Life on other planets


Is there extraterrestrial life? No answer to this eternally fascinating question currently exists, but astronomers have gathered a significant amount of relevant information. Bruce Jakosky argues that we have every reason to believe that there could be life elsewhere in the universe. Reviewing the development of life on Earth, he considers the likelihood of comparable processes having taken place on Mars and Venus, on moons around Jupiter and Saturn, and on planets orbiting other distant stars.  The argument suggesting that there is life on other planets is very simple and straightforward. It begins with life on Earth as that is, of course, the only life that we know about. The fossil record and the genetic record tell us that life on Earth is very old, having originated somewhere before 3.5 billion years ago (b.y.a.) and possibly before 3.85 b.y.a. The oldest known life forms were at once both very simple and remarkably complex. The simple aspect is that they consisted of microscopic single-celled bacteria and archaea. These were much less complex than the macroscopic life forms that exist today and that have existed for around a billion years.  Even the oldest life forms that we can find in the fossil record, however, dating back to 3.5 b.y.a., are very complex--they are very sophisticated organisms that relied on DNA and RNA to transfer genetic information, on ATP to store energy in a usable and accessible form, and, for some of them, on photosynthesis to get access to energy. These organisms are much more complicated than we imagine the first life forms would have been.  Life did not exist, however, for one-half to one billion years following the formation of the Earth. The earliest half-billion years on the Earth were marked by the continued influx of impacting objects left over from the formation of the planets. Even today, these objects are capable of dramatically affecting the terrestrial environment when they col