The oceans that formed from condensing water vapor (or incoming comets) were reservoirs of dissolved iron, pumped through hydrothermal vents on the ocean floor.
Then about 2.7 billion years ago, cyanobacteria, which have been called the most self-sufficient organisms on the planet because they can both photosynthesize and fix nitrogen, began bubbling oxygen into the atmosphere and shallow waters.
At first oxygen built up gradually in the atmosphere, but about 2.5 billion years ago there was a sudden spike upward, traditionally called the Great Oxygenation Event.
The oxygen killed off anerobes that didn't find refuge in sediments, the deep ocean and other airless environments and led to the evolution of aerobes that could use oxygen to spark their metabolism.
At roughly the same time iron began to precipitate out of the oceans, forming rocks peculiar to this period called banded iron formations that consist of alternating layers of gray and red rock.
Banded iron formations were created episodically from about 3 billion years ago until 1.8 billion years ago and almost never again.
The usual story is that iron was being swept from the oceans by increasing levels of dissolved oxygen.
And then, another two billion years after the Great Oxygenation Event, multicellular lifeforms finally put in an appearance. The first metazoans, as they are called, were the bizarre Edicaran fauna, sometimes unflatteringly compared to sacks of mud and quilted mattresses.
The assumption was oxygen levels were now high enough to support something more than a single cell in lonely solitude.
Of course, this story has holes you could drive a truck through.
Why did oxygen levels spike 2.5 billion years ago, and how much oxygen was there in the atmosphere really? Why are banded iron formations made of layers only a few centimeters thick, and why did they stop forming so
|Contact: Diana Lutz|
Washington University in St. Louis