Svante Pääbo did something most geneticists considered impossible: he recovered and sequenced DNA from organisms that had been dead for tens of thousands of years, eventually assembling the first Neanderthal genome and identifying a previously unknown human relative called the Denisovans. That work, which earned him the 2022 Nobel Prize in Physiology or Medicine, revealed that ancient interbreeding left a measurable stamp on the DNA of people alive today, reshaping how we understand human immunity, pain perception, sleep patterns, and even susceptibility to modern diseases.
Getting DNA Out of Bones Nobody Thought Had Any
The challenge Pääbo faced in the 1980s was not just scientific skepticism but chemistry. DNA degrades. Once an organism dies, water, oxygen, and microbes start chewing through its genetic material. By the time you are looking at a bone or tooth tens of thousands of years old, what remains is broken into tiny fragments, heavily damaged by oxidation, and contaminated with bacterial DNA and DNA from every modern human who has handled the specimen. Early in his career, Pääbo worked out that ancient DNA is characteristically short, riddled with chemical modifications to its building blocks, and prone to cross-linking between strands. These properties made standard cloning techniques unreliable but also provided a kind of fingerprint: if extracted DNA did not show these damage patterns, it was probably modern contamination rather than genuinely ancient material.
A turning point came with the polymerase chain reaction, the technique that copies specific DNA segments exponentially. Pääbo and colleagues demonstrated that PCR could amplify short stretches of mitochondrial DNA from remains ranging from 4 to 13,000 years old, spanning four species including two that were extinct.1PubMed Central. Ancient DNA: extraction, characterization, molecular cloning, and enzymatic amplification In a separate study, they managed to pull mitochondrial DNA sequences from a 7,000-year-old human brain, discovering that amplification efficiency dropped steeply as the target sequence got longer, a quirk that itself became a way to verify authenticity.2Nucleic Acids Research. Mitochondrial DNA sequences from a 7000-year old brain These early experiments established the basic ground rules for the entire field that would follow: ancient DNA work required extreme care about contamination, specialized extraction protocols, and a healthy paranoia about whether what you were reading was real.
The First Neanderthal DNA and What It Showed
In 1997, Pääbo’s team extracted DNA from the original Neanderthal specimen, the bones found in Germany’s Neander Valley back in 1856. They pieced together a previously unknown mitochondrial DNA sequence by amplifying short, overlapping fragments and sequencing each one.3PubMed. Neandertal DNA sequences and the origin of modern humans This was the first time anyone had read genetic material from a Neanderthal, and it immediately told researchers something important: the Neanderthal mitochondrial sequence was clearly distinct from all living humans, sitting outside the range of modern human mitochondrial variation.
A decade later, Pääbo’s group reconstructed a complete Neanderthal mitochondrial genome from a 38,000-year-old individual, pulling over 8,000 mitochondrial sequences from roughly 4.8 billion base pairs of raw DNA extracted from just a third of a gram of bone.4Cell. A Complete Neandertal Mitochondrial Genome Sequence Determined by High-Throughput Sequencing The complete sequence confirmed that Neanderthal and modern human mitochondrial lineages diverged roughly 660,000 years ago, give or take about 140,000 years. This was a deep split, but it was the nuclear genome that would deliver the real surprise.
The Draft Neanderthal Genome and the Discovery of Interbreeding
In 2010, Pääbo and a large international team published a draft sequence of the Neanderthal nuclear genome, assembled from more than four billion nucleotides extracted from three individuals. The finding that grabbed headlines was this: Neanderthals shared more genetic variants with present-day people in Europe and Asia than with present-day people in sub-Saharan Africa.5PubMed Central. A draft sequence of the Neandertal genome The simplest explanation was gene flow. Modern humans and Neanderthals had interbred, and the descendants of those unions went on to populate Eurasia. This overturned the prevailing view that Neanderthals were an evolutionary dead end with no genetic contribution to living people.
Subsequent work estimated that interbreeding most likely happened between about 47,000 and 65,000 years ago, fitting a scenario in which modern humans carrying new stone-tool technologies encountered Neanderthals as they expanded out of Africa.6PubMed Central. The Date of Interbreeding between Neandertals and Modern Humans The upshot is that people of non-African ancestry today carry roughly 1 to 4 percent Neanderthal DNA scattered across their genomes.7PubMed Central. The contribution of Neanderthal introgression to modern human traits That sounds like a small number, but collectively, different people carry different Neanderthal fragments, meaning a surprisingly large fraction of the Neanderthal genome still survives somewhere in the modern gene pool.
