To verify their discoveries weren’t just artifacts of their methodology, the researchers applied their model to data from other species—bats, dolphins, gorillas, and chimpanzees. The results varied considerably, with some species showing little evidence of ancient structure while others showed completely different patterns. This variation across species strengthens the credibility of the specific pattern found in humans.

The researchers also examined which genes had unusually high or low amounts of ancestry from the minority population. Genes rich in minority ancestry often had functions related to neural development, including neuron cell connections, startle response, and neurotransmitter transport.

Conversely, genes with little minority ancestry were often involved in RNA processing, cell structure organization, and immune functions. These patterns hint that the two ancestral populations may have adapted to different environments before reuniting, with certain genetic variants from each population offering advantages for specific biological functions.

This discovery joins other recent findings showing human evolution is more complex than we once thought. Rather than populations simply splitting from one another in a tree-like pattern, human evolution increasingly appears to involve repeated separation and remixing of populations.

The admixture event identified in this study is much older than previously detected interbreeding events, such as those between modern humans and Neanderthals or between modern humans and a proposed “ghost population” in West Africa. Unlike these more recent events that affected only certain human populations, the ancient admixture event described here is shared by all humans.

“The fact that we can reconstruct events from hundreds of thousands or millions of years ago just by looking at DNA today is astonishing,” says Scally. “And it tells us that our history is far richer and more complex than we imagined.”