Molybdenum and evolution

We tend to think of our current oxygen-rich atmosphere (21 kPa) as the norm, but for most of this planet's history, the atmospheric composition has been quite different. In fact, oxygen is a relative newcomer to our atmosphere. Until recently, it was thought that, as a result of photosynthesis, the level of oxygen gradually increased until it reached its present level. Now we realise that the situation was much more complex. We can look at the current state of knowledge.