The oxygen catastrophe 2.7 billion years ago led to the introduction of large quantities of oxygenated iron, creating the “waste product” of elemental oxygen. Over the next billion years, oxyphotosynthesizing organisms flourished, producing more and more elemental oxygen, resulting in today’s breathable air. The search for extraterrestrial planets with Earth-like air continues.
The origin of air as we know it begins with the oxygen catastrophe, also known as the great oxidation, which occurred about 2.7 billion years ago. Prior to this, the oxygen level in the air was about 1/50th of a percent. This is similar to the oxygen level in Mars’ atmosphere, about 1/5th of a percent. Like Mars today, the early Earth’s atmosphere was mostly carbon dioxide. Today, the atmosphere contains 20% oxygen and only 0.038% carbon dioxide, making the air completely breathable for oxygen-dependent organisms like us.
With the advent of oxyphotosynthesis in microorganisms, this carbon dioxide was progressively consumed, creating the “waste product” of elemental oxygen. The oxygen catastrophe is clearly demarcated in the geological record by the introduction of large quantities of oxygenated iron (rust). These relics are called banded iron formations. The event is called a “catastrophe” because the oxygen is toxic to anaerobic organisms, which the event wiped out in large numbers. There was a delay of about 300 million years before the evolution of the first oxygen-producing organisms and the real oxygen catastrophe.
Over the next billion years, oxyphotosynthesizing organisms flourished, producing more and more elemental oxygen. The history of air, from nearly zero oxygen to 20% oxygen, spans more than two billion years. During the Carboniferous period about 250 million years ago, when plants were thriving, oxygen levels were even higher than today. This allowed very large insects to exist, including a dragonfly, Meganeura, with a two-foot wingspan. Today’s air would be unbreathable for Meganeura, due to its relative lack of oxygen.
The search is ongoing for extraterrestrial planets with Earth-like air, with no luck so far. By closely examining the spectrum of a planetary body, astronomers can determine its chemical makeup, even if that body is extremely distant. This is the same technique used to determine the chemical composition of distant stars.
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