Veganism?

[Anomalocaris]

If humans were adopted to a purely or even largely vegetarian diet, humans would have small chests an an enormous distended belly with miles of digestive track in which plant matter can linger and ferment, sort of like cows, or like gorillas with their cone shaped rib cage sitting atop a big sack of a belly.

But instead humans have barrel shaped chest and rib cage adopted to accommodate an prodigious aerobic capacity needed to run down prey animals over long distances, and a small belly suitable mostly for high quality neutrient sources like meat.

Humans are not fast as far as runners in animal kingdoms go, so our running is not for running away from predators. But we are damn impressive long distance runners. almost no other animal can run win a marathon race with a fit human. It's not for chasing down plants that homo Sapiens have evolved to be the best long distance runner in mammalian world.

So next time when a vegan try to show off her trim stomach, tell her the only reason why she is genetically capable of a flat tummy is because she was not genetically suitable to be a vegetarian.