Probably nothing is taken more for granted than the air we breathe. At sea level our atmosphere has an air pressure of 15 pounds per square inch. It contains 21% oxygen by volume. This air pressure and this amount of oxygen is optimal for humans and for human civilization.

If Earth’s air pressure were three times greater or three times lesser, our lungs would cease to function and we would die. While other respiration systems exist in nature and still more conceivably could exist, none, at least for large animals, yield the respiratory efficiency of lungs. The air pressure at sea level is optimal as anyone who has been to the Dead Sea or climbed a high mountain can testify.

How Did We Get Our Optimal Air Pressure?
Earth’s air pressure is highly anomalous. A planet’s primordial atmosphere is determined by its surface gravity, its distance from its host star, and its host star’s effective temperature. These factors establish that Earth shortly after its formation possessed an atmosphere about a hundred times thicker than it has now.

As Earth aged, its atmosphere was augmented by the radiometric decay of potassium into argon. Presently, about 1% of Earth’s atmosphere is argon. Comets, large, small, and microscopic, bring additional amounts of water vapor and carbon dioxide to Earth’s atmosphere.

How, then, was Earth’s atmosphere reduced to just a percent of its primordial amount? As I explain in my book Improbable Planet, two events were responsible.1 The Moon-forming event rid Earth of all its atmosphere and all its surface liquid water as well. An event known as the late veneer (a shower of comets that occurred shortly after the Moon-forming event) delivered to Earth its present atmosphere and its present amount of surface liquid water.

Both the Moon-forming event and the late veneer are highly fine-tuned. If the impactor(s) that led to the formation of the Moon had a slightly greater impact energy, Earth would not have survived in its present form. If the impact energy were slightly less, Earth’s present atmosphere would be too thick for advanced life.

Astronomers note that most exoplanetary systems possess no comet or asteroid belts. Inward migration by the gas giant planets in these systems destroyed all the comets and asteroids. The remaining exoplanetary systems possess comet and asteroid belts hundred to thousands of times larger than the solar system’s. In these planetary systems the gas giant planets did not migrate at all. Unique, at least so far, to our solar system is an inward migration by Jupiter, Saturn, Uranus, and Neptune that stopped short of destroying all the solar system’s asteroids and comets.2 Our solar system had its asteroid and comet belts reduced by the just-right amounts to deliver the just-right late veneer to Earth at the just-right time.

Our air pressure is no accident of nature. Its highly anomalous value and its highly fine-tuned origin and historical development testify of God’s handiwork.

How Did We Get Our Optimal Oxygen Level?
As soon as Earth had large amounts of liquid water on its surface it had oxygen in its atmosphere. As I have mentioned in several of my books,3 Earth has hundreds of times more uranium and thorium than what astronomers calculate a rocky planet like ours should possess. As Earth’s uranium and thorium undergo radiometric decay, that decay splits some of the water molecules in Earth’s surface liquid water into hydrogen and oxygen. Some of the oxygen remains dissolved in the liquid water. Some escapes to the atmosphere above.

The resulting atmospheric oxygen, however, is only about 0.00001 of the amount of oxygen in Earth’s atmosphere today. That quantity of oxygen is enough to rule out a naturalistic origin of life but it is insufficient to sustain any life-forms except microbes.

Ever since life’s origin, though, Earth has possessed an enormous abundance of photosynthetic life. That life has continuously pumped enormous quantities of oxygen into Earth’s atmosphere. For billions of years, however, almost all the oxygen that photosynthetic life pumped into the atmosphere was absorbed by oxygen sinks—metals and minerals that became oxidized. The sucking up of oxygen by these mineral and metal sinks kept Earth’s atmospheric oxygen levels so low that only microbes and colonies of microbes could exist on Earth.

For the first 3.25 billion years of life history on Earth, Earth lacked the atmospheric oxygen to support animals either on land or in the oceans. However, 575 million years ago the oxygen sinks filled up enough that the atmospheric oxygen level suddenly jumped up from about 1% to 8%. This sudden rise coincided with the Avalon explosion of animals in the oceans. At 543 million years ago, the oxygen level jumped up again, rising from 8 to 10%. Coincident with this rise was the Cambrian explosion of complex animals in the oceans. At 205 million years ago, the oxygen level jumped up from about 10% to 18–20%. This level was high enough to sustain warm-blooded birds and mammals.

