ABSTRACT
Throughout the first few billion years of its history, microbial cells were the only life-forms on Earth and present the only life-forms that could have once existed or still exist on Mars. Our understanding of the formation of the signatures of microbial life-forms (biosignatures) comes from experimental fossilization studies, together with the detailed investigation of their remains throughout the preserved geological history of the Earth. We here resume mechanisms for the preservation of microbial biosignatures and review a selection of the highly diverse and well-preserved biota from the early history of the Earth. Our understanding of early life is informative for the search for life on Mars, presenting a paleontological and philosophical analogue against which putative signs of Martian life will need to be tested. Significant differences in the conditions of habitability between Mars and the Earth indicate that only primitive chemotrophic life-forms could have inhabited, or still inhabit, the Red Planet, and this may set a unique set of challenges for the proof of Martian biosignatures. We conclude by briefly outlining the findings of several decades of past and present missions of astrobiological interest to Mars and consider what additional information future missions could bring.
