Neanderthal DNA: The Ancient Human Inside You

For most of the twentieth century, the idea of recovering the genes of an extinct human seemed like fantasy. DNA breaks down after death, and a fossil tens of thousands of years old should hold only shattered, contaminated scraps. The triumph of paleogenomics was learning to read those scraps anyway. In 2010, a team led by the Swedish geneticist Svante Paabo published the first draft sequence of the Neanderthal genome, pieced together from bones found in Vindija Cave in Croatia. It was a landmark on the scale of the moon landing for human origins, and it delivered an immediate shock: comparing that Neanderthal genome to living people revealed that non-Africans carry Neanderthal DNA. Our ancestors had not merely competed with Neanderthals; they had interbred with them. Higher-quality genomes soon followed, from a Neanderthal toe bone in the Altai mountains, from Vindija, from Chagyrskaya Cave, each sharpening the picture. The work founded an entire scientific field, and in 2022 Paabo was awarded the Nobel Prize in Physiology or Medicine for it. An extinct human being's complete genetic blueprint could now be read from the dust of a cave floor.

The headline number deserves to be stated carefully. Most non-Africans alive today carry roughly 1.5 to 2.5 percent Neanderthal-derived DNA, with East Asian populations tending to hold a little more than Europeans. Present-day Africans carry small amounts too, generally read as the trace of later back-migration from Eurasia rather than interbreeding inside Africa. Two point five percent may sound minor, but it is not trivial: it is tens of millions of letters of an extinct human's code, riding along inside you. And here is a fact that changes how to picture the whole thing. No single living person carries much Neanderthal DNA, but different people carry different pieces of it, because natural selection and chance kept different fragments in different lineages. Add up all the surviving fragments scattered across all living humans, and researchers estimate that something like 40 to 50 percent of the entire Neanderthal genome is still out there, distributed across the human population like a shattered manuscript whose pages are held by millions of different readers. The Neanderthals are extinct as a people, but as a genome they are, in a strange and partial sense, still assembled among us.

When and where did it happen? The bulk of the Neanderthal DNA in living non-Africans traces to a single broad episode of interbreeding that occurred as modern humans expanded out of Africa and through the Near East, somewhere around 50,000 to 60,000 years ago. That timing is why all non-Africans, from Europe to East Asia to the Americas, share a common baseline of Neanderthal ancestry: their ancestors picked it up in that shared bottleneck before dispersing across the world. A very recent study, published in December 2024 and still settling into the field, used more than 300 ancient and modern genomes to argue the date can be narrowed much further, to a single shared window of roughly 50,500 to 43,500 years ago, a span of only about 7,000 years; a striking refinement, though a recent one that further work may yet adjust. Occasionally the genomes even catch the encounter almost in the act. A roughly 40,000-year-old individual from Pestera cu Oase in Romania carried 6 to 9 percent Neanderthal ancestry in enormous, barely-broken chromosomal blocks, meaning this person had a Neanderthal ancestor just four to six generations back, a great-great-grandparent or thereabouts. That particular family line left no descendants among modern Europeans, but its DNA is a snapshot of interbreeding caught red-handed.

Before going further into what Neanderthal DNA does, it is worth correcting the picture most people still carry of the people it came from. The popular image of the Neanderthal as a shambling, dim-witted caveman is not a finding; it is a mistake, and a traceable one. It stems largely from the French scientist Marcellin Boule, who in 1911 reconstructed an elderly, arthritis-riddled Neanderthal skeleton from La Chapelle-aux-Saints as a stooped, ape-like brute, mistaking one old man's disease for the anatomy of a whole species. The real evidence points the other way. Neanderthals made sophisticated tools using a multi-step technique that demands genuine planning. They buried their dead. They used red ochre and black pigment, and cave paintings in Spain have been dated to more than 65,000 years ago, before modern humans are thought to have even reached Europe, which would make them the work of Neanderthal hands. They cared for their injured and disabled. And their brains were not small: the average Neanderthal braincase, at roughly 1,500 to 1,600 cubic centimeters, was actually larger than the modern human average of about 1,350. Whatever became of them, it was not for lack of a mind.

So what is that 2 percent actually doing? Some of it turns out to be useful, kept and spread by natural selection because it helped, a process called adaptive introgression. The clearest cases involve the immune system: Neanderthal-derived versions of certain innate-immunity genes (the toll-like receptors TLR1, TLR6, and TLR10) are among the best-documented beneficial pieces of archaic DNA in living people, likely tuning how the body first detects pathogens. Other Neanderthal variants sit in genes governing skin and hair, part of a recurring pattern in which the biology of the body's outer surface was one of the domains where archaic DNA mattered most, perhaps helping newly-arrived Africans adapt to unfamiliar northern light and climate. One popular example must be corrected here, though, because it is almost always told wrong: the famous high-altitude adaptation that lets Tibetans thrive in thin mountain air, carried in a gene called EPAS1, is not Neanderthal at all. It came from the Denisovans, a different archaic human cousin with their own separate story. It is a genuinely spectacular case of inherited archaic DNA, but it belongs to a different chapter, not this one.

