3100 – 3000 million years ago

Year sixteen
Shark Bay, Western Australia

The first dome builders: blue-green bacteria

About 1600 million years after the formation of the Earth, another group of microscopic creatures emerged: blue-green cyanobacteria (sometimes called blue-green algae). These tiny organisms removed calcium carbonate from the sea water and secreted thin calcareous layers, which built up one on top of the other until they formed mounds and domes called stromatolites. The mounds were sometimes no more than a few millimetres high, but at times they grew to several metres in height. With the precipitation of this material, the first limestones were created.

Examples of these ancient stromatolites can be seen in many parts of the world, but perhaps the most famous are the Fig Tree Chert of South Africa (3000 million years old), the Gunflint Chert of Canada (about 2000 million years old) and Bitter Springs Chert in Australia (about 900 million years old).

Stromatolites

Blue-green cyanobacteria lived in vast numbers, forming a thin, sticky mat on the sea bed. The top layer of the mat was formed of interwoven threads of cyanobacteria that formed a skin-like upper surface and it was in this oxygen-rich upper layer that photosynthesis was going on.

Slowly — very slowly — the mats grew a new 'skin' on top of the older one. Fine particles of mud and silt were entrapped and incorporated into the mat. And layer on layer, millimetre by millimetre, the mats grew upwards. Some formed only as mats, but others formed mounds, domes, pillars and cones. Occasionally, fine tubes are present in the stromatolites, representing the calcified sheaths of filamentous cyanobacteria. The Bailadila rocks in India contain fossils of blue-green cyanobacteria in stromatolites.

Shark Bay

Proof that the ancient stromatolites were made by blue-green bacteria comes from modern examples such as those of Shark Bay, Western Australia. The water in the bay is very salty so that creatures such as molluscs, which normally eat blue-green bacteria, cannot survive. This means that there is nothing preventing the growth of the domes of limestone. Here then is proof that whilst some organisms evolve and quickly become extinct, others survive almost unchanged for billions of years.


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