1500 million years ago (1.5 billion years ago), a significant turning point occurred on Earth: the free iron in the oceans ran out, and oxygen began to be released into the atmosphere.
The Great Oxidation Event (Continued)
Before this point, any oxygen produced by early photosynthetic organisms was quickly absorbed by dissolved iron in the oceans, forming iron oxides (like rust) that precipitated out and formed banded iron formations. These formations serve as a geological record of this time. When the supply of dissolved iron was exhausted, oxygen levels began to rise in the oceans and, eventually, the atmosphere. This period is often referred to as the Great Oxidation Event (GOE) or the Oxygen Catastrophe.
Consequences of Increased Oxygen
The release of oxygen had profound consequences:
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Extinction of Anaerobic Organisms: For many anaerobic organisms (organisms that thrive in the absence of oxygen), oxygen was toxic. The rising oxygen levels led to a mass extinction of these life forms.
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Evolution of Aerobic Organisms: Conversely, the increase in oxygen paved the way for the evolution of aerobic organisms, which use oxygen for respiration and derive much more energy from food than anaerobic organisms. This provided a significant evolutionary advantage.
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Formation of the Ozone Layer: In the upper atmosphere, oxygen molecules (O2) were split by ultraviolet (UV) radiation, forming single oxygen atoms (O). These single atoms then combined with O2 molecules to form ozone (O3). The ozone layer effectively blocks harmful UV radiation from reaching the Earth's surface, making it safer for life to exist on land.
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Climate Change: The increase in atmospheric oxygen also reacted with methane, a potent greenhouse gas. This led to a decrease in global temperatures and possibly triggered one or more "snowball Earth" events.
Timeline Context
It's important to note that while 1500 million years ago marks the beginning of significant oxygen release, other major events occurred before and after:
- Around 700 million years ago: The first animals (simple, single-celled) began to evolve.
- Around 530 million years ago: The first vertebrates (fish) evolved.
Summary
The event 1500 million years ago was a pivotal moment in Earth's history, as the rise of oxygen in the atmosphere dramatically reshaped the planet's environment and set the stage for the evolution of more complex life forms. The depletion of free iron in the oceans allowed the by-product of photosynthesis to become a transformative force.
