Richard Zurawski

Take a deep breath. Feels good doesn’t it? No matter where you are on the face of the Earth you can always fill your lungs with life giving, sustaining, wonderful air. And the stuff that feels so good in each breath is oxygen. In fact, it is the second most common gas in the atmosphere. Twenty-one per cent of the entire atmosphere is made up of this remarkable gas. We are so used to having oxygen around us, ready for the breath of life or fires or any form of combustion from the myriad of engines that zip us from place to place, cut our lawns or blow our leaves.

The surprising thing about this gas is that it if it was not constantly being regenerated it would entirely disappear. Free oxygen is the product of biological activity. In fact there is a bit of irony that we can all learn about in a little oxygen story. In the course of the story we will also get a feeling for how entwined all life is. Mess with it at you own peril.

The Earth is about 4.65 billion years old give or take 150 million years. About half a billion years after the Earth was created, it is estimated that life had already gained a toehold in the relatively newly condensed oceans of the world. But if you could do a bit of time travel and seek yourself back to that nascent Earth, you would find it a very forbidding and alien place. What you would notice just before you expired because of a lack of oxygen to breathe is that the atmosphere was almost all carbon dioxide and the sky a dull brownish, the sun was much dimmer and there was nothing in either the oceans or on land that you could call life. The life that did exist was anaerobic, microscopic and and most closely resembled the life forms called stromatolites, today only found in ultra saline tropical pools.

For over three billion years the only life on the Earth were anaerobic single celled microscopic sludge-like colonies. They consumed the vast quantities of carbon dioxide available from the atmosphere and through energy from the sun used photosynthesis to live, multiply and dominate the Earth’s ecosystems like no other group of creatures have since. In numbers too large to believe they probably covered the oceans in thick sludge mats, sucking back CO2 and sunlight and excreting oxygen. Year after year they lived.

At first the oxygen the anaerobes excreted was absorbed by the environment. Oxygen is highly reactive and tries to bond with just about anything around it. First the iron bonded with oxygen to create rust. Then other elements absorbed the effluent. But eventually the environment became saturated. Even single celled microbes if they number in the countless quardrillions will eventually have an effect on the environment. And what an effect they had. Soon the oxygen began to pile up in the atmosphere. And the CO2 levels began to fall correspondingly to meet the need of the voracious anaerobes. This was twofold whammy.

Firstly oxygen is highly toxic to the anaerobes and where oxygen built up they died off. Secondly the CO2 was being replaced by a gas that wasn’t a very good greenhouse gas, oxygen. The temperature plummeted on the Earth and we had the first ever Ice Age. It took over three billion years to happen and in that time the humble little anaerobes had transformed the Earth. Oxygen levels had risen and carbon dioxide fallen so that when the Ice Age hit, you and I could breathe the air and the sky was clear and blue. But the anaerobes were buried under their own effluent.

Soon the entire Earth froze and for 200 million years the deep freeze lasted. The anaerobes almost died off and even the tropical oceans were covered in ice. From space the Earth must have been a spectacular glittering ice diamond. It was the first mass extinction and caused entirely by a species that had dominated the Earth for over three billion years. It was the first and only time, so far, that life itself had been a factor in climate change and in changing the ecosystem wholesale. The first, that is until humankind.

Out of the ice grew an entire new ecosystem, from which all the life that we call familiar is descended, plate tectonics released more CO2 and with a few thousands of years the ice melted and the great tapestry of life with creatures that could not only tolerate, but thrive in an oxygen environment came to dominate the world. But the legacy of oxygen and its catastrophic introduction echoed throughout the geologic ages. Today microbes, anaerobes and plants share the burden of producing the oxygen we all need to exist.

Every time you take a deep breath, think about the fate of those marvelous, over productive anaerobes and how they changed the ecosystem of a planet and destroyed themselves.