Why can't humans fly?
The density of a bird’s body is very less compared to the density of human body that is one reason they can fly. It is due to bird’s hollow bones that act as a part of avian respiratory system. These bones have air sacs filled with air up to one forth of bird’s weight. So they are very light-weighted to fly.Also, it took millions of years for birds to evolve from reptiles. Ancient reptiles that could leap gradually started gliding which in turn resulted into the development of wings. Their muscles and wings evolved aerodynamically and they now have a shape that makes possible for them to fly easily in sky.If along with ancient reptiles, apes tried to leap or fly, may be we would have evolved into flying homo sapiens. But our far ancestors were much more fascinated to run on two feet while holding their belongings with the help of thumbs. So we can run and write but not fly without technology. (Pun intended in last paragraph)