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A barren landscape devoid of obvious life, an atmosphere full of methane with little to no oxygen gas, and oceans that cover almost the entire surface of the planet. If we were to time travel three billion years into the past, these would be characteristics that our planet Earth would exhibit. These alien-like features of Earth’s past are vastly different from the features we see today. It begs the question: what changed?
The Earliest Life Forms
To begin answering this central question, we start with the introduction of life on Earth during the Archean Eon from 4 to 2.5 billion years ago. During this period of Earth’s history, the planet was essentially a water world. However, life was beginning to form. Tiny microscopic life was created by the complex chemical reactions that transformed carbon-containing molecules into simple, living cells not dependent on oxygen. These microscopic organisms, instead, used sulfur and other elements to gather their energy. They were the earliest records of life on Earth.
Cyanobacteria
It wasn’t until 2.7 billion years ago that a specific group of microbes known as cyanobacteria began to evolve. Their evolution allowed them to become the Earth’s first autotrophs as well as the earliest ancestors to many of the plants we see today. Cyanobacteria used sunlight and carbon dioxide to create sugar and oxygen waste which were then released into the atmosphere in vast quantities; this act of creating oxygen as a byproduct triggered what is known as the Great Oxidation Event.
The Great Oxidation Event
The Great Oxidation Event is aptly named because of how it changed Earth from a planet with a significant lack of oxygen to one that has an abundance of it. With the help of cyanobacteria, oxygen was being produced inside the seawater at a rate faster than it could react with other elements or get sequestered or trapped by minerals. As a result, the oceans became more and more oxygenated, and gradually, the oxygen escaped into the atmosphere, where it displaced the methane and thus became a dominant component of the atmosphere.
However, because oxygen became a huge portion in the atmosphere of Earth, many things started to change. For example, scientists have hypothesized that the Great Oxidation Event may have indirectly played a role in one of the earliest ice ages. Since methane is a greenhouse gas, it helps trap heat from sunlight and warms the planet. However, due to the Great Oxidation Event, more and more methane was being displaced by oxygen. Thus, global temperatures fell bit by bit until ice sheets were formed, covering the entire surface of the planet.
In addition to causing global climate changes, the Great Oxidation Event also played a role in the mass extinction of anaerobic life, while also helping with the emergence of aerobic metabolism. Billions of years ago, living organisms did not need oxygen to survive, therefore the sudden amount of oxygen appearing in the atmosphere became a poison and wiped out many anaerobic organisms. However, life found a way to survive. Organisms learned to adapt and use the rich potential of oxygen in respiration. In the end, the Great Oxidation Event was one of the main contributing factors that made life into what it is today, consisting of things such as multicellular organisms such as animals and plants who can use oxygen to live and thrive.
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