> I'd bet money that the increased caloric intake of our ancestors is entirely explained by their increased physical activity.
I'd bet that there were other factors involved, like parasites (worms) and lower quality food (inedible/rotten food tends to leave our intestines quickly).
Maybe. I bet, though, that if you made me do the work my grandfather did when he was a youngster, or that my great-grandfather did when he was my age, I'd burn those 4000 calories a day, just 'cause being a fat desk worker, I'm not going to be nearly as efficient, even if I get to keep my 21st century digital healthcare and hygiene.
I'd bet that there were other factors involved, like parasites (worms) and lower quality food (inedible/rotten food tends to leave our intestines quickly).