Between one and four percent of the genome of anyone with non-African ancestry traces back to Neanderthals, the result of interbreeding that took place roughly 45,000 to 49,000 years ago.1PubMed Central. The contribution of Neanderthal introgression to modern human traits2PubMed Central. Earliest modern human genomes constrain timing of Neanderthal admixture That small percentage does not sit there quietly. Neanderthal DNA fragments influence traits you can see in the mirror, diseases you may or may not develop, how you sleep, how you feel pain, and even how your body handles certain medications.
How Much Neanderthal DNA Is Actually in You
The one-to-four percent figure applies to any individual person with Eurasian roots. But those fragments are scattered across different chromosomes in different people. When researchers pool genomes from many modern humans, they can reconstruct up to about 35 percent of the entire Neanderthal genome from the small pieces each person carries.3Current Biology. The contribution of Neanderthal introgression to modern human traits – Section: Gene flow between Neanderthals and modern humans The segments in any one person tend to be short, usually less than a hundred kilobases, because tens of thousands of years of recombination have broken them into smaller and smaller chunks.4Nature. The genomic landscape of Neanderthal ancestry in present-day humans
Geography matters. East Asian populations carry roughly 12 to 20 percent more Neanderthal ancestry than European populations, a difference researchers have tried to explain through multiple episodes of interbreeding or differences in population size after the initial admixture event.5Nature Ecology & Evolution. Multiple episodes of interbreeding between Neanderthal and modern humans6PubMed Central. Higher levels of neanderthal ancestry in East Asians than in Europeans People of primarily sub-Saharan African descent have very little Neanderthal DNA, because the interbreeding happened after modern humans had already left Africa. And Neanderthals were not the only archaic group contributing DNA. Some Oceanian and South Asian populations carry up to roughly five percent Denisovan ancestry on top of their Neanderthal fraction, a bigger contribution from a separate ancient lineage.7PubMed Central. The Combined Landscape of Denisovan and Neanderthal Ancestry in Present-Day Humans
Skin, Hair, and Facial Features
Some of the most visible Neanderthal legacies involve the skin and hair. A gene called BNC2, which encodes a protein active in the outer skin cells, has been linked to variation in skin pigmentation and freckling in Europeans. Another introgressed region spans the gene POU2F3 on chromosome 11, involved in the growth and specialization of skin cells. That Neanderthal version is nearly absent in Europeans but present at about 60 percent frequency in East Asian populations, a striking example of how the same archaic inheritance can have very different fates in different parts of the world.8Current Biology. The contribution of Neanderthal introgression to modern human traits – Section: Skin and hair pigmentation A large biobank study in Europeans also found that Neanderthal alleles were strongly associated with red hair color.9PubMed Central. Denisovan and Neanderthal archaic introgression differentially impacted the genetics of complex traits in modern populations
Neanderthal ancestry also shapes the face. An introgressed region on chromosome 1 is significantly associated with increased nasal height, consistent with the taller noses seen in Neanderthal skulls compared with modern human ones.10Communications Biology. Automatic landmarking identifies new loci associated with face morphology and implicates Neanderthal introgression in human nasal shape A separate multi-ancestry analysis of facial features predicted that Neanderthals and Denisovans likely had wider but shorter noses and wider distances between the inner corners of their eyes, predictions that line up well with what fossil skulls show.11PubMed. A multi-ancestry GWAS meta-analysis of facial features and its application in predicting archaic human features
An Immune System Tuned by Ancient Infections
If there is one domain where Neanderthal DNA has had the clearest adaptive value, it is immunity. Neanderthals had been living in Europe and western Asia for hundreds of thousands of years before modern humans arrived, encountering local pathogens along the way. When the two groups interbred, modern humans picked up alleles for toll-like receptor genes (TLR1, TLR6, and TLR10) that had already been shaped by those encounters. These archaic alleles boost expression of those receptors in white blood cells, which ramp up the innate immune system’s ability to detect invaders.12PubMed Central. Introgression of Neandertal- and Denisovan-like Haplotypes Contributes to Adaptive Variation in Human Toll-like Receptors
The trade-off is real. The same heightened immune reactivity that helped modern humans fight off certain bacteria, including a measurable reduction in Helicobacter pylori infection rates, also appears to have increased susceptibility to allergies. In large present-day cohorts, carriers of the archaic TLR haplotypes show more allergic disease, likely because an immune system primed for aggressive pathogen surveillance also overreacts to harmless environmental triggers like pollen or dust.13American Journal of Human Genetics. Introgression of Neandertal- and Denisovan-like Haplotypes Contributes to Adaptive Variation in Human Toll-like Receptors
