Our diets changed. If we revert to eating what humans ate 100,000 years ago we do NOT need to count calories and we do not get fat. What happened is we started cooking and eating things that could not be eaten raw, like grains, potatoes and so on. If you eat only stuff that can be eaten raw your body will do a pretty good job of maintaining your ideal weight without counting calories (assuming you don't have a thyroid problem or something of course)