Stromatolite in the Old Red Sandstone, west of the Ness at Stromness, Orkney mainland. This is a sort of algal slime that stiffened itself with calcite (limestone). It's Earth's most successful organism ever, surviving for 3 billion years before the start of multi-celled lifeforms like us, responsible for creating the oxygen in our atmosphere out of its original carbon dioxide, and still around now in a couple of brackish lagoons in Australia. (We shall have to survive 800 times as long as we have so far, and 20 times as long as the dinosaurs, to match up with the algal slime.)

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