Neanderthal DNA and COVID-19
The immune legacy of archaic admixture came into sharp focus during the pandemic. A stretch of about 50 kilobases on chromosome 3, inherited from Neanderthals, turned out to be the strongest genetic risk factor for developing severe COVID-19. That segment is carried by about half the population of South Asia and roughly 16 percent of Europeans.14Nature. The major genetic risk factor for severe COVID-19 is inherited from Neanderthals But the story was not one-sided. A different Neanderthal-derived region on chromosome 12, encoding proteins that activate enzymes important during RNA virus infections, was found to be protective against needing intensive care.15PubMed Central. A genomic region associated with protection against severe COVID-19 is inherited from Neandertals A third Neanderthal haplotype, this one involving the OAS1 gene, was also shown to protect people of European ancestry against both susceptibility to infection and severity of disease by boosting levels of an antiviral protein.16Nature Medicine. A Neanderthal OAS1 isoform protects individuals of European ancestry against COVID-19 susceptibility and severity
The fact that different Neanderthal segments pushed in opposite directions for the same disease captures something important about archaic introgression. These were not genes designed for modern life. They were shaped by the selection pressures Neanderthals faced tens of thousands of years ago, and their effects today are sometimes beneficial, sometimes harmful, and often a bit of both.
Pain Sensitivity
Neanderthals may have experienced the world as a more painful place than most modern humans do. Three amino acid changes in the SCN9A gene, which encodes a sodium channel (Nav1.7) crucial for pain signaling in peripheral nerves, have been traced to Neanderthal introgression. Individually, the changes do not do much. But when all three appear together, the channel becomes slower to inactivate after firing, meaning the nerve stays excitable longer and pain signals persist. Laboratory work confirmed that this full Neanderthal variant makes peripheral nerves more responsive to painful stimuli.17Current Biology. A Neanderthal Sodium Channel Increases Pain Sensitivity in Present-Day Humans
About 0.4 percent of present-day Britons carry all three substitutions, and they report heightened pain sensitivity.18PubMed. A Neanderthal Sodium Channel Increases Pain Sensitivity in Present-Day Humans Further work found that Neanderthal ancestry in this gene region is associated specifically with a lower threshold for mechanical pain after the skin has already been sensitized, such as following a burn or inflammation, and that the effects of the individual variants appear to add up.19PubMed Central. Neanderthal introgression in SCN9A impacts mechanical pain sensitivity This is a narrow slice of the overall pain experience, not a claim that carriers are in constant agony. But it does illustrate how ancient genetic variation can influence something as personal as how a paper cut feels.
Why Many People Are Morning Larks
If you are naturally inclined to wake early and feel sharpest in the morning, some of that tendency could be Neanderthal in origin. Several studies have found that introgressed variants, especially those near genes involved in the body’s internal clock, disproportionately push toward morningness. One analysis found that the introgressed alleles most strongly associated with chronotype consistently increase the tendency toward being a morning person, and the pattern only gets stronger when the variants sit close to known circadian-rhythm genes.20Genome Biology and Evolution. Archaic Introgression Shaped Human Circadian Traits
The most likely explanation is adaptation to higher latitudes. Species that move to regions with longer seasonal swings in daylight often evolve toward earlier chronotypes. Neanderthals had been living at high latitudes for hundreds of thousands of years, so their circadian biology was presumably well tuned for it. When modern humans expanded north out of Africa, inheriting those alleles would have given them a ready-made circadian adjustment rather than waiting for new mutations to arise.21PubMed Central. Archaic Introgression Shaped Human Circadian Traits The picture is not perfectly one-directional. A more recent analysis confirmed the broad morningness signal but also identified specific archaic haplotypes on chromosome 2 that are associated with being an evening person, suggesting the inherited chronotype toolkit includes a few variants pushing the other way.22npj Biological Timing and Sleep. Adaptive introgression in modern human circadian rhythm genes
Fertility and Pregnancy
One of the more striking Neanderthal contributions involves reproduction. Two Neanderthal-derived haplotypes carrying the progesterone receptor gene entered the modern human population, and present-day carriers express higher levels of that receptor. Progesterone is essential for maintaining early pregnancy, and in a large cohort of present-day Britons, women who carried the Neanderthal version had more siblings, fewer miscarriages, and less bleeding during early pregnancy.23PubMed Central. The Neandertal Progesterone Receptor That is a rare case of a clear, unambiguously positive effect from archaic introgression on a trait modern humans care deeply about.
