To help focus the search, scientists can glean clues from the burgeoning field of synthetic, or artificial, life. Biochemists are currently attempting to engineer completely novel organisms by inserting additional amino acids into proteins. A pioneer of this research, Steve Benner of the Foundation for Applied Molecular Evolution in Gainesville, Fla., has pointed out that a class of molecules known as alpha-methyl amino acids look promising for artificial life because they can fold properly. These molecules, however, have not been found in any natural organism studied to date. As investigators identify new microbes, it would be a relatively simple matter to use standard tools for analyzing the composition of proteins, such as mass spectrometry, to learn which amino acids the organisms contain. Any glaring oddities in the inventory would signal that the microbe could be a candidate for shadow life.

If such a strategy were successful, researchers would face the difficulty of determining whether they were dealing with a genuine alternative form of life descended from a separate origin or with merely a new domain of known life, such as archaea, which were not identified until the late 1970s. In other words, how can scientists be sure that what seems like a new tree of life is not in fact an undiscovered branch of the known tree that split away a very long time ago and has so far escaped our attention? In all likelihood, the earliest life-forms were radically different from those that followed. For example, the sophisticated triplet DNA code for specifying particular amino acids shows evidence of being optimized in its efficiency by evolutionary selection. This observation suggests the existence of a more rudimentary precursor, such as a doublet code employing only 10, rather than 20, amino acids. It is conceivable that some primitive organisms are still using the old precursor code today. Such microbes would not be truly alien but more like living fossils. Nevertheless, their discovery would still be of immense scientific interest. Another possible holdover from an earlier biological epoch would be microbes that use RNA in place of DNA.

   
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