It is always reassuring when one of the most eminent specialists in your favoured field offers ideas that correspond to yours. Generously, Paul Davies has done just such a thing in his excellent SciAm article Are Aliens Among Us?
He starts by reflecting on the unresolved question of whether Earth is unique or whether life is prevalent everywhere. It seems bizarre to me that given the historical examination of meteorites, the astronomically observed clouds of cosmic dust, the clear evidence that basic life forms can survive in extreme conditions, the likelihood that water probably isn’t a prerequisite and the commonly held scientific view that life will be found on Mars, that there are still people who believe that Earth stands alone. It is simply mathematically impossible. Davies says;
How can scientists determine which view is correct? The most direct way is to seek evidence for life on another planet, such as Mars. If life originated from scratch on two planets in a single solar system, it would decisively confirm the hypothesis of biological determinism. Unfortunately, it may be a long time before missions to the Red Planet are sophisticated enough to hunt for Martian life-forms and, if they indeed exist, to study such extraterrestrial biota in detail.
My only point of disagreement with him here is his definition of “a long time”.
The remainder of his article reviews the possibilities of, using a term that he has borrowed from Carol Cleland and Shelley Copley of the University of Colorado, shadow life. This concept, which I believe to be a strong possibility, is that either alien life forms are already here or alternatively are the result of a previous genesis. When we talk of “alien life forms” we aren’t referring to ET in the shape of a gray, but to microbial life, so exciting it isn’t. It is more than conceivable that such forms exist and that science is simply not aware of them yet. There could even be one sitting on your sleeve at this minute.
Likewise with a previous genesis. There may be paleontological evidence of an earlier “start up” but discovering it is another matter.
This article is an outstanding overview of the areas where alien life might exist on Earth and how it might thrive. You may reason that if all we’re talking about are effectively specks of dust, does any of it really matter? But the central issue of how life originated has, as yet, eluded us and any clue that can help us on our way to that answer has to be worth the effort.