above sea level, the mummies were exceptionally preserved as well as the ectoparasites who shared their lives.

Perotti and his colleagues suspected that DNA may exist in the cement sheath that was used to stick each nit to a strand of hair on the mummies. Using a dye that binds to DNA and special imaging techniques, they revealed that the nuclei of human cells were in fact trapped and stored in ragweed. Then they inserted a tube and extracted this DNA for sampling.

DNA has shown genetic links between these mummies and individuals who lived in the Amazon 2,000 years ago. Evidence has shown that the region’s mountain dwellers, the Ansilta culture, were once native to the rainforest regions of what is now southern Venezuela and Colombia. Such information allows us to recreate the prehistory of South America, particularly complicated in Argentina where many indigenous groups were eradicated, assimilated or deported centuries ago.

To confirm their findings, the team also analyzed DNA from the nits themselves and compared it to other known lice populations. They found that the history of the parasites’ migration mirrored that of their human hosts from the Amazon to the Andes.

“All the nits we analyzed gave the same origin,” explains Perotti. ” It was very interesting. Completely independent of the host’s DNA, it has given us the same evolutionary history.

slow lice

A human hair with a slow attached with ragging cement.

Reading University

Because ragweed preserves everything it envelops, the team also found sources of environmental DNA that were neither human nor louse. With various strains of bacteria, they found the first evidence of the Merkel cell polymavirus. The virus, discovered in 2008, can cause skin cancer, and researchers now believe head lice may play a role in its spread.

The team also examined the morphology and attachment of nits to gain insight into the lives of their hosts. For example, lice lay eggs closer to the heat of the scalp in colder environments and the position of these nits, almost on the scalp of mummies, suggests that ancient humans were exposed to extremely cold temperatures which may have played a role in their deaths. .

“This work is remarkable on several levels”, says David reed a biologist from the Florida Museum of Natural History who was not involved in the study. “First, the authors were able to sequence the genome from such a small and seemingly insignificant starting material, and second, the lice on these heads have contributed to our understanding of human migrations.”

There is ample evidence to show that our ancestors lived with lice for millions of years. But scientists are only now exploring lice genomes to find out how the parasites moved, spread, and evolved with their primate and then human hosts around the world.

“Human lice have taught us so much about our history, from contact with archaic hominids to when humans first started wearing clothes,” Reed explains. “Lice seem to have more to say about our story. “

Surveys of mummies and archaeological sites confirm that many ancient groups were home to significant populations of head and clothing lice, which can still be found among their remains and artefacts of many types. Scientists even have discovered specialized combs that prehistoric South Americans used to try to get rid of pests. Fortunately for scientists today, these efforts have often failed.

The collections of museums and individuals are filled with lice, scattered among hair, textiles and clothing. Much of this archaeological material is now entirely out of context, collected generations ago from unknown sites and unrelated to particular places or times. But the nits that linger on these artifacts even long after their human hosts have faded into oblivion are now a newly discovered resource for learning much more about their former owners.

“The beauty of collecting information from nits is that they are stored for thousands of years, attached to hair or clothing,” says Perotti. “And now we can link them directly to a specific person. ”