Nearly a million years ago, a devastating event nearly wiped out the ancestors of mankind.
Genomic data from 3,154 modern humans suggests that the population was reduced from around 100,000 to just 1,280 breeding individuals around 900,000 years ago. This is a staggering 98.7% population decline that spanned 117,000 years and could have driven humanity to extinction.
The fact that we are here today, and that there are so many of us, proves that was not the case. But the findings, according to a team led by geneticists Haipeng Li of the Chinese Academy of Sciences and Yi-Hsuan Pan of East China Normal University in China, would explain a curious gap in the Pleistocene human fossil record.
“The discrepancy between the African and Eurasian fossil record can be explained by this bottleneck at the beginning of the Stone Age, as well as chronologically,” explains anthropologist Giorgio Manzi of the Sapienza University of Rome in Italy. “It coincides with this proposed period of significant loss of fossil evidence.”
Demographic bottlenecks, as is well known, are not uncommon in group memberships. When a species is devastated by an event such as war, famine or climate crisis, the resulting decline in genetic diversity can be traced back to the descendants of the survivors. This is how we know that there was also a human population bottleneck in the Northern Hemisphere much more recently, around 7,000 years ago.
However, the further you want to go back in time, the more difficult it becomes to give off a meaningful signal.
For this latest analysis, the research team developed a new method called the Fast Infinitesimal Time Coalescing Process (FitCoal) to circumvent the accumulation of numerical errors typically associated with trying to untangle these past events.
They used FitCoal to analyze genomic data from 3,154 people from around the world, from 10 African and 40 non-African populations, looking at how genetic lineages diverged over time. Their results showed a significant population-level bottleneck around 930,000 to 813,000 years ago, resulting in a current loss of genetic diversity of up to 65.85 percent.
As to what caused this bottleneck, we’ll never know 100% what all the contributing factors may have been, but there was one major event that happened at the time that could have played a role: the mid-Pleistocene transition, during which the Earth’s glaciation cycles changed dramatically.
It is possible that climatic upheavals produced conditions that were unfavorable to human populations struggling for survival at the time, resulting in famine and conflict that further reduced the population.
“This new discovery opens a new field in human evolution because it raises many questions,” says Pan, “such as where these individuals lived, how they overcame catastrophic climate change, and whether natural selection during the bottleneck throttling has accelerated the evolution of the human brain.”
This bottleneck seems to have contributed to another interesting feature of the human genome: the fusion of two chromosomes to form chromosome 2.
Humans have 23 pairs of chromosomes; all other hominids alive today – including the great apes – have 24. The formation of chromosome 2 appears to have been a speciation event that set humans on a different evolutionary path.
“These results are just the beginning,” Li says. tour will continue to unravel the mystery of early human ancestry and evolution.”
The research has been published in Science.
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