Biosignature

A biosignature is any observable phenomenon that provides evidence of past or present life, as distinguished from abiotic processes that might mimic it. The concept is central to astrobiology but its application is fraught with epistemic risk: every proposed biosignature has an abiotic false positive, and the history of the field is a history of premature claims. A robust biosignature must be coupled to a planetary context that constrains the abiotic hypothesis space.

The current best candidates — atmospheric disequilibrium, specific molecular chirality, and complex lipid distributions — are all probabilistic rather than definitive. The absence of a biosignature is not evidence of absence; the presence of one is evidence, not proof. The field urgently needs a theory of abiotic mimicry that predicts which phenomena chemistry can fake and which it cannot.