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Stromatolites

Stromatolites are one of the most exciting of life’s early forms – except that they are not actually alive and never were. They are inorganic structures made by bacteria. They’ve been found dating back to some 3½ thousand million years ago (see second picture). And they are still being made today, presumably by the same organisms. If you cut open a modern stromatolite and compare it with an ancient fossilised one, their structures are identical.

(I should warn you that I have seen the organic nature of ancient stromatolites dismissed as rubbish. But almost everything in Earth science is rubbished by someone. I think we’re entitled to ignore these naysayers until we see how the story develops.)

This picture of modern stromatolites is now widely known. It comes from Shark Bay in Australia, which is one of the few places where they still grow. It was clearly taken at low tide because stromatolites are made in shallow water. They are made by two types of cyanobacteria. They lie there soaking up the sun. They exude a layer of mucus to protect themselves from its UV content. When the wind stirs up the water, sand sticks to the mucus and blocks out the light. One of the cyanobacteria types is able to swim up over the layer of sand and start again. The other can’t, and dies. But before it dies it generates masses of small spores, some of which are wafted up past the sand to start a new colony.

The second picture shows the detailed structure of one of the earliest fossil stromatolites, from the “Fig Tree Group” in southern Africa. The picture comes from “History of Life” by Richard Cowen. As we’ve said, if you cut open a modern stromatolite, you would find a structure identical to this.

The heyday of stromatolites was 2-3 thousand million years ago, when they were everywhere and built up into huge structures.

But around the time of the Cambrian Explosion, molluscs and other creatures discovered how good they were to eat. They very nearly died out, and living stromatolites are only found in hostile environments, where their predators can’t go.