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According to Paul Davies in a NY Times article, the search for extraterrestrial intelligence is predicated, at the moment, on the assumption that life would emerge readily on Earth-like planets.
But the notion of life as a cosmic imperative is not backed up by hard evidence: the mechanism of life’s origin remains shrouded in mystery.
We need to test the idea that the transition from non-life to life is simple enough to happen repeatedly. And the most obvious and straightforward way to do so, according to Davies, is to search for a second form of life on Earth. (“No planet is more Earth-like than Earth itself…”)
So, if the path to life is easy, then life should have started up many times over right here.
There is excellent evidence that every kind of life so far studied evolved from a common ancestor that lived billions of years ago. But — says Davies — most of the life that exists on Earth has never been properly classified.
Microbes, invisible to the naked eye, make up the vast majority of species. Scientists have analyzed only the tiniest majority of them There could be microbes with other ancestral origins living literally under our noses.
Davies wants to call the denizens of the hidden “alien” biosphere Life 2.0. Life 2.0 creatures might employ radically different biochemical processes than the life we know about.
Because microbiologists’ methods are focused on the biochemistry of standard life, they could easily have overlooked their existence. (“If you go looking for A, you will find A and not B.”)
One way to go about tracking down Life 2.0 is to make educated guesses about what its biochemistry might be like. Alternative microbes might, for example, have different chemical elements. One shrewd suggestion, made by Felisa Wolfe-Simon of the United States Geological Survey, is that phosphorous — crucial to life as we know it — could be replaced by arsenic. She and her colleague Ron Oremland are dredging bugs from arsenic-contaminated Mono Lake in California in search of arsenic life.
Another focus is the “handedness” of molecules. In standard life, the key amino acids are always left-handed; the sugars are right-handed. It is not clear why standard life has made this particular choice, since nonliving chemical mixtures seem to contain equal amounts of both left- and right-handed molecules.
Perhaps if life started again it would select different handedness for its key molecules.
Perhaps a shadow biosphere of “mirror microbes” exist. Such organisms could be identified by culturing microbial samples in “mirror soup,” a cocktail of nutrients with the handedness reversed. Such material would be available from commercial suppliers.
Standard life would find the soup unpalatable, but “mirror life” would thrive on it. Experiments along these lines are being carried out by NASA.
And it would be easier to identify Life 2.0 if it inhabited distinct niches beyond the reach of regular life. For example, microbes are known to dwell in superheated water around volcanic vents in the deep ocean.
Others survive extremes of cold, salinity, acidity or radiation. All these so-called “extremophiles” which have been studied so far have been standard life forms. But there may be limits to how hardy and adaptable regular life is, and to their toleration for amazingly harsh conditions.
If Life 2.0 has a different chemical constitution, writes Davies, it may lurk in pockets at even more extreme temperatures or higher levels of radiation.
An argument often given for why Earth couldn’t host another form of life is that once the life we know became established, it would have eliminated any competition through natural selection. But if another form of life were confined to its own niche, there would be little direct competition with regular life. And, in any case, natural selection doesn’t always mean winner-takes-all.
Some years ago it was discovered that simple microbes actually belong to two very distinct domains — bacteria and archaea. Genetically, these groups differ from each other as much as they differ from humans. Yet they have peacefully co-existed in overlapping habitats for billions of years.
So if Davies’s theory turns out to be correct, he says, it will have sweeping consequences. A second form of life on our doorsteps would allow us to be confident that life is truly a cosmic phenomenon.
Paul Davies, the director of the Beyond Center for Fundamental Concepts in Science, at Arizona State University. He is the author of “The Eerie Silence: Renewing Our Search for Alien Intelligence.” Read his article at http://tinyurl.com/36paa7j
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