Neanderthal brains were large — in fact, slightly larger on average than modern human brains — but the architecture of those brains, inferred from the shape of the cranium, appeared different in ways that some researchers associated with different cognitive capabilities.

The model began to break down in the 1990s and accelerated into collapse in the 2000s and 2010s.

The most dramatic single blow came from ancient DNA.

The sequencing of the Neanderthal genome established definitively that Neanderthals and modern humans interbred — that gene flow had occurred between the two populations, in both directions though not symmetrically.

This was not the relationship of two species that had simply co-existed in the same regions without interaction.

It was the relationship of two closely related human populations whose members could and did form reproductive partnerships.