In a scene from the film Jurassic Park, by director Steven Spielberg, millionaire philanthropist John Hammond explains to a group of scientists how his team found a mosquito fossilized in amber. The discovery allowed them to extract DNA from dinosaurs, preserved intact for more than 65 million years.
Of course this is because it is a science fiction movie. “You can’t actually extract DNA preserved in amber,” says Maria Alejandra Perotti, a professor of invertebrate biology at the University of Reading in England.
But this does not mean that the technique itself is not viable. This is precisely what Perotti achieved. Like Hammond, she was able to extract DNA, not from dinosaurs and mosquitoes, but from 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 everyone asks: where did we come from?
But she takes the question to a more regional level: how was South America populated?
A group of scientists from five universities, coordinated by her, discovered that the “cement” used by lice to stick their nits (eggs) in people’s hair turned out to be a source of “very good quality” genetic information from mummies of up to 2,000 years old found and preserved in San Juan, Argentina, near the Andes.
Unlike modern humans, our ancestors did not have effective methods of getting rid of lice, which could cause actual infestations on the scalp and clothing.
The female laid the eggs and they adhered with unusual efficiency to their host, in such a way that, in some cases, they remained there for millennia.
Lice could be irritating to their former hosts, but for scientists like Perotti, the fact that they’re shown to have remained intact for so long is good news.
Knowing that it is possible to study the evolutionary history of humans through lice, she set out in search of samples of human remains that contained hair.
Due to its characteristics, the nit can remain intact for thousands of years, surrounded by its own “glue”. And within that material it is possible to find human cells.
“Yes, it’s a bit like Jurassic Park,” Perotti told BBC Mundo. “Of course the film is fictional, but we make the analogy because the objective is the same: to characterize the host through a parasite with a substance produced by the parasite itself.”
The human DNA found in these samples proved to be of very good quality, such as what can be extracted from teeth and from the petrous bone behind the ear.
Thanks to this, scientists are discovering more details about the ancient populations of South America and these mummies, such as migration routes on the continent, gender and even possible causes of death.
“It’s very interesting,” Perotti told BBC Mundo. “Lice 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 lice that affect humans, the louse The body (of the body) and the capitis (from the head).
Thousands of years ago, the body adapted to live, in addition to the epidermis, also in clothes. That’s when ancient humans started covering themselves with fur and fabric.
In itself, this is a demonstration of how the human louse evolved along with its hosts.
Perotti was aware of this relationship and began researching ancient collections held 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 at the University of Oxford (England), the University of Bangor (Wales) and the National University of San Juan (Argentina). In 2016, they hired scientists from the University of Copenhagen (Switzerland) to analyze human DNA.
The results were published in the specialized journal Molecular Biology and Evolution.
“I had been looking for samples from the original indigenous populations of South America for a long time. I did more research until I had access to collections of human remains that contained hair.”
These samples were from 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 this analysis, the team extracted no more than six nits per mummy.
Extracting DNA from samples this old is not easy, and is sometimes even considered controversial, especially in samples that are in good condition.
Genetic material is usually extracted from teeth or petrous bone, which implies their destruction.
The sample must also meet certain requirements and be in good condition. It is not always possible, and sometimes the material is destroyed or partially damaged.
With the study of lice, the scientists realized that they avoided several of these inconveniences and also preserved the samples for further study.
The original idea was to extract human DNA from the same preserved lice.
But they were surprised when they discovered that the “wrap” that covered the nit had trapped human DNA, not only “of the highest quality” but also very well protected thanks to the glue’s chemical characteristics, explains Mikkel Winther Pedersen of the University of Copenhagen. .
“Once the egg got stuck in the hair, it immediately absorbed 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 we still have these samples here”, he emphasizes.
Migration routes and diseases
What have scientists discovered?
One of the 2,000-year-old mummies was of an individual from northern Amazonia. They know this because the extracted DNA matches that of other previously analyzed indigenous populations from the southern areas of Colombia and Venezuela.
The others, in turn, more recent (between 1,300 and 1,500 years old), do not have the same genetic characteristics, so they have a different origin: from Patagonia, from the south.
All this reveals that there was a great migratory movement in the region thousands of years ago. Those from the north may have taken an eastern route, likely driven by climate change such as droughts. “But we didn’t know they had reached this far south,” confesses Perotti.
These routes had been suggested years ago through anthropological studies that did not involve genetics. But the DNA findings may provide more clues about how humans were distributed across the continent millennia ago.
They know this because in environments with much lower temperatures, nits are closer to the scalp, where there is more heat.
Because the nit’s glue holds everything around it, not just human DNA, the scientists also found genetic material that was neither from the louse nor the host. This is the oldest evidence of Merkel cell polyomavirus.
Discovered in the United States in 2008, this virus is suspected to cause the majority of cases of Merkel cell carcinoma, a rare but aggressive form of skin cancer. Experts now suspect that perhaps the lice have something to do with the spread of this virus.
To back up the 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, the United States and even Asia in knowing the history of South America”, adds Perotti. “South America received the last migrations of anatomically modern humans. Humans 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, I am proud to have carried out this research”, he concludes.
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