For example , at today 's rates of photosynthesis ( which are much greater than those in the land-plant-free Precambrian ) , modern atmospheric O2 levels could be produced in around 2,000 years .
At today 's rates of photosynthesis ( which are much greater than those in the land-plant-free Precambrian ) , modern atmospheric O2 levels could be produced in around 2,000 years .