As the American journalist and popularizer likes to remember Carl Zimmertoday there is more Neanderthal DNA on Earth than when the Neanderthals themselves existed, who disappeared about 40,000 years ago. This is because there are more than 8,000 million people and the vast majority of us carry between 1 and 2% of this genetic material that survives in us, after passing through shaker of evolution and as a consequence of a hybridization event that occurred after the arrival of the first Homo sapiens to Europe.
But when did this mixture between sapiens and Neanderthals occur and what paths did that DNA take in our species? This is a question about which until now we knew very little and about which a couple of studies published at the same time this Thursday in the journals Nature and Science They offer new and fascinating details.
The first, about the dates: the two studies agree in pointing out that most of the genetic flow that we receive from Neanderthals is attributable to a single period – between 43,000 and 50,000 years ago – from the sexual dalliances of a few thousand individuals.
The second, about the crossings between some human groups and others, with the discovery of lineages that crossed more than once with the Neanderthals and others that did not meet them again, but ended up facing the same fate: that of becoming extinct due to the path in an evolutionary branch that did not reach the present.
And finally, about the way in which Neanderthal genes passed through the sieve of natural selection; The result indicates that those that presented some type of adaptive value stayed with us, especially those related to skin pigmentation, immune response and metabolism.
Signs of a ‘lost’ lineage
For the work published in Naturea team led by Johannes Krausefrom the Max Planck Institute for Evolutionary Anthropology, has sequenced the genomes of the fossil remains of the oldest modern humans found in Europe, arriving in the north of the continent when the region was inhabited by Neanderthals, who were very well adapted to the harsh climatic conditions. .
The authors have scrutinized the DNA of seven individuals who lived between 42,000 and 49,000 years ago, six of them in ranis (Germany) and a seventh female skull very well preserved in Zlatý kůň (Czechia), at a distance of 230 km from each other. To their surprise, the result indicated that both groups were related but, after the first contact of their ancestors, they had not mixed again with other groups of Neanderthals, despite their presence in the area.
“The interesting thing is that we see that they did not have additional hybridization with Neanderthals and that it is a lineage that disappeared, just like them,” he explains. Vanessa Villalbaresearcher at the Institute of Evolutionary Biology (IBE UPF-CSIC) and co-author of the work, to elDiario.es. “This tells us that human history is not just a success story,” he says. Benjamin Peteralso co-author. “In fact, we became extinct several times and the Neanderthals also became extinct at that time, perhaps not because of us, but because of environmental factors.”
This tells us that human history is not just a success story. In fact, we went extinct several times and so did Neanderthals
Benjamin Peter
— Researcher at the Max Planck Institute for Evolutionary Anthropology
The other striking fact, Krause highlights, is that other ancient individuals of our species found further south, in Bacho Kiroin Bulgaria, and Oasein Romania, do show signs of a new additional mixture with other Neanderthals and did leave genetic traces later in Paleolithic populations. “Instead, it seems that human lineages [de los que tenemos restos fósiles] that were present in Europe at that time, including Ranis and the Neanderthals, became extinct about 40,000 years ago,” he says. “They did not contribute genetically to later populations, and this is a mystery.”
The slow evolutionary sieve
Thanks to the analysis of these genomes, the authors of the work in Nature have been able to calculate that this small group of Ranis, of just 200 individuals, had around 2.9% Neanderthal ancestry, which allows them to reconstruct the genealogical chronology and place the moment of hybridization between 45,000 and 49,000 years ago ( around 80 generations earlier), which coincides with the results published in Science.
This second study, led by Leonardo Iasialso by Max Planck, has consisted of the analysis of more than 300 already sequenced genomes, belonging to 334 modern humans, 59 ancient individuals between 45,000 and 2,200 years old and 275 current individuals from various global populations. And what they reveal is that modern humans acquired several Neanderthal genes and that rapid natural selection, both positive and negative, occurred within 100 generations of a single episode of gene flow, especially on the X chromosome.
We discovered several genes inherited from the Neanderthal that could have been beneficial, such as some that affect skin pigmentation and others that affect the immune system.
Leonardo Iasi
— Max Planck Institute for Evolutionary Anthropology
“We used our catalog of ancestry segments to also study the functional legacy of Neanderthal gene flow,” explains Iasi. “And we discovered several candidates inherited from the Neanderthal that could have been beneficial, such as some genes that affect skin pigmentation and are present with high frequency in modern humans, in addition to many immune system genes.”
At the same time, the authors located certain regions of the genome, known as archaic deserts, that completely lack Neanderthal ancestry, while others show high frequencies of Neanderthal variants, possibly due to beneficial adaptive mutations. And they found 91 candidate regions (169 genes) with high frequency in current individuals but not in ancient individuals, suggesting possible contributions of Neanderthal material that occurred later in this long history of intersecting human branches.
Mixing was an advantage
For Gemma Marfanyprofessor of Genetics at the University of Barcelona (UB), who has not participated in these studies, among the most interesting results of Nature This shows that the crossing with Neanderthals was prior to the crossing with Denisovans and that all current modern humans descended from the same crossings with Neanderthals.
These Neanderthal genes allowed greater survival to the wave of modern humans, new migrants in Eurasia
Gemma Marfany,
— Professor of Genetics at the University of Barcelona
Regarding the study of ScienceMarfany recalls that it shows that the fragments of our genome enriched with Neanderthal DNA have been selected, while those that have not remained have been purified because they had to compromise the survival of the resulting hybrids. “Which implies that these genes, selected in Neanderthals for thousands of years, allowed greater survival to the wave of modern humans, new migrants in Eurasia, which is why they were selected very quickly and have been maintained until today,” he points out.
Humans left by the way
Maria Martinondirector of the National Center for Research on Human Evolution (CENIEH), highlights that it has been possible to narrow down so well the time interval in which the mixtures between modern humans and Neanderthals occurred and believes that the two works show that “ The history of our species is full of lost civilizationsof cultures that have not survived to this day but are part of the legacy and memory of our history as a species,” he says.
However, Martinón points out, describing these populations as “failed” populations implies a very colonialist and directional narrative. “Those Homo sapiens Those of whom we have not yet been able to recover DNA (if there is any left) are part of our history even if they have not persisted genetically,” he says. “I think it is time to change the narrative of success or failure with which we construct the story of our past. There are many other side arguments that make our movie Much more interesting are the interactions with other human groups, or the way in which we and they face the same or different challenges depending on the time or place in which we live.”
These ‘Homo sapiens’ are also part of our history although they have not persisted genetically. It’s time to change the narrative of success or failure
Maria Martinon
— Director of CENIEH
The geneticist Carles Lalueza-Foxa specialist in ancient DNA and director of the Museum of Natural Sciences of Barcelona, insists on the same idea: many times we think of evolution as something linear, when in reality it is happening everything at once and everywhere. “Something that is derived from these results is the role of contingency, both due to the fact that there are many groups that do not survive to the present day and due to the fact that, apparently, a minority crossing between Neanderthals and modern humans left a sign that continues to this day,” he explains.
“Although there are genes that are selected because they represent an advantage, it is still a contingency,” concludes Lalueza-Fox. Or, put another way, those first groups that survived and left their genetic mark on us were also a little lucky.
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