Two eras, two diametrically opposed problems. If humanity today raised the issue of overpopulation from the height of its 8 billion representatives, our very distant grandparents would see their extinction very close, as an ordinary endangered species.
About 900,000 years ago, the number of ancestors of Homo sapiens suddenly dropped from 100,000 individuals to just over 1,000, according to a study by an international team of scientists published on Thursday, August 31, in the journal Science.
A total of 1280 people of childbearing age.
This “bottleneck” of humanity would last for a hundred thousand years, during which the survival of our species seemed to be hanging by a thread. “They are not the first to observe this phenomenon. Over a decade, using older methods, scientists have noted a significant decrease in the number of people, ”says Céline Bon, a paleogenetics specialist at CNRS and an employee of the National Museum of Nature. Story.
First of all, the new study provides an unprecedented level of accuracy. According to previous work, this famous “bottleneck” could have arisen at any time from 100,000 to a million years ago. The team of scientists actually gives a much more accurate estimate than the “thousand” individuals stated in the press release: for these specialists, the future of mankind then lay on the shoulders of 1280 individuals of an age capable of producing offspring.
An extremely accurate estimate for a population that lived so long ago. That’s the whole point of this article, “which uses a large amount of data using a new method to explore the very distant past,” sums up Antoine Balzot, a paleoanthropologist at the National Museum of Natural History.
It is this new method called FitCaol that these researchers are proud of. “It is brand new and we estimate it to be 95% accurate,” said Fabio Di Vincenzo, an anthropologist at the University of Florence, and Giorgio Manzi, a paleontologist at the Sapienza University of Rome, two authors of the study published in the journal Science. .
They selected genome samples from 3,154 people living today in about 50 populations around the world. They then traced this genetic baggage over the centuries to estimate the size of the populations from which these genetic characteristics originated.
To do this, “we have to look at the genetic diversity present in the populations where the ancestors of the selected individuals lived. The less genetic diversity, the smaller the population,” sums up Céline Bon.
It was by comparing all genetic mutations that the new algorithm of the authors of the study came to the conclusion that the most likely scenario is an extreme population decline, which almost deprived the Earth of Homo sapiens.
It’s not just our ancestors
But be careful not to take the number 1280 as the number of the only representatives of the human race on the whole Earth. Firstly, “these are only breeding individuals, i.e. this estimate does not take into account children, old people or those who, for one reason or another, are not going to breed,” Céline Bon insists on qualifications. In other words, the total population of our direct ancestors could have been much larger.
In addition, this type of genetic study “excludes all groups of people who may have lived at the time but are not our direct ancestors,” adds Antoine Balzot. The lineage that evolved into Homo Sapiens is not the only one to set foot on Earth. The authors of the study also recall this: “Other groups of people existed at that time in Asia and Europe, but they most likely belonged to other branches of human evolution,” Fabio Di Vincenzo and Giorgio Manzi point out.
But whatever the exact number of people who existed in this prehistoric time, the reality of this bottleneck is hardly in doubt, according to specialists interviewed by France 24.
According to the authors of the study, such an impressive reduction in the number of sapiens ancestors is due to … climatic changes of that time. “We know that there have been long periods of cooling since that time. In Africa, this was reflected, in particular, in a decrease in precipitation, 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 “if these climatic phenomena are undisputed, it is much less obvious to conclude with certainty that there is a causal relationship with a bottleneck,” says Antoine Balzot.
Is the climate to blame?
Firstly, because some experts have serious doubts about the date chosen by the authors of the study. One of the main criteria taken into account when calculating the date of occurrence of this bottleneck is the generation time, that is, the average age at which a person has a child. It is assumed that these data will make it possible to establish how many generations have passed since the appearance of genetic mutations, traces of which we still find today.
In this case, the researchers “choose a generation time of 24 years. But we are not at all sure that a million years ago, the average age for procreation was 24 years, emphasizes Céline Bon. A difference of several years can lead to dates that are very different. “The accuracy of the dates is highly questionable, perhaps 200,000 years more or less,” Thierry Grange, a geneticist who specializes in ancient populations at the Jacques Monod Institute in Paris, tells Le Figaro.
Under these conditions, it is difficult to argue that specific climatic phenomena are the cause of this bottleneck. This population decline can also be attributed to other causes, such as epidemics, volcanic eruptions, or changes in group dynamics.
But for Chris Stringer, “this article’s most daring hypothesis is to confirm that sapiens’ ancestors lived for over 100,000 years with only 1,280 individuals of childbearing age.” It would be a real miracle if such a small group managed to overcome the vagaries of nature of that time for so long, “which is why we are skeptical about the duration of this bottleneck,” concludes Chris Stringer.
Perhaps this method has its limits, but its merit is that it reminds us that the history of the emergence of modern man was far from being a long calm river and that it probably took very little time for us to not exist. For Céline Bon, this joins one of the most interesting questions: “How do you know how Homo sapiens managed to survive?”