What biology «fact» did you recently find out to be fake?

I've been getting into biology lately, and one thing that completely changed how I see the world. I always assumed that rainforests, trees and plants are the main source of oxygen... Turns out that's only half the story. There's a single-celled organism called Prochlorococcus that produces roughly half of all the oxygen in Earth's atmosphere. It's so small you could fit a million of them on a pinhead. What really got me is that we didn't even know it existed until 1988. This tiny organism responsible for every second breath we take, crazy! I’m sure there are even more fake «facts» that aren’t actually true, but a lot of people still believe them.
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