Not all of the inheritance is benign, and the most topical example is also a perfect illustration of why honesty about this subject matters. A stretch of Neanderthal DNA on chromosome 3 is the single strongest genetic risk factor known for severe COVID-19, raising the odds of serious illness by more than half; it is carried by roughly half of all people of South Asian descent, about 16 percent of Europeans, and only about 4 percent of East Asians. But here is the crucial twist, and the detail that popular accounts routinely garble. A completely separate piece of Neanderthal DNA, on chromosome 12, spanning genes called OAS1, OAS2, and OAS3 that help destroy viral RNA, is protective, lowering the risk of severe COVID-19 by around a fifth. Two different Neanderthal legacies, on two different chromosomes: one can hurt you, the other can help you, and which you carry is a matter of ancestral chance. Beyond COVID, Neanderthal DNA has been statistically linked to small shifts in risk for conditions from mood disorders to skin lesions to blood clotting. But these are population-level correlations with tiny individual effects, not genetic fortunes written for any one person, a distinction that becomes the article's most important refusal further down.

Neanderthal DNA is not scattered randomly through our genomes; it has been actively filtered, and reading that filter tells its own story. Across the genome there are large stretches, called 'deserts of introgression,' that are almost entirely swept clean of Neanderthal ancestry. The X chromosome carries about five times less Neanderthal DNA than the other chromosomes, and Neanderthal ancestry is especially depleted around genes active in the testes and in brain development. The interpretation is sobering and specific: some Neanderthal gene variants reduced the fertility of early male hybrids, and some were mildly incompatible with modern human biology, so natural selection quietly removed them over the generations. Models suggest that the average Neanderthal fraction in modern genomes fell from an initial level of perhaps 10 percent, right after admixture, down to about 2 percent today, a slow purge of the parts that did not work. A likely reason is that the Neanderthal population was small, which had allowed mildly harmful mutations to accumulate in it by chance; once those variants entered the far larger modern human population, selection could see them clearly and weed them out. What survives in us, in other words, is the Neanderthal DNA that was neutral or beneficial. The rest was edited away.

Which brings us to the deepest and most haunting gap of all, and the question buried in this article's own title. We carry Neanderthal DNA scattered across our ordinary chromosomes, but two crucial pieces of them are missing entirely. Not a single living human carries a Neanderthal Y chromosome, passed father to son, or Neanderthal mitochondrial DNA, passed mother to child. Both of these lineages, the pure paternal line and the pure maternal line, have vanished completely from the modern gene pool, even as the rest of the genome kept its Neanderthal fragments. Why? This is genuinely unresolved, and it is the honest open question at the center of the story. There are three leading ideas, and they may all be partly true. Perhaps the interbreeding ran mostly one direction, Neanderthal fathers and modern human mothers, which would leave no Neanderthal mitochondrial line and, combined with other effects, no surviving Neanderthal Y. Perhaps male hybrids with a Neanderthal father were simply less fertile, a common pattern when two long-separated populations mix, which fits the same X-chromosome and testis-gene evidence as the deserts of introgression. Or perhaps it was chance, the random loss of rare lineages from a small population over time. No one yet knows which of these dominated. Fittingly, the tidiest part of the Neanderthal legacy to state, that it is real and it is in us, sits right beside one of its most stubborn mysteries.

Two overclaims must be refused with equal firmness, because this subject attracts both. The first, already met above, is the old 'brute' myth, the idea that Neanderthals were unintelligent sub-humans. The tools, the burials, the pigment, the cave art, and the larger-than-average brain all refute it; it was a stereotype born of one misread skeleton, not a reading of the evidence. The second overclaim runs the opposite way and is subtler and more modern: the notion that carrying Neanderthal DNA 'makes' a person a certain way, that it gives you a particular temperament, intelligence, or character. It does not. The real trait associations, including the disease risks, are small-effect statistical patterns drawn from studying tens of thousands of people at once; they say nothing deterministic about any individual, and they are not a personality horoscope written in archaic genes. And one more, worth stating plainly: Neanderthal ancestry does not track with human 'races.' All non-African populations carry broadly similar amounts, the differences between them are minor, and there is no sense in which Neanderthals are the special ancestors of any one modern group. The genome is more interesting than any of these myths. It records a real meeting between two kinds of human, an inheritance that is partly useful and partly purged, and a maternal line that slipped away for reasons we still cannot fully name.