If the cavemen were here today, I think that they would jump on the easy food like the rest of us. With their children, they would probably go hog wild for awhile on foods new to them: potatoes in any form, soft white bread, lovely noodles, cheesy pizza. I imagine they would also gorge themselves on desserts—pastries, chocolate, ice cream.
Because it’s all so tasty and easy to get, they would get fat and feel terrible, and try to cut back to their original diet. Though they’d probably compromise on hunting and gathering it all, it would still be too hard to stick to in the face of so many other tempting possibilities. So they would give up, starting the whole cycle over again.
Somewhere between the two extremes is a broad and varied diet of Real Food