09-22-2015, 11:10 AM
If the planet got photosynthetic eukaryotes (or the equivalent) from somewhere else that would speed up the timeline considerably. However, it appears that it took quite a long time to oxygenate the Earth's atmosphere. There were huge volumes of dissolved minerals in the water and a large volume of exposed minerals on the surface that needed to be oxidized before any significant amount remained to add to atmospheric gases. This is one of the challenges of terraforming, too.
Stephen