Neanderthals. How many were there? Archaeologists and geneticists give different answers. The new research should help them come to a consensus and shed light on the forgotten history of these ancient people. Including rather early extinction. In 1856, three years before Charles Darwin's On the Origin of Species was published, a group of miners discovered human fossils in a limestone cave in the Neander Valley in northern Germany - later called Neanderthal 1, the first indication of another archaic human species. Since then, we have tried to understand as much as possible about our mysterious ancestors. To do this, experts collected two main lines of evidence: hundreds of bones and stone tools found scattered from Spain and England to the Altai Mountains, and very recent data and conclusions drawn from statistical models.
However, these approaches have painted strikingly different pictures of what Neanderthal populations should have looked like. Archaeological evidence suggests that approximately 150,000 individuals covered Europe and Asia, lived in small groups of 15-25 individuals, and their total number varied greatly with climate change (including severe ice ages), which occurred over an interval of half a million years - until the extinction of the Neanderthals 40,000 years ago.
Genetic sequencing tells a different story. Some of the gene-based estimates define the Neanderthal population as a mere 1,000; others specify a few thousand at most. There are several hypotheses that could explain such results: either the population was really so small, even at its peak, or it was larger, but decreased over time. In any case, the Neanderthals have always been in decline; their disappearance seemed to have been foretold from the very beginning.
“The fact that these two types of estimates are not the same is a problem that has yet to be resolved,” says John Hawkes, a paleoanthropologist at the University of Wisconsin-Madison.
Now, however, researchers led by Alan Rogers, an anthropologist and population genetics at the University of Utah, have proposed a new genetic model that could reconcile these differences. He believes that the Neanderthals were much more numerous than previous genetic studies have shown, and brings them under the obvious artifacts and fossils that support this. It also fills in the evolutionary history of Neanderthals from the time they first split from our ancestors in Africa and when they began to encounter modern humans. In many ways, Neanderthals have been much more successful as a species - and much more like us - than we used to think.
Contrary to consensus
In population genetics, effective population size is not a direct measure of the total number of people who lived at a given time. It is rather a measure of genetic diversity. Experts scan the DNA of individuals throughout history, looking for DNA sequence differences between two copies of his or her genome. In essence, they estimate how many sibling generations separate the maternal copy of the gene from the paternal copy. If the population is small, they expect to find a common ancestor quickly enough; if large, it takes more time. “It's amazing how much information can be learned from an individual,” Rogers says.
Promotional video:
Scientists have long noticed that Neanderthals had low levels of genetic diversity. In modern Africans, about 11 out of every 10,000 nucleotides are heterozygous, that is, they differ between two copies of a chromosome. In non-Africans, only 8 out of every 10,000 exhibit this behavior. That figure drops to 2 in 10,000 for Neanderthals and Denisovans, whom science has only identified in the past decade. "The theory of population genetics tells us that this must mean a small population size" in these archaic people, says Montgomery Slatkin, a biologist at the University of California, Berkeley, who was not convinced by Rogers' findings. This means that the population was aiming for a number of 2000-3000 individuals.
"But if there were really only 1,000 Neanderthals in the world," Rogers says, "it's hard to believe they left such a rich fossil record."
Yet it is the genetic evidence that Rogers and his colleagues are turning to, claiming that there were more Neanderthals, tens of thousands. The scientists' work was published in the Proceedings of the National Academy of Sciences last month.
The key to this new finding lies in the researchers' suggestion that Neanderthals had a much more diverse gene pool, but that they were divided into small, isolated inbred groups of genetically similar individuals. Such fragmentation would distort earlier genetic findings: past estimates would point to local populations and their regional history, leaving out the big picture.
Rogers decided to make up for this gap by adapting and extending the population mixing model that other researchers had used. Instead of analyzing the genome of one person, he and his team compared genetic variants common to modern Africans, modern Eurasians, Neanderthals, and Denisovans. An early version of this model was developed to assess how strongly modern humans and Neanderthals are crossed. Rogers' main innovation was to add Denisovans to this mixture and significantly increase the list of possible combinations and interweaving of populations. And it helped him answer questions that go far beyond crossbreeding about population size and other issues.
