Cambrian Explosion or Cambrian Conclusion?

I realized before I could post this, I had to make a few other posts.

The Cambrian Explosion was forty million years where multi cellular life seamed to explode out of nothingness to create almost every major body plan that exists today.  Seemed to explode out of nothing, but we know that is did not.  There are single celled fossils from almost three billion years ago, so it was from these cells that multicellular life evolved from, but that leads into the next question, why did it take so long?  As previously mentioned in an earlier post, multicellular life requires a higher metabolism and higher metabolisms require oxygen to fuel the fires of it. 

So the real question is why did it take so long for oxygen to become abundant on Earth?  That was also answered previously.  Oxygen creating organisms were probably developed fairly early in the Earth's history, but because oxygen is a very reactive element, every last atom of oxygen was used to oxidize all the available reactants, like iron and copper et al., so there was no accumulation of oxygen for many million years.  

When oxygen became abundant the first great extinction occurred, most of the anaerobic life died as it could not survive in an oxygen environment.  The ones that did survive lived where there was no oxygen like deep in the oceans or under ground.  People do not precisely know where all the life came from that entered into the Cambrian Explosion, but I propose a few thoughts on the matter.  First while most of the previous life in the oceans took energy from their surrounding chemical reactions, some organisms took there energy from the creatures that made the energy, i.e. they preyed upon them.  The first critters to do this would not be very abundant because they required the energy of the Chemotropes, critters who derive their energy from chemical reactions, as opposed to heliotropes like plants.

But as heliotropes became more numerous, I would assume that some would evolve to consume the heliotropes.  As oxygen started to become available some of these animals would likely evolve to use the oxygen.  Using oxygen means that the animals would be able to have a different metabolism, a more active environment; the difference between a fire, a quick chemical reaction and using the heat of steam, from a fire, to fuel a reaction.

What this would mean is that when the oxygen level hit a critical level, probably near 13%, but it might have been the Carbon Dioxide level that dropped done to less than 1%.  The Oxygen level probably hit a high of 30%, with Nitrogen and other trace gases making the other 70%.  

Thirty percent seems high today, but there has been a lot of oxygen sequestration since then in the form of coal and other fossil fuels.  Fossil fuels also represent Carbon stores, but they were formed through life and oxygen metabolism.  The second smaller limb of the carbon cycle.

My point is that when oxygen using life became the dominant form of life protoanimal life and protoplant life was already to expand into the world and was already expanding.  How many million years the evolution was moving to a take over of the biosphere was not important.  Scientists can't pin down to the exact time that oxygen became dominant but that it did, obviously, over 500 million years ago, and that within 40million years the were suddenly, insects, fish with back bones, sea creatures without backbones and plants.  And of coarse it did not stop there.