volumes than today and this carbon dioxide–rich atmosphere would have created hothouse-like conditions through a super greenhouse effect, with carbon dioxide partial pressures (measured as the actual amount of total gas pressure exerted by the atmosphere) 10,000 times higher than today.

There is abundant evidence that the present-day atmosphere is very different from that of the past. The most compelling lines are geologic. Today, the atmosphere contains so much oxygen that reduced metal species quickly oxidize: the familiar rusting of iron to a red color or the oxidation of copper to shades of green is evidence of this. In similar fashion, many metal-rich or organic-rich types of sediment quickly bind with atmospheric oxygen to produce oxidized minerals. Long ago in Earth’s history, however, minerals formed that are no longer seen on the planet’s surface. Before about 2.5 billion years ago the formation of “red beds,” sedimentary beds rich in oxidized iron minerals such as hematite did not form. Instead, there was formation of “banded iron formations,” composed of only partly oxidized iron species. Other rock types from this ancient time include uranium oxides and iron pyrites that cannot form in today’s atmosphere. This evidence strongly suggests that prior to 2.2 billion years ago there was no free oxygen in the atmosphere and little oxygen dissolved in seawater.

Even though there must have been, at most, only a few percent of oxygen in the gases making up Earth’s atmosphere as late as 2.2 billion years ago, soon after that the amount of oxygen began to climb rapidly. Where did all the oxygen come from? Some oxygen can be generated by photochemical reactions, where water high in the atmosphere is broken by sunlight into hydrogen and oxygen, but this process could account for only a small percentage of the oxygen rise. The most likely explanation is that most came from photosynthesis by single-celled bacteria. Life is known to have evolved on Earth by about 3.5 billion years ago, perhaps hundreds of millions of years earlier than that. Certainly, by 3.5 billion years ago, life had evolved to the point where cyanobacteria (informally and improperly known as blue-green algae) were widespread in the oceans.

The cyanobacteria were the first organisms to use carbon dioxide to produce free oxygen. They still exist and use carbon dioxide as a

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