Yes, humans and Neanderthals interbred. The evidence is now overwhelming, written into the DNA of virtually every living person. Most people of non-African descent carry between 1% and 4% Neanderthal DNA, and recent analysis has found that even African populations carry detectable Neanderthal ancestry, likely introduced through back-migrations from Eurasia. Remnants of Neanderthal genomes survive in every modern human population studied to date.
How We Know It Happened
The proof comes from ancient DNA. When scientists sequence the genomes of both Neanderthals and living humans, they find long stretches of matching genetic material that can only be explained by interbreeding. These shared segments are too large and too numerous to be coincidence or shared ancestry from a common ancestor millions of years ago. They reflect direct mating between the two groups.
Some of the most striking evidence comes from ancient fossils. A roughly 40,000-year-old jawbone found in Peștera cu Oase, a cave in Romania, belonged to a modern human whose genome was 6% to 9% Neanderthal. That’s far more than any person alive today. The Neanderthal DNA in his genome came in unusually large, unbroken chunks, which told geneticists something remarkable: this man had a Neanderthal ancestor just four to six generations back. That places a Neanderthal in his family tree less than 200 years before he was born, possibly a great-great-great-grandparent.
Similarly, four individuals from Bacho Kiro cave in Bulgaria, dating to around 44,000 years ago, had Neanderthal ancestors within the previous 10 to 20 generations. These weren’t isolated incidents separated by millennia. They were relatively recent family connections.
When and Where It Happened
The primary wave of interbreeding took place roughly 45,000 to 49,000 years ago, based on analysis of the earliest modern human genomes from Europe. This timing lines up with when Homo sapiens first spread into Eurasia in large numbers and overlapped geographically with Neanderthal populations.
But it wasn’t a single event. Evidence points to recurrent interbreeding over thousands of years, wherever the two species came into contact. A 44,000-year-old individual from Siberia (known as the Ust’-Ishim man) shows signals of an additional mixing event roughly 30 to 50 generations before he lived, separate from the main pulse of interbreeding. Genetic analyses have also detected gene flow between the two species as far back as 200,000 to 250,000 years ago, suggesting that earlier, smaller-scale encounters preceded the major mixing event.
Recent modeling work proposes that successive small waves of Homo sapiens migrating into Neanderthal territory, interbreeding each time, could have gradually diluted the Neanderthal gene pool over 10,000 to 30,000 years. Under this scenario, Neanderthals didn’t go extinct in the traditional sense. They were slowly absorbed into a much larger human population.
Patterns in Who Mated With Whom
The interbreeding wasn’t symmetrical. Modern humans carry noticeably less Neanderthal DNA on the X chromosome than on other chromosomes. Since women carry two X chromosomes and men carry one, this pattern is a fingerprint of sex-biased mating. It suggests that pairings between Neanderthal males and human females were more common than the reverse.
A 2024 study tested this idea by looking at it from the Neanderthal side. If mixing DNA between the two species produced unhealthy offspring regardless of the parents’ sexes, you’d expect to see gaps in Neanderthal genomes where human DNA was stripped out by natural selection, mirroring the gaps seen in human genomes. Instead, researchers found plenty of human ancestry on Neanderthal X chromosomes, including in regions unrelated to fitness. This confirmed that the missing Neanderthal DNA on human X chromosomes is best explained by a mating pattern, not by hybrid offspring being unhealthy.
Neanderthals Also Interbred With Denisovans
Humans weren’t the only species Neanderthals mated with. In 2018, geneticists sequenced DNA from a 90,000-year-old bone fragment found in Denisova Cave in Siberia. It belonged to a teenage girl whose mother was Neanderthal and whose father was Denisovan, another archaic human group. She was a first-generation hybrid.
Given how few ancient genomes have been sequenced, finding a direct hybrid among them suggests these encounters were not rare. As Svante Pääbo, who led the study, put it: Neanderthals and Denisovans may not have met often, but when they did, they mated frequently.
What Neanderthal DNA Does in Your Body
The Neanderthal DNA that persists in modern humans isn’t just genetic baggage. Some of it was actively useful. Neanderthals had lived in Europe and western Asia for hundreds of thousands of years before Homo sapiens arrived, adapting to local climates, diseases, and diets. When the two species interbred, some of those adaptations were passed along.
The clearest benefits involve the immune system. Multiple Neanderthal gene variants that survived in modern humans are linked to immune function, including genes involved in fighting bacterial infections, responding to influenza, and regulating white blood cell counts. Some of these variants appear to have been favored by natural selection, meaning they gave their carriers a survival advantage.
Neanderthal DNA also influences skin and hair. Variants inherited from Neanderthals affect how skin cells called keratinocytes develop and differentiate, which may have helped early humans in Eurasia adapt to lower levels of UV light. Neanderthal alleles contribute to variation in skin lesion risk, including sensitivity to sun damage.
Neanderthal DNA and Disease Risk
Not all inherited Neanderthal variants are helpful. Some increase the risk for conditions that matter today, in environments very different from those where the variants originally evolved. Neanderthal gene variants are associated with a measurable fraction of the risk for depression, coronary artery disease, obesity, and blood clotting disorders. One particularly well-studied variant, found in about 6.5% of Europeans, sits near genes that control how blood clots form and how the body recruits immune cells to injuries. Carrying this variant increases expression of proteins involved in coagulation and inflammation.
A Neanderthal-derived stretch of DNA on chromosome 17 has been linked to a roughly 20% increased risk of type 2 diabetes in Mexican and Latin American populations. Neanderthal variants have also been connected to both susceptibility to and protection against COVID-19 in Eurasian populations, a finding that drew widespread attention during the pandemic. Traits with disproportionate contributions from Neanderthal DNA include autoimmune diseases, balding, age at menopause, and respiratory and bone conditions.
Most Neanderthal variants in modern humans are thought to be neutral, neither helping nor hurting. But across the genome, Neanderthal DNA is slightly depleted compared to what you’d expect if it were completely harmless, suggesting that natural selection has been slowly pruning it over tens of thousands of years. The variants that remain are either beneficial, neutral enough to persist, or linked to traits where the evolutionary pressure wasn’t strong enough to eliminate them.
Neanderthal Ancestry in African Populations
For years, scientists assumed that people of African descent carried no Neanderthal DNA, since the interbreeding happened after humans left Africa. That turned out to be wrong. A 2020 study using a new analytical method called IBDmix, which doesn’t rely on comparing populations against each other, found that African individuals carry a stronger signal of Neanderthal ancestry than previously recognized.
This African Neanderthal ancestry has two sources. First, some early humans who had already mixed with Neanderthals migrated back into Africa, bringing Neanderthal DNA with them. Second, an early group of humans that left Africa before the main migration may have interbred with Neanderthals and introduced human DNA into the Neanderthal gene pool, complicating the genetic signals. The bottom line: every modern human population studied so far carries some trace of Neanderthal ancestry.