I offer a different (conspiracy) theory. Within each ice age, there are glacial (cold) and interglacial (warm) periods. Antecessor, heidelbergensis, neanderthalensis, denisova, sapiens, etc were a very tolerant bunch (i.e. not conservative) back in the day. During the glacial periods, there was housing shortage (caves got crowded). During interglacial periods, it was woodstock and free love. They wanted to be outside and partying. Marijuana was legal back then. Just like today when we have the big snow storms and the power goes out we have a mini baby boom 9 months later. Climate change was an enabler for, ahem, assimilation in a biblical sense. This (conspiracy) theory also explains sasquatchensis. Over time intolerance formed. The general trend was to not assimilate with something really big and hairy (can't say I blame them). They became outcasts living in the forest while the rest of the gang moved away from hunting and gathering.