The forefathers of humanity almost perished in a catastrophic tragedy nearly a million years ago.
The population was reduced from about 100,000 to just 1,280 breeding individuals around 900,000 years ago, according to genomic data from 3,154 modern humans. That’s a staggering population fall of 98.7% that lasted 117,000 years and might have led to the extinction of humanity.
The fact that there are so many of us here now is proof that it wasn’t. The findings, however, would, in the opinion of a team headed by geneticists Haipeng Li of the Chinese Academy of Sciences and Yi-Hsuan Pan of East China Normal University in China, explain a puzzling gap in the human fossil record in the Pleistocene.
According to anthropologist Giorgio Manzi of Sapienza University of Rome in Italy, “the gap in the African and Eurasian fossil records can be explained by this bottleneck in the Early Stone Age as chronologically.” It falls within the timeline of the supposed dramatic loss of fossil evidence.
Significant drops in a group’s numbers, or “population bottlenecks,” are a typical occurrence. The genetic diversity loss that occurs when a species is wiped out by an occurrence like a war, starvation, or climate disaster can be seen in the offspring of the survivors. This is how we know that the human population had a bottleneck much more recently, around 7,000 years ago, in the Northern Hemisphere.
However, it gets increasingly difficult to extract a significant signal the further back in time you want to investigate.
The study team created a novel technique for this most recent analysis dubbed the fast infinitesimal time coalescent process (FitCoal) in order to avoid the buildup of numerical inaccuracies that are typically brought on while attempting to reconstruct these historical occurrences.
They examined how gene lineages have diverged over time by using FitCoal to evaluate the genomic data of 3,154 persons from 40 non-African and 10 African groups. Their findings revealed a substantial population bottleneck that occurred between 930,000 and 813,000 years ago, which resulted in a loss of genetic diversity of up to 65.85% today.
We’ll never know for certain what exactly caused the bottleneck, but there was one significant event occurring at the time that might have had an impact: the Mid-Pleistocene Transition, a period in which the Earth’s glacial cycles underwent a significant change.
Climate change may have created unfavorable conditions for the human populations that were already struggling to survive, leading to hunger and conflict that further decreased population sizes.
According to Pan, “the novel finding opens a new field in human evolution because it evokes many questions,” including where these people lived, how they survived the catastrophic climate changes, and whether natural selection during the bottleneck sped up the development of the human brain.
Another intriguing aspect of the human genome that appears to have been influenced by the bottleneck is the joining of two chromosomes to form chromosome 2.
All other living hominids, including great apes, have 24 pairs of chromosomes in contrast to the 23 pairs that humans have. It appears that the origin of chromosome 2 was a speciation event that sent humans down a new evolutionary route.
These results are merely the beginning, claims Li. Future objectives using this knowledge “seek to paint a more complete picture of human evolution during this Early to Middle Pleistocene transition period, which in turn will continue to unravel the mystery that is early human ancestry and evolution.”
The research has been published in Science.