When researchers analyzed Denny’s genome, this is exactly what they found.

Looking at specific positions across her genome where Neanderthal and Denisovan populations are known to differ — positions where Neanderthals typically carry one variant and Denisovans typically carry a different variant — Denny’s genome consistently showed one chromosome carrying the Neanderthal variant and the other chromosome carrying the Denisovan variant.

This pattern — Neanderthal variant on one chromosome, Denisovan variant on the other, consistently across the genome — is the definitive signature of a first-generation hybrid.

It cannot be produced by any amount of ancient admixture within a single population.