Neanderthal die off caused by lack of skill

[Scythe13]

As usual, I'm confused.

How did they exist beyond a few generations? Something must have changed, or, they stopped teaching their children.

I don't get how they could survive long enough to reproduce and become 'a species' if they couldn't survive.

There must have been something that caused they to die. You don't exist for many years and then suddenly have no skills!