Parasites are really good at controlling their animal hosts’ behavior (think of the zombie ants letting themselves be eaten, rats with toxoplasmosis that lose their fear of cats, and so on) but it doesn’t work so well with humans. How did we miss out on the fun? This discussion of a paper that looks at the evolution of the human brain in terms of strategies to resist manipulation by parasites is really fascinating.
Throughout evolutionary history, parasites have been trying to manipulate host behavior and hosts have been trying to avoid manipulation, resulting in an eons-long arms race. The equilibrium is what we see today: parasite manipulation is common in insects, rare in higher animals, and overall of limited importance. But in arms race dynamics, the current size of the problem tells you nothing about the amount of resources invested in preventing the problem. There is zero problem with war between Iran and Saudi Arabia right now, but both sides have invested billions of dollars in military supplies to keep their opponent from getting a leg up. In the same way, just because mammals usually avoid parasite behavior manipulation nowdoesn’t mean they aren’t on a constant evolutionary war footing.
Note that we have no idea of any of this is actually true, or just crazy luck for Homo sapiens. But it’s pretty fascinating stuff. Scott Alexander talks about why he finds this particular paper so interesting:
But I like this paper because it takes the complexity of biology seriously. There’s a sense that science is stagnating, and biology is one of the worst offenders. In the 1800s and early 1900s, we were pinning down our mastery of anatomy, discovering all the major hormone systems, learning about microbes and inventing antibiotics. It seemed like the same kind of thing as physics, where you could go out into the world, observe things, and make difficult but fundamentally straightforward discoveries. But for the past fifty years, it’s been kind of a mess. Despite some amazing work by amazing people, we still don’t even understand questions as basic as what depression is. Everything seems bogged down in a million different opaque signaling cascades that fight off any effort to untangle or shift them.
What I find fascinating about these areas of inquiry is that for a sizable chunk of modern history, humans thought that there was some basic duality between other animals and ourselves: we were complex creatures endowed with free will and a “soul” that animated us, whereas animals were essentially biological machines running on relatively simple programs without self-awareness. In reality, it seems we are running a lot of very complex programs that are hard-wired into us (and can indeed be hijacked), along with our sense of self and our ability to make choices… and so all the other animals, with various degrees of self-awareness and complexity and intelligence. We’re sitting on a continuum with them, and some of them are very, very close to us.