Something perhaps interesting about the gut is that it used to be much larger when we were more ape-like, and then it shrank when we started eating meat, and our brains got bigger. Brains are expensive, and meat provided us with a more nutrient-dense diet but not more calories (also, eating too much meat is toxic). So if the calories we consumed stayed more or less the same, but our brains got bigger, it means something else must've gotten smaller, so we could save calories from that. It appears that the gut is the thing that got smaller, since we didn't need a really big gut now that we ate meat.