Why Did Homo sapiens Interbreed With Neanderthals Around 50,000 Years Ago? — Here’s What DNA Reveals About Their Union

Around 70,000 to 50,000 years ago, the modern human species, Homo sapiens, was finally able to successfully migrate out of Africa into other continents like Asia and Europe. In these new environments, they met an already established population of different human species, such as the Neanderthals.

Today, about 1 to 4% of the DNA of people outside of Africa came from the Neanderthals. This is proof that they not only met, but they also interbred and exchanged genes. But why exactly did this mating between two different human species occur? And could this phenomenon have influenced the eventual rise of Homo sapiens and the disappearance of the Neanderthals? Let’s explore these questions deeper.

Geographical Proximity – Why They Met

One of the reasons why interbreeding between Homo sapiens and Homo neanderthalensis occurred was geographical proximity. If the two human species had continued to live on different continents, they never would have met and interacted with each other. However, Homo sapiens’ searches for a new habitat that could support them led them to follow animal herds and vegetated areas until they were able to migrate to a new environment.

This is most likely driven by climate change and has made the natural habitats of humans in Africa less hospitable. Around 70,000 to 50,000 years ago, these shifts could have made forests recede and grasslands or savannas expand. There were also some fluctuating rainfall and periods of cold and drier seasons. These factors could have influenced vegetation in the area, and made animal food sources also migrate to new habitats. If this happened, the two main sources of diet for humans, which are meat and plants, would be scarce, and they would need to move to survive.

This is probably one of the leading reasons why Homo sapiens arrived at the places where Neanderthals were already occupying, such as the Middle East, Europe, and western Asia. Then, when two groups live in the same environment and share the same dietary food sources, the chances of meeting and interacting would go up. They both hunted big game, but also collected wild plants and other edible materials. During these hunting and scavenging phases, smaller groups of Homo sapiens and Neanderthals could have met, shared resources, and interbred.

Read more: Natural History Museum

Genetic and Adaptive Benefits for Early Humans

Proof that Homo sapiens bred with Neanderthals | Human - BBC

If the mating between Homo sapiens and Neanderthals is disadvantageous to the population and does not present any benefits, this would most likely reduce the chances of their DNA persisting even in the modern world. However, that was not the case. Because until today, we still have a considerable amount of their genes, so it most likely presented our ancestors with some key evolutionary advantage that helped them thrive in the new environments.

One of the possible advantages that their interbreeding offered is the gene variant of Neanderthals that helped modern humans adapt to the new habitat they had just migrated to. Because, unlike them, neanderthals were already adapted and had been surviving in these areas for hundreds of thousands of years. They could probably deal with the different challenges that the new environment presented, such as the colder climate, unique pathogens, and new dietary sources.

There are also some studies indicating that the Neanderthal genome influences humans’ skin color, hair, and even immune system response to certain diseases. In other words, the interbreeding of these two human species may have been beneficial for their survival, especially for modern humans who migrated into an unfamiliar environment and climate.

Shared Social, Survival, and Population Contexts

Another factor that could have led to the two different species interbreeding was their shared social and survival challenges. Both of them are highly likely to live in smaller groups when they interact in the new continent. The Homo sapiens may have had more numbers, but in the early stages, the difference is probably not too high.

This is because the nomadic lifestyle of humans back then could only support a limited number of people. So, their groups are small, but they were mobile. However, some fossils of Neanderthals showed signs of inbreeding and a genetic bottleneck, suggesting that their population in a group may have been too small. This scenario would have made them more vulnerable to diseases and genetic problems at birth.

With these factors in mind, when they met and interacted with the Homo sapiens, their shared survival challenges may have prompted both groups to become allies or at least establish a network of relationships that allowed them to interbreed with each other. This mating and sharing could have led to decreased chances of inbreeding and made their gene pools more diverse and resilient to survival challenges like pathogens and natural disasters. But, overall, their relationship is most likely much more complex than this picture.

Read more: PubMed Central

Why Scientists Think It Mattered for Modern Humans

When We Met Other Human Species

So, why does this interbreeding still matter in today’s world? Well, from DNA studies, we know that about 1 to 4% of the DNA of people outside of Africa came from Neanderthals. It is a clear sign that we still carry their legacy within us today, even if they are gone now. But why exactly do those genes remain, especially in a large group of population?

Currently, some of the possible answers are that they are evolutionary advantageous to us, or they are neutral. The evolution of humans is not a single ladder that leads to a goal; instead, it is like a branching tree, where individuals who possess a trait or characteristic that is advantageous to their environment or circumstances are the ones who are most likely to survive and pass on their genes to the next generation.

In other words, the genes of Neanderthals being passed on from our genetic lineage and still being persistent today means that they could have some important functions more than we realized, or they might be completely neutral, not affecting our survival. It may not be just about the color of our skin and hair, or the responses of our immune system. There could be something else deep down we have not yet uncovered, a deeper part of the Neanderthal inheritance that continues to shape who we are today.

Author's Final Thoughts

So, why did Homo sapiens interbreed with the Neanderthals around 50,000 years ago? It is most likely because of a combination of factors, which could include their geographic proximity, beneficial sharing of traits, and dealing with the same environmental and genetic problems.

Moreover, their mating most likely did not occur with just two groups interacting with one another. Studies suggest there could have been several interbreeding events that left a mark on our biology and lineage. So, it may not have been just a one-time accident of two human species exchanging genes, but rather a complex interaction that helped us to become who we are today, and who we could be in the future.

Read next: How Did Early Humans Survive the Ice Age That Began Around 2.6 Million Years Ago? — Here’s What Experts Discovered

References & Further Reading

Green, R. E., et al. (2010). A draft sequence of the Neandertal genome. Science. https://doi.org/10.1126/science.1188021

Prüfer, K., et al. (2017). A high-coverage Neandertal genome from Vindija Cave in Croatia. Science. https://doi.org/10.1126/science.aao1887

Sankararaman, S., et al. (2014). The genomic landscape of Neanderthal ancestry in present-day humans. Nature. https://doi.org/10.1038/nature12961

Hajdinjak, M., et al. (2021). Initial Upper Palaeolithic humans in Europe had recent Neanderthal ancestry. Nature. https://doi.org/10.1038/s41586-021-03335-3

Dannemann, M., & Kelso, J. (2017). The contribution of Neanderthals to phenotypic variation in modern humans. American Journal of Human Genetics. https://doi.org/10.1016/j.ajhg.2017.09.010

Zeberg, H., & Pääbo, S. (2020). The major genetic risk factor for severe COVID-19 is inherited from Neanderthals. Nature. https://doi.org/10.1038/s41586-020-2818-3

Villanea, F. A., & Schraiber, J. G. (2019). Multiple episodes of interbreeding between Neanderthal and modern humans. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-018-0735-8