Stromatolites

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Layered sedimentary structures built when a microbial mat traps and binds sediment and precipitates carbonate, growing upward in millimetre laminae over years to centuries. They are the most abundant fossil evidence for the first three billion years of life, and almost all of it is a structure rather than an organism: the mat itself is rarely preserved, so the case for biogenicity usually rests on lamina geometry, on textures inconsistent with pure chemical precipitation, and on the absence of a plausible abiotic mechanism. That is a weaker kind of evidence than a body fossil and it is why several of the oldest claims remain contested. Living stromatolites still form in a small number of settings where grazing animals are excluded, most famously in the hypersaline waters of Hamelin Pool in Shark Bay, Western Australia. Their scarcity today is itself informative: stromatolites declined sharply through the later Proterozoic, and the most economical explanation is that animals learned to eat microbial mats.