I retract my Great Filter hypothesis: I realized this predicts an ever smaller population of ever smarter hominids, who still have a good quality of life, making up in smarts what they miss in numbers. But the simple fact is, hominid populations were not dwindling. They were pretty steadily taking over the planet, migrating out of Africa and all that.

Well, unless it happened before and caused the mitochondrial Eve bottleneck and then things turned different after that, but that is adding too much detail and courting a conjunction bias, so I don't propose that until more evidence is unearthed.