Most people alive today carry a little Neanderthal in their DNA
For decades the textbook story said modern humans left Africa and simply replaced the Neanderthals. Then, on 7 May 2010, a draft Neanderthal genome showed that the two had interbred. Everyone whose ancestry lies outside Africa carries roughly 1 to 4 percent Neanderthal DNA, and some populations also carry traces of Denisovans.
The 2010 study, built from three Neanderthals found at Vindija, noticed that Neanderthals shared more genetic variants with Eurasians than with sub-Saharan Africans. The simplest explanation was gene flow after modern humans migrated out of Africa. Later estimates have wobbled, from 1.5 to 2.1 percent in one 2013 analysis to 3.4 to 7.3 percent in another, but the geneticist David Reich puts it at around 2 percent for everyone outside sub-Saharan Africa. A fossil jaw known as Oase 1 had hinted at mixing long before the genetic proof arrived.
Denisovans, a second archaic group, left their strongest mark in Oceania, where they account for about 4 to 6 percent of people's genomes. East Asians carry somewhat more Neanderthal ancestry than Europeans, which models explain with an extra pulse of mixing after the two lineages split. Africa is not untouched either. Migrations back from Eurasia carried Neanderthal DNA into North Africa, and a 2020 study found about 0.3 percent in African genomes, though Reich calls that signal really weak.
Traffic ran both ways. A 2023 study concluded that modern humans passed genes to Neanderthals around 250,000 years ago, making up roughly 6 percent of one Altai Neanderthal's genome. Spread across living people, a large share of the Neanderthal genome survives, about 50 percent among people in India and 41 percent among Icelanders.
Not every inherited gene was welcome. Some helped newcomers adapt to fresh environments, including immune genes, variants affecting skin and hair, and adaptations to high altitude. Yet large stretches of the X chromosome and genes active in the testes contain almost no archaic ancestry, suggesting that natural selection weeded out variants that reduced fertility in male hybrids. The picture that emerges is assimilation rather than clean replacement.
Source: Interbreeding between archaic and modern humans
