So did we or didn't we? Last week saw the publication of two new papers with diametrically opposed conclusions about whether non-African people have Neanderthal-human hybrids among their ancestors—a result of at least some interspecies dalliance in the distant past.

That non-Africans share 1% to 4% of their genomes with Neanderthals is not in doubt, thanks to the pioneering work of paleo-geneticists led by the Max Planck Institute's Svante Paabo. At issue is how to interpret that fact. Dr. Paabo originally...