The oxygen level was on the rise some 400 million years ago. And this was majorly was due to the bodies of vascular plants (that also appear on the earth's landscape around that time) decompose with difficulty, making it easier for organic carbon to be buried in sediments. When that happens, the organic carbon -- produced by photosynthesis -- is not available for reaction with oxygen. The consequence is a rise in the amount of oxygen in the environment. The vascular plants led to more carbon burial, and therefore to more oxygen. Then, the rise in oxygen made it possible for larger animals to evolve. (for more scientific facts, read on - http://www.sciencedaily.com/releases/2010/10/101008121348.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+%28ScienceDaily%3A+Latest+Science+News%29)
Now the question is,
I presume nobody could have predicted then that such a rise would lead to more complex life forms.....& in turn some positive twists to the entire evolutionary story!
Likewise, the carbon level is on the rise today. And our predictions on its effect on humanity and earth at a universal scale is at best patchy. So, why not assume the rise in carbon level is gonna bring in some good to the earth and the evolution as a whole??
Well, if the question sounds quite naive, pardon me. I'm curious to know.....