<P> Before photosynthesis evolved , Earth 's atmosphere had no free oxygen ( O ) . Photosynthetic prokaryotic organisms that produced O as a waste product lived long before the first build - up of free oxygen in the atmosphere , perhaps as early as 3.5 billion years ago . The oxygen they produced would have been rapidly removed from the atmosphere by weathering of reducing minerals , most notably iron . This `` mass rusting '' led to the deposition of iron oxide on the ocean floor , forming banded iron formations . Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event . This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen . At current rates of primary production , today 's concentration of oxygen could be produced by photosynthetic organisms in 2,000 years . In the absence of plants , the rate of oxygen production by photosynthesis was slower in the Precambrian , and the concentrations of O attained were less than 10 % of today 's and probably fluctuated greatly ; oxygen may even have disappeared from the atmosphere again around 1.9 billion years ago . These fluctuations in oxygen concentration had little direct effect on life , with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period , 541 million years ago . The presence of O 2 provided life with new opportunities . Aerobic metabolism is more efficient than anaerobic pathways , and the presence of oxygen undoubtedly created new possibilities for life to explore . Since the start of the Cambrian period , atmospheric oxygen concentrations have fluctuated between 15 % and 35 % of atmospheric volume . The maximum of 35 % was reached towards the end of the Carboniferous period ( about 300 million years ago ) , a peak which may have contributed to the large size of insects and amphibians at that time . Whilst human activities , such as the burning of fossil fuels , affect relative carbon dioxide concentrations , their effect on the much larger concentration of oxygen is less significant . </P>
<P> Before photosynthesis evolved , Earth 's atmosphere had no free oxygen ( O ) . Photosynthetic prokaryotic organisms that produced O as a waste product lived long before the first build - up of free oxygen in the atmosphere , perhaps as early as 3.5 billion years ago . The oxygen they produced would have been rapidly removed from the atmosphere by weathering of reducing minerals , most notably iron . This `` mass rusting '' led to the deposition of iron oxide on the ocean floor , forming banded iron formations . Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event . This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen . At current rates of primary production , today 's concentration of oxygen could be produced by photosynthetic organisms in 2,000 years . In the absence of plants , the rate of oxygen production by photosynthesis was slower in the Precambrian , and the concentrations of O attained were less than 10 % of today 's and probably fluctuated greatly ; oxygen may even have disappeared from the atmosphere again around 1.9 billion years ago . These fluctuations in oxygen concentration had little direct effect on life , with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period , 541 million years ago . The presence of O 2 provided life with new opportunities . Aerobic metabolism is more efficient than anaerobic pathways , and the presence of oxygen undoubtedly created new possibilities for life to explore . Since the start of the Cambrian period , atmospheric oxygen concentrations have fluctuated between 15 % and 35 % of atmospheric volume . The maximum of 35 % was reached towards the end of the Carboniferous period ( about 300 million years ago ) , a peak which may have contributed to the large size of insects and amphibians at that time . Whilst human activities , such as the burning of fossil fuels , affect relative carbon dioxide concentrations , their effect on the much larger concentration of oxygen is less significant . </P>
where does most of earths oxygen come from today
where does most of earths oxygen come from today
photosynthetic organisms
photosynthetic organisms