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They were the oldest organisms called Stromatolites which did not need oxygen themselves but produced it by photosynthesis.

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Volcanoes

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Q: Which of these was the first source of oxygen in our atmosphere?
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Related questions

What is inferred to be the main source of the free oxygen that first entered earth's atmosphere?

oxygen producing organisms


Which atmosphere gas is photosynthesis the major source of?

oxygen


What is the major source of oxygen for the earths atmosphere is?

Plants


The primary source of oxygen for the earth's atmosphere?

Plants


What is the source of the oxygen that plants give off as oxygen gas?

The oxygen in water is the oxygen that is released into the atmosphere.


What is the major source of free oxygen in atmosphere?

Green Plants


What is the source the ozone layer?

Oxygen in the atmosphere, and UV-C from the Sun.


What is hypothesized to be the early source of free oxygen in Earth's atmosphere?

cyanobacteria


Hypothesized to be the early source of free oxygen in Earth's atmosphere?

photosynthesis by cyanobacteria


What source removes carbon dioxide from the atmosphere?

Plants and trees, they convert it to oxygen


What is the source of much of the oxygen the atmosphere?

plant-like plankton in the ocean put out most of the oxygen in the atmoshere as a waste-product of photosynthesis.


What source of much of the oxygen in the atmosphere?

plant-like plankton in the ocean put out most of the oxygen in the atmoshere as a waste-product of photosynthesis.