New research by scientists from China, Italy and the United States has uncovered a period in human history during which 98.7% of the ancestral population was lost. Published in the journal Science on August 31, the study found that, over the course of 117,000 years, the number of human ancestors could have reached as few as 1,280 reproductive individuals.

Study may explain large gap in fossil record

Using a new method called fast infinitesimal time coalescence (FitCoal), researchers were able to determine ancient demographic inferences with modern human genomic sequences. From this data, FitCoal was able to detect an "ancient bottleneck," calculating what this ancient loss of life and genetic diversity would look like.

According to the new information, the team was able to determine that, during a period between 800,000 and 900,000 years ago, only 1.3% of our ancestors survived. And this would explain a large gap in the African and Eurasian fossil record, exactly during the transition between the beginning and the middle of the Pleistocene.

It is believed that one of the main causes of this population decline was extreme climatic events, such as severe droughts, temperature changes and the consequent decrease in food sources, with the extinction of some animals. In addition to causing thousands of deaths among human ancestors, the new conditions forced them to migrate.

"The gap in the fossil records of Africa and Eurasia can be explained chronologically by this bottleneck at the beginning of the Stone Age," Giorgio Manzi, co-author of the study and anthropologist at Sapienza University, explained in a statement. "This coincides with this proposed period of significant loss of fossil evidence."

The period may have lost genetic diversity, but it may also have "created" more

According to the research, during this "bottleneck," an estimated 65.85% of the current genetic diversity may have been lost. This even prolonged the period with a minimum number of humans able to successfully procreate. It is believed that this number only reached 1,280 living reproductive individuals, which posed a great threat to the existence of the species.

However, the researchers also stress that the combination of all these events may have contributed to a process of speciation. In other words, the creation of two new species from a single lineage. As the study explains, the result of the conversion of two ancestral chromosomes may have been chromosome 2, the second largest human chromosome in modern humans. Understanding this split is a new step toward identifying the last common ancestor of Denisovans, Neanderthals and Homo sapiens (modern humans).

Co-author and specialist in evolutionary and functional genomics at East China Normal University, Yi-Hsuan PAN, explained in a statement that the new discovery opens up a new field in human evolution because it raises many questions, "such as where these individuals lived, how they overcame catastrophic climate change, and whether natural selection during the bottleneck accelerated human brain evolution," he said.

For LI Haipeng, also a co-author of the study and a theoretical population geneticist and computational biologist at the Shanghai Institute of Nutrition and Health, the new findings are just the beginning: "Future goals with these insights aim to paint a more complete picture of human evolution during this transitional period from the early Pleistocene to the middle Pleistocene, which in turn will continue to unravel the mystery that is early human ancestry and evolution," he said.

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