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How Oxygen Came into the Atmosphere

We see the process of photosynthesis all around us – green plants use the energy of the sun, along with carbon dioxide and water, and convert it into chemical energy. This energy is later used for their self-sustenance and growth. Moreover, plants also produce oxygen, which is a very vital by-product of this process. However, plants were not the first photosynthetic organisms on earth and oxygen did not exist for a very long time in the earth’s history.

A Time Before Plants

According to radiometric dating, the age of the earth is estimated to be around 4.54 billion years old. And the earth was devoid of life for almost another billion years. Fossil evidence suggests that the very first forms of life formed 3.4 billion years ago. But newer evidence suggests that the first lifeform evolved much earlier than that.

These ancient lifeforms were microscopic extremophiles, which thrived in the hellish environments of the primitive earth. They essentially survived on the chemical energy found in their environments, converting the energy in the environment into energy-rich molecules that are able to sustain the organism. And unsurprisingly, we see their modern-day descendants even today – in places such as hot springs, radioactive pools, glaciers, and acidic lakes.

The Event

However, the very first evidence of the photosynthesis reaction is still unclear. But unlike modern-day photosynthesis, the by-product was not oxygen, and the reducing agent was probably hydrogen rather than water. Hence, geological records show that prior to 2.3 billion years ago, the earth’s atmosphere was devoid of oxygen.

So, scientists speculate that an “event” took place shortly after, and this event is responsible for introducing oxygen into the atmosphere. This event was known as the Great Oxygenation Event, where molecular oxygen was biologically induced into the earth’s atmosphere. And the group of organisms responsible was thought to be cyanobacteria, doing the process of photosynthesis. This event occurred over a very long period of time – from nearly 3 billion years ago to 1 billion years ago. Since then, the earth’s atmosphere gradually started to accumulate oxygen and the evolution of a much more complex life began to take off.

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