900,000 years ago, 98.7% of human ancestors would have disappeared

Lman on the verge of extinction? A few weeks after the publication of a study in the journal Science, the controversy intensifies. According to a team of Chinese researchers, the human line escaped extinction 900,000 years ago. However, many genetics experts severely question these claims and publicly express “skepticism” about the conditions under which this study was conducted.

The study, published at the end of August in the journal Science and signed by a Chinese team, was based on a genetic analysis model to determine that the existence of the ancestors of modern humans was threatened for at least 120,000 years.

But many critical voices were raised against this publication, like Aylwyn Scally, researcher in human evolutionary genetics at the University of Cambridge. “There was an almost unanimous response in the community of geneticists, saying that this study is not convincing,” he told Agence France-Presse.

Computer models

No one doubts the possibility of a bottleneck in the evolution of the population in question, that is to say a phenomenon of severe reduction in the number of humans. But these other experts question the supposed precision of the model used. They thus mention the time scale considered and the fact that similar methods have previously led to different results.

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Furthermore, it is very difficult to extract DNA from rare fossils of the human lineage that are a few hundred thousand years old. Scientists therefore use mutations observed in the genome of modern humans to deduce, using computer models, changes that occurred in the past.

In the case of the Chinese study, the team of researchers used the genomes of 3,150 modern humans. To conclude that “around 98.7% of human ancestors disappeared” at the start of the bottleneck (930,000 years ago), according to Haipeng Li, co-author of the study, from the Institute of nutrition and health of Shanghai. Over 120,000 years, the population collapsed, reaching just 1,300 individuals, according to the study. “Our ancestors came close to extinction and had to cooperate to survive,” he explained to AFP.

This episode would have been caused by climatic cooling, and would have lasted until 813,000 years ago, before a rebound in the population coinciding with a rise in temperatures and, perhaps, the “control of fire”, adds the searcher.

Statistical uncertainties

This bottleneck would have led to strong inbreeding, resulting in lower genetic diversity in the human species than in other closely related species, such as the chimpanzee. The bottleneck may even have contributed to the parallel evolution of the Neanderthal, Denisova and modern humans, which are thought to have potentially split from a common ancestor around the same time, according to the study.

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This would finally explain why we find so few fossils of the human lineage at this time. An argument, however, refuted by paleontologists who report discoveries in Kenya, Ethiopia, Europe and China: “The hypothesis of a global collapse [de population, NDLR] does not fit with archaeological discoveries and human fossils,” notes an official from the British Museum, Nicholas Ashton, interviewed by the magazine Science.

For Haipeng Li, on the contrary, the hominids then living in Eurasia and East Asia were not necessarily the ancestors of contemporary humanity. “The former small population [qui a frôlé l’extinction, NDLR] is the ancestor of all modern humans, otherwise we would not carry traces of it in our DNA,” he continues to assert.

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For his part, Stephan Schiffels, specialist in population genetics at the German Max-Planck Institute for Evolutionary Anthropology, expresses his “extreme skepticism”. He criticizes the researchers for not having taken into account the statistical uncertainties in the matter. It will “never be possible,” he says, to use genomic analysis of modern humans to arrive at a figure as precise as 1,280 individuals who lived so long ago.

The expert from the Germanic Institute adds that the data used had been known for years, and that methods of estimating past populations have never concluded that there was virtual extinction. Under these conditions “it is difficult to accept their conclusion”, comments Pontus Skoglund, of the British Francis-Crick Institute.

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