Identifying Exofossils

An interesting post on Microbeworld asks the question, how could scientists identify whether organic compounds found elsewhere in the Solar System are derived from decomposed life forms? It’s all about chirality.

Many organic molecules, including sugars and amino acids, can be built in mirror-image forms. Two forms that are mirror images of each other cannot be substituted for each other, no matter how you turn them, very much like having a left and a right hand. Your left and right hands are very similar, but they have chiral mirror-symmetry.

Chirality is useful in biology, because fitting molecules together to make moving parts is how the work of life gets done. This becomes clear when I, as a left-hander, try to use a right-handed pair of scissors. It can be done, but it’s clumsy. In biochemistry, such substitutions often don’t work at all. Life on Earth has evolved to mostly use one chirality of sugars and one chirality of amino acids, probably because other combinations don’t work as well. Early life that indiscriminantly used left- and right-handed molecules would have been a jack of all trades, in that respect, but probably would not have been as efficient at self-replication as specializers who only built themselves using molecules of a single chirality. Over time, as more organisms made and used only one mirror form of each necessary amino acid or sugar, microbes using both mirror forms would quietly fade away.

It’s entirely possible that very early life on Earth might have used different chiral forms regionally. Life appears to be an emergent phenomenon of geochemistry, so it’s likely that multiple ‘startup’ biochemistries arose around the same time in different parts of the global ocean. But eventually only one would survive; the one we see today. On other worlds the same outcome is likely, simply as a by-product of time and parsimony.

Life forms on other worlds need not end up with the same result we did on Earth, but it’s most likely that their molecules will end up as either one mirror form or the other, and not both at once. Looking for organic molecules on other worlds, and looking to see if such molecules are chirally skewed, would be a good approach to locating molecular fossils of exobiology.


~ by Planetologist on September 22, 2008.

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