<P> The increased production of oxygen set Earth's original atmosphere off - balance . Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have destroyed most such organisms at the time . </P> <P> A spike in chromium contained in ancient rock - deposits formed underwater shows the accumulation had been washed off from the continental shelves . Chromium is not easily dissolved and its release from rocks would have required the presence of a powerful acid . One such acid, sulfuric acid (H SO), might have formed through bacterial reactions with pyrite . Mats of oxygen - producing cyanobacteria can produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice; before oxygen started accumulating in the atmosphere, these organisms would already have adapted to oxygen . Additionally, the free oxygen would have reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest episode of glaciation in Earth's history and called "snowball Earth". </P> <P> Eventually, the evolution of aerobic organisms that consumed oxygen established an equilibrium in its availability . Free oxygen has been an important constituent of the atmosphere ever since . </P> <P> The most widely accepted chronology of the Great Oxygenation Event suggests that free oxygen was first produced by prokaryotic and then later eukaryotic organisms that carried out photosynthesis more efficiently, producing oxygen as a waste product . The first oxygen - producing organisms arose long before the GOE, perhaps as early as 3,400 million years ago . </P>

When did the level of oxygen in earths atmosphere become high enough to sustain aerobic respiration