Oxygen: The Killer We Can’t Live Without
When you play the game of life, you are playing with fire.
The Earth’s history is punctuated by mass extinction events – calamities that annihilated a large proportion of all life on Earth. Sixty-six million years ago, an asteroid the size of a city hit the Earth causing global devastation that wiped out the dinosaurs and their kin. Two hundred and fifty million years ago, rapid global warming caused by volcanic activity caused an even greater catastrophe. But over two billion years ago, a more peculiar extinction event is hypothesised to have occurred. Unlike later events, this cataclysm cannot be blamed on natural disasters from the Earth or space. Instead, the culprit for this assault on life was life itself.
It began when a remarkable new life form evolved that was capable of harnessing the energy of sunlight to create its own food out of nothing but air and water. Unfortunately, this process unleashed an extremely toxic, and hitherto incredibly rare, gas as a by-product. Over hundreds of millions of years, this toxic gas accumulated in the atmosphere until it reached such a level that it is believed that very little of the life on Earth could have survived.
This toxic gas was never eliminated. It surrounds us, and we breathe it in every minute of our lives. But we do not fear it. In fact, we crave it more than anything else. If we are denied it, we panic in mere seconds and die in a matter of minutes. Remarkably, the gas that once threatened to end life on Earth is oxygen! What was once a deadly poison has become the indispensable breath of life. This is the story of how.
Fire and Rust
How can the flick of a child’s hand destroy a mighty forest? By lighting a match! Under the right conditions, a single spark can grow into a massive flame that spreads uncontrollably and rapidly devours everything in its path. We take the destructive power of fires for granted, but they are truly a remarkable phenomenon, capable of reshaping landscapes and remaking environments in a way very little else can.
Fire is the observable manifestation of the energy produced when a substance reacts with oxygen. Most commonly, the reacting substance is organic matter. With heat from a spark, oxygen can tear apart the carbon backbone that runs through organic matter, reacting with it to become carbon dioxide gas, and leaving behind only water vapour and some charred remains. This process releases a vast quantity of stored energy, which we see as light and feel as heat. Crucially, this released heat allows oxygen to destroy yet more organic matter, creating a chain reaction that spreads and grows. This is the terrifying power of oxygen; it can quickly turn a whole forest into ash and vapour, releasing the energy that sustained an entire ecosystem in a terrifying blaze.
Of course, the fact that fire needs a spark to start protects life. Provided we are not ignited, our bodies will not be torn apart by oxygen in that way. But even without a flame, oxygen finds other ways to work its power. A solid piece of iron is extremely strong and seemingly impervious to damage, but if it is left for long enough oxygen will turn it into a pile of iron oxide, which we call rust. Similarly, over time oxygen can attack organic matter like DNA, perniciously degrading it until the life that depends on it cannot survive. The ultimate reason oxygen causes this degradation is that it is very reactive. This means it has a strong tendency to form bonds with a wide range of other atoms. To form these bonds, it must first break existing bonds that bind together other substances — like iron or organic matter — so that it can replace these bonds with its own. In pursuit of this goal, it relentlessly attacks the structure of matter, exploiting any weaknesses it can find in order to tear it apart.
This poses an interesting question: if oxygen is so reactive, why is there so much of it in the air? Every time iron rusts or wood burns, oxygen is taken from the air and converted into other compounds like iron oxide or carbon dioxide. Over the vast span of Earth’s history, the oxygen in the air should have been depleted until almost none remains. Indeed, this is the fate of Mars. Mars appears red because it is covered in rust. There is no oxygen to breathe in Mars’ atmosphere; whatever oxygen Mars has is permanently bound up with iron into a solid.1 This prompts the question: why isn’t the same true of Earth?
How the Earth Got Its Oxygen – And What It Cost
The reason the Earth is different is life. Just like Mars, the early Earth had essentially no free oxygen in its atmosphere. It was all bound up in other compounds like rust, water or carbon dioxide. This is lucky; had oxygen been present, it quickly would have broken down any complex organic compounds that formed and it is very unlikely that life could ever have arisen. For over a billion years, these organic compounds, and then the earliest lifeforms, could emerge and prosper in a safe environment free of oxygen.
