In recent years astronomers and biologists have discovered that many of the molecules essential for life - purines, amino acids, etc. - seem to be widely distributed in the universe, and hence might gain a foothold on other suitable planets. Some maintain that these seeds of life, "transpermia," wafting through the universe are the most likely reason for the origin of life on earth. But whether life on this planet began by a unique, fortuitous combination of elements or was jump-started by transpermia from afar, bear in mind that there is no evidence of any form of life during earth's first billion years. Only at about 3.8 billion years ago have we found evidence for the first life forms, prokaryotes - simple cells without an organized nucleus.    
        From one point of view we are minor actors in a scheme of which we are largely unaware. From another point of view bacteria illustrate how very simple organisms can become spectacularly successful - aided immensely, of course, by invading complex organisms, and bending these hosts to their own purposes.