Denisovans, a Species Identified by DNA Alone
In 2010, the same year the Neanderthal genome paper appeared, Pääbo’s lab announced something stranger. A tiny finger bone fragment from Denisova Cave in Siberia yielded DNA that did not match modern humans or Neanderthals. It belonged to a previously unknown group of archaic humans, quickly named Denisovans.8Nature. Fossil finger points to new human species This was an unprecedented event in the history of biology: a new type of human identified not from its skull shape or its stone tools but from its genome sequence. The physical remains were so fragmentary that paleontologists would never have recognized Denisovans from bones alone.
Further analysis revealed that Denisovans, too, had interbred with modern humans. Their genetic legacy shows up most prominently in present-day populations across East Asia, Siberia, and the Americas, as well as at higher levels in people of Melanesian and Australian Aboriginal ancestry.9Cell. Recovering signals of ghost archaic introgression in African populations The Denisova Cave itself later produced one of the most dramatic finds in human evolution: a bone fragment from an individual whose mother was Neanderthal and whose father was Denisovan, a first-generation hybrid.10PubMed Central. The genome of the offspring of a Neanderthal mother and a Denisovan father That such a hybrid turned up among the tiny number of archaic specimens sequenced suggested that mixing between these groups was not a freakish accident but a relatively ordinary occurrence when they met.
Why the Neanderthal DNA in Your Genome Actually Does Something
One of the most productive lines of research flowing from Pääbo’s work has been figuring out what all that inherited archaic DNA does in living people. The answer is not “nothing.” Neanderthal-derived DNA influences skin tone, hair color, height, sleeping patterns, mood, and even smoking behavior in present-day Europeans.11PubMed Central. The Contribution of Neanderthals to Phenotypic Variation in Modern Humans Separate analyses using electronic health records linked Neanderthal gene variants to measurable differences in risk for depression, sun-induced skin lesions, and a tendency toward hypercoagulation, where blood clots more easily.12PubMed Central. The phenotypic legacy of admixture between modern humans and Neandertals
The immune system is where archaic introgression may have mattered most. A cluster of genes involved in the innate immune response, the body’s first line of defense against pathogens, carries higher Neanderthal ancestry than the rest of the protein-coding genome.13American Journal of Human Genetics. Evolutionary History of Human Innate Immunity Genes These introgressed immune gene variants boost the expression of certain toll-like receptors in white blood cells, which enhances surveillance against certain pathogens. The trade-off appears to be an increased susceptibility to allergies, as the same heightened immune reactivity can overreact to harmless substances.14American Journal of Human Genetics. Introgression of Neandertal- and Denisovan-like Haplotypes Contributes to Adaptive Variation in Human Toll-like Receptors This is a vivid example of evolution’s tendency to recycle whatever works: modern humans arriving in new environments picked up locally adapted immune defenses by mating with archaic humans who had spent hundreds of thousands of years there, but those defenses came with side effects we still live with today.
Archaic DNA and COVID-19
The pandemic brought Neanderthal genetics to an unexpectedly wide audience. A stretch of DNA on chromosome 3, inherited from Neanderthals, was linked to a higher risk of severe COVID-19 infection and hospitalization.15PubMed Central. How Our Neanderthal Genes Affect the COVID-19 Mortality: Iran and Mongolia, Two Countries with the Same SARS-CoV-2 Mutation Cluster but Different Mortality Rates But the story turned out to be more complicated than a single risk variant. A different Neanderthal-derived segment on chromosome 12 was found to be protective against severe disease, reducing the likelihood of needing intensive care.16PubMed Central. A genomic region associated with protection against severe COVID-19 is inherited from Neandertals So archaic introgression handed modern humans both a genetic vulnerability and a genetic shield against the same virus, depending on which fragments a person happened to carry. The two haplotypes vary in frequency across world populations, which contributed to geographic patterns in COVID-19 severity that could not be fully explained by healthcare infrastructure or public health policy alone.
Pain Sensitivity and the Neanderthal Ion Channel
A particularly vivid illustration of archaic inheritance at work involves how people experience physical pain. The gene SCN9A encodes a sodium channel called Nav1.7 that sits on the peripheral nerves responsible for detecting painful stimuli. The Neanderthal version of this protein carried three amino acid substitutions compared to the modern human version. When researchers expressed the full Neanderthal variant in the lab, they found it showed reduced inactivation, meaning the channel stays open longer, which would make peripheral nerves more responsive to painful inputs.17PubMed. A Neanderthal Sodium Channel Increases Pain Sensitivity in Present-Day Humans About 0.4 percent of present-day Britons carry all three Neanderthal substitutions, and those individuals report heightened pain sensitivity.