However, about 2.7 billion years ago, the first cyanobacteria evolved. They had a powerful mechanism for obtaining the energy needed for life – photosynthesis. Specifically, they could harness the energy of sunlight to break apart water into its constituent hydrogen and oxygen. This released energy that could then be stored in the chemical bonds of sugars and other organic matter, to later be consumed by the cyanobacteria as required. The hydrogen produced in the split was incorporated into the organic matter, but the oxygen was simply emitted into the surroundings as a waste product. Much of this oxygen reacted with other compounds to become locked up again in iron oxide, carbon dioxide and other compounds. However, eventually, photosynthetic organisms became so abundant that oxygen was produced more quickly than it could be consumed. This resulted in the accumulation of oxygen in the atmosphere.
Today, photosynthesis remains the source of virtually all of the oxygen in the Earth’s atmosphere. Oxygen is constantly being consumed by reactions in the physical world, and by life, but it is replenished by cyanobacteria and their descendants, such as plants.2 In this way, life has fundamentally reshaped the chemistry of the Earth. While oxygen was once an extremely rare gas, it now makes up more than 20% of the Earth’s atmosphere. In fact, this abundance of oxygen is a sign of life on Earth; an alien remotely studying the chemical composition of the Earth’s atmosphere would observe it as an anomaly that could only be explained by the presence of life. Indeed, astrobiologists on Earth hope to use the same process to discover life on distant planets!
However, the initial result of this sign of life was death. Prior to the initial accumulation of oxygen in the atmosphere, there had been no reason for life to evolve defences against the aggressive reactivity of oxygen. So, we can expect that oxygen would have been a deadly poison for virtually all life on Earth. For such life, being exposed to an oxygen-rich world would have been like us finding ourselves surrounded by sulfuric acid. Organic matter would have been broken down en masse and whole species could have been wiped out by this inescapable new toxin. This is why it is believed that a mass extinction event took place; for the life of the time, the Earth would have become just as uninhabitable as it became for the dinosaurs.
Playing With Fire
Fortunately, the destruction was not complete. Some life forms evolved mechanisms to protect themselves from the harmful effects of oxygen, by inhibiting oxygen reactions or reversing them. We have inherited these detoxifying, antioxidant molecules and enzymes, which make us adapted to live in an oxygenated world. Even now, these systems sometimes fail, allowing oxygen to do damage that can contribute to cancer and dementia. In fact, the damaging effects of oxygen are hypothesised to be a major contributor to aging itself. Nevertheless, we do not wish for a world without oxygen.
This is because life evolved to do more than just tolerate oxygen; it learnt to thrive on it. Fire shows oxygen at its most devastating. But early humans learnt that fire is also a tool. If its power can be contained, its immense ability to release energy can be used to meet human needs like cooking or heating. Billions of years earlier, life performed the same trick. Bacteria evolved that could perform a type of extremely controlled burn called aerobic respiration. The net effect of this process is identical to that of burning organic matter, but it is performed in such a way that the energy released can be captured as useful chemical energy, instead of being released as destructive heat. The descendants of these bacteria are mitochondria, which live in every cell in your body, and in all animals, plants and other complex life. Through aerobic respiration, an abundance of energy can be accessed, which has allowed incomparably larger, more complex and more dynamic life to evolve than would otherwise be possible.
The destructive power of oxygen turns out to be a double-edged sword. Even after billions of years of adaptation, we are still threatened by the ability of oxygen to destroy life every time a fire rages out of our control. Yet this same ability to release energy by burning organic matter powers our bodies, from our brains to our toes. It is what makes our rich, energetic lives possible. For all of the harm that fire can do to life, we cannot wish it away, because the same chemistry of burning is what makes our lives possible. The truth is: When you play the game of life, you are playing with fire.
In fact, any gaseous oxygen on Mars would have been lost to space because Mars’ gravitational pull is not strong enough to keep it in its atmosphere. But even on other planets with atmospheres, such as Venus, very little free oxygen is present for the reason discussed.
More precisely, it is not whole plants that are descended directly from cyanobacteria, but the chloroplasts within their cells which perform photosynthesis.