A mathematical model has shown that the disappearance of Neanderthals was not necessarily caused by wars, epidemics, or climate disasters. The study suggests that even a small but steady influx of Homo sapiens migrants into Neanderthal groups over thousands of years could have led to their complete assimilation, leaving only a minor share of Neanderthal genes in the DNA of modern humans.

The History of Neanderthals and Theories of Their Disappearance

Neanderthals and their ancestors—the Heidelberg people, close relatives of Homo sapiens—inhabited Europe and Asia for about 400,000 years. However, around 40,000 years ago, roughly 10–15 thousand years after the mass migration of modern humans out of Africa, Neanderthals vanished. The reasons for their extinction remain debated, and several main hypotheses exist.

One theory suggests that Homo sapiens were more skilled hunters and had intellectual advantages, allowing them to displace or even eliminate Neanderthals. Another hypothesis links the disappearance of Neanderthals to demographic problems: their population was small, and inbreeding may have reduced their viability. However, this does not explain why the demographic crisis occurred only after contact with modern humans and not over the previous hundreds of thousands of years.

According to a third theory, Neanderthals could not adapt to climate change, natural disasters, or contracted new diseases from Homo sapiens. Yet another hypothesis proposes that Neanderthals simply blended in with the newcomers, as evidenced by the fact that 1 to 4% of the genes of modern non-African people are of Neanderthal origin.

The Mathematical Model of Assimilation

The authors of the study, published in Nature, tested the latter hypothesis using mathematical modeling. They applied a population genetics model known as the “continent-island” model, where a small and relatively isolated population (“island”—the Neanderthals) receives a constant influx of genes from a much larger population (“continent”—Homo sapiens).

The model simulated what would happen to the Neanderthal gene pool if, in each “cycle” (every few generations), a small number of Homo sapiens migrants joined them. Calculations showed that if 1 to 8% of migrants joined the population per cycle, then after 10–30 thousand years, the group’s gene pool would be 99% composed of Homo sapiens genes. The calculations used the maximum estimated Neanderthal population—about 70,000 individuals living in small groups of 25–50 people, while the Homo sapiens population was 10 to 100 times larger.

Key Findings of the Study

The authors emphasize that their model was neutral: it did not account for any innate advantages of Homo sapiens, wars, disasters, or diseases—only population sizes and migration intensity.

The results indicate that rare but regular contacts, when small groups of modern humans (possibly individual families or hunters) joined Neanderthals, would have been enough for Neanderthals to disappear. Due to the significant demographic advantage of Homo sapiens, this genetic flow was almost one-way and, in the long run, proved fatal for Neanderthals as a distinct species.

Although conflicts, diseases, and competition may also have played a role, the calculations show that ordinary migration and natural assimilation over several tens of thousands of years could have led to the disappearance of Neanderthals. These conclusions align well with the known timelines of contact between Homo sapiens and Neanderthals, as well as with estimates of their population sizes.