Key takeaways
- Neanderthals died out around 40,000 years ago, possibly due to climate change and competition and interbreeding with modern humans, but no one knows for sure.
- New research on one Neanderthal population in Belgium right before extinction shows that it had healthy levels of genetic diversity and no signs of stress from interbreeding with modern humans.
- The findings suggest that extinction played out differently across the vast range that Neanderthals occupied across Europe and Central Asia.
Neanderthals thrived across Europe and the Middle East for hundreds of thousands of years. They occupied vast distances, ranging from Europe through the Altai Mountains in Central Asia. They survived large variations in climate and the arrival of modern humans until around 40,000 years ago, when they died out. The exact factors that resulted in their disappearance remain unknown, with possible explanations including climate change, resource competition and interbreeding with anatomically modern humans.
A new study published in Nature reveals that a Neanderthal population around 45,000 years ago in Belgium and France was doing well, with no signs of inbreeding or pressure from or genetic mixture with modern humans, who lived in the area at that time. The finding suggests that localized Neanderthal groups in this area were large and well-connected enough that individuals could have children with partners who were not closely related. Nonetheless, around 2,000 years later, even this population vanished.
"In other, earlier Neanderthal populations, close relatives were interbreeding, leading to unhealthy levels of genetic diversity similar to what we see today in some endangered species. But this population in Belgium and France does not seem to be dying out, even though we know that they will die out in the end," said UCLA computational geneticist Benjamin Peter, who is one of the paper's corresponding authors.
The research examined DNA extracted from the bones of 27 individuals who lived between 49-40,000 years ago in the Meuse Basin, which spans Belgium and France, using established computational techniques for studying ancient DNA and a new technique the researchers developed for these particular samples. They found no close relatives among any of the individuals up to third-degree relatedness, in which individuals share about 12.5% of their DNA and was the limit of what their methods could detect. This is comparable to the amount of DNA shared between first cousins.
The mitochondrial DNA, which is inherited only from the mother, showed that most individuals belonged to a large lineage common to late Neanderthals in Southern and Southwestern Europe. This means that all these individuals shared female ancestors. One individual, however, belonged to a different maternal lineage found in a handful of other Neanderthals, suggesting that more than one of these mitochondrial lineages coexisted. On the other hand, data from the Y-chromosome of three male individuals, which is passed on from father to son, showed that they did not belong to a single lineage, meaning that they each had different male ancestors.
To study the amount of genetic diversity, the researchers looked for stretches of DNA that contained identical base pairs. This happens when a person inherits copies of the same genes from both parents, which is much more likely when the parents are related. When close relatives reproduce over time, a situation likely when a population is small and isolated, eventually most of the population will end up having similar genes, which will show up as long stretches of identical pairs on the DNA strands.
This reduction in genetic diversity, which population geneticists call "inbreeding depression," means that harmful genes are more likely to be passed on, which reduces the potential to adapt to changing environments or respond to new diseases and may cause lower fertility. All of these factors make it harder for a population to survive and reproduce and, without an influx of new genes, can lead to the population's extinction.
"I think the most interesting finding we made is that these Neanderthals are genetically relatively healthy, with no strong signs that there was inbreeding depression," said Peter, who conducted the research as a group leader at the Max Planck Institute for Evolutionary Anthropology and is now a UCLA assistant professor of ecology and evolutionary biology and the Institute for Quantitative and Computational Biology. "It's also interesting that we didn't find evidence that they have ancestry from anatomically modern humans, even though we know that at least they must have overlapped in time."
A final point of interest was the finding that all of the remains from one site, Goyet, were of unrelated females, juvenile males and one newborn baby.
Taken together, the genetic analysis shows that although this Belgian population was more closely related to one another than to other Neanderthal groups, the difference wasn't substantial, suggesting there may have been considerable movement among these Western Neanderthal groups.
"Over the very long time scale that Neanderthals existed in Eurasia, their entire range was not inhabited consistently. There were probably some areas where Neanderthals could survive through climatic events like ice ages, and they expanded into other areas when conditions were suitable for human habitation," said Peter. "It seems that Western Europe at this time was a good place to be a Neanderthal. I think a climate shift later on, and maybe competition from modern humans, or a combination, led to the extinction very soon after the individuals that we studied."
