On Slate Star Codex (previously), Scott Alexander breaks down Invisible Designers: Brain Evolution Through the Lens of Parasite Manipulation, Marco Del Giudice's Quarterly Review of Biology paper that examines the measures that parasites take to influence their hosts' behaviors, and the countermeasures that hosts evolve to combat them.
Diligent readers will know that parasites manage some incredible feats of behavior modification (one of Scott Westerfeld's best novels looks at vampirism as a form of parasitic behavior modification and it's just great).
It's a truism that the predator carves the prey and the prey carve the predator. Del Giudice's investigations into parasite tactics and countermeasures lead him to hypothesize that perhaps human variation is driven by responses to parasites' attempts at behavior modification (for example, humans have a lot of variability in our major histocompatibility complex genes, which mean that our immune systems can readily distinguish between our cells and invasive ones).
It's a super-interesting paper, and Alexander's breakdown is a great path into it.
Sixth, you use antiparasitic drugs as neurotransmitters. This is the kind of murderous-yet-clever solution I expect of evolution, and it does not disappoint. Several neurotransmitters, including neuropeptide Y, neurokinin A, and substance P are pretty good antimicrobials. The assumption has always been that the body kills two birds with one stone, getting its signaling done and also having some antimicrobials around to take out stray bacteria. But Del Giudice proposes that this is to prevent parasites from hijacking the signal; any parasite that tried to produce or secrete an antiparasitic drug would die in the process.
Dopamine is mildly toxic. The body is usually pretty good at protecting itself, but the mechanism fails under stress; this is why too much methamphetamine rots your brain. Why would you use a toxic chemical as a neurotransmitter? For the same reason you would use antiparasitic drugs – because you want to kill anything smaller than you that tries to synthesize it.
People always talk about the body as a beautiful well-oiled machine. But sometimes the body communicates with itself by messages written with radioactive ink on asbestos-laced paper, in the hopes that it's killing itself slightly more slowly than it's killing anyone who tries to send it fake messages. Honestly it is a miracle anybody manages to stay alive at all.
All these features together are a pretty effective way of dealing with parasite manipulation. There are a few parasites that can manipulate human behavior – rabies definitely, toxoplasma maybe – but overall we are remarkably safe.
Maybe Your Zoloft Stopped Working Because A Liver Fluke Tried To Turn Your Nth-Great-Grandmother Into A Zombie [Scott Alexander/Slate Star Codex]
Invisible Designers: Brain Evolution Through the Lens of Parasite Manipulation [Marco Del Giudice/Quarterly Review of Biology]