Comment by jjk166

> IMO the simpler hypothesis here is that this isn’t anything adaptive, it’s just another example of how biology is like spaghetti code; changing one chemical signal will affect an essentially random set of downstream systems, some of which can be causally connected to psychology/behavior.

That's not a competing hypothesis. Obviously the change to the gut affects other things, some of which happen to impact psychology and behavior.

The question is why does this particular change to gut microbiome have such a similar impact on two different species that diverged many tens of millions of years ago? "It just does" is not a very satisfying answer.