Did Humans and Neanderthals Interbreed?

Our ancestors coexisted with the Neanderthals for thousands of years. These two species occupied shared territories in Europe and parts of Asia until 40,000 years ago, when the Neanderthals went extinct. Homo sapiens and the Neanderthals are genetically similar species that could interbreed. Contemporary genetic research demonstrates that there was indeed interbreeding between these groups; in fact, today, human populations have around 1-4% of the Neanderthal genes.

Homo neanderthalensis, commonly known as the Neanderthals, represent a distinct species of early humans discovered in the 19th century. They are probably the most famous of all extinct human species, even though we still do not know enough about them.

Luckily, genetic research helps uncover the differences and similarities between the Neanderthals and our own species, Homo sapiens. We know that our ancestors shared habitats with the Neanderthals in Europe, Western Asia, and parts of the Middle East up until the extinction of the Neanderthals around 40,000 years ago. However, we don’t know much about their life together, and we are still unsure if our own species contributed to the extinction of the Neanderthals.

We also know that these were genetically very similar groups of early humans. While our species, Homo sapiens, is the only one left on Earth today, it was just one of the many human species to have existed. The first members of our genus Homo (humans), appeared around 2,4-1,5 million years ago. These early species, now extinct, include Homo habilis, Homo ergaster, and Homo erectus. Later, around 700-600 000 years ago, new species of humans appeared, often called “archaic Homo sapiens”. These probably evolved from Homo erectus, and it is believed that archaic Homo sapiens, such as Homo Heidelbergensis, are direct ancestors of both modern humans and the Neanderthals. It is estimated that the Neanderthals genetically diverged from our species around 550-500 000 years ago.

The earliest members of our species originated in Africa, around 200-150,000 years ago (or even earlier, around 300,000 years ago, according to some data). However, they did not have a chance to encounter the Neanderthals straight away, since the Neanderthals did not live in Africa. Only after Homo sapiens migrated out of Africa into Asia and Europe did they make first contact, around 100,000 years ago.

Physically, the Neanderthals were very similar to Homo sapiens, except for some specific features. The Neanderthals had a pronounced brow ridge, and they were somewhat shorter and stockier than anatomically modern humans. Such body shape was likely an adaptation to cold environments. They also had a bit larger brains than our species. In many other ways, they were very similar to us, which is not surprising since our species are very close genetically. This is crucial since interbreeding would not be possible with a larger genetic difference.

Early humans and the Neanderthals were so similar genetically that scientists wondered about the possibility of their interbreeding since the second part of the 20th century. This also opened up a question of whether Homo sapiens and the Neanderthals were actually just different variations of the same species since members of the same species can interbreed successfully.

According to the biological species concept, a species is a group of individuals that can interbreed and produce fertile offspring. “Fertile offspring” is the key term because we know that some very close but different species can produce live offspring, but such offspring is generally infertile. A good example is horses and donkeys. They can interbreed and produce infertile offspring – mules and hinnies. In some rare cases, these animals can be fertile (this is mainly true for female mules), but it is not the norm. Their general infertility is one of the reasons for classifying donkeys and horses as different species.

Of course, the biological species concept is just one of the many ways to define a species. What makes a separate species is a matter of scientific consensus, and it depends on complex criteria. Nature itself is often more elusive than we like to think, and it is not always easy to precisely define the boundaries between species.

At the same time, the question of fertile vs infertile offspring can help understand the relationship between Homo sapiens and Neanderthal populations in prehistory. If these were the same species, they would be able to reproduce and leave fertile offspring. Consequently, today’s human populations would have a mix of Homo sapiens and Neanderthal genes.

On the other hand, if the Homo sapiens and the Neanderthals were different species, there should be no Neanderthal genes in today’s human populations. This would mean either that there was no interbreeding or that even if there was, it did not produce fertile offspring.

So, which one is the case? As it turns out, the answer is more complex than a simple “yes” or “no”. Genetic studies reveal that today’s human populations indeed have a percentage of Neanderthal genes. This proves that our ancestors interbred with the Neanderthals. However, the percentage of these genes is relatively small, around 1-4%. This complicates the interpretation since this relatively small percentage can mean different things: either interbreeding was rare, or only a small percentage of offspring was fertile (which would suggest close but different species). Finally, the small percentage of the Neanderthal genes could be because of the relatively small number of Neanderthal individuals. We know that their population was smaller compared to Homo sapiens.

