For geneticists, our ancestors did not fear extinction – 09/15/2023 – Science

Could it be true that humanity was on the verge of disappearing 900,000 years ago, when there were only around 1,280 individuals spread across the entire planet?

That’s the theory behind a recent study by Chinese scientists who used genetic analysis models to determine that our ancestors were on the brink of annihilation for almost 120,000 years.

However, scholars who were not involved in the investigation criticize this statement, and one of them told AFP that there was “practically unanimous” agreement among population geneticists that the theory was not convincing.

Although it is plausible to infer that the ancestors of humans may have been on the brink of extinction at some point — a phenomenon known as a population “bottleneck effect” — experts have raised doubts that the study can be as precise in terms of dates and quantities of species that survived.

Estimating such ancient demographic changes is extremely complicated, given the few fossils of human ancestors available that date back hundreds of thousands of years, making it difficult to extract their DNA and learn more about their lives.

These critical scientists reinforced that other similar methods had not been able to detect this significant drop in the population.

However, with advances in genetic sequencing, scientists can now analyze mutations in modern humans, and then use a computational model that works backwards in time to understand how populations changed, even in the more distant past.

99% of ancestors annihilated?

In early September, a study published in the journal Science analyzed the genomes of more than 3,150 modern humans.

The team led by Chinese researchers developed a calculation in which the number of reproductive human ancestors fell to approximately 1,280 individuals around 930,000 years ago.

“Around 98.7% of human ancestors were lost” at the beginning of this “bottleneck effect,” said co-author Haipeng Li of the Shanghai Institute of Nutrition and Health of the Chinese Academy of Sciences.

“Our ancestors almost became extinct and had to help each other to survive”, he analyzed.

Potentially caused by a period of global cooling, this funnel extended until about 813,000 years ago, according to the study. Then, there was a demographic spike, possibly caused by warming terrestrial temperatures and the “dominance of fire”, he adds.

The researchers suggested that inbreeding during the bottleneck effect could explain why humans have a significantly lower level of genetic diversity compared to many other species.

Population reduction could then have contributed to the separate evolution of Neanderthals, Denisovans, and modern humans. They are all believed to have been born from a common ancestor at that time, according to the study.

It could also explain the difficulty in finding fossils of human ancestors dating from that time, a finding, according to archaeologists’ reports, in Kenya, Ethiopia, Europe and China and which could suggest that our ancestors were more spread out than the bottleneck effect would allow.

“The hypothesis of a global collapse does not fit with the archaeological and human fossil evidence,” Nicholas Ashton of the British Museum told Science magazine.

In response, the study authors indicated that hominins that lived in Eurasia and East Asia may not be ancestors of modern humans.

Today’s humanity is descended from those who miraculously survived, explains Li. “This small population is the common ancestor of all modern humans. Otherwise, we would have no trace of it in our DNA,” he added.

‘Extremely skeptical’

Stephan Schiffels, president of the population genetics group at the Max Planck Institute for Evolutionary Anthropology in Germany, said he was “extremely skeptical” about the study’s accuracy.

For him, “it will never be possible” to use the analysis of modern human genomes to obtain a number as precise as 1,280, as estimated by the group, which said it calculated that this population would be between 1,270 and 1,300 individuals.

Schiffels further stated that the data used by the study had already existed for years and that previous analyses, based on this information, had not identified any event that could lead to extinction.
The authors of the Chinese research simulated the bottleneck effect, using some of these previous models, but this time they detected a large population drop.

“It is difficult to be convinced by this conclusion”, says Pontus Skoglund, from the Francis Crick Institute, in the United Kingdom.

Aylwyn Scally, a human evolutionary genetics researcher at the University of Cambridge, told AFP that “a fairly unanimous response among population geneticists and people working in this field is that the paper is not convincing.”

According to her, the ancestors of modern humans may have been on the brink of extinction at some point, but the ability of modern genome data to infer such an event is “very weak.”
“This is probably one of those questions we won’t be able to answer,” he concluded.