Two eras, two diametrically opposed problems. If today humanity is asking the question of overpopulation from the top of its 8 billion representatives, our very distant grandparents would have seen their extinction very closely, like a vulgar endangered species.

About 900,000 years ago, the ancestors of Homo sapiens went from about 100,000 individuals to just over 1000,31, says an international team of scientists in a study published Thursday, August <> by the journal Science.

Just 1280 individuals of childbearing age

This "bottleneck" of humanity would have lasted a hundred thousand years, during which the survival of our species seems to have hung by a thread. "They are not the first to see this phenomenon. For about ten years, with older methods, scientists had noticed a significant decline in the number of humans," notes Céline Bon, a specialist in paleogenetics at the CNRS and attached to the National Museum of Natural History.

Above all, the new study brings a new level of precision. In previous work, this famous "bottleneck" could have occurred any time between 100,000 and a million years ago. The team of scientists actually provides a much more accurate estimate than the "thousand" individuals announced in the press release: for these specialists, the future of humanity then rested on the shoulders of 1280 individuals of childbearing age.

An extremely accurate count for a population that lived so long ago. This is the interest of this article "which exploits a large amount of data using a new method to explore very far in the past," summarizes Antoine Balzeau, paleoanthropologist at the National Museum of Natural History.

It is this new method, called FitCaol, that is the pride of these researchers. "It is completely new and we estimate that it is 95% accurate," say Fabio Di Vincenzo, an anthropologist at the University of Florence, and Giorgio Manzi, a paleontologist at Rome's Sapienza University, two of the authors of the study published in Science.

They selected genome samples from 3,154 people living today in about fifty population groups around the world. They then traced this genetic background through the ages to estimate the size of the populations from which these genetic characteristics originate.

To do this, "we must look at the genetic diversity present in the populations where the ancestors of the selected individuals lived. The lower the genetic diversity, the smaller the population," says Céline Bon.

It is by comparing all the genetic mutations that the new algorithm of the authors of the study came to the conclusion that the most likely scenario is an extraordinary population tightening that almost deprived the Earth of Homo sapiens.

It's not just our ancestors

But be careful not to take the number of 1,280 as that of the only representatives of the human race on the whole Earth. First, "it is only breeding individuals, that is to say that this estimate does not take into account children, elders or those who, for one reason or another, will not reproduce," nuances Céline Bon. In other words, the total population of our direct ancestors could be significantly larger.

Then, this type of genetic investigation "excludes all human groups that could have lived at that time but who are not our direct ancestors," adds Antoine Balzeau. The lineage that evolved into Homo sapiens is not the only one to have walked the ground of the Earth with its two feet. The authors of the study also recall it: "At that time, there were other groups of humans in Asia and Europe, but they most likely belonged to other branches of human evolution," say Fabio Di Vincenzo and Giorgio Manzi.

But whatever the exact number of humans who existed at that time in prehistory, the reality of this bottleneck is hardly in doubt according to the specialists interviewed by France 24.

For the authors of the study, this dramatic decline in the number of sapiens ancestors is due to... climate change of the time. "It is known that from that time on, there were prolonged periods of cooling of the climate. In Africa, this has resulted in lower rainfall, which may have led to the formation of deserts and made survival more difficult," says Chris Stringer, a paleoanthropologist at the Natural History Museum in London.

But "if these climatic events are indisputable, it is much less obvious to conclude with certainty a causal link with the bottleneck," says Antoine Balzeau.

The fault of the climate?

First, because some experts have serious doubts about the date chosen by the authors of the study. One of the main criteria taken into account to calculate the date on which this bottleneck occurred is the duration of a generation, i.e. the average age at which an individual has a child. This data is supposed to establish how many generations there have been since the appearance of the genetic mutations that are still found today.

In this case, the researchers "retained a generation duration of 24 years. But we are not at all sure that a million years ago, the average age to procreate was 24 years, "says Céline Bon. And a difference of a few years can greatly vary the dates. "The accuracy of the dates is very questionable, there are perhaps 200,000 years more or less," Thierry Grange, a geneticist specializing in ancient populations at the Jacques Monod Institute in Paris, told Le Figaro.

Under these conditions, it is difficult to say that specific climatic events are at the origin of this bottleneck. Other causes, such as epidemics, volcanic eruptions or changes in group dynamics could explain this population decline.

But for Chris Stringer, "the most daring hypothesis of this article is to assert that the ancestors of sapiens survived for more than 100,000 years with only 1280 individuals of reproductive age". It would be a real miracle if such a small group managed to overcome the vagaries of nature for so long at the time, "which is why we are skeptical about the advanced duration of this bottleneck," concludes Chris Stringer.

The method may have its limits, but it has the merit of reminding us that the history of the advent of modern man is far from having been a long quiet river, and that it probably took little for us not to exist. This joins, for Céline Bon, one of the most interesting questions, "that of knowing by what chance Homo sapiens managed to survive".

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