Metabolic Risks and Connective-Tissue Disease
Not all Neanderthal heritage confers advantages. A haplotype on the SLC16A11 gene is present at about 50 percent frequency in Native American populations and around 10 percent in East Asian populations, but is rare in Europeans and Africans. It was traced to Neanderthal introgression and carries four amino acid changes that alter lipid metabolism in the liver, increasing intracellular fat levels.24Nature. Sequence variants in SLC16A11 are a common risk factor for type 2 diabetes in Mexico Follow-up work showed that this disruption increases type 2 diabetes risk through changes in fatty acid transport.25PubMed Central. Type 2 Diabetes Variants Disrupt Function of SLC16A11 through Two Distinct Mechanisms The variant’s high frequency in populations with elevated diabetes rates makes it a meaningful contributor to modern metabolic disease, not the whole story, but a piece that clinicians increasingly recognize.
A different and more obscure condition, Dupuytren’s disease, a progressive thickening and contraction of connective tissue in the hand, also has major Neanderthal roots. A meta-analysis of nearly 8,000 cases and over 645,000 controls identified 61 genomic regions linked to the disease. Three of those regions harbored alleles from Neanderthals, including the second and third most strongly associated loci in the entire study.26Molecular Biology and Evolution. Major Genetic Risk Factors for Dupuytren’s Disease Are Inherited From Neandertals Dupuytren’s is much more common in people of European descent than in those of primarily African ancestry, consistent with its archaic genetic origins.
Brain Shape and Psychiatric Traits
Neanderthal skulls were longer and less globular than modern human skulls, and that difference left a mark in the genome. Two Neanderthal-derived alleles, on chromosomes 1 and 18, are associated with reduced endocranial globularity in living people. These alleles influence genes involved in the development and insulation of nerve cells.27PubMed Central. Neandertal Introgression Sheds Light on Modern Human Endocranial Globularity Separately, a higher overall load of Neanderthal genetic variants correlated with deeper folding in a specific brain region, the right intraparietal sulcus, located directly beneath the area of greatest Neanderthal-influenced skull shape change.28Scientific Reports. Neanderthal-Derived Genetic Variation Shapes Modern Human Cranium and Brain These are subtle structural effects. Nobody can look at a brain scan and decide someone has “more Neanderthal brain.” But they confirm that archaic introgression reaches beyond surface traits into the organ most central to who we are.
On the psychiatric side, a large study linking genetic data to electronic health records found that Neanderthal alleles collectively explained a significant fraction of the variation in risk for depression, along with associations with skin lesions from sun exposure, blood clotting tendencies, and even tobacco use.29PubMed Central. The phenotypic legacy of admixture between modern humans and Neandertals A separate analysis in East Asian populations echoed the psychiatric signal, finding Neanderthal alleles associated with traits related to depression and bipolar disorder.9PubMed Central. Denisovan and Neanderthal archaic introgression differentially impacted the genetics of complex traits in modern populations These are population-level statistical associations, not diagnoses. Many factors contribute to psychiatric conditions, and carrying a Neanderthal risk allele does not make depression inevitable any more than lacking one makes you immune.
How Neanderthal DNA Affects Medication Response
One of the more practically relevant discoveries is that a clinically important haplotype involving two drug-metabolizing genes, CYP2C8 and CYP2C9, was inherited from Neanderthals. This haplotype, spanning about 300 kilobases on chromosome 10, changes how the body processes several medications. Most significantly, it reduces the metabolism of warfarin (a widely prescribed blood thinner) and phenytoin (an anti-seizure drug), leading to a risk of toxicity at doses that are otherwise considered safe.30The Pharmacogenomics Journal. The clinically relevant CYP2C8*3 and CYP2C9*2 haplotype is inherited from Neandertals
More broadly, researchers cataloging variants in key drug-metabolizing enzyme genes across archaic genomes identified several mutations shared between Neanderthals, Denisovans, and modern humans that could alter how medications are broken down in the body.31PubMed Central. Pharmacogenetic Variation in Neanderthals and Denisovans and Implications for Human Health and Response to Medications This is still an emerging area. Most pharmacogenomic testing panels already screen for these CYP variants for dosing purposes. The Neanderthal origin of those variants is a new layer of understanding, not a change in clinical practice, but it does add an evolutionary dimension to why certain populations have higher frequencies of slow-metabolizer alleles for specific drugs.
