Everyone knows that humans die without adequate oxygen and many know that oxygen comprises about 20% of Earth’s atmosphere. As a kid, I always wondered why Earth’s atmosphere contained only that small fraction, if oxygen was so important to our survival. After taking a few chemistry classes, I realized how violently oxygen reacts and that too much atmospheric oxygen would be bad for Earth’s habitability.
Until the Cambrian explosion, living organisms (particularly photosynthesizing cyanobacteria) generated virtually all the oxygen in Earth’s atmosphere. However, while evidence points to abundant cyanobacteria activity at least 2.7 billion years ago (and maybe back to 3.5 billion years), Earth’s atmosphere was essentially devoid of oxygen until 2.4-2.5 billion years ago (although geological evidence indicates sporadic episodes of atmospheric oxygen somewhat earlier). Scientists have worked to understand what might have caused this lag between the advent of abundant oxygen-producing organisms and a sustained presence of atmospheric oxygen.
A recent article in Nature highlights one potential solution. Anyone who has tried to fill a bucket with water when the bucket has a hole understands the basic idea. Given a large enough hole, all the water put into the bucket drains before any can accumulate. However, reduce the size of the hole and the bucket now starts to fill up.
The authors propose that a spurt of continental crust formation filled the “hole” draining all the oxygen from Earth’s atmosphere. As the amount of continental crust increased the number of undersea volcanoes decreased, as more volcanoes formed on land. Land-based volcanoes emit different gases than submarine volcanoes. In particular, the gases are less reactive with oxygen, meaning that a non-negligible amount of the cyanobacteria-produced oxygen makes it into the atmosphere. Once the atmospheric oxygen crosses a threshold, ozone forms, which shields the oxygen from destruction by the sun’s ultraviolet radiation. This ozone shield essentially ensures a stable abundance of oxygen in Earth’s atmosphere.
Additionally, shortly after the establishment of a stable abundance of oxygen in the atmosphere, more-complex organisms that rely on oxygen are introduced into Earth’s biosphere. RTB’s creation model understands the timing of such events as evidence of a supernatural Creator’s preparation of Earth for humanity.