Humans
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Despite surviving for hundreds of thousands of years and conquering much of Eurasia, Neanderthals were actually pretty few and far-between. Living in tiny, isolated groups spread out across vast distances, this ancient hominin often had to resort to mating with close relatives, leaving the population highly inbred.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Evidence for this somewhat miserable state of affairs comes from two new studies. The first of these compared the genome of a 110,000-year-old Neanderthal from Denisova Cave in Siberia to a slightly older individual from the same site, as well as an 80,000-year-old Neanderthal from Croatia.
Amazingly, the level of genetic differentiation between the Siberian and European Neanderthals was found to be greater than the most differentiated pairs of modern-day humans, such as the Mbuti of Central Africa and the Papuan Highlanders of New Guinea. This is particularly startling given that the Mbuti and the Papuans are estimated to have been isolated from one another for up to 220,000 years, while the two Neanderthal populations had only been separate for about 80,000 years.
Such a finding indicates that Neanderthals reached higher levels of differentiation than modern human populations, over much shorter time periods. This, in turn, suggests that they must have lived in smaller groups that were completely isolated from one another.
Furthermore, such clear genetic distinctions between Neanderthal groups can only have been achieved so quickly if they were routinely inbreeding. Overall, the results of this study paint a picture of a small, highly fragmented Neanderthal population, consisting of just a few thousand breeding pairs that often had to resort to mating with their own kin.
The fragility of the global Neanderthal collective is reinforced by the findings of a second new study, which examined the mitochondrial DNA of Neanderthals from France, Germany, Belgium and Serbia. Results hint at a bottleneck event starting around 73,000 years ago, when a glacial maximum forced the Neanderthal population to abandon much of its range and seek refuge in southern France and the Iberian Peninsula.
By the time things warmed up and Neanderthals were able to expand their range again, the vast majority of their genetic diversity had been replaced by a single mitochondrial lineage. According to the study authors, this sole surviving line descended from a population that existed in France about 65,000 years ago before spreading across Eurasia once the ice melted.
Yet even as Neanderthals spread out again, their effective population size remained small, with a relatively low number of fertile adults. This figure then crashed between 45,000 and 42,000 years ago, as a combination of climate change and the arrival of modern humans in Neanderthal territory sent this ancient hominin spiralling towards extinction.
Taken together, these two studies help to reveal the population dynamics that caused Neanderthals to teeter on the brink of oblivion for tens of thousands of years. Ultimately, our intervention may have been the factor that tipped the balance, as Eurasia became genetically swamped with Homo sapiens DNA and the small remaining Neanderthal gene pool was engulfed.
The two studies have been published in the Proceedings of the National Academy of Sciences.