How Neanderthal Variants Actually Influence Your Genes
A natural question is: if Neanderthal DNA makes up such a small fraction of the genome, how can it have so many effects? The answer lies in gene regulation. A study using data from dozens of human tissue types found that about a quarter of Neanderthal-introgressed variants they tested showed significant differences in how actively the gene was read from one chromosome versus the other. In other words, when a person carries a Neanderthal version on one chromosome and a modern human version on the other, the two versions often produce noticeably different amounts of the gene’s product.32PubMed Central. Impacts of Neanderthal-introgressed sequences on the landscape of human gene expression This regulatory effect was found across 767 genes and dozens of tissues. Neanderthal DNA does not need to change the protein itself. It can change how much protein is made, in which tissue, and when, which is often more than enough to shift a trait.
Some Neanderthal variants act even further from the genes they affect. Research has identified cases where archaic variants alter the activity of transcription factors, proteins that control the expression of other genes elsewhere in the genome, creating ripple effects across distant regions.33Genetics. Long-range regulatory effects of Neandertal DNA in modern humans This helps explain why even small amounts of introgressed DNA can touch so many different traits. A single regulatory variant in the right spot can influence a cascade of downstream genes.
Why Natural Selection Has Not Removed It All
If Neanderthal DNA sometimes increases disease risk, you might wonder why natural selection has not simply cleaned it out over the past 45,000 years. It has, partially. Both Neanderthal and Denisovan ancestry are depleted near genes and especially depleted on the X chromosome and near genes highly expressed in testes, a pattern consistent with archaic alleles being harmful to male fertility in particular.7PubMed Central. The Combined Landscape of Denisovan and Neanderthal Ancestry in Present-Day Humans The depletion is strongest in regulatory regions and in the most conserved parts of protein-coding sequences, the stretches of DNA where even small changes tend to matter most.34PubMed Central. Limits of long-term selection against Neandertal introgression
But the average strength of selection against deleterious Neanderthal alleles is very weak, on the order of a few ten-thousandths per generation. That is enough to slowly push Neanderthal ancestry down over millennia, but nowhere near strong enough to purge it quickly.35PLOS Genetics. The Strength of Selection against Neanderthal Introgression Meanwhile, some Neanderthal alleles have been actively favored by selection, including the immune and skin-related variants discussed above. The genome that results is a mosaic: some archaic segments are slowly vanishing, some are being preserved because they are useful, and many are simply drifting, too weakly deleterious for selection to act on them efficiently over the timescales involved.
Neanderthal Ancestry and the Nose You Breathe Through
Returning to craniofacial morphology, nasal shape is one of the traits where Neanderthal introgression has the most direct and visible influence. The introgressed region at 1q32.3 does not just correlate with nasal measurements in a subtle statistical sense. The effect is directionally consistent with what we see in the fossil record: Neanderthal skulls have markedly higher nasal bridges than modern human ones, and people who carry the introgressed tract at this locus show measurably taller noses.10Communications Biology. Automatic landmarking identifies new loci associated with face morphology and implicates Neanderthal introgression in human nasal shape This does not mean having a tall nose makes someone “more Neanderthal.” Nasal shape is polygenic, influenced by dozens of genes, climate, and developmental factors. But the archaic contribution is real and measurable, and it is one of the few cases where an introgressed variant maps neatly onto a physical feature visible in both ancient fossils and living people.
The research also highlights something worth keeping in mind when you see consumer DNA tests offering Neanderthal ancestry percentages: the number you get is a genome-wide average. It does not tell you which specific traits, if any, are influenced by your particular collection of archaic fragments. Two people with identical Neanderthal percentages could carry completely different segments, one inheriting the taller-nose variant and the other inheriting an immune-related segment with no visible phenotype at all. The percentage is a statistical summary, not a trait map.