Saturday, December 4, 2010

Arsenic-Based Life

This week, NASA geobiologist Felisa Wolfe-Simon announced the discovery of arsenic-based microbes in Mono Lake in Yosemite National Park. This is a major scientific bombshell that is causing biologists to re-examine much of the conventional wisdom about what life is. All life, from the smallest bacteria to the largest redwood tree, was thought to be based on five elements: Carbon, hydrogen, oxygen, nitrogen, and phosphorus. These elements are crucial ingredients in life’s software – DNA and RNA molecules – as well as life’s fuel – ATP molecules. But now scientists have discovered organisms that use arsenic instead of phosphorus in their biochemistry, which was thought to be impossible. Most organisms are poisoned by arsenic because it is highly reactive, destroying the DNA and ATP found in living cells. But this new species of extremophile not only thrives in a lake with an arsenic concentration 700 times what the EPA deems safe, but actually manages to incorporate it into its DNA and ATP. This is more than just a strange new species with an interesting quirk; many of those are discovered every year. This is a radical redefinition of what “life” is.

Although these microbes appear to have evolved from more traditional forms of life, their mere existence opens up the possibility of a "shadow biosphere" on earth. All known living things have descended from a common ancestor, but what if organisms with other biochemistries are living right under our noses undetected? Might they provide evidence of a second genesis on earth? If life has developed twice, with two different biochemistries, it would prove that the evolution of life on earth was not just a one-in-a-trillion fluke, and would greatly increase the likelihood of life developing on any suitable world.

The discovery of arsenic-based life has important applications in the search for extraterrestrial life, which is where NASA comes in. For ages, scientists have pondered about the possibility of completely new types of life. A handful of astrobiologists (and a slew of science fiction writers) have speculated that extraterrestrial life might be too alien for us to even recognize as life. In fact, this is one possible explanation for Fermi's Paradox, which questions why we haven't already found life if it is commonplace in the universe. In light of this week's discovery, this explanation has become a lot more plausible. Perhaps DNA, reliance on water, and cells are just unique traits of life on earth, and we are barking up the wrong tree if we focus solely on finding them elsewhere.

Others have believed that “life as we know it” was the only type of life possible. This has been the dominant mindset of NASA for several decades, and is the basis of NASA’s search for life. NASA has concentrated its efforts on locating worlds similar to our own, where the conditions exist to permit the development of life as we know it. This generally means finding worlds with water on them, located in the “Goldilocks Zone” of their solar systems where they are neither too hot nor too cold to sustain life. This, of course, has been premised on the assumption that any extraterrestrial life is probably not too different from the life we know.

The discovery of arsenic-based life has cast doubt on this approach. If a new form of life can be found in Yosemite National Park, we can scarcely imagine how different extraterrestrial life must be. NASA’s obsessive search for earth-like planets may be overlooking a huge number of worlds where life may exist, in forms unknown to us earthlings. If it is possible for life to exist with a completely different biochemistry from our own, then it’s equally possible that it could thrive on worlds far different from our own, under conditions that have traditionally been regarded as hostile to life.

NASA will need to do a lot of soul-searching in light of this week’s discovery, and reevaluate how it determines if a world is potentially suitable for life. For the first time in history, a long-standing astrobiological question has been answered: Is “life as we know it” the only type of life possible? We now have our answer: It is not.

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