The boundary between Neanderthals and Denisovans — groups that had been diverging for perhaps 400,000 years — was not an absolute reproductive barrier.

Children could exist at that boundary.

The broader question — what the existence of such children means for how we categorize ancient humans, and for what “species” means in the context of slowly diverging evolutionary lineages — remains philosophically open.

But the factual ground for that question has been permanently changed.

Before Denny: interbreeding between ancient human groups was inferred from population genetic signals in modern genomes.

After Denny: it is directly observed in the genome of a single ancient individual.