Hadean Period (4600-4000 mya), Molten Earth
During the formation of the Solar System the collisions between planetesimals and protoplanets would have released a lot of heat and the implication is that at its beginning Earth was probably molten. As Earth cooled, the heavier molten iron sank to the centre and the lighter rock rose to the surface.
Sometime during the first 800 million years of Earth’s history its surface solidified. Gases trapped in the rocks when Earth was molten were released to form an atmosphere. It is thought that these gases would have been similar to those released during the volcanic emissions of today, i.e. water vapour, carbon dioxide, nitrogen and several trace gases; crucially, it is unlikely that free (uncombined) oxygen would have been present.
As the planet continued to cool, vast amounts of water vapour formed clouds and then great rains began to fall. At first the water either evaporated in the hot air or quickly boiled away on contact with Earth’s hot surface. When the surface had cooled below water’s boiling point, torrential rains filled the low areas and formed the oceans.
Archaean Period (4000-2500 mya), Stromatolites
Evidence for the beginning of life has been found in the 3500 million-year-old Warrawoona rocks in northwest Australia, which contain the most common of Precambrian fossils, stromatolites. Stromatolites are laminated structures built mainly by cyanobacteria (blue-green bacteria). Stromatolites are still found today in environments such as saline lakes or hot springs that other organisms that would eat them cannot tolerate. In a world with little or no oxygen, bacteria were probably the first living things. Like most bacteria, cyanobacteria secrete slime.
At Shark Bay in Western Australia the salinity is high and cyanobacteria thrive. When the tide comes in, some of the sediment thrown up by the turbulent water sticks to the slime and covers the bacteria. The bacteria grow through the sediment towards the light and form a new layer of living matter and so trap the layer of sediment. The cycle repeats itself until the mound reaches the height of the maximum spring tide. Cyanobacteria break down water by photosynthesis, a process in which light splits water and produces oxygen. In the beginning oxygen would have been toxic to cyanobacteria but they eventually evolved a new process, biological oxidation, or respiration in which organisms take oxygen from the surroundings and give out carbon dioxide. Photosynthesising takes place in the light and respiration in the dark.
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