No place on the planet has escaped the influence of Homo sapiens, from rainforests cleared for plantations to deep oceans full of microplastics to climate-altering jet streams. Last November, the world’s population reached 8 billion.
But even though humans are everywhere today, a team of scientists is now claiming that our species came very close to not appearing at all.
Researchers in China have found evidence indicating that 930,000 years ago, the ancestors of modern humans suffered a massive population crash. They point to the drastic change in climate that occurred at that time as the reason.
Our ancestors remained 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.
The scientists wrote: “About 98.7% of human ancestors lost at the beginning of the bottleneck, which threatens our ancestors with extinction.” they Stady Published Thursday in the journal Science.
If the research holds, it will have provocative implications. It raises the possibility that a climate bottleneck helped split early humans into two evolutionary lineages — one that eventually gave rise to Neanderthals, and one to modern humans.
But outside experts said they were skeptical about the new statistical methods the researchers used in the study. “It’s a bit like inferring the volume of a stone that falls in the middle of a large lake only from the ripples that reach the shore after a few minutes,” Stefan Scheffels, 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 of the studies take advantage of the same basic facts of our biology: every baby is born with dozens of new genetic mutations, and some of those mutations can be passed down 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 around and intermarried. They can even infer the size of this population 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 3 billion genetic letters of DNA, each passed down over thousands or millions of years – making up a vast record of our history. To read this history, researchers are now using 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 named the method FitCoal (short for Fast Infinitesimal Time Coalescent). FitCoal enables scientists to slice history into precise time slices, allowing them to create a model of a million years of evolution broken down into month periods.
“It’s a tool we’ve created to learn the history of different groups of organisms, from humans to plants,” said Dr. Lee.
At first 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. And they ended up with a scenario involving an 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 of the study.
The scientists concluded that before the bottleneck, our ancestors numbered about 98,000 breeding individuals. Then it shrank below 1,280 and stayed at that size for 117,000 years. Then the population rebounded.
In their paper, Dr. Hu and colleagues argue that this bottleneck is consistent with the fossil record of our human ancestors.
Our branch of the evolutionary tree separated from that of other apes about seven million years ago in Africa. Our tall, big-brained ancestors evolved in Africa about a million years ago. After that, some of these early humans spread to Europe and Asia, evolving into 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 is still 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.
Brenna Henn, a geneticist at the University of California, Davis who was not involved in the new study, said the bottleneck was “one plausible explanation”. She added that the genetic diversity that exists today may have resulted from a different evolutionary history.
For example, humans may have diverged into separate populations and then gathered together again. “Testing alternative models would be more robust,” Dr. Henn said.
Dr. Hu and his colleagues suggest that the global climate shift led to the population crash 930,000 years ago. They point to geological evidence that the planet became cooler 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, points out that a number of remains of ancient human relatives dating back to the time of the Bottleneck have been found outside Africa.
He argued that if a global catastrophe had caused population collapse in Africa, it should have made human relatives more scarce elsewhere in the world.
“The number of sites in Africa and Eurasia that date back to this period suggests that they only affected a limited number of populations, who may have been the ancestors of modern humans,” he said.
Dr. Lee and colleagues also drew attention to the fact that modern humans seem to have separated from Neanderthals and Denisovans after the proposed population crash. 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 merger of two groups. After the accident, scientists suggest that a set of fused chromosomes may have arisen and spread through the young population.
“All humans with 24 pairs of chromosomes became extinct, while the isolated small group with 23 pairs of chromosomes fortunately survived and passed from generation to generation,” said Ziqian Hao, a bioinformatics researcher at Shandong First Medical University and author of the book. Stady.
But Dr. Shivels didn’t buy the bottleneck story yet: “The finding is very surprising indeed, and I think the more surprising the claim, the better the evidence.”
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