Sir Martin Rees in a TED lecture. He suggests that humans have evolved to this scale, an almost beautiful mean between stars and atomic particles, because we must be large enough to permit massive complexity in structure while small enough to experience minimal gravitational effects.

This idea reminds me of Schrödinger’s amazing explanation of why the fundamental components of human life -particularly DNA- are sized as they are.

It always makes me feel rather happy to think that everything had to be just so for our world, as we know it, to occur. Rees calls this quality of the universe its biophilia and describes it more here.