How Do We Know What Comes First?

The first time you've got cut marks on an animal that's too big to eat by yourself, it's probably sharing. That happens around two to an half billion years ago. Extra calories don't automatically mean big brains, right? City pigeons and rats have access to more easy calories than ever before. But I'm guessing we're not seeing birds or rodents with bigger brains just because they can get pizza or big tubs of fat from behind restaurants.