Beginnings of life

Genesis – Pre-Cambrian Blue-green algae and their surviving structures We might be fascinated by a movie about extinct dinosaurs, but many of our prehistoric animals are still with us. Some survive merely as tiny remnant populations, while others survive as copies of ancient body forms. In the beginning there was light… but not much else to write home about. Billions of years ago the atmosphere was unbreathable (1% oxygen) and shallow saline seas covered many areas with a milky soup of organic slush. High levels of dissolved iron gave the water a greenish tinge. The moon was closer than it is today, causing huge tides. In that soupy mix God, Allah, Shiva, Zeus, chance, physics or whomever you may wish to use to describe these things, sparked off something. Chains of molecules evolved into complex compositions that became the first simple living things able to process sunlight into food. Life burst forth from this liquid soup. One of the earliest forms of life was cyanobacteria, also known as toxic blue-green algae. You can still find a film of this algae on rocks in intertidal pools. It is rarely noticed except when we add pollution to a warm, shallow waterway. That creates conditions a bit like the early history of earth and it blooms into a ‘toxic tide’ that taints drinking water and irritates the skin of swimmers. Algal Reefs Three and a half billion years ago it was one of the dominant lifeforms on the planet This simple plant was soon creating complex structures that would rival the Great Barrier Reef in grandeur. The algae trapped sediments on their sticky surfaces and excreted limestone. This acted as a ‘glue’ binding more sediments together around these clumps of algae called stromatolites. It’s a slow process as a one metre high clump might take a thousand years to grow. Three billion organisms can live on this one metre tall algal clump. Over the millennia they became prominent structures in the fossil record. There was so much algae that it converted the huge amounts of carbon dioxide in the atmosphere into oxygen. If you visit Shark Bay you can see the oxygen fizzing out of the structures. This combined with the iron sludge in the ancient seas and caused iron oxide to settle. The seas began to clear. In places like the Pilbara of W.A. this process created the immense bands of iron deposits that we mine today. About 1.8 billion years ago the iron became completely oxidised and the excess oxygen now escaped into the atmosphere. Stromatolites sowed the seeds of their own decline. Oxygen became fuel for the evolution of more complex animals like snails and worms that eventually outcompeted cyanobacteria and ate away at the giant structures. After a three billion year reign they disappeared from the fossil record about 500 million years ago. This type of microbe can still 16 be found in our oceans today, but now they can’t form these giant stromatolite ‘reefs’. In the modern era geologists started to unearth fossil stromatolites and quickly put them into the ‘extinct’ category, but that underestimated the resilience of this simple but amazing collection of organisms. Modern Stromatolites of Shark Bay In 1956 stromatolites were rediscovered in the Hamelin Pool, a small area in Shark Bay, Western Australia. Here the topography and odd tides have created an isolated pocket of water so hot and saline that stromatolites can form and survive free from competition. Although the stromatolites are only a few thousand years old and a bit different biologically, one species of microbe that was found on a stromatolite is so unchanged it has been traced to a fossil 1.5 billion years old. It is thought to be the oldest known surviving species on earth. Since then salt water stromatolites, mostly pretty small, have been found in other places. The really big ones exist only in Western Australia and two locations in the Bahamas. Some freshwater stromatolites have been found in Brazil, Mexico, Turkey and Canada. You can visit one of the world’s great natural wonders at Shark Bay, a 10 hour drive north of Perth. Make a week of it and do some of the local diving, kayaking or snorkelling. The other attractions aren’t a billion years old, but there are dolphin encounters and historic wrecks, turtles, mantas, dugongs and lots of other ‘younger’ stuff you would predictably find in a World Heritage Area. Thrombolites of Lake Clifton Stromatolites form in annual layers like tree rings in salty water, but they aren’t much different from thrombolites that form slightly differently in brackish water. Thrombolites don’t settle evenly but clot in uneven lumpy concretions. Once again this ancient lifeform can still be found in Australia. Lake Clifton in the Yalgorup National Park, south of Mandurah is well known for its thrombolites. They grow at an average of 1mm a year. The thrombolites are dominant on the east side of the lake, because their calcium source is in the fresh groundwater which passes through the sand dunes on this side. These peculiar structures are most easily seen in March and April. There’s an observation walkway that allows you to get up close without causing damage. Microbial mounds, which are the remains of thrombolites, can be seen at nearby Lake Preston. Thrombolites are the most common form of microbialites (microbial structures) and are formed by a variety of micro-organisms. Lake Clifton is one of the few places in the State where living thrombolites survive and it is only one of two places in the world where they grow in brackish water.