A Plan For Detecting Evidence of Hadean Life

Abstract:

The paucity of an early (>3.8 Ga) Earth rock record and the intense metamorphism of what little is left makes challenging the definitive detection of Hadean signs of life and thus an empirical test of molecular clock predictions that the last universal common ancestor (LUCA) of Eubacteria and Archaebacteria emerged between 4.2 and 4.4 Ga (Betts et al., 2018, Nature ecol. & evol. 2, 1556). Evidence that pushes back the established minimum age of life’s origin is typically met with asymmetric criticism and argues for guidelines from which to judge the merit of a claim. The Ladder of Life Detection (Neveu et al., 2018, Astrobiology 18, 1375) is a set of criteria meant to guide design of robotic space missions to detect signs of life elsewhere, but this system also has application to studies of early Earth. Seven criteria build toward an increasingly robust interpretation of life detection: Sensitivity, Contaminant-free signal, Repeatability, Context, Detectability, Preservation, Compatibility, and Definitiveness. An 8^th criterion, the Last Resort Hypothesis, is the ultimate hurdle: does the signal “compellingly preclude an abiotic origin”. Evaluation of those cases in which light C isotopes were measured in situ on graphitic inclusions within an armoring mineral shell in >3.8 Ga samples (i.e., Akilia, Jack Hills) meet the first 6 criteria. While exotic abiotic mechanisms capable of creating light C isotope signals might appear to preclude Definitiveness and the Last Resort Hypothesis, the failure of those mechanisms to globally mimic the sign and magnitude of the vital effect of photosynthesis on C isotopes (Schidlowski, 2001, Precamb. Res. 106, 117) supports the view that these 3.8-4.1 Ga materials meet those tests as well. The path forward is acquisition of a large global database of C isotopes in carbonaceous inclusions (along with geochemical clues as to the origin of their hosts) from the 15 localities known to host >4 Ga zircons. We describe a field and analytical protocol that should permit clear selection between their biogenic vs. abiogenic origin and thus provide an observational test of molecular clock predictions of when LUCA arose.