Modern humans—aka Homo sapiens—emerged about 300,000 years ago after evolving from human ancestors. There’s a lot we still don’t know about who these ancestors were and where they lived. But according to an August 2023 study, our ancestors may have come close to extinction some 900,000 to 800,000 years ago.

During this period, our human ancestors lost 98.7 percent of their population, according to the study published in Science. The authors estimate that the reduced population had an average of 1,280 breeding individuals, and remained this diminished for over 100,000 years.

With such a dramatic population loss, the authors suggest that these human ancestors may have come close to extinction, noting that the number of remaining human ancestors is comparable to populations of other endangered mammals. However, other scholars have raised doubts about the strength of the study’s conclusions.

Extreme Cooling May Have Led to Decline

Before this dramatic population loss,human ancestors had between 58,600 and 135,000 breeding individuals, the authors estimate. So what could have caused their numbers to dwindle to 1,280, and to remain so low for over 100,000 years?

They argue that a major factor may have been extreme cooling that began around 900,000 years ago, according to geologic evidence. This cooling period coincided with severe drought in Africa and the decline of other species that human ancestors may have used as a food source.

To survive these changes, “we believe that our ancestors must have been well-united to fight against the harsh environment,” co-authors Yi-Hsuan Pan, an evolutionary and functional genomicist at East China Normal University, and Haipeng Li, a population geneticist at the University of Chinese Academy of Sciences, said in an email.

The nine study authors—who are based in China, Italy and the United States—argue that this dramatic population decline could explain an apparent gap in the fossil record in Africa. The human ancestors whose population dropped likely lived in Africa, though some may have been in Europe.

If correct, this would also mean that our human ancestors experienced a decline in genetic diversity, since there were fewer potential partners for reproduction. This may have had implications for the evolution of Homo heidelbergensis, a human ancestor that emerged around 700,000 years ago. This population decline may also have played a role in the evolution of Neanderthals and Denisovans by narrowing the genetic pool.

The authors of the study came to their conclusion that human ancestors experienced a severe population “bottleneck” by analyzing modern genetic data from over 3,000 modern humans. They used this data to track gene variants and draw conclusions about the early evolution of human ancestors. But some in the scientific community have questioned whether the results of this research are strong enough to back up the claims.

Other Scholars Question Study Results

In the same issue that Science published the new study claiming human ancestors came close to extinction, the journal also published a comment by Nick Ashton, a curator at the British Museum, and Chris Stringer, a research leader in human origins at the Natural History Museum in London, that seemed to accept the results.

But others are not so convinced.

“This paper was met with quite a bit of skepticism within the scientific community,” says Aaron Ragsdale, a population geneticist at the University of Wisconsin–Madison, who was not involved with the study.

“There are critiques of both the methodology and interpretation of this study,” he says. “I would like to see these results corroborated by independent methods and for the results to be validated with different features of genetic data.”

If other studies are able to reproduce these results, Ragsdale says that extreme cooling is one possible explanation for why the population of human ancestors may have dropped so precipitously at this point in time. Even so, he thinks there is not enough research to make a strong argument that this population decline happened.

“Altogether, I think it is a stretch to conclude definitively that human ancestral populations experienced such a severe decline in actual population size for that extended period of time,” he says.

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