No place on the planet has escaped the influence of Homo sapiens, from rainforests cleared for plantations to deep oceans filled with microplastics to climate-changing jet streams. Last November, the world population reached eight billion people.
But although humans are everywhere today, a team of scientists now claims that our species has come very close to not existing at all.
Researchers in China have found evidence suggesting that 930,000 years ago, the ancestors of modern humans suffered a massive population collapse. They point to the radical change in climate that occurred at that time as a cause.
Our ancestors survived in low numbers—less than 1,280 individuals—during a period known as the bottleneck. It lasted for more than 100,000 years before the population rebounded.
“About 98.7% of hominid ancestors were lost at the beginning of the bottleneck, threatening our ancestors with extinction,” the scientists wrote. they Stady It was published Thursday in the journal Science.
If the research holds up, it will have provocative implications. It raises the possibility that a climate-induced bottleneck helped split early humans into two evolutionary lineages — one that eventually gave rise to Neanderthals, the other to modern humans.
But outside experts said they were skeptical of the new statistical methods the researchers used in the study. “It’s a bit like inferring the size of a stone that falls into the middle of a large lake just from the ripples that reach the shore a few minutes later,” Stefan Schiffels, a population geneticist at the Max Planck Institute for Evolutionary Anthropology, said in 2015. Leipzig – Germany.
For decades, scientists have reconstructed the history of our species by analyzing the genes of living people. All studies tap the same basic facts of our biology: every child is born with dozens of new genetic mutations, and some of these mutations can be transmitted over thousands or even millions of years.
By comparing genetic differences in DNA, scientists can trace people’s ancestors back to ancient populations that lived in different parts of the world, moved and interbred. They can even infer the size of these populations at different times in history.
These studies are becoming more complex as DNA sequencing technology becomes more powerful. Today, scientists can compare the entire genomes of people from different populations.
Each human genome contains more than three billion genetic letters of DNA, each passed down over thousands or millions of years – forming a vast record of our history. To read this history, researchers now use increasingly powerful computers that can perform the huge numbers of calculations required for more realistic models of human evolution.
Haiping Li, an evolutionary genomics researcher at the Chinese Academy of Sciences in Shanghai, and his colleagues spent more than a decade devising their own method for reconstructing evolution.
The researchers called the method FitCoal (short for Fast Infinitesimal Time Coalescent). FitCoal allows scientists to slice history into precise time slices, allowing them to create a model of a million years of evolution divided into periods of months.
“It’s a tool we created to learn the history of different groups of organisms, from humans to plants,” Dr. Lee said.
Initially, he and his colleagues focused on animals such as fruit flies. But once they had sequenced enough genetic data from our species, they turned to human history, comparing the genomes of 3,154 people from 50 populations around the world.
The researchers explored different models in order to find the one that best explains the current genetic diversity among humans. They ended up with a scenario that involved a near-extinction event among our ancestors 930,000 years ago.
“We realized we had discovered something big about human history,” said Wangji Hu, a computational biologist at the Icahn School of Medicine at Mount Sinai in New York and an author on the study.
The scientists concluded that before the bottleneck, our ancestral population included about 98,000 breeding individuals. It then shrank to less than 1,280 and remained at that size for 117,000 years. Then the population rebounded.
Dr. Hu and his colleagues argue in their paper that this bottleneck is consistent with the fossil record of our hominid ancestors.
Our branch of the evolutionary tree separated from that of other apes about seven million years ago in Africa. Our ancestors evolved to be tall and with large brains in Africa about a million years ago. Some of these early humans then spread to Europe and Asia, evolving into the Neanderthals and their cousins, the Denisovans.
Our lineage continued to evolve into modern humans in Africa.
After decades of fossil hunting, the record of ancient human relatives remains relatively scarce in Africa between 950,000 and 650,000 years ago. Dr. Hu said the new study offers a possible explanation: There weren’t enough people to leave so many remains behind.
The bottleneck was “one plausible explanation,” said Brenna Henn, a geneticist at the University of California, Davis, who was not involved in the new study. She added that the genetic diversity found today may have resulted from a different evolutionary history.
For example, humans may have diverged into separate populations and then come together again. “Testing alternative models will be more robust,” Dr. Henn said.
Dr. Hu and his colleagues suggest that global climate change led to the population collapse 930,000 years ago. They point to geological evidence that the planet became colder and drier around the time of the proposed bottleneck. These conditions may have made it difficult for our ancestors to find food.
But Nick Ashton, an archaeologist at the British Museum, pointed out that a number of remains of ancient human relatives dating back to the time of the Bottleneck have been found outside Africa.
He said that if a global catastrophe caused the population to collapse in Africa, it should have made human relatives even rarer elsewhere in the world.
He said: “The number of sites in Africa and Eurasia dating back to this period suggests that it only affected a limited number of populations, which were probably the ancestors of modern humans.”
Dr. Lee and his colleagues also drew attention to the fact that modern humans appear to have separated from Neanderthals and Denisovans after the proposed population collapse. They speculate that the two events are related.
The researchers noted that most monkeys have 24 pairs of chromosomes. Humans only have 23, thanks to the merging of two groups. After the accident, scientists suggest that a set of fused chromosomes may have arisen and spread among the young population.
“All humans with 24 pairs of chromosomes became extinct, while the small isolated group with 23 pairs of chromosomes fortunately survived and were passed on from generation to generation,” said Ziqian Hao, a bioinformatics researcher at Shandong First Medical University and author of the book. Stady.
But Dr Shuffles doesn’t believe the bottleneck story just yet: “The finding is very surprising indeed, and I think the more surprising the claim, the better the evidence.”
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