New discoveries about humans approaching extinction raise doubts

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The writer is a scientific commentator

Despite being the dominant species on the planet, we Homo sapiens should consider ourselves fortunate to exist at all. Our ancestors were on the brink of extinction about 900,000 years ago, according to scientists, with just over 1,000 individuals breeding and living solitary lives for more than 100,000 years.

This supposed “big bottleneck” in our evolutionary history, mapped using a complex combination of genetic analysis and computer modeling, may explain gaps in the fossil record (minimal populations will leave minimal remains). It also roughly coincides with a period of climate change that could have wiped out our ancestors’ chances of survival. The population collapse could have promoted inbreeding – which may also explain why humans exhibit 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 species. The further away academics try, the more elusive their conclusions become. In the absence of well-preserved DNA from ancient humans, it is entirely possible that our true origin story would never have been told.

The research, which was co-led by Haiping Li of the Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences, and Yi Hsuan-Pan of East China Normal University, is based on the assumption that genetic mutations accumulate in the population at an almost constant rate. Tracking them through the generations and observing how they converged or “merged” allows estimating the size of the population at any given time. In general, the higher the coalescence rate, the lower the population size.

By counting and tracking mutations in more than 3,000 contemporary genomes, taken from Africa and beyond, the researchers concluded that the number of our ancestors declined about 930,000 years ago. They wrote in the journal Science that nearly 99 percent of human ancestors were lost in the accident. The reproductive number has been reduced to 1280 individuals, give or take; Subsequent inbreeding led to the dramatic decline in human genetic diversity that we see today. “When we first got this result six or seven years ago, it was also hard to believe,” said Li Li, adding that the team had spent the intervening years checking it out.

It was probably the long-term global cooling, for which there is corresponding climatic evidence, that led to the bottleneck that lasted for about 120,000 years. They then speculate that controlling the fire could lead to a population explosion. The researchers add that the genetic crisis may have led to the family tree eventually splitting into Neanderthals, the mysterious Denisovans, and modern humans. All three Homo (Homo) species are thought to share a common ancestor – possibly Homo heidelberg – with Homo sapiens appearing around 200,000 to 300,000 years ago.

While Lee and his colleagues claim that sporadic African and Eurasian fossil records support their account, paleontologist Chris Stringer of the Natural History Museum in London is more cautious. He notes that several countries, including Kenya, Ethiopia, Spain and China, show tentative evidence of human occupation during the bottleneck, although these lineages may be unrelated to our lineage and therefore irrelevant to the analysis.

Pontus Skogland, who leads the paleontological genomics lab at the Francis Crick Institute in London, also has reservations, noting that other models do not show the same dramatic population pressure. “Most in the field were a little surprised to see such a different outcome,” Skogland said. “It would be nice if it could be repeated.” Lee says he welcomes such attempts. He thinks other models treat time slightly differently, which leads them to capture more recent population fluctuations, but perhaps miss older fluctuations.

The most straightforward answer as to whether our ancestors braved annihilation lies in ancient human DNA, but our ancestors in hot Africa, rather than in cooler, conservation-friendly climates, do not bode well. While massive DNA more than a million years old has been found in Siberian permafrost, the oldest human DNA found dates back to only about 400,000 years old.

Even so, we can never be sure of the full story of Homo sapiens. Instead, we can contemplate each new tentative chapter that emerges, including the incredible story of how more than eight billion people alive today carry the genetic flame of 1,280 of the most powerful souls that ever lived.

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