We are so accustomed to the presence of oxygen on Earth, that we take its presence for granted. After all, oxygen is the third most abundant element in the universe, and is forged in the fiery interiors of stars. It is also the most abundant element in the Earth’s crust, and makes up one fifth of the atmosphere. We cannot imagine our planet supporting life without oxygen. Therefore, it is easy to overlook the fact that oxygen was virtually absent for a significant period on Earth, and it took a series of phenomenal chemical events to give birth to oxygen. The rise of oxygen in Earth’s water and atmosphere is one of the defining events in Earth’s history and has shaped life the way we know it.
When the Earth was formed 4.5 billion years ago, the composition of its atmosphere was very different to what it is today. The atmosphere then consisted mainly of carbon dioxide, methane and water vapour. The sun was still in its infancy, and its temperature was much cooler. Under those conditions, the oceans would have frozen but for the presence of carbon dioxide and methane (greenhouse gases), which trapped sufficient heat. Oxygen was absent from earth.
So how did oxygen arise and make its way into the atmosphere? Let us look into the chemical reactions that were occurring in the atmosphere. The energy present in sunlight split water vapour in the earth’s atmosphere into hydrogen and oxygen. Being the lightest element, Hydrogen escaped into space. Oxygen, however, is a highly reactive element, and is known to form compounds with just about every single element in the periodic table. Being highly reactive, the oxygen that was generated quickly reacted with methane and got locked into the Earth’s crust, leaving very little traces in the atmosphere. Thus, though chemical reactions in the atmosphere did play a part, they were not sufficient to generate oxygen to the levels that it currently exists. There was another mysterious force that was at play, silently releasing oxygen at a steady rate until the composition of the very atmosphere changed. That mysterious force, most interestingly, happened to be life!
Life originated in the ocean around 3.8 billion years ago. At that time, oxygen wasn’t present in the Earth’s atmosphere, and the early life forms survived on minerals present in the ocean without ever requiring oxygen. However, around 2.7 billion years ago, there arose a peculiar group of microbes – known as cyanobacteria. These creatures were remarkable and unlike any of the other primitive life forms. They had the ability to carry out photosynthesis, i.e. they could generate energy from sunlight and carbon dioxide without depending on anything else. Most significantly, the by-product of photosynthesis was oxygen!
But how could such tiny bacteria affect the composition of such a vast atmosphere? It may sound surprising, but these bacteria were very efficient at photosynthesis. The rate of oxygen release gradually increased over a span of time (200 – 300 million years), and oxygen was eventually produced at a faster rate than it could react with other elements. Oxygen released from cyanobacteria steadily accumulated over large regions of the ocean, thus oxygenating the water. Gradually, oxygen also escaped into the atmosphere, displacing methane and increasing in concentration till it became a major component of the atmosphere. This event is known as the “Great Oxidation Event”, and occurred between 2.4 to 2.1 billion years ago.
The great oxidation event had several consequences. Firstly, the chemistry of the earth’s atmosphere changed due to presence of oxygen. Oxygen was responsible for formation of the ozone layer that acts as a natural sunscreen to filter out harmful UV radiation from the sun. Accumulation of oxygen also displaced methane from the atmosphere. Since methane was a greenhouse gas and warmed the planet, a drastic fall in its levels meant that the earth was no longer getting heated up sufficiently. This lead to one of the earliest ice ages, with ice sheets extending all the way from the poles to the tropics! Also, since life was totally anaerobic (not needing oxygen) at that point of time, the release of oxygen proved fatal to most of life. It was a poisonous gas, and lead to a great extinction event that wiped out much of life! No wonder that the great oxidation event is also known as the “Oxygen Holocaust”. However, life found a way to evolve, and another series of fantastic events led to the appearance of different life forms that required oxygen for their metabolism (I’ll try to tell that story as well). This completely changed the course of evolution, leading to complex, multicellular life. Oxygen is the one molecule that makes Earth what it is today, far more hospitable and beautiful than the early Earth which was akin to a violent, alien planet. The great oxidation event is an epochal event in Earth’s history and is intricately linked with the evolution of complex life.
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