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What Is Life—and How Do We Search for It in Other Worlds?

Public Library of Science
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PLBI0209_1243-1277.indd PLoS Biology | 1260 Open access, freely available online September 2004 | Volume 2 | Issue 9 | e302 I need a “tricorder”—the convenient, hand-held device featured on Star Trek that can detect life forms even from orbit. Unfortunately, we don’t have a clue how a tricorder might work, since life forms don’t seem to have any observable property that distinguishes them from inanimate matter. Furthermore, we lack a defi nition of life that can guide a search for life outside Earth. How can we fi nd what we can’t defi ne? An answer may lie in the observation that life uses a small, discrete set of organic molecules as basic building blocks. On the surface of Europa and in the subsurface of Mars, we can search for alien but analogous patterns in the organics. Life As We Know It The obvious diversity of life on Earth overlies a fundamental biochemical and genetic similarity. The three main polymers of biology—the nucleic acids, the proteins, and the polysaccarides— are built from 20 amino acids, fi ve nucleotide bases, and a few sugars, respectively. Together with lipids and fatty acids, these are the main constituents of biomass: the hardware of life (Lehninger 1975, p 21). The DNA and RNA software of life is also common, indicating shared descent (Woese 1987). But with only one example of life—life on Earth—it is not all that surprising that we do not have a fundamental understanding of what life is. We don’t know which features of Earth life are essential and which are just accidents of history. Our lack of data is refl ected in our attempts to defi ne life. Koshland (2002) lists seven features of life: (1) program (DNA), (2) improvisation (response to environment), (3) compartmentalization, (4) energy, (5) regeneration, (6) adaptability, and (7) seclusion (chemical control and selectivity). A simpler defi nition is that life is a material system that undergoes reproduction, mutation, an

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