Neanderthals vanished as a distinct population tens of thousands of years ago, but they never fully disappeared. Most people whose ancestry traces outside of Africa still carry a small but measurable amount of Neanderthal DNA in their genomes — usually around two percent — and that inheritance is not inert. It continues to shape traits as fundamental as how the immune system reacts and when a person naturally feels sleepy.
The picture that has emerged from more than a decade of genetics is not one of a few harmless leftovers, but of a scattered set of ancient gene variants that were absorbed into the human line and then kept, discarded or reshaped by evolution depending on whether they helped or hurt. Some of those variants turned out to be surprisingly useful, which is part of why they are still here.
About two percent, and where it came from
The Neanderthal DNA in living people is the legacy of interbreeding that occurred after modern humans expanded out of Africa and encountered Neanderthal populations across Eurasia. Because that mixing happened outside Africa, people of non-African descent carry the largest share — roughly two percent of their DNA — while populations that remained in Africa carry far less, though not zero. Researchers have since worked out how strongly that inheritance influences physical traits; one analysis quantifying the contribution of Neanderthal ancestry to the heritability of complex traits found that these introgressed variants make a real, measurable contribution to differences in a range of characteristics among people today.
Neanderthal variants tune the immune system
Some of the most consequential Neanderthal genes involve immunity. As modern humans moved into new environments, they met pathogens their ancestors in Africa had never faced, and Neanderthals — who had lived in those environments for hundreds of thousands of years — carried immune adaptations already suited to them. Inheriting those variants gave the newcomers a head start against local diseases. The tradeoff is that the same immune-tuning genes are associated with a heightened tendency toward allergies and autoimmune reactions, a reminder that a variant helpful against infection can carry a cost when the immune system overreacts.
Genes that shape sleep and the body clock
The sleep connection is subtler but well documented. Studies of large genetic databases have found that Neanderthal-derived variants are enriched near genes involved in the circadian system, the internal clock that governs when a person feels alert or tired. Work on adaptive changes in human circadian rhythm genes has linked this ancient inheritance to chronotype — the tendency to be a “morning person” or an “evening person.” The interpretation has shifted over time: an earlier study tied certain variants to being a night owl, while a broader later analysis found that the introgressed variants most strongly associated with chronotype actually push toward morningness, a trait that may have suited populations adapting to the shorter days and different light cycles of high-latitude Eurasia.
From skin tone to mood, a long list of traits
Immunity and sleep are only part of the inheritance. A large study drawing on more than 112,000 participants in a national health database mapped Neanderthal contributions to phenotypic variation in modern humans and connected the ancient DNA to traits including skin tone, hair color, mood and even the likelihood of taking up smoking. None of these are simple one-gene switches; each reflects many variants contributing small effects, and the Neanderthal fraction is one influence among many. But the breadth of the list is striking — it shows the archaic inheritance reaching into everyday biology rather than sitting in some silent corner of the genome.
Why some Neanderthal DNA survived and much did not
The two percent that remains is a fraction of what was originally acquired. In the generations after interbreeding, natural selection weeded out many Neanderthal variants that were mildly harmful in a modern human background, which is why large stretches of the genome are now almost free of Neanderthal ancestry. What survived tended to be either neutral or beneficial — the immune and circadian variants that helped humans settle into non-African environments being prime examples. General overviews of the species and its genetic legacy, including a broad reference summary of Neanderthal biology and ancestry, describe this as a filtering process that left a curated remnant rather than a random sample.
The result is a kind of living archaeology. Every non-African genome carries a faint record of an encounter between two human lineages, preserved not as fossils but as functioning code that still nudges immune responses, sleep timing and appearance. Understanding which variants do what has practical value for medicine and for the study of human adaptation, and it reframes an extinct cousin as an ongoing, if quiet, presence in modern biology.
This article was produced with AI assistance and reviewed by the Morning Overview editorial team.
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