The pupil in the eye. The sun through the clouds. The craters after an impact. The waves when you drop a rock in water. The section of a bamboo stick... And it gets more evident when you go to microscopic world. Some eggs are perfect spheres, as small bubbles in water. I think circles are a real natural shape. Not an invention. Nature brings us all kind of shapes, some of them are discovered studying living creatures, like the scutoid.