What factor caused the appearance of oxygen? What is one form of evidence used to support this theory?

The appearance of free oxygen in the Earth's atmosphere led to the Great Oxidation Event. This was triggered by cyanobacteria producing oxygen which developed into multicellular forms as early as 2.3 billion years ago

Explanation:

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How did oxygen first appear?

Whiffs in the air is how oxygen first appeared billions of years ago.

For the most part, scientists agree that oxygen, though lacking in the atmosphere, was likely brewing in the oceans as a byproduct of cyanobacterial photosynthesis as early as 3 billion years ago

When was oxygen found?

2.33 billion years ago

Since its first appearance 2.33 billion years ago, oxygen accumulated in high enough concentrations to have a weathering effect on rocks just 10 million years later.

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fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.