Last I remember, the top candidates for GRB's were super massive stars, like Eta Carinae, going supernova and having their jets pointed directly at us, 2 nuetron stars colliding and forming a singularity (black hole), or 2 singularities colliding. These events occured much more frequently when the universe was new and more compact. Again, as far as I know, all GRB's originate billions of light years away. For the cosmologically challenged, the farther away something is, the older it is, and the younger the universe was when the event occured. Since GRB's are all billions of light years away, they are all very old and occurred when the universe was very young. Where you are in the universe has nothing to do with that. Folks 5 billion lights years away see our galaxy as very young, but we are actually the same age. Our galaxy is the same age, give or take a little, as the galaxy 3 billion light years away. So, my point is, a GRB has had plenty of time to blast and disappate. That leaves plenty of time for life to evolve. Yeah, evolve. I do not believe in creationism, but that's a debate for another time! I will concede that a GRB could wipe out chemicals in a comet near it that could have carried building blocks for life. In that respect, yes, the GRB had an effect. I guess I'm just disagreeing with the significance of the GRB in the development of life in a galaxy. There's no doubt that billions of years ago, our very own galaxy produced GRB's and we developed just fine. I think life is the greatest enemy to itself in the universe. In other words, humans developing nukes, or worse, is much more hazardous than a GRB that happened billions of years ago. I think aliens have themselves to worry about, just like we do. Not GRB's.