My theory ( although I am sure it has been proposed before ) is that life can not exist on a planet until sufficient organic matter is already present having arrived there by natural chemical processes like those described earlier by Lyndon Tiu.
In a similar manner as what came first, the chicken or the egg; to me it seems clear that organic matter had to be here first and in great quantities. Natural processes can convert matter, but nature itself can not create matter, at least from a scientific view. I would not be opposed to the belief that advanced intelligent species interfered with the natural processes on earth or any other planet, however, there seems to be almost no visible sign that took place, unless they took great care to hide their efforts, or it happened so many millions of years ago, that their equipment has weathered away or sunk into the earth due to plate tectonics or other great natural events. David
