Informational polymers as unambiguous biomarkers for aqueous-based life
2018
The National Aeronautics and Space Administration (NASA)defines life as a “self-sustaining chemical
System capable of Darwinian evolution,”yet a NASA-developed ladder of life detection suggests that
measuring such evolution is not feasible within the resource constraints of currently-envisioned life
detection missions. If true, such missions cannot strictly verify whether life beyond Earth exists. We
propose to resolve this issue in two ways. First, we argue that it is possible to make this measurement. By
way of example, we report measurement of evolution in Bacillus subtilis 168 subjected to UV exposure,
made solely with a small portable nanopore-based single molecule sequencing device, which is broadly
compatible with the constraints of life detection missions, especially for Mars. Detected genetic differences
suggest an inherent lifestyle tradeoff between adaptation to oxidative stress and growth rates, consistent
with an observed slow-growth phenotype. Second, we argue that we can be confident in detecting life
without measuring Darwinian evolution per se. For example, amino acid abundance distributions and lipid
carbon chain length distributions have previously been proposed as unambiguous biomarkers. Here we
focus on informational polymers, e.g. nucleic acids, which form the basis for heredity that enables
evolution in all known life. Indeed, long charged polymers may be universal features of aqueous-based life
due to their ability to separate information storage from physicochemical properties. The goal to detect not
only DNA and RNA, but related or unrelated polymers, will require further development of nascent
technologies to enable a broad-based search for life on Mars, Enceladus, and Europa. Informational
polymers, in combination with other“unambiguous” biomarkers,can provide a high confidence in
detection of life as we know it or as we don’t know it.
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