Vague Beginnings
Life on earth began nearly 4.6 billion years ago. However, scientists think life may have evolved even further back. During this time, the earth was barren, and any forms of life that existed then would have had to prepare its own food. Ancient earth’s atmosphere was filled with gases that were toxic to today’s lifeforms. However, these ancient organisms must have utilized these gases along with the sunlight to produce food. This process was reminiscent of the photosynthesis process. Moreover, oxygen did not exist after the formation of the earth. In fact, atmospheric oxygen was almost non-existent for another 2.15 billion years.
Great Oxygenation Event
Scientists theorize that the oxygen in the atmosphere was formed as a result of Cyanobacteria. These microorganisms used photosynthesis to produce oxygen, which then accumulated in the earth’s atmosphere. The event is called the Great Oxygenation Event. However, this event may also have caused one of the very first mass extinction events on earth. The microorganisms that existed before the cyanobacteria were anaerobic in nature; this meant oxygen was very toxic. When these cyanobacteria pumped even more oxygen into the atmosphere, these anaerobic life forms were poisoned and eventually went extinct. Hence, this event is also known as the Oxygen Catastrophe.
Aftermath
Cyanobacteria are responsible for the oxygen levels that we observe today. Having more oxygen in the atmosphere catalyzed the evolution of life, kickstarting the rise of multicellular life. Eventually, conditions on earth became very favourable for life to start becoming very complex. Early water-dwelling organisms transitioned onto land, which opened up a whole new environment. Land plants became more abundant, gradually evolving into more advanced forms. If not for the Great Oxygenation Event, life on earth would have been entirely different.
Alternate Theories
The introduction of oxygen into the atmosphere poisoned the anaerobes, but this may not have been the only cause for their extinction. A cosmic event called gamma-ray-burst may have acted as a catalyst too. The event is caused when a type of star explodes in a distant galaxy. Gamma rays from these explosions may have showered the earth with deadly radiation. This may have proved fatal for the lifeforms that were directly exposed. Hence, aerobic microorganisms may have got an opportunity to fill in the empty niche left by these organisms. However, this event is based purely on speculation – such events, though plausible, lack any sort of evidence to support their occurrence. Another alternate theory is the earth faced a global “cooldown” – temperatures dropped due to the gases in the atmosphere blocking out the sun. Anaerobes may have found it difficult to carry out their basic life processes in this new environment. However, this is just a speculation as there is no evidence to support this claim.
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