Psychologist Marco del Giudice has written a paper arguing that the arms race between parasites and hosts has driven our biological systems and especially our brains toward ever great complexity. Scott Alexander explains:
So if you’re an animal at constant risk of having your behavior hijacked by parasites, what do you do?If this is true, it would explain a lot about us. On the other hand, evolution seems to do a lot of things by crazily complicated systems, even extremely fundamental actions like cellular respiration. I have always thought that this is what happens when you build a system one random change at a time. So while this idea is interesting it will take a lot to convince me that it is true.
First, you make your biological signaling cascades more complicated. You have multiple redundant systems controlling every part of behavior, and have them interact in ways too complicated for any attacker to figure out. You have them sometimes do the opposite of what it looks like they should do, just to keep enemies on their toes. This situation should sound very familiar to anyone who’s ever studied biology.
Del Giudice compares the neurosignaling of the shrimp-like gammarids (small, simple, frequently hijacked by parasites) to rats (large, complex, hard to hijack). Gammarids have very simple signaling: high serotonin means “slow down”, low serotonin means “speed up”. The helminths that parasitize gammarids secrete serotonin, and the gammarids slow down and get eaten, transferring the parasite to a new host. Biologists can replicate this process; if they inject serotonin into a gammarid, the gammarid will slow down in the same way.
Toxoplasma hijacks rats and makes them fearless enough to approach cats. Dopamine seems to be involved somehow. But researchers injecting dopamine into rats don’t get the same result; in fact, this seems to make rats avoid cats more. Maybe toxoplasma started by increasing dopamine, rats evolved a more complicated signaling code, and toxoplasma cracked the code and now increases dopamine plus other things we don’t understand yet.
Aside from the brain, the immune system is the most important target to secure, so this theory should predict that immune signaling will also be unusually inscrutable. Again, this situation should sound very familiar to anyone who’s ever studied biology.
Second, you have a bunch of feedback loops and flexibility ready to deploy at any kind of trouble. If something makes dopamine levels go up, you decrease the number of dopamine receptors, so that overall dopaminergic neurotransmission is the same as always. If something is making you calmer than normal, you have some other system ready to react by making you more anxious again. . . .