We've linked to this site before, about a year ago, I guess, but it's worth another visit. Notice that as you view the earth's position in the Milky Way that it's actually situated between two spiral arms. Guillermo Gonzalez points out in Privileged Planet that this location is absolutely essential not only for intelligent life like us to exist on earth but also for that life to be able to do significant science.
This is because if our solar system were in a spiral arm there would be so much dust and debris in our immediate neighborhood that it would have a chaotic effect on the earth's surface and consequently on any life trying to evolve there. It would also obscure our view of the sky which would have prevented us, if we somehow could have emerged in such an uncongenial environment, from ever really learning anything about our universe. This in turn would have impeded the development of modern physics.
Notice, too, how far earth is from the galactic center (about 27,000 light years). This puts us pretty much in the middle of the galactic habitable zone, a band around the galactic center in which conditions are suitable for life. If our solar system were much closer or much further away from the center, intelligent life would be impossible.
Perhaps we should give thanks to random chance and the laws of physics for accidentally producing such a serendipitous bit of good fortune.
