We function on about 1,500-2,000 kcal/day now, in an extremely sedentary culture. I’d not be surprised at early cultures (as noted, “safe” ones where food was abundant & reliable, and threats rare) consuming several times that due to no transportation (for self nor goods), close involvement with farming & hunting, and labor-intensive everything every day all the time. Non-temperate weather also necessitates high calorie burn just to maintain safe body temperature, both in just existing and in actively managing environment (hauling wood to burn, wearing heavy animal hides, sweating profusely, etc.).
Today, Olympic athletes et al are known to burn upwards of 5,000kcal/day - and that’s concentrated activity surrounded by sedentary behavior (sitting while transported, other such relaxed behaviors). When every waking moment involves labor, I can see common folk burning that much easily.
I recall someone computing that a naked man just standing in a blizzard would need 30,000kcal/day to survive. That’s pretty extreme, yes, but does establish an upper limit to caloric intake; if you’re living Survivorman-like in extreme cold with primitive insulation & heat, you’re gonna be eating a LOT.
The 20,000 number may also cover _available_ calories. People may not have consumed/burned that much daily, but did have it available and may have reached that occasionally. Our current available calories are at least on par with that.
Just some thoughts; I’m no expert.
I hear what you are saying.
Gone are the days when farmer brown would awaken before dawn, consume five eggs, some biscuits and sausage gravy, home fries, toast, and BACON , and then spend the entire day farming.
I get up at six, walk the dog, have breakfast, spend the morning on the computer, practice four instruments, and then go and teach from 2:00 PM till 9:00 at night.
Then I get on the computer again till midnight.
Probably the bass is my most “calorie-burning” thing.