Scientific American is a popular science magazine. It has a 60-second science blog that explains scientific features for laypeople. Recently, Ted Alvarez reported biogeochemist Graham Logan’s hypothesis of the reason for complex life. Logan speculates that in the beginning bacteria gobbled up all the oxygen.
According to Alvarez, “Plankton produced oxygen slowly, but bacteria would consume most of it in order to digest dead plankton. The dearth of oxygen didn't allow for much multicellular development.” 
Then poop came to the rescue. Creatures that produced feces consumed plankton instead of bacteria. Thus, bacteria populations dwindled as they had less plankton. This turned out to be a blessing since there was more oxygen for all other creatures.
Alvarez points out that Logan’s hypothesis is not based on wild guesses. Logan studied the carbon 12 –carbon 13 isotope ratio in Cambrian rocks, suspecting that animals that ate other animals had more C-13 in them.
Logan thinks that C -13 levels were very high before the Cambrian period because “bacteria were eating large amounts of dead plankton. Once crapping animals arrived, however, C 13 levels dropped since there was less food for bacteria to eat.”
Logan’s idea is reminiscent of the explanations that have been given for the extinction of dinosaurs. The Natural History Museum in London, for instance, gives some more or less tongue-in-cheek hypotheses, all the way from mass suicide to slipped disks. But does science cease being science when scientists have to rely on storytelling?
It seems that since evolution is not supported by facts, scientists have to resort to “just so” stories. No wonder the late creationist pioneer Henry Morris gave his last book the title Some Call it Science.
 Alvarez, Ted. 2007. What gave rise to complex life on earth? Poop! (maybe). Scientific American. 60-second science. http://www.60secondscience.com/archive/biology-news-articles/what-gave-rise-to-complex-life.php?sc=WR_20071204
 Morris, Henry. 2006. Some Call It Science. The Religion of Evolution. Santee, CA: Institute for Creation Research.