Even with these unknowns, we can safely conclude that humans and Neanderthals did interbreed. In fact, this happened more often than previously thought.

On average, today’s humans have between 1% and 4% of the Neanderthal genes, although the percentage is not the same in all populations. For a long time, it was believed that people tracing their ancestry exclusively to Africa had no Neanderthal admixture because the Neanderthals did not live in Africa. However, recent studies showed that this is not necessarily the case — there is some Neanderthal DNA in African populations. Why? This is due to the complexity of human migrations around the world. Homo sapiens groups initially left Africa and migrated around the world. However, some later returned to Africa and a percentage of them had interbred with the Neanderthals during their time out of Africa, something that is reflected in the genes of today’s African populations.

The gene flow occurred in the opposite direction, too. We know of Neanderthal individuals with some Homo sapiens gene admixture. Some of these events happened relatively as early as 100,000 years ago.

Another thing that we know about human and Neanderthal interbreeding is that it happened numerous times. Some of the earliest examples of interbreeding occurred among populations that died out. In other words, no humans living today are the descendants of those people. Since today’s humans do have some Neanderthal admixture, this is proof of later interbreeding events, indicating that such events were more frequent than once thought.

Mitochondrial DNA

It is also interesting that modern humans don’t have any mitochondrial DNA (mtDNA) from the Neanderthals. To understand the implications of this, we need to understand the patterns of mtDNA inheritance. Most of the DNA is located in the nucleus of the cell, but a small portion is located in the mitochondria, part of the cell responsible for energy production. This DNA is inherited solely from the mother, so it is useful for tracing the genetic lineage from the mother’s side. According to present knowledge, humans inherited mtDNA solely from the Homo sapiens’ side and none from the Neanderthals.

The implications of this are not completely understood. Would that mean that men contributed more of the Neanderthal genes to modern humans than women? If yes, this could tell us something about the social organization between humans and the Neanderthals. On the other hand, it is possible that there was once a Neanderthal admixture in our mtDNA but that these lineages died out at one point. Another possibility is that humans and the Neanderthals are indeed different species (based on the biological species concept) and that only human women and Neanderthal men could produce fertile offspring (but not human men and Neanderthal women). Further research will hopefully shed light on these questions.

It is also important to note that the Neanderthals are not the only prehistoric humans who interbred with our ancestors. Another species, the Denisovans, was discovered in 2010 in Siberia. We still don’t know much about them, but they are identified as a new species of humans based on their DNA. According to current knowledge, the Denisovans were closely related to both modern humans and the Neanderthals. There are genetic indicators that they were closer to the Neanderthals than Homo sapiens, but we still don’t have enough Denisovan specimens to fully understand this species.

What we do know is that Denisovans, too, interbred with our species. In some human populations, like those from Oceania and Papua New Guinea, the Denisovan admixture is more significant than the Neanderthal, going to around 6%. In addition to this, the Denisovans also interbred with the Neanderthals — genetic research found proof of shared DNA between all three species (or subspecies) of humans.

The Influence of the Neanderthal Genes?

Recent research has proved that the Neanderthals did interbreed with Homo sapiens. The proof is in our genes: there is a Neanderthal admixture in modern human populations. Even a small percentage tells an interesting story about our ancestors and their relationship with other, now extinct, human species.

What does it mean for us? The fact that some of us have Neanderthal and/or Denisovan admixture does not imply the level of evolutionary progress. No person is “less evolved” for having a higher Neanderthal or Denisovan admixture than somebody else.

Scientists are working on understanding how this genetic admixture might affect living humans. Do Neanderthal genes affect health (and if yes, in what way?) Are there certain advantages or disadvantages that this admixture can bring?

We still don’t have concrete answers, although there are indicators that these genes might make a certain difference. Scientists have found a link between the Neanderthal genes and the immune system, which might reflect allergies. Another study found that women with a higher percentage of Neanderthal genes might have better fertility. Scientists also found a connection between Neanderthal genes and the risk of developing serious COVID-19. Neanderthal genes could also contribute to lower pain thresholds in modern humans.

These studies are fascinating, but there is still so much we don’t know. We are only starting to discover details about the interbreeding events between our ancestors and other human species. Further research and discoveries will help us uncover more secrets from the prehistory of our species and its relationship with other populations.