The study of the genome of today’s and yesterday’s humans, as well as of the archaic human lineages that have disappeared, reveals to us the great migrations that Homo sapiens and the multiple interbreeding that resulted.
With the exception of the original African populations, those who today populate Europe, Asia, Australia, Oceania and America contain in their genome the traces of the multiple crosses that occurred during the great journey what has undertaken Homo sapiens when he left Africa and which allowed him to conquer the surface of the entire Earth.
Twenty years ago, the sequencing of the human genome provided access to a wealth of information on the history of human populations. By comparing DNA sequences from present-day populations with those extracted from fossil bones of humans who lived thousands of years ago and other now extinct human forms, such as Neanderthals and Denisova’s man, geneticists were able to reconstruct the history of human populations. They were able to trace the DNA segments that these populations exchanged during their encounters and which helped Homo sapiens to adapt to the new environments he has taken over.
The study of the genomes of human populations has confirmed that our ancestors would have left Africa around 60,000 years ago. “All humans today, [hormis les Africains de souche], are descendants of those first migrants who colonized Europe, then Asia and Australia, 50,000 years ago, who crossed the Bering Strait and entered America, about 20,000 years ago . And who, much later, populated the islands of distant Oceania, including Polynesia and Melanesia, only 1000 years ago”, summarizes in an interview with To have to population geneticist Lluis Quintana-Murci, who gave a lecture on Wednesday, May 18 at the Cœur des sciences at UQAM.
While colonizing Europe, these representatives of the species Homo sapiens first encountered Neanderthals who were already living on the European continent and cohabited with them. Members of these two populations even mated, allowing the new arrivals to acquire Neanderthal mutations that facilitated their adaptation to the cold northern climate and to new pathogens.
It is these crosses that are at the origin of the presence of genetic material (approximately 2% of the human genome) originating from Neanderthal man in the genome of all modern humans, with the exception of ethnic Africans, whose the ancestors never rubbed shoulders with Neanderthals.
According to M. Quintana-Murci, who directs the Human Evolutionary Genetics Unit at the Institut Pasteur, “these interbreeding which took place during the migrations ofHomo sapiens have greatly facilitated its adaptation to the various environments of our planet. They allowed humans coming from Africa to adapt quickly to different climates, new nutritional resources and pathogens to which they had never been exposed”.
“Two options were available to the first Sapiens who arrived in Europe from Africa: either they waited for mutations allowing them to better cope with the cold and better resist the pathogens circulating in Europe to arise spontaneously, or they interbred with the populations of Neanderthals who were adapted to the cold. European environment, because they had lived there for 300,000 years. Of course, it turned out to be easier to interbreed with local populations, because this interbreeding made it possible to acquire advantageous and adaptive mutations,” explains Mr. Quintana-Murci.
His laboratory has shown that this crossbreeding between the first Homo sapiens Europeans and Neanderthals was particularly advantageous in the fight against viral diseases. Last year, German and Swedish researchers discovered that a DNA fragment inherited from Neanderthal man and which is present in nearly 50% of the world’s population, except in Africa, provided protection against the severe form of COVID-19.
Population genetics has also revealed that when Homo sapiens continued his migration to Asia and Australia, he met in these eastern countries another archaic human form now extinct, Denisova’s man, with whom he would have interbred. As proof, the genome of current Asian populations includes not only a component of Neanderthal origin, but also a Denisovan contribution. The latter reaches between 3.5 and 5% of the genome in the inhabitants of Papua New Guinea.
This interbreeding with Denisovans has notably endowed the Tibetans of the Himalayas with physiological characteristics allowing them to live at high altitude, where the quantity of oxygen present in the air is 40% less than at sea level. Denisova’s man possessed mutations implicated in genetic adaptation to hypoxia and he transmitted them to modern humans — including Tibetans — through interbreeding. These mutations increase the secretion of erythropoietin (EPO), which stimulates the production of red blood cells and thus increases the concentration of hemoglobin and therefore oxygen in the blood, explains Mr. Quintana-Murci in his book The people of humans, published by Odile Jacob.
These interbreedings which took place during the migrations of Homo sapiens greatly facilitated its adaptation to the various environments of our planet.
Mr. Quintana-Murci’s laboratory has also shown that interbreeding between different human populations can also contribute to better adaptation to the environment. These researchers have discovered that a mutation involved in resistance to malaria would have appeared, approximately 25,000 years ago, in the ancestors of the Bantu populations of village farmers in Africa. However, when these farmers have undertaken deforestation over the last 5000 years, they have at the same time introduced malaria to the pygmy populations of hunter-gatherers living in the forest. But by crossing with these Pygmies, they would have transmitted to them the mutation which makes it possible to resist it.
“We are all, to varying degrees, half-breeds, since our genomes are made up of multitudes of DNA segments from extremely varied sources. A historical and geographical patchwork, where peoples and generations mingle, ”he writes.
The European population, for example, carries in its genome the vestiges of the interbreeding of four different groups: that of the first hunter-gatherers from Africa, that of the Neanderthals, then that of the Neolithic farmers who came from the Middle East 10 years ago. 000 years and, finally, that of the peoples who brought the Indo-European languages from Eastern Europe 4000 years ago.
Certain mutations acquired during this interbreeding proved to be advantageous at the time of their introduction, but could have become deleterious with the metamorphosis of lifestyles and the emergence of new diseases, points out Mr. Quintana-Murci, however.
For example, while a mutation inherited from Neanderthals protects us from the severe form of COVID-19, another genetic variant transmitted to us by this extinct line, which is present in 16% of Europeans and 65% of the South Asian population, this time increases the risk of developing a severe form of COVID-19.
In addition, “several mutations that were once advantageous in defending against infectious pathogens are now responsible for an increased risk of developing autoimmune, allergic, or inflammatory diseases”, continues the geneticist. Since its appearance, our species has almost always lived surrounded by pathogens and without any medical help to deal with them. In such a context, she developed and selected mutations that made our immune system extremely reactive and very combative. But by reducing the risk of infections, the discovery of vaccines and antibiotics, as well as the adoption of better hygiene, at the beginning of the XXand century, has at the same time favored the increase of autoimmune and inflammatory diseases in people with these mutations, he explains.
“We are the descendants of those who survived the great epidemics of the past. Studying the genome of populations not only tells us about their history, it will allow us to practice precision medicine, medicine that is better suited to each individual”, underlines the researcher.