“If you ask the question—how complicated can non-biological chemistry get?—the answer is biological chemistry, because biological chemistry comes from non-biological chemistry,” says Sean McMahon, an astrobiologist who leads the Planetary Paleobiology Group at the University of Edinburgh.
Indeed, there are structures on Earth that don’t clearly fall into biotic or abiotic categories. Frances Westall, emeritus director of research and former director of the Exobiology Group at the Center for Molecular Biophysics in France, says it took her nearly 20 years to confirm the biological origin of Australian fossilized microbes that date back 3.45 billion years. In a study published in September, her team said these microbes, known as chemolithotrophs, could be a common form of alien life that “are notoriously difficult to detect and identify.”
(The is the best evidence yet for ancient life on Mars)
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“I had to wait until there was a suitable instrument that could measure these small things and detect the very small amounts of carbon,” Westall says. “It just took ages and ages, but finally, we managed to get the smoking gun results that we really wanted.”
The possibility of abstract or unexpected forms of life, as well as the presence of novel geological processes, could make it hard to even pin down a standard for biosignatures.