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Despite being the dominant species on the planet, Homo sapiens should consider ourselves lucky to exist. Our ancestors were on the brink of extinction about 900,000 years ago, and according to scientists, more than a thousand breeding individuals lived in isolation for more than 100,000 years.
This so-called „super flatneck” in our evolutionary history, mapped using a complex combination of genetic analysis and computer modeling, may explain gaps in the fossil record (minimum populations leaving minimal remains). This roughly coincides with a period of climate change that may have destroyed our ancestors’ chances of survival. Population declines would have encouraged inbreeding—which may explain why humans show relatively low genetic diversity compared to other mammals.
But the discovery has been met with some skepticism, highlighting the challenge of reconstructing the story of our own species. The further back in time academics try to go, the more slippery their deductions become. In the absence of well-preserved DNA from ancient humans, it is entirely possible that our true origin story will never be told.
The research—co-led by Haipeng Li at the Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences, and Yi-Hsuan Pan at East China Normal University—relies on the assumption that genetic mutations evolve at a roughly constant rate in a population. Tracking them from generation to generation and tracking how they converge or „cohere” allows us to estimate population size at any given time. Broadly speaking, the higher the fusion rate, the smaller the population size.
By counting and tracing mutations in more than 3,000 modern-day genes taken from Africa and beyond, the researchers deduced that our ancestral population collapsed around 930,000 years ago. About 99 percent of human ancestors were lost in an accident, they write in the journal Science. The breeding population fell to 1,280 individuals, give or take; Subsequent inbreeding led to the dramatic decline in human genetic diversity seen today. „When we first got this result six or seven years ago, it was hard to believe,” Li told me, adding that the team has verified it in the intervening years.
Long-term global cooling, for which there is climatic evidence, may have been responsible for the disturbance, which continued for about 120,000 years. After that, they speculate, the mastery of fire may have triggered a population explosion. A genetic crisis may have triggered the eventual split in the family tree between Neanderthals, the mysterious Denisovans and modern humans, researchers say. All three Homo (human) species are thought to share a common ancestor — perhaps Homo heidelbergensis — from which Homo sapiens appeared about 200,000 to 300,000 years ago.
While Li and colleagues say the African and Eurasian fossil record supports their story, paleoanthropologist Chris Stringer of the Natural History Museum in London is more cautious. Several countries, including Kenya, Ethiopia, Spain, and China, show temporal evidence of human occupation during the Barrier, although these lineages may be unrelated to our own and therefore irrelevant for analysis.
Pontus Skoglund, who directs the Ancient Genetics Laboratory at the Francis Crick Institute in London, has reservations, noting that other samples do not show the same dramatic population pressure. „Most people in the field are a little surprised to see such an odd result,” Skoglund said. „It would be great if it could be duplicated.” Li says he welcomes such efforts; He believes other models treat time slightly differently, leading them to capture more recent population fluctuations but potentially miss ancient ones.
The direct answer to whether our ancestors survived extinction lies in ancient human DNA, but our ancestry in hot Africa is no better for conservation than in cooler climates. Although mammoth DNA more than a million years old has been found in Siberian permafrost, the oldest human DNA recovered is about 400,000 years old.
Even so, we can never be sure of the complete story of Homo sapiens. Instead, we can contemplate each new temporal chapter that unfolds, including the incredible story of how more than 8 billion people today carry the genetic torch for the 1,280 hardiest souls who ever lived.
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