How did we get Oxygen in the first place?

[Laudenum]

Origin of life question.

I was speaking to a friend recently who was very excited that we might soon be able to read the atmospheric contents of planets in other solar systems.

Apparently if we find a planet with a very reactive gas, that means that there is life there - otherwise the gas would have reacted with something else and vanished into a compound...(or something alon those llines).

If this is true, how did we get life in the first place, assuming life needs something combustable to survive?

[Colibri]

The first life forms no doubt used anaerobic respiration (http://en.wikipedia.org/wiki/Anaerobic_respiration), that is, respiration in which the final electron receptor is something other than O2, such as sulfate, nitrate, or sulfur. O2 is not the only possible electron receptor. Anaerobic metabolism is still used by many organisms today (including us at times, as in our muscles during heavy exercise).

O2 first became prevalent in the Earth's atmosphere after about 2.4-1.7 billion years ago, after dissolved iron in the oceans became depleted and could no longer absorb most of the O2 produced by photosynthesis. The increase in this toxic gas created the Great Oxygenation Event (http://en.wikipedia.org/wiki/Oxygen_catastrophe), or Oxygenation Catastrophe, that may have killed off many of the anaerobic organisms of the time.

[Squink]

The increase in this toxic gas created the Great Oxygenation Event (http://en.wikipedia.org/wiki/Oxygen_catastrophe), or Oxygenation Catastrophe, that may have killed off many of the anaerobic organisms of the time.The needs of the many outweigh the needs of the few: (http://edmall.gsfc.nasa.gov/aacps/news/Photosynthesis.html)
Global thermal fluxes were greater in the distant geologic past (11, 12), but the onset of oxygenic photosynthesis most probably increased global organic productivity by at least two to three orders of magnitude. This enormous productivity resulted principally from the ability of oxygenic photosynthetic bacteria to capture hydrogen for organic biosynthesis by cleaving water. This virtually unlimited supply ofhydrogen freed life from its sole dependence upon abiotic chemical sources of reducing power, such as hydrothermal sources and weathering. Communities sustained by oxygenic photosynthesis could thrive wherever supplies of sunlight, moisture, and nutrients were sufficient.

[dracoi]

If only the other organisms had introduced cap and trade on the cyanobacteria early enough, the Earth could have been spared from complex life forms. :)

More seriously... one of the reasons people are looking at deep-sea geothermal vents is that they're relatively stable sources of sulfur, which can (and still is) used by anaerobic bacteria for energy. They're pretty good candidates for the conditions scientists expect to find the origins of life. If that's the case, photosynthesis wouldn't have been possible until life started migrating away from the vents and saw light for the first time.

[Lemur866]

And it took between 1 and 2 billion years between when life first arose, and when the first photosynthetic bacteria evolved. So for more than a billion years, all life on earth was anaerobic and chemosynthetic.

[Uncertain]

The first life forms no doubt used anaerobic respiration (http://en.wikipedia.org/wiki/Anaerobic_respiration), that is, respiration in which the final electron receptor is something other than O2, such as sulfate, nitrate, or sulfur. O2 is not the only possible electron receptor. Anaerobic metabolism is still used by many organisms today (including us at times, as in our muscles during heavy exercise).

Note that fermentation and anaerobic respiration are not the same thing. Production of lactic acid by muscles is not anaerobic respiration. I don't think it's clear that the first life forms used anaerobic respiration rather than fermentation.

[Colibri]

Note that fermentation and anaerobic respiration are not the same thing. Production of lactic acid by muscles is not anaerobic respiration.

I didn't say it was the same. I said anaerobic metabolism in the second case, not respiration. Both processes are similar in being anaerobic.

I don't think it's clear that the first life forms used anaerobic respiration rather than fermentation.

Why would it be more likely that they would use organic electron receptors than inorganic ones?