Two epochs, two diametrically opposed problems. If humanity faces the question of overpopulation today at the peak of its 8 billion representatives, our distant grandparents would have witnessed their extinction up close, like any ordinary endangered species.
About 900,000 years ago, the number of ancestors of Homo sapiens suddenly grew from around 100,000 to just over 1,000, says an international team of scientists in a study published Thursday, August 31, in the journal Science.
Only 1280 people of childbearing age
This “bottleneck” of humanity would have lasted for hundreds of thousands of years during which the survival of our species seems to be hanging by a thread. “You are not the first to observe this phenomenon. For about ten years, scientists using older methods have detected a significant decrease in the number of humans,” notes Céline Bon, specialist in paleogenetics at the CNRS and collaborator at the National Museum of Natural History.
Above all, the new study offers an unprecedented level of precision. In previous work, this famous “bottleneck” could have occurred anywhere from 100,000 to 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 rested on the shoulders of 1,280 individuals of reproductive age at the time.
An extremely accurate count for a population that lived so long ago. That’s what this article is about, “which uses a large amount of data with a new method to go very far back in time,” summarizes Antoine Balzeau, paleoanthropologist at the National Museum of Natural History.
It’s this new method called FitCaol that these researchers are proud of. “It’s completely new and we estimate that it’s 95% accurate,” assure Fabio Di Vincenzo, anthropologist at the University of Florence and Giorgio Manzi, paleontologist at the University of Sapienza in Rome, two of the authors of the study. published in Science.
They selected genome samples from 3,154 people living in around fifty population groups around the world today. They then tracked this genetic background over time to estimate the size of the populations from which these genetic traits originated.
To do this, “we need to look at the genetic diversity that is present in the populations in which the ancestors of the selected individuals lived. The lower the genetic diversity, the smaller the population,” sums up Céline Bon.
By comparing all genetic mutations, the study’s authors’ new algorithm concluded that the most likely scenario is an extraordinary population decline that nearly deprived the Earth of Homo sapiens.
It’s not just our ancestors
But be careful and do not take the number 1,280 as the number of the only representatives of humanity on the whole Earth. First: “This is only about reproductive individuals, that is, this estimate does not take into account children, the elderly or those who will not reproduce for one reason or another,” Céline Bon wants to specify. In other words, the total population of our direct ancestors could be significantly larger.
Then this type of genetic study excludes “all human groups that may have lived at that time but are not our direct ancestors,” adds Antoine Balzeau. The lineage that evolved into Homo sapiens is not the only one to have set foot on Earth. The authors of the study also point out: “At that time there were other groups of people in Asia and Europe, but they most likely belonged to different branches of human evolution,” emphasize Fabio Di Vincenzo and Giorgio Manzi.
But regardless of the exact number of people who existed at that point in prehistory, according to the experts interviewed by France 24, there is little doubt about the reality of this bottleneck.
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 this point onwards there were longer periods of climate cooling. In Africa, this led to a decrease in rainfall, which could lead to the formation of deserts and make survival more difficult,” says Chris Stringer, a paleoanthropologist at the Natural History Museum (London).
But “when these climatic events are undeniable, it is much less obvious to conclude with certainty that there is a cause-and-effect relationship with the bottleneck,” assures Antoine Balzeau.
Is the climate to blame?
Firstly, because some experts have serious doubts about the date chosen by the study authors. One of the main criteria for calculating the timing of this shortage is the generation time, i.e. the average age at which a person has a child. This data will be used to determine how many generations have passed since the appearance of genetic mutations, traces of which we can still find today.
In this case, the researchers “assumed a generation time of 24 years. “But we are not at all sure whether the average reproductive age was 24 years ago a million years ago,” emphasizes Céline Bon. And a difference of a few years can cause dating to fluctuate wildly. “The accuracy of the data is very questionable, it is perhaps 200,000 years more or less,” assures Le Figaro Thierry Grange, a geneticist specializing in ancient populations at the Jacques Monod Institute in Paris.
Under these circumstances, it is difficult to claim that specific climatic events are the cause of this shortage. Other causes such as epidemics, volcanic eruptions or changes in group dynamics could explain this population decline.
But for Chris Stringer, “the boldest hypothesis of this article is to claim that the ancestors of sapiens survived for more than 100,000 years with only 1,280 individuals of childbearing age.” It would be a true 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 that this bottleneck will last for so long,” concludes Chris Stringer.
The method may have its limitations, but it has the virtue of reminding us that the story of the emergence of modern humans was far from a long, calm river and that it was probably only a matter of time. We don’t exist. For Céline Bon, this is about one of the most interesting questions: “Knowing by what chance Homo Sapiens survived”.