THE SEARCH FOR ANCIENT LIFE ON MARS FROM PAST AND FUTURE STUDIES OF SAMPLES: A FRAMEWORK FROM PALEONTOLOGY

Calendar year 2021 marked both the 25th anniversary of the publication of McKay et al. (16 August 1996, Science; the "evidence of microbial life in a Mars meteorite?") and the landing of Mars 2020 rover Perseverance on Mars 18 February 2021. Scientific and public (stakeholder) interest in the search for life on Mars was renewed by the vigorous scientific debate that followed the publication of McKay et al. Over the next several decades NASA sent missions to "Follow the (present, if any and, especially, past) Water" and more recently "Follow the (past) Habitability". The Mars 2020 Science Team seeks host rocks of preserved potential biosignatures for sampling and caching until return to Earth laboratories.

In arriving at their interpretation, McKay et alia reasoned that the confluence of four observable phenomena in ALH 84001 was parsimoniously explained by a single cause - ancient microbial life on Mars - rather than by a confluence of four different causal processes yielding the same four observable phenomena. Fractious debate ensued. Still unresolved is the matter of how to unify interpretation of suites of multiple observations - each with its own alternative hypothesis for its occurrence in a sample - into an evidentiary foundation for a compelling interpretation that the observations are evidence for ancient life.

Paleontologist and historian of geology Martin Rudwick identified criteria - form, matter, and position - used for centuries to distinguish natural materials that are remains of ancient life (fossils in the modern sense) from natural materials that are not. There are useful similarities and differences between the foundational paleontological criteria identified by Rudwick and recent and current ideas and criteria (including those invoked by McKay et al.) for attributing observed phenomena in Mars samples to past life.