Microbial life may have migrated from Earth\'s oceans to land by 2.75 billion years ago, producing oxygen earlier than previously thought, U.S. scientists say. Research reported from the University of Washington suggests the land-based microbes were producing oxygen well in advance of what geologists refer to as the \"Great Oxidation Event\" about 2.4 billion years ago that initiated the oxygen-rich atmosphere and the creation of life as we know it. In addition to producing oxygen, the microbes were weathering pyrite, an iron sulfide mineral, which released sulfur into the oceans, researches said. In turn, that influx of sulfur probably enhanced the spread of life in the oceans, Eva Stueken, a doctoral student in Earth and space sciences, said. The added sulfur might also have allowed marine microbes to consume methane, setting the stage for atmospheric oxygenation. Before that, large amounts of oxygen would have been destroyed by reacting with methane that rose from the ocean into the air, the researchers said. \"It supports the theory that oxygen was being produced for several hundred million years before the Great Oxidation Event. It just took time for it to reach higher concentrations in the atmosphere,\" Stueken said.