[omarchowdhury]

Wouldn't oxygen producing organisms such as plants and trees be a cause for oxygen levels taking off?

[adrianratnapala]

I thought even today phytoplankton produced more oxygen than all the multicellular plants put together.

I might just be imagining that factoid though.

[HCIdivision17]

That was my understanding as well. The idea being that there is a lot of volume in the first few meters of ocean surface area churn. It's not hard to imagine that does better in surface area than (even a lot) of plants.

[ethbro]

Not forgetting the fact that it seems like it would be substantially easier for a faintly mobile fluid filled bag(s) of DNA to survive and reproduce in an aqueous environment!

[dredmorbius]

You're getting ahead of yourself a bit. By a billion or two years.

The lifeforms that oxygenated the atmosphere were rather more primitive than trees. Generally cyanobacteria. Technically, yes, plants.

[ori_b]

Most oxygen (even today) is produced by single-celled algae in the ocean.

[Udo]

They're certainly sufficient but not necessary. There's no reason why single celled organisms couldn't do that. Maybe not at 20% but close.