We may find the evidence we need within the frigid hydrocarbon lakes of Titan. Or perhaps we shall locate it by tracing the source of the methane hot spots on Mars to their deep underground origin. It may be easier though to sample in depth the contents of the water vapor and ice jets that erupt from “tiger stripe” cracks near the south pole of Enceladus. Least expensive of all would be to explore closer to home - in forsaken regions of Earth that are so hot, or so acidic, or so lacking in some vital nutrient necessary to life as we know it, that no creatures built of such life would deign to inhabit it. So many promising leads have appeared that it seems likely that only our desire and our finances stand as obstacles to our gaining the prize.
The prize in this case would be a sample of truly alien life. Despite the great legacy from pulp science fiction magazines and expensive Hollywood box office productions, aliens need not be green men, or menacing fanged monsters. Even humble microbes dismissed as “shower scum” in the New York Times would do nicely, provided that they met one key requirement. They must differ enough from us at the biochemical level so that it would be clear that they had started up and evolved on their own. Two separate origins in the same Solar System would imply that the universe is liberally sprinkled with life. Why would a discovery of this type change everything? It would not put food upon our dinner tables or shorten our commute to work. The change would come in our perception of the universe (which does contain everything that we know) and the place of life within it. We would learn that life, like art, can take on many forms and be constructed in countless ways and that we appear to be residents of a universe built to encourage such diversity.
We have always understood, of course, that living things came in many sizes and shapes: bacteria and whales, octopuses and centipedes. But we took it for granted that they were all made of one substance, one flesh. When they were ferocious, they could devour us. When they were domestic, we could make meals of them. This expectation was extended to alien life in fiction, myth and imagination. The Martian invaders of H.G. Wells "War of the Worlds" were ultimately subdued by infection by Earthly microbes. The creatures of the “Aliens” film series could incubate in humans, and draw nourishment from them. Humans could have sexual encounters with ancient Greek gods as well as with intruders in flying saucers.
Such events would have provoked little surprise in the 19th century, when the basic substance of life was thought to be a vital, gel-like protoplasm, which presumably would be the same everywhere. We now understand that life’s basic structure is much more intricate, but that the same building materials, nucleic acids, proteins, carbohydrates and fatty substances are used by all known life forms here. Some scientists have extended this conclusion to alien life. To quote Nobel Laureate George Wald "…So I tell my students: learn your biochemistry here and you will be able to pass examinations on Arcturus." This view carries practical consequences today: proposals for instruments to be flown to Mars and elsewhere include antibodies, probes, and other methods designed to detect the molecules familiar to us on Earth today.
If the microbes that we discoved on a nearby world had traveled from Earth within a meteorite, then this expectation would be valid.. By studying them, we might learn a lot about the earlier stages of the evolution of life here, and about the ability of Earth life to adapt to a very different environment. Such information would be valuable, but it would not change everything.
When we discover separate origin life, we will hit the jackpot. It will truly be made of a different flesh. Biochemists will be fascinated to learn how life functions such as energy capture, information storage and catalysis can be carried out by materials different from the life we know. The field of biology will be greatly enriched, and a host of new technical innovations may arise from the new knowledge, but even this would not change everything. The largest impact would take place in the way that we view our existence and plan for our future.
For stability and comfort, most human beings appear to require a narrative that provides meaning and purpose to their lives. In many religions, our behavior as individuals here determines our fate in a hereafter. Our actions are crucial in the grand scheme of things. Prior to the Copernican revolution, the Earth was naturally placed at the very center of the stage in the theater of existence. As suitable decorations, the various heavenly bodies were embedded in spheres that rotated above us.
Now we understand that our home world occupies only a minute fragment of an immense array of planets, stars, and galaxies. Our species has experienced only a sliver of the great expanse of time that has passed since the Big Bang, and much more is yet to come. The playing field has become immense.
Traditional religions have generally ignored this huge expansion of the cosmic scheme and cling to an essentially pre-Copernican view of existence. In doing so, they reduce their narratives to cherished folk tales, with a message as relevant today as the science of Aristotle. By contrast, some Nobel Laureate scientists have regarded the Universe as meaningless and pointless, with our life representing an accidental anomaly that will disappear sooner or later. Fortunately, another interpretation exists; one that is fully compatible with science though it extends beyond it.
Eric Chaisson, Paul Davies and others have described a viewpoint often called "Cosmic Evolution". The successive appearance of galaxies, stars and planets, atoms and molecules, life and intelligence are all seen as inherent in the laws that have governed our universe since the Big Bang. Minor alterations in many of the fundamental constants that are embedded in those laws would have made this succession of events impossible. For whatever reason, our universe is (to use Paul Davies' word) "bio-friendly".
If a separate origin of life were encountered within our own Solar System, the credibility of this viewpoint would be strengthened immensely. We could see ourselves as active participants in a vast cosmic competition that required all available space and time to play itself out to the fullest extent. To advance in the game and ultimately grasp its point, our mission would be to survive, prosper, evolve to the next stage and to expand into the greater universe (which would not be bad goals under any circumstance). By liberating humanity from a choice between obsolete dogma and unrelenting pessimism, this discovery would ultimately change everything.