Genetic patterns thought to prove that modern humans interbred with Neanderthals actually stem from variations in human mutation rates. This finding challenges the widely accepted view that people outside Africa carry surviving pieces of the Neanderthal genome. The mathematical test used to detect ancient interbreeding assumes mutation rates stay constant between groups, but changing rates generate the exact same genetic signal.
When modern humans separated into distinct populations, differences in genetic diversity altered how frequently chemical changes occurred in their DNA. Rather than swapping large segments of genetic text like shared chapters in a book, individual DNA letters mutated independently at separate positions. These recurrent mutations occurred repeatedly at identical locations in both Neanderthals and modern humans. Three-letter DNA sequences show wide variation in these signals, confirming that the process acts at single chemical bases rather than across inherited chromosome blocks.
The researchers evaluated genome data to test whether mutation rates and population diversity drive the standard interbreeding score across different sequence motifs. They discovered that the signal concentrates at sites where the Neanderthal DNA variant is already fixed in modern humans outside Africa. Under true interbreeding, Neanderthal gene variants should remain rare, but differences in genetic diversity correlate with the signal across African and non-African comparisons.
Geneticists can now use models where genetic diversity modulates mutation rates to re-evaluate human evolutionary history. This framework enables scientists to test whether traits attributed to archaic human ancestors arose from interbreeding or from standard human mutation patterns.