An extremely thin bottleneck is emerging: around 900,000 years ago, the population of today's ancestors may have collapsed to only around 1,280 individuals and then remained so threateningly small for over 100,000 years. This bottleneck is reflected in the results of a new genome analysis of modern humans. Further investigations should now confirm the finding and clarify the precise circumstances of the population collapse and its effects on human development, say the scientists.
Around eight billion people live on earth today. One thing is clear: the beginnings of this gigantic population development were modest - our ancestors only formed comparatively small populations for a long time. Stocks were characterized by considerable fluctuations. Researchers have been trying to track down these processes for some time using certain clues in the genome of modern humans. This has already resulted in evidence that certain 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 drastic bottleneck in a potentially crucial phase of human evolution.
A population bottleneck in the genetic mirror
Their results are based on their newly developed investigation method called FitCoal (Fast Infinitesimal Time Coalescent Process). In doing so, they use patterns in the genetic differences between modern human population groups in a special way. Through model calculations, conclusions can be drawn about former population sizes and their changes. Based on typical mutation rates that occur in generation sequences, time windows for an established population development can also be determined. The researchers have now applied these methods to genome sequences from 3154 people from 50 different populations of modern humans.
As they report, their results revealed a drastic population collapse in the human family tree that has so far remained undetected. Accordingly, the ancestors of today's humans emerged from a tiny “founding population” that existed on the verge of extinction around 900,000 to 800,000 years ago. Specifically, the model calculations based on the genetic evidence show that the number of these representatives of the genus Homo had collapsed by over 98 percent to only around 1,280 individuals. Only after the bottleneck, which lasted over 100,000 years, was the human species able to multiply again and develop stable populations again, according to the results.
As the time window suggests, the common ancestors of modern humans, Neanderthals and Denisovans, may have been affected by the bottleneck effect, the researchers argue. Specifically, they could have been hominins that can be assigned to Homo heidelbergensis. Interestingly, according to the researchers, the result also fits with a certain aspect of the fossil record from the genus Homo: "The bottleneck in the Early Stone Age can now explain a gap in the African and Eurasian fossil finds chronologically," says co-author Giorgio Manzi von the University of La Sapienza in Rome.
A potentially decisive phase
According to scientists, the main reasons for the decline in the human ancestor population are climatic factors. As is well known, significant temperature changes occurred on Earth during the relevant time window in the Pleistocene. It is possible that around 900,000 years ago they led to severe droughts in the habitat of the ancestors of modern humans and thus to the loss of their food resources.
“The new finding opens a new field in the study of human evolution because it raises many more questions,” says co-author Yi-Hsuan Pan from East China Normal University in Shanghai. "Like the places where these individuals lived and how they survived the catastrophic climate changes." The hardship may also have triggered groundbreaking intellectual or cultural developments, the researchers say: "The question arises as to whether natural selection accelerated the development of the human brain during the shortage," says Pan. In addition to an improvement in climatic conditions 813,000 years ago, control over fire may also have contributed to the rapid population growth after the bottleneck.
In conclusion, senior author LI Haipeng from 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 transitional period from early to middle Pleistocene, which is crucial to early human evolutionary history could illuminate,” said the population geneticist.
Source: Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences, Article: Science, doi: 10.1126/science.abq7487