Prehistory is largely a mystery. Without writing, he has left no stories, only ruins, dust and bones. But events large and small were recorded inside these bones. They tell stories that humanity did not learn to read until just 10 years ago, when modern genome sequencing techniques began to work. It was then that scientists from the University of Copenhagen began to investigate the genetic material of 1,600 human remains, mainly from the Neolithic and Mesolithic. They then began a titanic investigation that crystallized this Wednesday in the publication of four studies in the magazine Nature. “We have had many surprises,” explain the authors. “And we are sure there are many more waiting to be discovered.” The results provide insights into European history over the past 15,000 years and highlight how past migrations explain who modern Europeans are and why they get sick. Genetics has opened a window to a remote and unknown past that sheds light on the present.
It is known that Europeans today have up to 80% more likely to develop multiple sclerosis than people of Asian origin. That northern Europeans are more likely to suffer from this autoimmune disease of the nervous system than those in the south. Now, we also know why. This predisposition began to form about 5,000 years ago, when shepherds from the Pontic steppe arrived on the continent, an area that extends through part of modern-day Ukraine, southern Russia and the border of Kazakhstan. It is believed that the genetic variants associated with sclerosis had an immunological advantage for these herders, at a time when infectious diseases were increasing due to their direct and prolonged contact with livestock.
“It is a very nice example of how changes in lifestyle can increase or decrease the risk of getting sick or even introduce new pathogens,” he explained in the virtual presentation of the study. Astrid Iversen, virologist at the University of Oxford. “Our lifestyle has changed thanks to hygiene, we have less contact with animals and we see less benefits in these variants, which can degenerate into autoimmune responses, but it is important to realize that this is something new, from the last 200 years ”.
The same genes that began to protect us from new diseases millennia ago have been turning against us for a handful of decades. But it is worth thinking that in the future, if the environment does not change, these mutations will be discarded to adapt to the new reality. “One of the things we have seen is that the genes related to the immune system are selected depending on the pathogens it has to deal with, the genetic variations depend on the ecosystem,” explained the virologist. This idea, she acknowledged, has already been used as a hypothesis before. “But it has not been demonstrated as clearly as we have seen, we have solid evidence.”
Another analysis has tracked genetic variants associated with the risk of diabetes and Alzheimer's. And he has concluded that they are related to the ancestry of Western hunter-gatherers. Once again, this genetic mutation, which today has negative effects, once made some sense from an evolutionary point of view. “The combination of genes that predisposes to Alzheimer's also has protective effects in pregnancy,” said Dr. Iversen. “There is a huge advantage, especially in populations where many children die young and where life expectancy is not going to be very high, so Alzheimer's would not have much impact.”
You cannot treat something without knowing where it comes from, and these studies give a very clear idea of the origin of diseases, he summarized in the presentation. Rasmus Nielsen, biologist at the University of Berkeley. The analysis, he explained, shows that genetic evolution involves accepting a certain balance: “There are mutations that can be protective in one environment and negative in another. This gives us a global vision that can demystify the origin of these diseases.”
Why are they lower in southern Europe?
The genetic study also explains more trivial, but equally interesting, aspects. The average height of a man in Spain is 1.76 centimeters, while in Holland it is around 1.84. These height differences between northern and southern Europe are well known, but thanks to this analysis we know that they began to take shape thousands of years ago. They would be associated with a differential steppe ancestry, not so much with issues of selection or current feeding. The study also reveals how the human genome changed by modifying the diet, with the arrival of agriculture. What is interesting, experts point out, is not so much what it reveals about our past, but rather putting it in context with current genetic maps.
“In these we see that there are these differences between northern and southern Europe; or from the east and the west,” explains the CSIC geneticist in a telephone conversation. Carles Lalueza-Fox, who also participated in the study. “But we are not able to know how far back these go back, nor to understand why they have occurred.” Having a time scale of genomes allows us to do this. Before we had a photograph, now previous frames have been added, composing a film. “In recent years we have been rebuilding migrations. But now, based on them, an explanation of the origin of the disease is being provided.”
Three major migrations are believed to have shaped the genetic diversity of modern Western Eurasian populations: the arrival of hunter-gatherers about 45,000 years ago, the expansion of Neolithic farmers from the Middle East about 11,000 years ago, and the arrival of the steppe shepherds about 5,000 years ago. “These three great ancestors have different frequencies of genes that predispose to certain diseases,” says Lalueza-Fox. “Then you can look at them and compare them.” The study, in fact, compared the ancient genome with that of about 410,000 current white Europeans in the United Kingdom. Thus, the researchers were able to quantify the proportion of genetic material from these prehistoric populations in modern Europeans and its relationship with certain diseases.
The geneticist highlights as a strength of this study the fact that he sequenced the entire genomes. “Most jobs are limited to genotyping a million variable positions in the genome, of which you get half. With that you already have enough to do population genetics,” he explains. “But here the entire genome has been sequenced, which gives more capacity to track phenomena.”
The weakest part of the study, on the other hand, is due to an archaeological limitation. DNA is better preserved in cold climates than in warm, temperate ones, especially if thousands of years have passed. “There is a clear bias towards northern areas, because few samples have been preserved in southern Europe,” laments the expert. In fact, one of the four studies is limited to Denmark, analyzing the demographic, cultural and dietary changes of the Danish population in prehistory.
In any case, this study adds 1,600 prehistoric genomes to a base that already exceeds 10,000. “And in a few years, the figure is going to double,” says L
alueza-Fox. “It is an exponential progression, limited only by the number of individuals available.” The race to start the story of prehistory passes through ancient sites and modern laboratories. Tools from the past and technology from the future that combine to read stories that were never written in books, but in the hearts of bones.
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