Fertility is one of many factors in the evolutionary equasion. In evolutionary time human populations have been very scattered At the center of a slightly isolated population this lineage reproduces normally. On the periphery of the isolated population there are more miscarriages, but adequate reproduction for introgression of useful genes into the population. It's probable Neandertal/Sapiens hybrids and Denisovan/Sapiens hybrids were significantly less fertile, yet their genes made it into our pool. Surprisingly the ancestral trees of individual genes are often very different than the species tree.

Stoicism (and it's variants like epicureanism) and Buddhism are similar, but I was under the impression that they arose independently. Since they are on the same landmass, I guess two traditions could have influenced each other. On the other hand I'm (almost) sure I read somewhere that Aztecs philosophers had the same sort of ideas: The world is slippery slick. One can't hold onto anything.