Many people no doubt take for granted that we enjoy a 21% oxygen level at Earth’s surface. Variable rates in Earth’s history have coincided with ocean animal introductions and extinctions that ultimately provided the oxygen humans would need for advanced civilization.
Oxygen Content in Earth’s History
The atmospheric oxygen content at Earth’s surface is 20.946% by volume and 23.14% by mass. It has not always been that high. When life originated 3.8 billion years ago (bya), the atmospheric oxygen level was less than 0.001%. Not until 800 million years ago (mya) did oxygen in the atmosphere rise above 0.2%.1 (The only exception is the epoch 2.45–2.2 bya, when the atmospheric oxygen level rose to 1.0–2.5%.)
Geochemists studying atmospheric oxygen levels from 800–50 mya have found that direct chemical isotope proxies (substitutes) are scarce and reconciling indirect proxy records has also proven challenging.2 Nevertheless, geochemists have determined, with some certainty, when atmospheric oxygen levels surged and by how much. According to a 2018 study, levels rose:
To about 8%, 575 mya (just before the Avalon explosion);
To 10%, 540 mya (just before the Cambrian explosion);
To greater than 14%, 423–419 mya (during the Late Silurian period); and
To greater than 16%, 383–359 mya (during the Late Devonian period).3
The geochemists also determined that there were likely more than a few rapid brief leaps, up and down, in atmospheric oxygen levels from 800–50 mya.
Marine Animal Extinction
The fossil record reveals a decline in extinction rates for marine animals throughout the past 540 million years. The reason for this decline has mystified both paleontologists and evolutionary biologists.
Now, a team of five geologists and biogeologists at universities in California and France have suggested a cause for extinction decline.4 The group, led by Richard Stockey, hypothesizes that the decline in marine animal extinction rate is driven largely by the increase in the oxygenation of the atmosphere and oceans that occurred during the early part of the Phanerozoic eon (541 million years ago until the present). In support of their hypothesis, they pointed out that the onset of generally lower marine animal extinction rates coincides with both Earth’s atmosphere and oceans becoming oxygenated to near-modern levels about 380 million years ago. They reasoned that limited surface oxygenation would increase the volume of anoxic ocean water and make marine animals more vulnerable to extinction. They then launched a project to test their hypothesis against competing explanations for the decline in extinction rates.
The team’s test was made possible thanks to recent advances in Earth system modeling. They used the cGENIE model5 “to generate a suite of three-dimensional realizations of potential marine environmental conditions.”6 They included in these models a range of atmospheric oxygen and carbon dioxide concentrations.
Stockey and his colleagues discovered in their models that as one moves from the equator to the poles, temperature gradients decrease and the oxygen concentration increases relative to the ocean depth. When the team combined the cGENIE models of ocean biogeochemistry with the known physiology of marine animals, they were able to establish unequivocally that the atmospheric oxygen level is the primary factor determining the vulnerability of marine animals to extinction.
The researchers also identified several secondary factors affecting marine animal extinction:
The sizes, configuration, and distribution of the continents
The efficiency of the biological carbon pump in the oceans (sequestration of carbon from the atmosphere by plankton, which die and sink to the ocean bottom where their organic matter becomes trapped in sediments), which plays a role in the distribution of oxygen in the upper water column7
The initial climate state
Sizes of continental shelves
Stockey’s group showed that even when these five factors are combined they still play a lesser role in governing marine animal extinction than the atmospheric oxygen level.
As I explain in my book Improbable Planet, it took several billion years of Earth being packed with as much photosynthetic life as is physically and chemically possible for the atmospheric oxygen level to rise to where it is today. For much of Earth’s history, minerals in the crust and mantle soaked up oxygen as fast as photosynthetic life produced it. It took over 3 billion years for these oxygen “sinks” to fill up.
The repeated introductions of just-right life-forms at just-right times and in just-right amounts and diversity explains how Earth’s atmosphere and oceans attained the precise oxygen levels required for global human civilization. These introductions occurred at the exact time when the Sun entered into its narrow time window when its flaring activity and its luminosity stability were both at ideal levels. Anything less than a 21% atmospheric oxygen level would deliver a lower agricultural output and lower work output from humans and their domesticated animals. Any higher than a 21% atmospheric oxygen level would result in more grass and forest fires and shorter human lifespans.
Stockey’s team established another design feature in Earth’s oxygenation history that benefits humanity. Presently, the atmospheric oxygen level is optimal to maximize marine animal biomass and biodiversity. It is also optimal for minimizing marine animal extinctions. The bottom line is that only a Mind that knows and understands the future physics of the Sun and Earth and the future needs of humans that he intends to create would also know which geological events to bring about and which life-forms to introduce at the required times and places for humans to exist and thrive.
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Rosalie Tostevin and Benjamin J. W. Mills, “Reconciling Proxy Records and Models of Earth’s Oxygenation during the Neoproterozoic and Paleozoic,” Interface Focus 10, no. 4 (August 6, 2020): id. 20190137, doi:10.1098/rsfs.2019.0137.
Tostevin and Mills, “Reconciling Proxy Records,” id. 20190137; Alexander J. Krause et al., “Stepwise Oxygenation of the Paleozoic Atmosphere,” Nature Communications 9 (October 4, 2018): id. 4081, doi:10.1038/s41467-018-06383-y.
Tostevin and Mills, “Reconciling Proxy Records,” id. 20190137.
Richard G. Stockey et al., “Decreasing Phanerozoic Extinction Intensity as a Consequence of Earth Surface Oxygenation and Metazoan Ecophysiology,” Proceedings of the National Academy of Sciences USA 118, no. 41 (October 12, 2021): id. e2101900118, doi:10.1073/pnas.2101900118.
A. Ridgwell et al., “Marine Geochemical Data Assimilation in and Efficient Earth System Model of Global Biogeochemical Cycling,” Biogeosciences 4, no. 1 (January 25, 2007): 87–104, doi:10.5194/bg-4-87-2007.
Stockey et al., “Decreasing Phanerozoic Extinction,” id. e2101900118.
K. M. Meyer, A. Ridgwell, and J. L. Payne, “The Influence of the Biological Pump on Ocean Chemistry: Implications for Long-Term Trends in Marine Redox Chemistry, the Global Carbon Cycle, and Marine Animal Ecosystems,” Geobiology 14, no. 3 (May 2016): 207–209, doi:10.1111/gbi.12176.