No place on the planet has escaped the influence of Homo sapiens, from rainforests cleared for farms to microplastic-laced deep oceans to climate-altered jet streams. In November, the world’s population reached 8 billion.
But as omnipresent as humans are today, a group of scientists now claim that our species has almost ceased to exist.
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 a drastic climate change that occurred during that time as the cause.
Our ancestors remained in low numbers — fewer than 1,280 breeding individuals — during what is known as a bottleneck. It took over 100,000 years for the population to rise again.
“About 98.7 percent of human ancestors were lost at the beginning of the bottleneck, so our ancestors are threatened with extinction,” the scientists wrote. Their study was published Thursday in the journal Science.
If the research continues, it will have tantalizing implications. This raises the possibility that a climate-driven bottleneck helped split early humans into two evolutionary lines – one that eventually gave rise to Neanderthals, the other to modern humans.
But outside experts say they are skeptical of the novel statistical methods the researchers used for the study. “It’s a bit like inferring the size of a rock that falls into the middle of a large lake just from the waves that come ashore a few minutes later,” said Stephan Schiffels, a population geneticist at the Max Planck Institute for Evolutionary Anthropology. in Leipzig, Germany.
For decades now, scientists have been reconstructing the history of our species by analyzing the genes of living people. All 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 on to thousands or even millions. year.
By comparing genetic variations in DNA, scientists can trace people’s ancestors to ancient populations that lived in different parts of the world, moved around and interbred. They can even indicate the size of populations at different times in history.
These studies have become more sophisticated as DNA sequencing technology has grown 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 – creating a vast record of our history. To read that history, researchers now use more powerful computers that can perform the vast number of calculations required for more realistic models of human evolution.
Haipeng Li, an evolutionary genomics researcher at the Chinese Academy of Sciences in Shanghai, and his colleagues spent more than a decade creating their own method for reconstructing evolution.
The researchers named the method FitCoal (short for Fast Infinitesimal Time Coalescent). FitCoal lets scientists cut history into fine slices of time, allowing them to create a model of a million years of evolution divided into phases of the moon.
“This is a tool we created to learn the history of different groups of living things, from humans to plants,” said Dr. Li.
At first he and his colleagues focused on animals such as fruit flies. But when enough genetic data from our own species was sequenced, they turned to the history of humans, comparing the genomes of 3,154 people from 50 populations around the world.
Researchers have explored different models to find one that best explains today’s genetic diversity in humans. They ended up with a scenario that included a near-extinction event in our ancestors 930,000 years ago.
“We realized that we had discovered something huge about human history,” said Wangjie Hu, a computational biologist at the Icahn School of Medicine at Mount Sinai in New York and an author of the study.
Before the bottleneck, the scientists concluded, the population of our ancestors included about 98,000 breeding individuals. It then dwindled to fewer than 1,280 and remained that small for 117,000 years. Then the population rebounded.
Dr. argues. Hu and his colleagues in their paper that this bottleneck is consistent with the fossil record of our human ancestors.
Our branch of the evolutionary tree split from other apes about seven million years ago in Africa. Our ancestors evolved to be tall and big-brained in Africa about a million years ago. Then, some of the early humans spread to Europe and Asia, becoming the Neanderthals and their cousins, the Denisovans.
Our own race continued to evolve into modern humans in Africa.
After decades of fossil hunting, the record of early human relatives remains relatively scarce in Africa for the period between 950,000 and 650,000 years ago. The new study offers a potential explanation: there aren’t enough people to leave a lot of remains, Dr. Huh.
Brenna Henn, a geneticist at the University of California, Davis, who was not involved in the new study, said a bottleneck is “a reasonable interpretation.” But today’s genetic diversity may have been produced by a different evolutionary history, he added.
For example, people may have separated into separate populations then reunited. “It will be more powerful to test alternative models,” said Dr. Henn.
Dr. suggests Hu and his colleagues found that a global climate change caused a population collapse 930,000 years ago. They point to geological evidence that the planet became colder and drier at the time of their proposed bottleneck. Those conditions may have made it difficult for our ancestors to find food.
But Nick Ashton, an archaeologist at the British Museum, noted that some remains of early human relatives dating back to the bottleneck period have been found outside of Africa.
If a global catastrophe caused the human population to collapse in Africa, he said, then it must have made human relatives rarer elsewhere in the world.
“The number of sites in Africa and Eurasia that have been dated to this period suggests that it affected only a limited population, which may have been the ancestors of modern humans,” he said.
Dr. Li and his colleagues also drew attention to the fact that modern humans appear to have diverged from Neanderthals and Denisovans after their proposed population collapse. They assume that the two events are related.
Researchers have noticed that most monkeys have 24 pairs of chromosomes. Humans only have 23, thanks to the merger of the two sets. After the crash, the scientists suggest, a combined set of chromosomes may have emerged and spread through the small population.
“All people with 24 pairs of chromosomes disappeared, while only a small isolated population with 23 pairs of chromosomes survived and was passed from generation to generation,” said Ziqian Hao, a bioinformatics researcher at Shandong First Medical University and an author of the study.
But Dr. hasn’t bought it yet. Schiffels the story of the bottleneck: “The finding is really surprising, and I think the more surprising the claim, the better the evidence.”
