The Ability to Overcome Gravity

For every life form, life is lent. If it were not, they would not evolve. So i am sure the reason why certain animals can fly is there was a need for them. There must have been situations when they had to over there was an absolute need to overcome gravity. When you look at a bird, the alertness of a bird is so wonderful to see. Why is the bird so alert? Because it could not have survived. Let's go to human beings. How do you grow a stronger body? You need to exercise. But at what does that mean? He's weight, and you have to lift that weight, or whatever you do, on a treadmill. That