[dead]

Yes, I see that the sentence could mean it only happened once as far as earth is concerned. I took it to mean it only occurred once anywhere and happened to be on our planet.

What mechanism would reveal it to be a singular event in earth history? Seems like a speculative call.

[Moonman62]

Many years of study indicate that all photosynthetic organisms are related to each other, so that they all have a common ancestor, and they are all on a single branch of the tree of life.

Then there are articles like this that indicate geologic evidence, too.

http://www.dailygalaxy.com/my_weblog/2013/07/great-oxidation-event-a-new-discovery-of-evolutionary-singularity-that-transformed-the-planet.html