Life, then Oxygen, then Fire

by Alexander Bastidas Fry
image of firebreather by flickr user margaretmeloanThe Earth is on fire, but it was not always this way. Billions of years ago at the time of primordial life's genesis the Earth lacked free oxygen in the atmosphere. The evolutionary rise of blue-green algae in the oceans led to the advent of oxygen. And so today every creature burns a little when it breaths. Oxygen is an extremely reactive element. From oxygen's perspective the earth is a pile of fuel waiting to burn. Consider what a single spark would ravage with no human intervention. Cities are piles of neatly stacked kindling and forests are scattered matchsticks. So it is somewhat of an amazement the entire thing doesn't just catch fire. Perhaps one day it will.
The power of fire is transformational: creative and destructive. Our ancestors took control of fire and took control of their environment. Yet, our bodies had already harnessed the biggest trick of fire—extracting its transformational energy—long ago when primordial organisms started breathing oxygen in the atmosphere. The quintessential energy releasing chemical reaction is oxidation. It is not quite fire. It is mere oxidation. When oxidization springs into full fledged fire, flames cast light into darkness. A flame is what you see when something burns, but oxidation is what occurs constantly to nearly every substance in an atmosphere of rich oxygen.

Long before the world was burning it was merely turning. Two and a half billion years ago there was essentially no free oxygen on earth. Any free oxygen in the atmosphere was quickly chemically captured by organic matter or minerals (in particular in iron via rust). During this time life on earth was simple and anaerobic. Most organisms did not require oxygen for growth and in the presence of oxygen these organisms would die. Around this time blue-green algae or cyanobacteria began to flourish in the oceans. Cyanobacteria is a simple prokaryotic form of life that has thrived on Earth. It has accomplished a feat that humans are also on the verge on accomplishing: the transformation of the entire atmosphere of Earth. Cyanobacteria are by some measure most successful group of microorganisms on Earth. Cyanobacteria are photosynethizers, nitrogen fixers, and carbon fixers. They convert the energy in sunlight into useable energy in their cells by taking in carbon dioxide and water to produce oxygen. The stupendous success of cyanobacteria created a deluge of oxygen on Earth. The transformation from an oxygen poor to an oxygen rich atmosphere on Earth is known as the Great Oxygenation Event. The consequence was that all previous life forms accustomed to anaerobic environments were pushed into the depths and shadows of Earth: free oxygen will destroy organisms unprepared for it. A new phase of evolution on Earth occurred where creatures could survive in oxygen rich environments. The new creatures had protective skins on the outside so oxygen wouldn't destroy them and on their inside they used oxygen as an essential part of their survival through cellular respiration. Cellular respiration was a wonderful adaptation because it allowed the rapid release of energy. The pace of life on Earth from the lunge of a lion to the quick thoughts in your brain are the result of relatively efficient cellular respiration process.
Evolution of organisms which could survive in oxygen was rapid, but organisms which could thrive took a bit longer. The rise of the animal kingdom occurred 600 million years ago. At this time cells also started clumping into living creatures like sponges in the sea. They grew backbones and structure. Collagen, the duct tape of living organisms, needs oxygen to be created. The very first animals absorbed dissolved oxygen in the oceans through the skin of their soft bodies. Some creatures breathed through pores in their bodies and had a myriad of adaptations as seen in Aysheaia and its modern ancestor the velvet worm which may have been similar to some of the first creatures to transition to a terrestrial existence 500 million years ago. Larger complex moving creatures improved their method of getting oxygen with gills. During the Carboniferous period 350 million years ago oxygen in the Earth's atmosphere peaked at 35% as vast forests covered the land (today the oxygen content of our atmosphere has leveled off at 21% by volume).
There was a moment on Earth when the first forest fire occurred. The forests seeded their own destruction as they pumped out more oxygen into the atmosphere and made fire more viable. Wide spread forest fires have likely has existed on Earth for some 450 million years. Some plants adapted, the pyrophytes, for which fire is their means by which they thrive. As early as 1.5 million years ago an ancestor of humans was using fire in some capacity. During the Pleistocene starting 11,000 years ago humans mastered fire.
Today during the Anthropocene we burn fossil fuels on a global scale, but just as fire went from the surface of cells into their core, fire has moved from external on the land to inside the machine. Or so humans have tried to make it so. Today the only forest fires that we allow are the ones that occur during the least opportune conditions; these are the ones we cannot control. There are natural global cycles of forest fire that follow from rainfall, dryness, and lightning that we cannot completely control. There are always fire somewhere. Historical evidence in North America shows that old growth forests see fires every few decades. Brush and organic debris on the forest floor is burnt up and big trees are left mostly unaffected during most forest fires. However, forest fire suppression has led to a build up of fuel so when fires do occur they are larger than ever. This problem is recognized, but social constraints continue to push us closer to the flames of wildfires.
Oxygen is magic because of its physical chemistry. It is the third most abundant element in the Universe after hydrogen and helium. It is the quintessential oxidizer (oxygen is one of the most electronegative elements just behind fluorine and helium). This means that it rips away electrons from other atoms or molecules in a myriad of chemical reactions such as rusting iron and burning fire. The oxygen found in our atmosphere comes in pairs – two oxygen atoms together as a stable molecule. The oxygen molecule is stable in its usual ground state. The molecule is extremely reactive in its excited state, however there is an internal energy barrier to oxygen achieving this state. This barrier can is overcome under conditions of high heat or in the presence of catalytic molecules and atoms such as iron. When this barrier is overcome, energy is released and chemical transformations occur.
Complex life on Earth begins with photosynthesis. During photosynthesis the energy in sunlight is captured, carbon dioxide undergoes reduction into sugars, and water is oxidized into molecular oxygen Photosynthesis is a similar (but not quite) reverse of the redox process of respiration. During cellular respiration sugars are turned into carbon dioxide and water. In chemistry the accepting of electrons is called oxidation. Cellular respiration is the oxidation of glucose to carbon dioxide and the reduction of oxygen to water. Respiration releases energy quite slowly compared to burning. Ultimately the difference between respiration and fire seems arbitrary. When it happens in a cell, it is respiration. When it happens outside a cell, it is burning.
The crux of life is oxygen. It is the turning point for positive and negative creations. While it is necessary it is also damaging. For example, free radicals, which spur the mutation of molecules they encounter, are created when certain compounds interact with oxygen. Antioxidants are molecules which inhibit the creation of these free radicals. Do not take this as advice to go out and buy ‘antioxidants' (studies indicate contradictory results as to the efficacy of antioxidant dietary supplements). Do transform your food with fire. Some scholars argue that cooking with fire is crucially what made humans: cooking had the profound evolutionary effect of increasing the efficiency of food to the point that energy was made available for larger brain growth, by warming us, and by helping us fend off predators. Fire really was transformational for humans.
We reach back to a genetic and cultural inheritance with fire. The story of life, oxygen, and fire takes us from the stars to our cells. Oxygen was created through the fusion of lighter elements in the cores of massive stars billions of years before the Earth formed. It is produced on Earth through the collective action of a myriad of plant life forms. Finally, its energy is released through fire. Our collective human mythologies of fire tell us that in its flames is the origin of mystical transformation. It is all true. Fire, and thus oxygen, is the genesis of revolutions.
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