[AndrewC]

and may have increased the earth's oxygen levels to a degree that made the development of multicellular life possible...

And here all this time I've been told that photosynthesis was the cause of the oxygen.

[FDNYRHEROES]

And here all this time I've been told that photosynthesis was the cause of the oxygen.

Don't believe 20.8% of everything you hear...

[SunkenCiv]

And here all this time I've been told that photosynthesis was the cause of the oxygen.

On a cellular basis, plants are more complicated than animals, so one possibility that has presented itself (from the materialist/reductionist perspective) is that unicellular animals appeared first, and then plants came along sometime later. That was difficult for the just-so school, who figured that plants woudl have to arise first. Extremophile bacteria of some sort are the leading candidates for the first Earthly life, but having a nice healthy catastrophe come along means having something to saddle on right away, y'know, for people who think that everything must have some connection with gradualism. :')