I heard this story from one of my friends:

Cyanobacteria can convert carbon dioxide into oxygen through photosynthesis, so the concentration of oxygen in the atmosphere kept going up over a long period after their appearance. Finally, those oxygen poisoned cyanobacteria through some mechanism, which effectively killed many of them. As a side effect, the oxygen produced lead to the evolution of fishes and other fantastic creatures, including human beings.

I wonder whether it is true. After some searching, I found that cyanobacteria can perform photosynthesis. However, I can't tell whether oxygen is poisonous to them.