Hmmm, another possibility could have been ...
Global warming.
Low oxygen retards evolution?
Al Franken explained.
You mean complex life as we know it requires oxygen?!?
The things you learn from science...
As someone else wisely stated on a thread last month, "may" can also mean "may not". It's a study that didn't reach a conclusion, only a guess.
“If Planavsky’s estimation of these oxygen levels are correct, it could mean that simple organisms were ready to evolve into complexity for ages before they actually got the chance.”
So in other words, their DNA contained complexities before they evolved. Like fish contained feet DNA years before they grew feet.
It would not make any sense to me how anything could evolve traits in their DNA that would not benefit them immediately.
It has long been suspected that the real culprit in keeping Earth’s atmosphere low oxygen was iron.
http://paleobiology.si.edu/geotime/main/htmlversion/proterozoic4.html
“In the absence of free oxygen, iron dissolves in water. This resulted in ocean waters that contained a great deal of dissolved iron. The dissolved iron bonded with oxygen released into ocean water by photosynthesizing cyanobacteria to form magnetite (Fe3O4). This magnetite was then deposited on the ocean floor.
“A vast amount of iron dissolved in the oceans was available to react chemically with oxygen, which kept oxygen from accumulating in the ocean and atmosphere. Once all of the dissolved iron was used up, the oxygen released by photosynthetic organisms could escape directly into the atmosphere. As gaseous oxygen built up, the atmosphere began to change from one that was chemically reducing to one that was oxidizing (i.e., rust-forming), like today’s.
“(This is seen today in) Banded iron formations in rock, which are silica-rich and iron-rich sedimentary layers in what was sea floor.”
Ya, that's science.