Where Did the Molecular Oxygen in the Earth's Atmosphere Originate?
Today, oxygen is the second most common gas in the atmosphere by volume, with argon a distant third. However, oxygen in the early Earth's atmosphere was bound up in water. What changed? As oxygen-breathers, we have an interest in how our planet became habitable to us. The answer is simple: the invention of photosynthesis.
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Early Atmosphere
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The early atmosphere of the Earth was probably made up of hydrogen and methane, with a little carbon dioxide and water vapor expelled by volcanoes. Volcanoes also expelled ammonia, which sunlight broke down into nitrogen, which over billions of years accumulated to today's levels. The earliest life likely used the methane for metabolism.
How We Know This
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There is a great deal of evidence that oxygen levels were low more than 2.5 billion years ago. Evidence from the geological record includes, among other evidence, the lack of oxidation of iron and the occurrence of minerals that don't form in oxygen-rich environments (pyrite, uraninite).
Biological evidence includes the observation that oxygen prevents the growth of the most primitive life forms in existence today. Also, the chemical reactions that produce amino acids, the building blocks of life, are inhibited by even small amounts of oxygen.
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Cyanobacteria
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A photosynthesizing bacterium arose no later than 2.7 billion years ago that expelled oxygen as part of its metabolism. This organism is called cyanobacteria, and still exists today. (Bacteria appeared about 3.5 billion years ago.) Their photosynthetic ability means they are able to split water molecules to produce oxygen using the sun's energy. These organisms were the driving force in filling the atmosphere with oxygen.
Chloroplasts
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Approximately 1.6 to 1.7 billion years ago, some cyanobacteria were absorbed by eukaryotes (cells with nuclei) to benefit from their ability to harness the sun's energy. These are the ancestors of the organelles in green plant cells that we call chloroplasts. Their bacterial origin is the reason that chloroplasts have their own genetic code.
Algae are the first organisms believed to have used chloroplasts. They therefore were huge contributors to the oxygenation of the early atmosphere as well, though what scientists call the Great Oxidation Event had already occurred several hundred million years before.
A Paradox
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Although cyanobacteria arose 2.7 billion years ago, atmospheric oxygen levels didn't begin to rise until 2.5 billion years ago. Why the lag? A 2007 study in "Nature" indicates that submarine volcanism removed the oxygen, acting as a sink. By 2.5 billion years ago, though, continental land masses arose, providing the needed stability for terrestrial volcanoes to appear. Soon the submarine-to-terrestrial difference switched. Previous research had found that submarine volcanoes erupt at lower temperatures, producing gasses that were much better at locking up any atmospheric oxygen than terrestrial volcanoes.
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