The difference in our guts is reversed with our heads.
Humans have huge heads. Apes have small heads.
The Expensive
Tissue Hypothesis explains this critical tradeoff.
Our brain weighs 5X what you’d expect for a mammal our size. It uses 20% of our daily energy expenditure, burning calories at 10X the rate of the body as a whole.
(...it takes a lot of energy to fire nerve cells and pump
ions across cell membranes)
But humans don’t burn more calories than is expected for a mammal our size.
We have an energy budget that we must balance, and the brain takes a huge chunk of that balance.
Thus, we have a conundrum, the brain's huge energy expenditure must be offset by something else.
The gut took the fall.
If you add our gut and our brain together, their summed weight is what you'd predict for a mammal our size.
The energy saved by a smaller gut, made possible by energy dense food (i.e. meat) compensates for that required by our large brains.
Humans sacrifice gut size and the ability to process large amounts of plant material (the reason apes have large guts) for a big brain that facilitates hunting success
via intelligence, language, tool- / weapon-making, and cooperation.
TL;DR
We aren't the biggest, fastest, or strongest animal, but by sacrificing our large plant-processing gut we could fuel an outsized brain with energy dense, nutrient-rich meat — the tradeoff (gut for brain) that
catapulted us to the top of the food chain.
Meat was the fork that made us human.
