But, as noted, for this case there is very little in the way of cost. Nature did, in fact, explore the winged-bipeds body plan to full fruition twice, once in

But, as noted, for this case there is very little in the way of cost. Nature did, in fact, explore the winged-bipeds body plan to full fruition twice, once in mammals, and did winged quadrupeds before and again in mammals. She is progressing on a another as I write this. And, humans did end up conquering the air without growing wings. Nature isn't picky about methods.