We think of Earth orbiting the Sun, floating in the splendid isolation of space. That image is blemished a bit by the currently heady notion of asteroid impacts. There is no question that without Bruce Willis to save them, an asteroid impact helped to do in the dinosaurs. There are other things of more immediate danger to worry about, many man-made, but there are reasons to be prudent and to keep an eye out for near-Earth asteroids. There is also the recently highly publicized exchange of rocks from Mars to Earth, the fascinating question of whether one carried primitive life from Mars, and the bigger issue of whether life even arose on Mars and traveled to Earth.

Despite these hints of exchange within the solar system, there is still a widespread assumption, even among astrobiologists, that we are here and the rest of the galaxy is there, and never the twain shall meet -- until we get a signal from some other happy-go-lucky space-exploring civilization. This sense of isolation is also probably a fiction of our limited perspective and short history.

A hint of this came when a neutron star erupted in a gigantic magnetic flare 20,000 years ago, sending out a burst of gamma rays that impacted Earth for a brief moment in August of 1998, causing brilliant auroras and interrupting radio communications. That direct impact on Earth that occurred in our lifetimes came from a star 20,000 light-years away -- a fair fraction of the way across our galaxy. How many other such events could have affected this planet? And what effect might they have had over the eons Earth has been around, before and after it harbored evolving life?

The answer is -- lots. Exploding stars, supernovae, happen every hundred years or so in the Milky Way Galaxy. Some of them will be near enough to disrupt the chemistry of the atmosphere and flood us with ultraviolet and gamma-ray radiation sufficient to cause mutations. Such relatively nearby events probably occur about once every million years or so. That means several thousand in the time since the first RNA memorized and reproduced itself, since the first bacteria formed and divided. We are not talking about the rare events that are so nearby they could cause a massive die-off. Much more interesting is the question of whether these impulses of mutation-inducing astronomical radiation could help to drive evolution.

How did that adaptation occur? Some bacteria are remarkably resistant to radiation. How did they evolve? Many of the mechanisms of genetic radiation repair are very similar to those of "gene swapping" that drove evolution in ancient times; tangling the roots of the tree of life that separate now into the various kingdoms of plants and animals. There are even strong similarities between these processes of genetic transplantation and the combining of zygotes in sexual reproduction. There are suggestions that the development of sexual reproduction, so much more efficient in creating genetic diversity than simple cell mitosis, was a variation on the theme of radiation repair.

All these issues involve radiation in some intimate way. Can they be addressed in any coherent manner without considering that significant jolts of biologically significant radiation from supernovae, gamma-ray bursts and passing stars bathe Earth on time scales that are short compared to the sweep of evolutionary and geological time? I suspect not. Just as we have found that our planet represents a tightly intertwined ecology involving all living things, I believe that the future of astrobiology will reveal that Earth is part of a galactic ecology and that life here, and elsewhere, is intermeshed with and dependent upon astronomical events throughout the galaxy.