Researchers have found evidence that early modern humans in Africa interbred with an unidentified extinct human lineage, whose genetic contribution remains detectable in present-day DNA. The study matters because it suggests human ancestry was more complex than the well-known mixing with Neanderthals and Denisovans, and because the source population has no known recovered genome.

The team used a new tool, TRACE, to identify DNA segments that appear genetically old but show relatively little recombination, a pattern expected when DNA entered modern human genomes from a separate lineage. The proposed interbreeding occurred before modern humans left Africa; unlike Neanderthal DNA, which averages about 2 per cent in many populations, this contribution is attributed to a still-unidentified “ghost lineage”.