Challenges in evidencing the earliest traces of life.

Earth has been habitable for 4.3 billion years, and the earliest rock record indicates the presence of a microbial biosphere by at least 3.4 billion years ago-and disputably earlier. Possible traces of life can be morphological or chemical but abiotic processes that mimic or alter them, or subsequent contamination, may challenge their interpretation. Advances in micro- and nanoscale analyses, as well as experimental approaches, are improving the characterization of these biosignatures and constraining abiotic processes, when combined with the geological context. Reassessing the evidence of early life is challenging, but essential and timely in the quest to understand the origin and evolution of life, both on Earth and beyond.