Nice to measure it but wasn't there already proof of larger amounts of oxygen in the past and the earliest life (one of from what we know) was a bacteria that produced the first oxygen in abundance and explains the higher levels and is indeed thought why, or least factor in why early mammals were larger and during the dinosaur days (sorry but easier to call it that for many) the oxygen levels were higher than today by far.
Well this is talking about 200,000 years before the explosive emergence of life on Earth. Before this discovery it was thought that there was much less oxygen at that time (the Neoproterozoic era according to the article). And that life had to wait until more oxygen came along before it could happen. Now they are basically saying the oldest fossils we have found are probably not the oldest we could find. Time to look for older signs of life!
There were lots of anaerobic bacteria, however. And photosynthetic ones produced the oxygen. What oxygen enabled was evolution of complex multicellular life.
Nice article to matchup with this http://www.scientificamerican.com/article/timeline-of-photos...