Could the lives of the 8 billion people on Earth have depended on the resilience of just 1,280 human ancestors who nearly went extinct 900,000 years ago?
That is the finding of a recent study that used genetic analysis modelling to postulate that our ancestors teetered on the brink of annihilation for nearly 120,000 years.
However, scientists not involved in the research have criticized the claim, with one saying that there was “pretty much unanimous” agreement among population geneticists that it was not convincing.
None denied that the ancestors of humans could have neared extinction at some point, in what is known as a population bottleneck, but experts expressed doubts that the study could be so precise, given the extraordinarily complicated task of estimating population changes so long ago.
They said that similar methods had not spotted this massive population crash.
It is difficult to extract DNA from the few fossils of humans dated from more than a couple of hundred thousand years ago, making it hard to know much about them, but advances in genome sequencing mean that scientists are now able to analyze genetic mutations in modern humans, then use a computer model to infer how populations changed — even in the distant past.
The study, published in the journal Science earlier this month, looked at the genomes of more than 3,150 modern-day humans.
The Chinese-led team of researchers developed a model to crunch the numbers, finding that the population of breeding human ancestors shrank to about 1,280 about 930,000 years ago.
“About 98.7 percent of human ancestors were lost” at the start of the bottleneck, said coauthor Li Haipeng (李海鵬), a professor of evolutionary genomics at the Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences.
“Our ancestors almost went extinct and had to work together to survive,” Li told reporters.
The bottleneck, potentially caused by a period of global cooling, continued until 813,000 years ago, the study said.
Then there was a population boom, possibly sparked by a warming climate and “control of fire,” it said.
The researchers said that inbreeding during the bottleneck might explain why humans have a significantly lower level of genetic diversity compared with many other species.
The population squeeze might explain why so few fossils of human ancestors have been found from the period, the study said.
However, archeologists have said that some fossils dating from the time have been discovered in Kenya, Ethiopia, Europe and China, which might suggest that our ancestors were more widespread than such a bottleneck would allow.
“The hypothesis of a global crash does not fit in with the archeological and human fossil evidence,” British Museum curator Nicholas Ashton told Science.
In response, the study’s authors said that hominins then living in Eurasia and East Asia might not have contributed to the ancestry of modern humans.
“The ancient small population is the ancestor of all modern humans, otherwise we would not carry the traces in our DNA,” Li said.
Stephan Schiffels, group leader for population genetics at Germany’s Max Planck Institute for Evolutionary Anthropology, told reporters that he was “extremely skeptical” that the researchers had accounted for the statistical uncertainty involved in this kind of analysis.
It would “never be possible” to use genomic analysis of modern humans to get such a precise number as 1,280 from that long ago, Schiffels said, adding that there are normally wide ranges of estimations in such research.
Li said that the range in the study was 1,270 to 1,300 individuals.
Schiffels also said that the data used for the research had been around for years, and previous methods using it to infer past population sizes had not spotted any such near-extinction event.
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