My seventh-grade science teacher asked the class to list “the three most essential needs of human life.” The “correct” answer—water, food, and sleep—illustrates how easily people take for granted the air we breathe, specifically its oxygen content. Most humans can live a few days without water, food, and sleep, and yet we can’t go more than a few minutes without oxygen. I understood that much in junior high, but at the time I had no idea that Earth’s oxygen-rich atmosphere represents—and facilitates—a miracle.
What’s so special about oxygen? In humans, as in any creature larger than about a millimeter in length, oxygen powers virtually all its life functions. Oxygen typically releases an abundance of energy when it attaches to another element or compound, and that’s how it fuels the metabolic process. Some life-forms called anaerobes do not rely on oxygen for their metabolism, but these creatures are tiny, no larger than unicellular filaments, with relatively low rates of metabolism. Organisms that approach even one millimeter in length require orders of magnitude more energy to survive. Their metabolism is aerobic, or oxygen-based, like our own, and with incremental increases in size come hugely increasing power demands. For example, species averaging about a meter in length (as adults) need at least 70% as much oxygen as we humans do for their bodies to do the “work” of living. Two other elements, fluorine and chlorine, also release significant amounts of energy, but they are relatively rare and highly volatile by comparison. Of the three reactive elements, only oxygen is sufficiently stable to accumulate in Earth’s atmosphere and provide for life’s metabolic needs. Today it constitutes about 20% of Earth’s atmosphere. (Nitrogen makes up most of the remaining 80%.)
How did oxygen get here? Inorganic chemical reactions and radioactivity passing through water produce only miniscule amounts. Nothing but photosynthetic life (such as cyanobacteria)—and lots of it—can possibly generate enough atmospheric oxygen to sustain aerobic life in any abundance. However, because oxygen is so highly reactive, most of what photosynthesis produced throughout Earth’s history lasted only briefly in the atmosphere. As soon as these tiny life-forms (powered by sunlight) converted carbon dioxide and water vapor into sugar and oxygen, the oxygen was swallowed up by enormous “oxygen sinks” (oxygen-absorbing chemicals and decaying organic matter) in Earth’s mantle and crust. Not until these oxygen sinks began to fill up could the atmosphere hold onto significant quantities of oxygen. The oxygen history of Earth’s atmosphere has been difficult to trace, but breakthroughs are coming. In a series of seven research papers, a team of chemists and physicists were able to write the early chapters of that history. They described two great oxygenation events: the first occurred roughly 2.4 billion years ago, and the second in three episodes between 635 and 545 million years ago.1 Additional research shows that these two events set the stage for a third event that occurred about 200 million years ago.2 While scientists continue to investigate the exact causes of these three great oxygenation events, they agree that catastrophic upheavals played a significant role. These disruptions buried large quantities of decaying organic matter, thereby preventing the carbon in this material from gobbling up oxygen. This circumstance would allow a major buildup in atmospheric oxygen if two crucial conditions were met. First, photosynthetic life would have to be extremely abundant and diverse to stay ahead of oxygen consumption by other oxygen sinks. Second, at least one of the other oxygen sinks would have to be filled up by the time the burials occurred.
What does science show? Both conditions appear to have been met with precise timing and with even more remarkable results. The earliest oxygenation event (2.4 billion years ago) provided for the sudden and widespread appearance of eukaryotic bacteria (cells with definite nuclei) existing both as individual cells and as mats of cells. The second oxygenation event (from 635 to 545 million years ago) precipitated the appearance of the first large animals. The last great oxygenation event (200 million years ago) coincided with the appearance of the first birds and mammals. This profile of available oxygen followed immediately by the appearance of creatures equipped to exploit it defies the assumptions of naturalism. A biblical creation model, on the other hand, can explain why the fossil record looks the way it does.3 The Creator of the universe orchestrated and timed these events on purpose, as part of a larger plan that includes you and me.
- D. A. Fike et al., “Oxidation of the Ediacaran Ocean,” Nature 444 (2006): 744-47; Richard A. Kerr, “A Shot of Oxygen to Unleash the Evolution of Animals,” Science 314 (2006): 1529; Don E. Canfield, Simon W. Poulton, and Guy M. Narbonne, “Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life,” Science 315 (2007): 92-95; David C. Catling and Mark W. Claire, “How Earth’s Atmosphere Evolved to an Oxic State: A Status Report,” Earth and Planetary Science Letters 237 (2005): 1-20; David C. Catling et al., “Why O2 Is Required by Complex Life on Habitable Planets and the Concept of Planetary ‘Oxygenation Time,’” Astrobiology 5 (2005): 415-38; James F. Kasting, “Ups and Downs of Ancient Oxygen,” Nature 443 (2006): 643-45; Colin Goldblatt, Timothy M. Lenton, and Andrew J. Watson, “Bistability of Atmospheric Oxygen and the Great Oxidation,” Nature 443 (2006): 683-86.
- Paul G. Falkowski et al., “The Rise of Oxygen Over the Past 205 Million Years and the Evolution of Large Placental Mammals,” Science 309 (2005): 2202-04. 3. Hugh Ross, Creation as Science (Colorado Springs: NavPress, 2006): 125-47.