The living rocks in the Hamelin Pool lagoon of Shark Bay in Western Australia convey a vivid impression of how Earth may have looked like at the beginning of life 3500 million years ago.
The living rocks are stromatolites. They consist of thin mats of blue-green algae, the cyanobacteria. These bacteria enclose themselves with sugar-rich slime. Small sediment particles floating in the current are trapped in the sticky substance and calcium carbonate, which precipitates from the saturated seawater, causes solidification. Over very long periods of time, stromatolites grow layer by layer into columnar, humpbacked reef structures. The name stromatolite literally means layered rock.
In Shark Bay they thrive in water depths down to three meters (9 feet) and fall dry during low tide. With a diameter of up to one meter (3 feet), they can grow up to 50 cm (20 inches) high. In the tidal area, they settle darkly against the white sand.
The sands at the bottom of the turquoise lagoon water are of purely biological origin. They consist of organic shells, especially the tiny foraminifers, but also shells, snails and corals. The permanent movement of the waves grinds the material into fine sand.
The hot and dry climate of Shark Bay produces extremely high evaporation rates of the seawater. A sand barrier densely covered with seagrass meadows to the north of the lagoon additionally hinders the influx of new seawater. This doubles the salinity in the lagoon. Only very few organisms can tolerate such harsh living conditions. Cyanobacteria are among them, which is why they are protected from a multitude of predators and therefore thrive only here.
The significance of stromatolites is that they are the oldest known organisms and that they still exist today. Their 3500 million year old fossils are found north of Shark Bay in the Pilbara region of the Karijini National Park.
In the early days of Earth, when life slowly conquered the planet about 3550 million years ago, bacteria were the dominant life form. The earliest bacteria were anaerobic photosynthesizers, which did not yet produce free oxygen as a waste product of their metabolism. They fed mainly on sulphur-containing compounds and formed the banded iron ores of Karijini. Meanwhile, a new group of bacteria, the cyanobacteria, built the first reef structures in the form of stromatolites. They were also the first to carry out aerobic photosynthesis, releasing oxygen, that is so vital for us, first into the oceans and later into the atmosphere. At that time, this free oxygen was a catastrophic environmental toxin that caused a mass extinction among the remaining species of bacteria. It was not until evolution created hemoglobin to protect against the aggressive oxygen that the first animals on the ocean floor were able to use this oxygen for respiration. This happened only 500 million years ago. Thus, life on earth consisted exclusively of microorganisms for 3000 million years!
With their oxygen production, the cyanobacteria initiated another chain reaction vital for future life. The oxygen accumulated in the atmosphere over time made its way to altitudes of more than 20 km (12 miles). There the ultraviolet radiation of the sun is so intense that it splits the molecular oxygen (O2) into two oxygen atoms each (O+O). Each of these highly reactive oxygen atoms immediately recombines with a molecular oxygen to form trivalent oxygen, the ozone (O2+O=O3). The ozone layer thus formed prevented the life-threatening hard UV radiation from reaching the ground. This made it possible to colonize the surface waters. Without the cyanobacteria, there would therefore be no higher organisms and thus no us. They provided the basis on which the animals could first conquer the oceans and later also the continents. They paved the way for us.
With this fundamental environmental change caused by the cyanobacteria, came another stunning effect. The sky, which until then had appeared green, changed its color to the blue we are so familiar with. This is because ozone effectively scatters those wavelengths of visible sunlight that appear blue to us. However, the maximum of visible sunlight is in the green spectral range. The ozone thus shifted the color of the sky from green to blue. This effect can even be seen today. In spring, when the ozone hole over Antarctica reaches its maximum extent, the blue sky there takes on a slightly greenish hue.
The vivid sunset colors of Shark Bay emphasize the epic importance of these stromatolites for the development of life on earth. These fascinating living rocks take us on a geological time travel back to the origin of life more than 3500 million years ago.
