From ScienceDaily:
By examining rocks at the bottom of ancient oceans, an international group of researchers has revealed that arsenic concentrations in the oceans have varied greatly over time. But also that in the very early oceans, arsenic co-varied with the rise of atmospheric oxygen and coincided with the coming and going of global glaciations, researchers say.
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The authors infer — from the way modern photosynthetic organisms react to changing marine arsenic concentrations — that this event was due to widespread ocean toxicity resulting from the release of toxic elements into the oceans when the ice melted.
A similar low and high arsenic content accompanied the coming and going of global glaciations at around 0.7 billion years ago, which is when Earth first saw the appearance of complex life. While the low marine arsenic concentrations again coincide with a rapid rise in atmospheric oxygen content to near modern day levels at this time, the subsequent increase when the ice melted is not accompanied by atmospheric oxygen decline.More.
See also: Darwin’s Doubt
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