Did our ancestors almost die one day?

A very thin bottleneck is emerging: about 900,000 years ago, the population of today’s ancestors may have declined to only about 1,280 individuals, and then remained dangerously small for more than 100,000 years. This bottleneck is reflected in the results of the analysis of the new genome of modern humans. Scientists say further investigations must now confirm the findings and clarify the precise circumstances of the population collapse and its effects on human development.

Today there are about eight billion people on Earth. One thing is clear: the beginnings of this massive population evolution were humble – our ancestors formed relatively small populations for a long time. Stocks witnessed significant fluctuations. Researchers have been trying to trace these processes for some time using specific clues in the modern human genome. This has already given rise to evidence that some environmental changes in the history of human development have led to severe population declines. The study by an international team of researchers now points to a particularly severe bottleneck at a potentially crucial stage in human evolution.

Population bottleneck in the genetic mirror

Their findings are based on their newly developed research method called FitCoal (Fast Micro Coalition Process). In doing so, they take particular advantage of patterns found in genetic differences between human populations today. Using model calculations, conclusions can be drawn about past population sizes and their changes. Based on typical mutation rates occurring in generative sequences, time windows for specific population evolution can also be determined. The researchers have now applied these methods to sequence the genomes of 3,154 people from 50 different groups of modern humans.

As they report, their findings revealed a radical population collapse in the human family tree that has until now remained undiscovered. Accordingly, the ancestors of today’s humans emerged from a small “founder group” that was on the verge of extinction about 900,000 to 800,000 years ago. Specifically, model calculations based on genetic evidence show that the number of these representatives of the human race has collapsed by more than 98% to only about 1,280 individuals. Only after a bottleneck, which lasted more than 100,000 years, were humans able to reproduce again and develop stable populations again, according to the findings.

As the time window indicates, the common ancestors of modern humans, Neanderthals and Denisovans, were affected by the bottleneck effect, the researchers say. Specifically, they could be a hominin that can be attributed to Homo heidelbergensis. According to the researchers, the finding also fits interestingly with a particular aspect of the fossil record of the genus Homo: “Because of the bottleneck in the Early Stone Age, the gap in African and Eurasian fossil discoveries can now be explained chronologically.” Co-author Giorgio Manzi, La Sapienza University of Rome.

Potential critical stage

According to scientists, the main reasons for the decline in the number of human ancestors are climatic factors. As is known, significant temperature changes occurred on Earth during the relevant time period in the Pleistocene era. About 900,000 years ago, it likely led to severe droughts in the habitat of today’s ancestors, and thus to the loss of their food resources.

“The new discovery opens a new field in the study of human evolution because it raises many questions,” says co-author Yi-Hsuan Pan of East China Normal University in Shanghai. “For example, about where these individuals lived and how they survived catastrophic climate changes.” This hardship may also have led to groundbreaking intellectual or cultural developments, say the researchers: “The question arises as to whether natural selection accelerated human brain evolution during the shortage,” Pan says. In addition to improved climatic conditions 813,000 years ago, fire control may also have contributed to rapid population growth after the bottleneck.

Finally, lead researcher Li Haiping of the University of the Chinese Academy of Sciences in Shanghai says, “The new evidence could be used in the future to paint a more complete picture of human evolution during the transition period from the Early Pleistocene to the Middle Pleistocene,” which the population geneticist said. “Very important for the history of early human evolution.”

Source: Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences, specialized article: Science, doi: 10.1126/science.abq7487

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