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Planetary News: Astrobiology (2007)Panel Recommends Searching for “Life as We Don’t Know It"By Amir AlexanderJuly 13, 2007
One day, perhaps hundreds of years from now, perhaps much sooner, a human-made space probe will encounter alien life forms. We do not know what these beings will be like, but it is reasonable to assume that this first encounter will not be with the intelligent extraterrestrials that SETI is searching for. Far more likely they will be simple organisms, perhaps microbial, that have found shelter in a friendly environment from the harshness of space. They might have found a home deep below the surface of Mars, in the subterranean ocean of Europa, a completely different place in the solar system or, just as likely, light-years away from our Sun. We know nothing about these extraterrestrial creatures except that some day, somewhere, we are sure to encounter them. But when we do, will we recognize them? That is the question considered in a National Research Council report by a panel of experts led by John A. Baross of the University of Washington, the entitled The Limits of Organic Life in Planetary Systems. The report, published July 7, argues that our search for life in the universe is modeled very closely on Earthly, or “terran,” life. Such an approach, according to the panel, is perfectly natural and not unreasonable, given that this is the only kind of life that we know. For the fact is that Earthly life, for all its apparent rich diversity, is in fact surprisingly uniform. Amoebas and elephants, elm trees and human beings, may seem to us extraordinarily different, but when observed at the molecular level they are all, in fact, very similar. All life on Earth is carbon-based, relies on water as a solvent, and uses DNA to encode genetic information and build proteins from amino acids. This uniformity might lead to some unwarranted assumptions about the nature of life in general. After all, the panel quotes Carl Sagan, “it is not surprising that carbon-based organisms breathing oxygen and composed of 60 percent water would conclude that life must be based on carbon and water and metabolize free oxygen. The problem with this approach, according to the panel, is that if we only look for terran-like life, we might overlook other viable life forms that are built differently from us. The “terran” approach is particularly misleading, the authors suggest, since the reason that all Earthly life is so similar is almost certainly because it derives from a single common ancestor. Alien life, in contrast, most likely has a very different ancestry, and might therefore be structured very differently. If we only search for life as we know it, we might never see such “weird life” at all. “Nothing would be more unfortunate,” the panel writes, “than to expend considerable resources in the search for alien life and then not recognize it if it is encountered.” To prevent such a tragic outcome, the panel encourages scientists to expand their view, look beyond the familiar, and try and imagine what truly alien life might be like. And the best place to start such a study of “weird life” might be right here on Earth. “There is much about Earth life we don’t understand” said Baross, and studying it might alert us to possible alien life-forms as well. For example, according to panel member Steven A. Benner of the Foundation for Applied Molecular Evolution, there are good grounds to suppose that the earliest life on Earth did not make use of the DNA “double helix” but of single-stranded RNA. Some scientists believe that remnants of such RNA life may still exist on our planet in extremely remote and inaccessible locales, such as deep beneath the ocean floor. “It’s an incredibly primordial world down there” said Baross. “If you’re going to look for the remnants of an RNA world, those are the environments you want to go to. Our genetic material might hold the key to other types of “weird life” as well. DNA, used by all known Earthly creatures to transmit hereditary information, uses phosphorus as its backbone. But from a purely chemical standpoint, arsenic would do just as well. Could alien creatures use arsenic-based “DNA” to transmit their genes? Maybe, maybe not, but if we don’t keep an open mind about this possibility we will never discover such beings.
Then there is the question of that elixir of terran life – water. “Follow the water” has been the mantra of the search for life in the solar system, and has led us to remarkable discoveries of ancient seas on Mars and subterranean oceans on Jupiter’s moon Europa. But it is possible, the report suggests, that such an approach is too narrow. What if alien life forms do not use water as their solvent, but ammonia or methane instead? In that case, instead of looking for life on Mars and Europa, we might want to focus on Saturn’s moon Titan. The Huygens probe, which landed on Titan in 2005, discovered a world in which liquid methane rains down on a landscape dotted with pools of water and ammonia seeping up from the ground. Hostile as such an environment is to us, it might be heaven to a life form using methane or ammonia as its solvent. But why stop there? According to the panel, scientists should explore even more radical alternatives to life as we know it. Life on Earth is carbon-based, but does it have to be? Probably, the panel notes, but not certainly. There could potentially be “organic” molecules based on silicon rather than carbon, but it is far from clear how they would have emerged spontaneously to support life, whereas the case for carbon is much stronger. If we move even further away from familiar life we could ask whether life is life can exist in a totally different state, such as gas? And can life exist is solids, such as ice? More fundamentally, all life on Earth is engaged in Darwinian evolution, which appears to be a fundamental principle of all conceivable life. But is it? Can there be life that emerges and evolves through altruistic principles rather than a Darwinian struggle for survival? It is very hard to imagine a positive answer to any of these questions, or what a gas-based or non-Darwinian life-form would be like. But according to the study’s authors that is no reason not to look. Chemists must investigate “the chemical possibilities of what forms life might take” said Benner.
The panel did not limit itself to warning against current assumptions and suggesting radical alternatives, but also made concrete recommendations. It suggested, for example, the NASA and the National Science Foundation (NSF) redouble their efforts to study the origins of life, and make every effort to locate any remnants of the lost RNA world. In Mars exploration NASA should broaden its scope beyond its “follow the water” strategy, and detectors of small organic molecules should be included in missions to Mars as well as the Jupiter’s and Saturn’s Moon. NASA, the report suggests, should also reevaluate its exploration priorities and consider whether Titan should be revisited in light of the Huygens probe’s discoveries. Another intriguing target in the search for life, according to the panel, is another of Saturn’s moons, Enceladus, where water geysers were recently observed. But the report’s main intent was not to call for specific projects but to change the attitudes and assumptions of scientists working on all facets of the search for life in the universe. “Nothing would alter our view of humanity or our position in the cosmos more than the discovery of alien life” the authors note, and we should therefore always be on the watch for this cosmic treasure. We must not assume that alien life will resemble our own, they warn, for we will very likely be wrong. We must keep an open mind as to what life can be like, and look for it even in seemingly unpromising places. And then, when we finally encounter these alien beings, we just might recognize them for what they are: fellow living inhabitants of the universe. |
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