2023-09-01 17:21:51
Two eras, two diametrically opposed problems. If today humanity arises the issue of overpopulation from the height of its 8 billion representatives, our very distant grandparents would have seen their extinction very closely, like a common endangered species.
About 900,000 years ago, the ancestors of Homo sapiens went quite abruptly from around 100,000 individuals to just over 1,000, says an international team of scientists in a study published Thursday August 31 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 held by a thread. “They are not the first to see this phenomenon. For ten years, with older methods, scientists had noted a significant drop in the number of humans”, notes Céline Bon, specialist in paleogenetics at the CNRS and attached at the National Museum of Natural History.
Above all, the new study provides an unprecedented level of precision. In previous works, this famous “bottleneck” might have occurred anytime between 100,000 and a million years ago. The team of scientists actually provides a much more precise estimate than the “thousand” of individuals announced in the press release: for these specialists, the future of humanity then rested on the shoulders of 1280 individuals of age to procreate.
An extremely accurate count for a population that lived so long ago. This is the whole point of this article “which uses a large amount of data using a new method to explore very far into the past”, summarizes Antoine Balzeau, paleoanthropologist at the National Museum of Natural History.
It is this new method, called FitCaol, which is the pride of these researchers. “It is completely new and we estimate that it is 95% accurate”, assure Fabio Di Vincenzo, anthropologist at the University of Florence and Giorgio Manzi, paleontologist at the Sapienza University in Rome, two of the authors of the study. published in Science.
They selected genome samples from 3,154 people living today in around 50 population groups around the world. They then traced this genetic baggage through the ages to estimate the size of the populations from which these genetic characteristics originated.
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”, summarizes 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 probable scenario is an extraordinary tightening of the population which very nearly 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, “these are only breeding individuals, meaning that this estimate does not take into account children, elders or those who, for one reason or another, will not reproduce”, insists on qualifying Céline Bon. In other words, the total population of our direct ancestors might be significantly larger.
Then, this type of genetic investigation “excludes all human groups who might live at that time but who are not our direct ancestors”, adds Antoine Balzeau. The lineage that evolved into Homo sapiens isn’t the only one to have set foot on Earth. The authors of the study also recall this: “At that time, there were other groups of humans in Asia and Europe, but they most likely belonged to other branches of human evolution” , point out Fabio Di Vincenzo and Giorgio Manzi.
But whatever the exact number of humans who existed at this time of 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 spectacular decline in the number of sapiens ancestors is due to… climatic changes at the time. “We know that from that time there were prolonged periods of cooling of the climate. In Africa, this was reflected in particular by a drop in precipitation, which may have led to the formation of deserts and to make the more difficult to survive,” says Chris Stringer, a paleoanthropologist at the Natural History Museum (London).
But “if these climatic events are indisputable, it is much less obvious to conclude with certainty that there is a cause and effect link with the bottleneck”, assures Antoine Balzeau.
Blame it on the climate?
First, because some experts have serious doubts regarding the date chosen by the authors of the study. One of the main criteria taken into account to calculate the date when this bottleneck occurred is the generation time, that is, the average age at which an individual has a child. This data is supposed to make it possible to establish how many generations there have been since the appearance of the genetic mutations of which we still find traces today.
In this case, the researchers “have chosen a generation time of 24 years. But we are not at all sure that a million years ago, the average age to procreate was 24 years”, emphasizes Céline Bon. And a difference of a few years can cause dating to vary greatly. “The accuracy of the dates is very questionable, there are perhaps 200,000 years more or less”, assures the Figaro Thierry Grange, geneticist specializing in ancient populations at the Jacques Monod Institute in Paris.
It is difficult, under these conditions, to affirm that specific climatic events are at the origin of this bottleneck. Other causes, such as epidemics, volcanic eruptions or changes in group dynamics might explain this drop in population.
But for Chris Stringer, “the most daring hypothesis of this article is to affirm that the ancestors of sapiens survived for more than 100,000 years with only 1280 individuals of childbearing age”. It would be a real miracle if such a small group managed to overcome the vagaries of nature at the time for so long, “which is why we are skeptical regarding 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 calm river, and that it probably took little so that we do not 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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