Emergence of biosignatures on Earth and implications for life detection

Earth's organic chemical evolution (OCE) from abiotic to prebiotic to biotic provides critical insights that may have implications for how we interpret observations made by life-detection planetary missions. To search for life on other worlds, astrobiologists seek biosignatures: features that indicate the presence of extant or extinct life as we know it. However, because the age of putative extraterrestrial biospheres is unknown, biosignatures present at or shortly after life's emergence may have been diluted with prebiotic or abiotic signatures. Thus, the emergence of biosignatures and their formation context are critical considerations for mission interpretations, especially where there is no obvious pervasive biosphere. Here, we provide a structured approach for interpreting chemical states across the continuum of OCE on a planetary body. We distinguish between universal biosignatures of a world vs. specific biosignatures that are tied to environmental or temporal conditions of that world. We then consider how the evolution of a planetary body would impact the expression of these signatures over time. The detection of active prebiotic chemistry-that is, signatures that fall in the ambiguous transition between prebiotic and living systems-would be a hallmark discovery, providing insight into life's origins and justifying investment in further exploration of such worlds. Leveraging Earth as an example, we suggest that while mission observations may be more challenging to interpret than previously thought, they may also open doors to a more productive exploration of our planetary neighbors.