In an iconic scene from Steven Spielberg’s “Jurassic Park,” millionaire philanthropist John Hammond and an entourage of scientists find a mosquito fossilized in amber. The finding serves to extract the DNA of the dinosaurs preserved intact for more than 65 million years.
Of course that’s a science fiction movie. “In fact, you can’t extract DNA preserved in amber,” says María Alejandra Perotti, professor of invertebrate biology at the University of Reading, England.
But that doesn’t mean the technique itself isn’t feasible. This is precisely what Perotti has achieved, who like Hammond was able to extract DNA, but not with dinosaurs and mosquitoes, but with ancient humans and lice.
Perotti, an Argentine who has been working in England for almost 20 years, studies the scientific and historical importance between invertebrates (more specifically lice) and humans to answer one of the questions we have all asked ourselves: where do we come from?
But it takes the issue to a more regional level: how was South America populated?
A group of scientists from five universities coordinated by her discovered that the “concrete” that lice use to attach their nits (eggs) to people’s hair turned out to be a source of genetic information “of very good quality” of mummies up to 2,000 years old found and preserved in San Juan, Argentina, near the Andes mountain range.
Unlike modern humans, our ancestors did not have effective methods of getting rid of lice, which could cause real infestations on the scalp and clothing.
The female deposited the eggs and these adhered with unusual efficiency to their host in such a way that in some cases they have even remained in them for millennia.
Lice could be annoying to their former hosts, but for scientists like Perotti, the fact that they show they have remained intact for so long is good news.
knowing that it is possible study the evolutionary history of humans through these insects, set out to search for samples of human remains that contained hair.
Due to its own characteristics, the nit can remain intact for thousands of years, surrounded by its own glue. And within this “cement” it is possible to find human cells.
“Yeah, it’s a bit like Jurassic Park”, Perotti tells BBC Mundo. “Of course the film is fictitious, but we make the analogy because the objective is the same: to characterize the host through a parasite with a substance produced by the same parasite.”
The human DNA found in these samples turned out to be of very good quality, as much as the one that can be extracted from the teeth and the petrosal bone that is located behind the ear.
Thanks to this, scientists are discovering more details about the ancient populations of South America and these mummies, like the migration routes on the continent, sex and even possible causes of death.
“It’s very interesting,” Perotti tells BBC Mundo. “Lice have always caught my attention because they live very close to the host and act like a mirror. I started using them to interpret what had happened to the host.”
“Thanks to them we can study thousands of years of history. They are a mirror of evolutionary history.”
A reflection of ancient humans
There are two subspecies of louse that affect humans, the louse: the body (of body) and the capitis (of the head).
Thousands of years ago, the body it adapted to live from the epidermis to clothing when ancient humans began to cover themselves with fur and cloth.
In itself, this is a demonstration of how the human louse has evolved alongside its hosts.
Perotti was aware of this relationship and began to investigate ancient collections kept in natural history museums to study the history of our ancestors from parasites.
The project includes the University of Reading (England), the Natural History Museum of the University of Oxford (England), the University of Bangor (Wales) and the National University of San Juan (Argentina). In 2016 they engaged scientists from the University of Copenhagen to do human DNA analysis.
The results were published in the specialized journal Molecular Biology and Evolution.
“I had been looking for samples of the original indigenous populations of South America for a long time. I investigated further until I had access to collections of human remains that contained hair.”
These samples were mummies preserved in Argentina, in addition to other human remains, with different dates between 1,300 and 2,000 years found in the caves of Calingasta, in the province of San Juan. For analysis, the team extracted no more than six nits per mummy.
Extracting DNA from such ancient samples is not easy, and is sometimes even considered controversial, especially in samples in a good state of preservation.
Normally the genetic material is extracted from the teeth or the petrous bone, which in turn implies its destruction.
The sample also has to meet certain requirements and be in good condition. It is not always possible, and sometimes the material is destroyed or partial.
With the study of the lice, the scientists saw that they avoided several of these inconveniences and also preserved the samples for later studies.
The original idea was to extract human DNA from the same preserved lice.
But they were surprised when they discovered that the “packaging” that covered the nit had trapped human DNA not only “of very high quality”, but also it was very well protected thanks to the chemical characteristics of the glue, as explained by Mikkel Winther Pedersen, from the University of Copenhagen.
“Once the egg was attached to the hair, it immediately absorbed the skin cells, probably from the scalp,” he says. “The interesting thing here is that (the material) was protected from degradation.
“Everything degrades, including me. We disappear over time. And yet, here we have these samples”, he emphasizes.
Migration routes and diseases
What did the scientists find?
One of the mummies, the 2,000-year-old, was of an individual who came from the north of the Amazon. They know this because the extracted DNA coincides with that of other previously analyzed indigenous populations from the southern areas of Colombia and Venezuela.
The others, on the other hand, more recent (between 1,300 and 1,500 years), do not have the same genetic characteristics, so their origin is different: from the Patagonia, from the south.
All this reveals that there was a great migratory movement in the region thousands of years ago. Those coming from the north may have taken an eastern route, probably driven by climatic changes, such as droughts. “But we didn’t know they had come so far south,” Perotti confesses.
These routes had been suggested years ago through anthropological studies that did not involve genetics. But the DNA findings may provide further clues to how humans were distributed on the continent millennia ago.
Scientists also believe that the individuals became very cold and that this may have been the cause of death.
They know this because in environments with much lower temperatures, the nits are closer to the scalp, where there is more heat.
Because the nit’s glue traps everything around it, not just human DNA, the scientists also found genetic material that was neither from the louse nor from the host. This is the earliest evidence of polymorphic target of Merkel.
Discovered in the United States in 2008, it is suspected this virus causes most cases of Merkel cell carcinoma, a rare but aggressive form of skin cancer. Experts now suspect that perhaps head lice had something to do with the spread of this virus.
To check their findings, the team also analyzed the DNA of the lice themselves, and what they found was that they had the same migration pattern as their hosts.
“There is a lot of interest from Europe, from the United States and even from Asia to learn about the history of South America,” adds Perotti.
“South America received the last migrations of anatomically modern humans. Human beings have been studied in all parts of the world, but there is a lack of focus and more precise studies to find out what happened in South America”.
“For me, who is from Latin America, it is a pride to have carried out this investigation,” he concludes.
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