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Living Rocks

Living Rocks

At first glance, the lumpy grey mounds in the shallows of Hamelin Pool, in Western Australia's Shark Bay, look like nothing more than rocks. Visitors who walk out along the boardwalk sometimes wonder what all the fuss is about. Yet these mounds, called stromatolites, are among the most remarkable structures on Earth, because they are alive, or at least their surfaces are.

Each stromatolite is built by communities of microbes, living things too small to see without a microscope. The microbes produce a sticky film. Grains of sand and sediment washed in by the tide become trapped in this film, and the microbes slowly grow up through the new layer to reach the sunlight they need. Layer by layer, a mound forms. The process is extraordinarily slow: a stromatolite may grow less than half a millimetre in a year, so a mound the height of a school ruler may have taken many hundreds of years to build.

Stromatolites were once common in shallow seas around the world. Today they survive in only a few places, and Hamelin Pool holds some of the best examples. The reason is the water. A bank of sand and seagrass at the entrance to the pool slows the flow of water from the open ocean, and the hot sun evaporates the shallows, leaving the pool about twice as salty as ordinary sea water. Few of the snails and other grazing animals that feed on microbes can survive in such conditions, so the builders are left in peace.

Scientists value these mounds for another reason. Fossil stromatolites billions of years old are among the earliest evidence of life on Earth, and the oxygen produced by ancient microbes like these helped make the air breathable for the animals that came later. Standing on the boardwalk, then, a visitor is looking at something very like the first chapter of life's story.