They were seen as a dead-end lineage — cognitively limited compared to anatomically modern humans, behaviorally rigid, unable to innovate or adapt to new circumstances.

The eventual replacement of Neanderthals by modern humans was understood within this model as essentially inevitable: a smarter, more flexible, more creative population had outcompeted a duller, more constrained one.

This model was not arbitrary.

It was based on real observations.

The stone tool technology associated with Neanderthals — the Mousterian — appeared to be relatively stable across its long history, without the rapid innovation and diversification that characterized the Upper Paleolithic technologies associated with modern humans in Europe.

The symbolic behavior visible in the modern human record — cave painting, portable art, personal ornamentation — was largely absent or ambiguous in the Neanderthal record.