The increase in genetic diversity that Rogers and colleagues found corresponds to a roughly tenfold increase in effective population size. While there is no way of knowing how many more Neanderthal individuals may fall out of this number, the fossil estimates will have to be seriously revised.
“The study provides DNA evidence for what we've seen in archaeological records,” says Joshua Aki, an evolutionary biologist at Princeton University.
From Africa - twice
By working with genetic sequences and their revised model, scientists have gained new insights into how Neanderthals, Denisovans, and modern humans grew, shrank, split, and merged periodically throughout history. “We want to create a good family tree so that we can tell accurate stories about how the two groups are related,” says Stephen Churchill, an anthropologist at Duke University. "But it's clear that this relationship is much more complicated."
About 750,000 years ago, Rogers said, the predecessors of the Neanderthals and Denisovans left the ancestors of modern humans in Africa to move into the vast territory of Eurasia. This alone almost destroyed them; genetic evidence suggests that the population has gone through severe trials that research has not previously identified. Whatever caused this catastrophe, the ancients survived it and just a few thousand years later - 744,000 years ago - they split into two separate lineages, Neanderthals and Denisovans. The former then split into smaller regional groups that fascinated Rogers.
The dating of this split between Neanderthals and Denisovans is surprising in itself, because previous studies put it later: in a 2016 study, for example, the split was identified as an event that happened 450,000 years ago. An early separation means we need to find even more fossils from both groups. You will also have to revise the fossils already found. Take, for example, the bones of the brain of a hominid of the species Homo heidelbergensis that lived in Europe and Asia about 600,000 years ago. Paleoanthropologists cannot agree with how he relates to other groups of people; some believe that they were the ancestors of both modern humans and Neanderthals, others that they were non-ancient species replaced by the Neanderthals that roamed Europe.
Rogers' findings imply that H. heidelbergensis must have been an early Neanderthal. "We've set the time of separation so early that a European hominid 600,000 years ago must almost certainly be a Neanderthal," he says, "at least genetically, even if it didn't look like a Neanderthal at all."
Either way, this new reconstruction of the complex early history of Neanderthals is very similar to what we have learned about the anatomically modern human populations that first spread across Europe and Asia. About 50,000 years ago, the Eurasians split from the Africans, went through a difficult period during which their population was very low, and then split into regional populations throughout Eurasia - the so-called non-African theory of human migration. “It looks like the same thing happened 600,000 or 700,000 years ago,” with the Neanderthals and Denisovans, Rogers says. "There was another non-African diaspora that no one had previously suspected."
Not so bad after all
It's no secret that the Neanderthals were doing badly: the ice ages they went through and the fragmentation of their populations left them unable to sustain sustainable social or technological growth. “But one of the misconceptions is that the way we think of progress is that modern humans are better and Neanderthals are worse,” Hawkes says. “When it came to hunting and relying on high-energy food resources in marginalized environments, Neanderthals were second to none. They solved problems that we do not face today at all. How did they live with such a low population density for hundreds of thousands of years? We never understood that."
Before embarking on his research, Rogers believed that Neanderthals were on the verge of extinction when modern humans entered their territory, that their populations were already depleted and rife with genetic diseases. He doesn't think so anymore.
Understanding the true structure of the Neanderthal population can help scientists delve deeper into the dynamics of these ancient people and their interactions with us. For example, I wonder if there were any peculiarities when crossing humans and Neanderthals. Did we include an equal amount of Neanderthal maternal and paternal DNA, or was there a shift?
Rogers' work and related research by other groups can be a powerful highlight for modern genetics too. Their analytical model can be applied to dogs and horses - indeed to any species that exhibit structured reproduction in their populations, rather than random gene flow. Neanderthal genes may have been linked to an increased risk of depression, diabetes, heart disease and other disorders, and this remains to be seen.
Ilya Khel