Scientific have tried to explain the rate of evolution over time by means of

where in your body is there an enzyme that does function at a high or low ph? what is the name of that enzyme and its function?

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.