The present atmospheric oxygen level of 21% is kept stable by a delicate balance between photosynthetic plants that produce atmospheric oxygen and animals, fires, and decaying organic matter than consume oxygen. This level is optimal for humans and human civilization for the following reasons:

  1. Even a slightly higher atmospheric oxygen level would ignite many more wild fires, destroying valuable timber and grass resources and filling the atmosphere with pollutants, ash, and dust that impair respiration.
  2. Even a slightly lower atmospheric oxygen level would ignite so few wildfires that too many dead leaves would remain on the ground and insufficient charcoal would be released into Earth’s soils. Both outcomes would very substantially lower the productivity of Earth’s forests and grasslands.
  3. Even a slightly higher atmospheric oxygen level would cause human bodies to wear down faster, thus shortening human life spans.
  4. Even a slightly lower atmospheric oxygen level would limit the physical and mental activity levels of humans.

It is thanks to God creating an abundance of photosynthetic life as early as he did (3.825 billion years ago) and maintaining that extreme abundance for billions of years that Earth’s oxygen sinks finally got filled and atmospheric oxygen levels were able to rise to levels to support first primitive animals and later advanced animals. Today, thanks to a delicate balance between animals and photosynthetic life, Earth’s atmospheric oxygen level is sustained at level that is optimal for us humans. Have you thanked God today for your oxygen?


  1. Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016), 47–57, 58–60.
  2. Ibid., 64–75.
  3. Ibid., 167–68.

Subjects: Animals, Atmosphere, Comets, Early Earth, Exoplanets, Fine-Tuning, Gas Giant Planets, Moon & Its Formation

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About The Author

Dr. Hugh Ross

Reasons to Believe emerged from my passion to research, develop, and proclaim the most powerful new reasons to believe in Christ as Creator, Lord, and Savior and to use those new reasons to reach people for Christ. I also am eager to equip Christians to engage, rather than withdraw from or attack, educated non-Christians. One of the approaches I’ve developed, with the help of my RTB colleagues, is a biblical creation model that is testable, falsifiable, and predictive. I enjoy constructively integrating all 66 books of the Bible with all the science disciplines as a way to discover and apply deeper truths. 1 Peter 3:15–16 sets my ministry goal, "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect, keeping a clear conscience." Hugh Ross launched his career at age seven when he went to the library to find out why stars are hot. Physics and astronomy captured his curiosity and never let go. At age seventeen he became the youngest person ever to serve as director of observations for Vancouver's Royal Astronomical Society. With the help of a provincial scholarship and a National Research Council (NRC) of Canada fellowship, he completed his undergraduate degree in physics (University of British Columbia) and graduate degrees in astronomy (University of Toronto). The NRC also sent him to the United States for postdoctoral studies. At Caltech he researched quasi-stellar objects, or "quasars," some of the most distant and ancient objects in the universe. Not all of Hugh's discoveries involved astrophysics. Prompted by curiosity, he studied the world’s religions and "holy books" and found only one book that proved scientifically and historically accurate: the Bible. Hugh started at religious "ground zero" and through scientific and historical reality-testing became convinced that the Bible is truly the Word of God! When he went on to describe for others his journey to faith in Jesus Christ, he was surprised to discover how many people believed or disbelieved without checking the evidence. Hugh's unshakable confidence that God's revelations in Scripture and nature do not, will not, and cannot contradict became his unique message. Wholeheartedly encouraged by family and friends, communicating that message as broadly and clearly as possible became his mission. Thus, in 1986, he founded science-faith think tank Reasons to Believe (RTB). He and his colleagues at RTB keep tabs on the frontiers of research to share with scientists and nonscientists alike the thrilling news of what's being discovered and how it connects with biblical theology. In this realm, he has written many books, including: The Fingerprint of God, The Creator and the Cosmos, Beyond the Cosmos, A Matter of Days, Creation as Science, Why the Universe Is the Way It Is, and More Than a Theory. Between writing books and articles, recording podcasts, and taking interviews, Hugh travels the world challenging students and faculty, churches and professional groups, to consider what they believe and why. He presents a persuasive case for Christianity without applying pressure. Because he treats people's questions and comments with respect, he is in great demand as a speaker and as a talk-radio and television guest. Having grown up amid the splendor of Canada's mountains, wildlife, and waterways, Hugh loves the outdoors. Hiking, trail running, and photography are among his favorite recreational pursuits - in addition to stargazing. Hugh lives in Southern California with his wife, Kathy, and two sons.

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