Further work confirmed the association in experimental settings: people with Neanderthal ancestry in SCN9A showed a significantly lower threshold for mechanical pain after their skin was sensitized, and the effects of the individual variants appeared to be additive.18PubMed Central. Neanderthal introgression in SCN9A impacts mechanical pain sensitivity The more Neanderthal variants you carry in this gene, the more sensitive to certain kinds of pain you tend to be. This is a case where an archaic genetic variant has a concrete, measurable effect on a daily human experience, not just a statistical blip in a genome-wide association study.
Altitude Adaptation and Denisovan DNA in Tibet
The Denisovan genetic legacy found its most celebrated example on the Tibetan Plateau. Tibetans can thrive at altitudes above 4,000 meters, where oxygen levels are roughly 40 percent lower than at sea level. A key gene underlying this adaptation is EPAS1, which regulates the body’s response to low oxygen. The version of EPAS1 that Tibetans carry shows strong signatures of both positive natural selection and introgression from Denisovans.19PubMed Central. The history and evolution of the Denisovan-EPAS1 haplotype in Tibetans In practical terms, Tibetans inherited a physiological solution to extreme altitude from a long-extinct relative, and natural selection then spread it through the population because people who carried it survived and reproduced more successfully at high elevation.
This is probably the clearest example of archaic introgression providing a direct, life-or-death adaptive advantage. Rather than waiting for a brand-new beneficial mutation to arise by chance, a modern human population gained a ready-made adaptation through interbreeding and then kept it because it worked. The Tibetan EPAS1 story changed how evolutionary biologists think about adaptation itself: sometimes the fastest route to a new environment is not mutation but inheritance from a neighbor.
Circadian Rhythms and the Archaic Clock
Sleep and wakefulness patterns also carry traces of archaic ancestry. A genome-wide scan found that introgressed variants affecting circadian-clock genes are significantly more likely to be associated with measurable traits in living people than other introgressed variants, and they associate with a wider range of traits per variant.20Genome Biology and Evolution. Archaic Introgression Shaped Human Circadian Traits Beyond chronotype, which is whether you tend to be a morning or evening person, these variants showed connections to blood-related traits and other phenotypes. One interpretation is that when modern humans migrated into higher latitudes with dramatically different seasonal light cycles, archaic variants that shifted circadian biology offered a ready advantage, much as Denisovan EPAS1 did for altitude. The circadian system touches nearly every organ, so even modest changes in its tuning could ripple into multiple visible traits.
What Makes Modern Humans Different
Pääbo’s genome comparisons also flipped the question around: instead of asking what we inherited from Neanderthals, researchers began asking what genetic changes are unique to modern humans and absent from all archaic genomes. Two genes have drawn particular attention. TKTL1, which plays a role in brain development, differs between modern humans and Neanderthals by a single amino acid. When researchers tested the modern human version in developing brain tissue, it increased the production of a specific type of neural stem cell that generates more neurons in the neocortex. The Neanderthal version did not produce this effect, suggesting that modern humans may generate more neocortical neurons during brain development than Neanderthals did.21PubMed. Human TKTL1 implies greater neurogenesis in frontal neocortex of modern humans than Neanderthals
Another gene, NOVA1, regulates how neurons develop and connect. When researchers introduced the archaic version of NOVA1 into human stem cells and grew them into small brain-like structures called cortical organoids, those organoids developed more slowly, had different surface structures, and showed altered electrical activity compared to organoids with the modern human version.22PubMed Central. Reintroduction of the archaic variant of NOVA1 in cortical organoids alters neurodevelopment These experiments do not mean that one or two gene changes explain the cognitive differences between modern humans and Neanderthals; brain development involves thousands of genes working in concert. But they offer the first molecular candidates for changes that may have contributed to how our species thinks and processes information differently.
