The earliest morphological traces of life on Earth are often highly controversial, both because non-biological processes can produce relatively similar structures and because such fossils have often been subjected to advanced alteration and metamorphism. Stromatolites, layered organo-sedimentary structures reflecting complex interplays between microbial communities and their environment, have long been considered key macrofossils for life detection in ancient sedimentary rocks; however, the biological origin of ancient stromatolites has frequently been criticized.
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Source: ScienceDaily