We might be developing some confusion here. I don’t mean leftism works. I mean the individual strategy that liberal\weak people use works.
Specifically that weak/nonproductive people use to influence people who are productive and strong to help them, to give them a share of the kill. Today that equivalent would be to provide them with welfare benefits etc.
I imagine Stone Age people used ideas like pleasing the mountain gods while today they use ideas like equity or social justice. Both eras had different people, different strategies but the same goal. That the numbers on welfare,tax paid benefits like food stamps, free heath care etc are swelling sort of proves it works.
Interestingly Ayn Rand’s Novels get into this sort of philosophical questions and she wrote them nearly a century ago.
As to stone-aged people, Will Durant, in his "Story of Civilization," posited that the collectivist demand for sharing (not necessarily trying to please the gods) was evident as soon as men invented fire (and probably before).
In other words, weak, non-productive people wanted more than their earned portion and pushed collectivism (sharing equally) as a means to get it.