Reading DNA from Dirt
One of the more recent innovations to come out of Pääbo’s group pushed ancient DNA work beyond bones and teeth entirely. Many archaeological sites are rich in artifacts and animal remains but contain no identifiable hominin fossils. The team developed methods to enrich and analyze nuclear DNA directly from cave sediments, successfully recovering genetic information from deposits in western Europe and southern Siberia that dated between 200,000 and 50,000 years ago.23PubMed. Unearthing Neanderthal population history using nuclear and mitochondrial DNA from cave sediments Earlier attempts had managed to pull mitochondrial DNA from dirt, which was useful but too limited to reveal much about population relationships. Getting nuclear DNA from sediments meant that researchers could study which hominin groups occupied a site, and how those populations were related, without ever finding a bone.
This technique has already expanded the geographic and temporal range of paleogenomic research. Sites that were previously “genetically silent” because they lacked hominin fossils can now yield population-level data. For archaeological contexts where preservation is poor or where hominins left traces of their presence but not their bodies, sediment DNA effectively opens a new archive.
Ghost Populations and the Complexity of African Ancestry
The focus on Neanderthal and Denisovan introgression in Eurasian populations initially gave the misleading impression that archaic admixture was mainly a story about people outside Africa. Recent computational work has corrected that picture. Analyses of West African genomes found statistical evidence of gene flow from an archaic “ghost” population, one that diverged from the common ancestor of Neanderthals and modern humans somewhere between 360,000 and over a million years ago and later interbred with the ancestors of present-day Africans, contributing an estimated 2 to 19 percent of their ancestry.24PubMed Central. Recovering signals of ghost archaic introgression in African populations No fossils or ancient DNA from this group have been identified yet, hence the term “ghost.” Its existence is inferred entirely from the patterns it left in living genomes.
A 2025 study applying a new computational method to over a thousand modern genomes not only confirmed known Neanderthal and Denisovan introgression but also uncovered evidence of ghost archaic ancestry in both African and non-African populations.25PubMed. Recovering signatures of archaic hominin introgression using ancestral recombination graphs That study also found ghost ancestry persisting in genomic regions that had been called “ancestry deserts,” areas once thought to be free of archaic DNA and therefore uniquely modern human. The finding challenges the interpretation that those deserts represent parts of the genome where archaic variants were simply incompatible with modern human biology. The reality may be that multiple, overlapping waves of admixture from different archaic sources left a more tangled record than earlier models assumed.
What the Nobel Prize Recognized
The 2022 Nobel Prize in Physiology or Medicine recognized Pääbo for three interlocking accomplishments: sequencing the Neanderthal genome, discovering the Denisovans, and establishing paleogenomics as a scientific discipline.26PubMed Central. The Nobel prize in physiology and medicine – 2022 The “discipline” part of that citation is easy to underappreciate. Before Pääbo’s work, the idea that you could reconstruct the genome of a species that went extinct 40,000 years ago was not a research program; it was closer to science fiction. The contamination controls, the library-preparation chemistry, the computational methods for separating tiny fragments of ancient DNA from overwhelming quantities of microbial and modern human contamination — all of this infrastructure had to be invented or adapted from scratch. Much of it came from Pääbo’s lab or from close collaborators working within the framework he established.
The practical reach of that framework now extends well beyond Neanderthals and Denisovans. Ancient DNA methods developed for archaic hominins are routinely applied to trace human migrations during the Bronze Age, the spread of farming, the population history of the Americas, and the origins of epidemic diseases. The tools Pääbo built to answer one question, who were the Neanderthals, turned out to be general-purpose instruments for reading the deep history embedded in degraded biological material of almost any kind.
The Evolving Map of Archaic Ancestry
One of the more surprising developments in the years since the draft Neanderthal genome is how much messier the picture of human evolution has become. The clean branching tree that textbooks once drew, with modern humans, Neanderthals, and Denisovans splitting neatly from common ancestors, now looks more like a braided river. Groups diverged, migrated, re-encountered each other, interbred, and diverged again. The first-generation Neanderthal-Denisovan hybrid from Denisova Cave suggests that these encounters were not one-off events but a recurring pattern whenever archaic populations came into geographic contact.10PubMed Central. The genome of the offspring of a Neanderthal mother and a Denisovan father Ghost populations in Africa add further complexity, implying that even in the continent where our species originated, encounters with deeply diverged relatives left lasting genetic marks.
The field Pääbo founded is now generating data faster than anyone can fully interpret it. New computational tools are detecting increasingly subtle signals of ancient admixture, revealing layers of interbreeding that happened at different times and places. Some of these signals point to archaic groups for which we have no fossils and may never find any. What started as one scientist’s determination to extract DNA from old bones has turned into a reshaping of the human family tree into something far more interconnected, and far more interesting, than anyone had imagined.