It takes a lot of calories (and protein) to maintain lean muscle. The hunter-gatherer lifestyle that shaped much of our evolution has these, but not in a reliable continuous stream. It's more of a boom-bust feast-famine thing.
We store fat for fuel reserves in good times, but in bad times we not only dip into the fat but into the muscle. Some details of mechanism make that necessary, mostly the way we run out of carnitine in the mitochondria during prolonged exercise. You can burn fat for a while in prolonged exercise or starvation, then you have to cannibalize some protein to replenish carnitine to allow your mitochondria to process fat.
That turns out not to be a bad idea, since burning some muscle gives you less mitchondrial mouths to feed in the first place and saves some fat for later. That's the adaptation most of us have.
It's hard to say how the kid's super-muscle mutation would have worked out had it surfaced back in the last Ice Age. The kid might have lorded it over his skinny pack mates, or he might never have made it.
Elsie IS in shape!
(Round is a shape.)
I'm leaning now toward the idea he'd succumb to the toxic effects of ketosis if very lean times ever hit. Breaking down protein for calories is hard work for the liver and kidneys even for people who have some fat reserves in addition.
That's probably why we have myostatin in the first place, to prevent muscle buildup from blocking fat buildup so we don't go into lean times too lean.
Wonderful as muscle is, it's expensive. It costs fat. That never comes up, never matters, now for most of us. This knowledge, once essential, is now hidden.
Fat is cheap. Fat is easy. Fat takes nothing but sitting around watching the game and munching potato chips.
Fat used to be precious. The literature is still replete with references. "Living off the fat of the land." "The fatted calf." Heck, "Fat City" and that's only from the 70s. The "cream" of the crop.
Well, fat used to be harder to do. We have it pretty nice, when you think about it.