Once upon a time I read a science fiction story that suggested a sentient species acquired intelligence through neurochemical interactions between their brains and a parasitical worm. The worm made them physically ill, but it also gave them self awareness, comprehension, and radically changed their behavior as a species. The story ends with a punchline that this alien species gained the knowledge of Good and Evil by eating Adam & Eve’s forbidden fruit, but in their case their apple has a worm in it.
It seems something similar might be going on in our human brain, and in our case (at least for men) the parasite seems to make us a little more evil. I read a fascinating discussion on Toxoplasmosis and the effect this parasite has on the mammalian limbic system.