When the results first popped up, paleogeneticist Viviane Slon didn't believe it. “What went wrong?” she recalls asking herself at the time. Her mind immediately turned to the analysis. Did she make a mistake? Could the sample be contaminated?

The data was telling her that the roughly 90,000-year-old flake of bone she had tested was from a teenager that had a Neanderthal mom and Denisovan dad. Researchers had long suspected that these two groups of ancient human relatives interbred, finding whiffs of both their genes in ancient and modern human genomes. But no one had ever found the direct offspring from such a pairing.

Slon, a postdoc at the Max Planck Institute in Leipzig, sampled the bone from another spot;