Deciphering the early record and evolution of life is crucial to characterize plausible and reliable biosignatures of microbial life and understand the evolution of the Earth biosphere. We can then address questions regarding the conditions for life to appear and develop on a planetary body (habitability), or the probability for an extraterrestrial biosphere to develop complex metabolism or complex life. This research is also critical to develop life detection strategies, instruments and missions applicable to other planets of the solar system such as the ongoing and future Martian missions, and to atmospheres of rocky exoplanets, and to samples returned to Earth, as space agencies have recently come to appreciate.

Considerable debates still exist regarding the age and origins of the three domains of life (Archaea, Bacteria, Eucarya), as well as the evolution of cellular life before LUCA. Possible isotopic, biosedimentary, molecular and morphological traces of life suggest the presence of microbial communities in diverse environments. However, these traces may in some cases also be produced by abiotic processes or later contamination, leaving a controversy surrounding the earliest record of life on Earth. Before a microstructure can be accepted as a microfossil, a series of approaches need to be employed to prove its endogenicity, syngenicity, and biological origin, as well as to falsify an abiotic explanation for the observed morphologies or chemistries. These micro- to nano-scale analyses complement the macro-scale characterisation of the geological context, as the environmental conditions will determine the plausibility of ancient habitats and the conditions of fossilisation.  

Experimental taphonomy also helps understanding the processes of decay and preservation of biosignatures during fossilization. Interpreting the identity and paleobiology of unambiguous traces may also be challenging. However, regardless of taxonomy, the paleobiological record can provide direct evidence for extinct clades and/or for the minimum age of evolution of biological innovations. Reassessing the evidence of early life is challenging but essential and timely for the quest of life’s first traces and evolution, both on Earth and beyond.