Human Ancestors Almost Died Out, Genetic Studies Suggest

No place on the planet has been spared the impact of Homo sapiens, from rainforests cleared for farms to plastic-laden deep oceans to climate-altered jet streams. Last November, the world’s population reached eight billion.

But even though humans are everywhere today, a group of scientists now say that our species is too close to ever being seen at all.

Chinese researchers have found evidence that 930,000 years ago, the ancestors of modern humans were hit by a massive human crash. They point to a major change in climate that occurred at the time as the cause.

Our ancestors lived in low numbers – less than 1,280 births – during the so-called bottleneck. It lasted for more than 100,000 years before the population became more numerous.

The scientists wrote: “About 98.7 percent of human ancestors disappeared at the beginning of the crisis, thus threatening our ancestors with extinction.” Their study was published Thursday in the journal Science.

If the research holds up, it will have a provocative effect. It raises the possibility that a climate-driven bottleneck helped separate ancient humans into two evolutionary paths – 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 in the study. “It’s like looking at the size of a rock that falls into the middle of a large lake only to have the waves reach the shore a few minutes later,” said Stephan Schiffels, a population geneticist at the Max Planck Institute for Evolutionary Anthropology. Leipzig, Germany.

For decades now, scientists have been retracing the history of living things by analyzing the genes of living people. The studies all take advantage of the same basic facts of our biology: every baby is born with a new genetic mutation, and some of those mutations can be passed down over thousands or millions of years.

By comparing genetic variations in DNA, scientists can trace human ancestry back to ancient people who lived in different parts of the world, moved around and intermingled. They may even estimate the size of those nations at different times in history.

These studies have become more sophisticated as DNA sequencing technology has advanced. Today, scientists can compare entire genomes of people from different places.

The entire human genome contains more than three billion genetic letters of DNA, each of which has been passed down over thousands or millions of years – creating a vast record of our history. To study that history, researchers are now using more powerful computers that can perform the many calculations required for more realistic models of human evolution.

Haipeng Li, an emerging genomics researcher at the Chinese Academy of Sciences in Shanghai, and his colleagues have spent more than a decade developing their own method for reconstructing evolution.

The researchers call the method FitCoal (short for Fast Infinitesimal Time Coalescent). FitCoal allows scientists to slice history into chunks of time, allowing them to model millions of years of evolution broken down into lunar periods.

“It’s a tool we’ve created to trace the history of different groups of organisms, from humans to plants,” said Dr.

At first he and his colleagues focused on animals such as fruit flies. But after enough genetic data from our species had been sequenced, they turned to human history, comparing the genomes of 3,154 people from 50 people around the world.

Researchers are examining different models to find the one that best explains today’s genetic diversity among people. They concluded a scenario involving a near-extinction event among our ancestors 930,000 years ago.

“We realized that we had discovered something great 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 ended, the scientists concluded, our ancestral population consisted of about 98,000 reproductive individuals. It then shrank to less than 1,280 and remained that small for 117,000 years. After that, the population increased again.

Dr. Hu and his colleagues argue in their paper that this phenomenon is consistent with the fossil record of our human ancestors.

Our branch of the evolutionary tree split from the other apes about seven million years ago in Africa. Our ancestors evolved to become taller and have larger brains in Africa about a million years ago. Then, some of these early humans spread to Europe and Asia, evolving into the Neanderthals and their cousins, the Denisovans.

Our lineage continued to evolve into the modern people of Africa.

After decades of fossil hunting, records of early human relatives in Africa between 950,000 and 650,000 years ago are rare. The new study offers a possible explanation: there weren’t enough people to leave behind more remains, said Dr. Hu.

Brenna Henn, a geneticist at the University of California, Davis, who was not involved in the new study, said the barrier was “one logical path.” But today’s genetic diversity may have resulted from a different evolutionary history, he added.

For example, people may split up into different personalities and then reunite. “It would be more powerful to test other models,” said Dr. Henn.

Dr. Hu and his colleagues suggest that global climate change caused the population collapse 930,000 years ago. They point to geological evidence that the planet has become colder and drier right around the time of their proposed shutdown. Those conditions may have made it difficult for our human ancestors to find food.

But Nick Ashton, an archaeologist at the British Museum, noted that a number of remains of prehistoric human relatives that existed during the closure of the crisis have been found outside of Africa.

If a global crisis has caused the collapse of humanity in Africa, he said, then it must have made human relatives rare elsewhere in the world.

“The number of sites in Africa and Eurasia that date back to this period suggests that it affected only a few people, who may have been the ancestors of modern humans,” he said.

Dr. Li and his colleagues also drew attention to the fact that modern humans seem to have diverged from Neanderthals and Denisovans after their proposed crash. They speculate that the two events are related.

Researchers note that most monkeys have 24 pairs of chromosomes. Humans only have 23, due to the combination of two sets. After the crash, scientists suggest, a combined set of chromosomes may have evolved and spread in a small population.

“All individuals with 24 pairs of chromosomes perished, while only a few individuals with 23 pairs of chromosomes fortunately survived and were passed down from generation to generation,” said Ziqian Hao, a bioinformatics researcher at Shandong First Medical University and the author of the study.

But Dr. Schiffels isn’t buying it yet: “This finding is really surprising, and I think the more surprising the claim, the better the evidence should be.”