The long-term persistence of microbial biospheres introduces a temporal bias 1 into the Drake equation: implications for the search for extraterrestrial life

Dan Roizman, José María Gómez-Gómez, Alexandro Rodriguez-Rojas

The Drake equation was designed to estimate the number of communicative technological civilisations, not the prevalence of life. Its lifetime term therefore describes a detectable technological phase that may represent only a small part of a planet’s biological history. This emphasis introduces a temporal bias when Drake-style reasoning is extended from technological civilisations to life more generally, because microbial biospheres have persisted throughout nearly the entirety of Earth's inhabited history. In contrast, technological detectability is a very recent state. Insights from non-equilibrium thermodynamics and microbial energetics provide a mechanistic basis for understanding how biological systems can persist under extremely low energy flux. Geological evidence further shows that microbial biospheres arose early and persisted across major planetary transitions long before the emergence of complex ecosystems and technological civilisation. Here, we use the lifetime term as a residence-time variable describing the duration of a biological state. Under this interpretation, microbial-biosphere occupancy could account for most of the time during which an inhabited planet’s biological history, including intervals in which complex ecosystems or technological civilisations coexist with microbial life. Biological prevalence, residence time and remote detectability are therefore not equivalent quantities. Planets such as Proxima Centauri b provide an important test case. If they retain habitable environments and develop life, they can potentially support long-lived microbial biospheres whose signatures remain weak or intermittent. We hypothesise that microbial biospheres may occupy a larger fraction of inhabited planetary histories than technological states on inhabited planets, producing a systematic temporal bias when Drake-style reasoning is extended from technological civilisations to life more generally. Microbial biosignatures and technosignatures should therefore be treated as complementary observational targets in the search for extraterrestrial life.