Oxygen is an essential element for life. Almost all animals, including humans, take oxygen from the air and use it to create energy. If we cannot breathe for even just one minute, we feel pain. Failure to breathe for more than 4-5 minutes can result in permanent brain damage or even death.

Oxygen makes up about a fifth of the air we breathe. Animals consume oxygen, but plants make oxygen through photosynthesis. This is why we worry that we will not be able to breathe due to a decrease in oxygen whenever we hear that a large area of rainforest is disappearing. Fortunately, we can still breathe. Do you know how the oxygen concentration in the Earth's atmosphere is maintained?

While oxygen is the most common element in the Earth's crust, the atmosphere of the primordial Earth, which formed 4.5 billion years ago, contained few molecules of oxygen. Oxygen in the Earth's atmosphere began to increase about 3 billion years ago. Even in the days when there was little oxygen, various microorganisms were living on our planet. These were anaerobic microorganisms that can live in an oxygen-free environment.

About 3 billion years ago, the first photosynthetic microorganisms appeared: cyanobacteria. Cyanobacteria used sunlight and carbon dioxide to create energy and oxygen to thrive in the ocean. The oxygen they made started to fill the oceans and the atmosphere. Fossils of cyanobacteria are now found in many parts of the world. A rock called stromatolite is a fossil formed from sediments of cyanobacteria and soil.

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About 2.5 billion years ago, the Great Oxygenation Event occurred, causing a rapid increase in oxygen. This event also wiped out a lot of anaerobic microorganisms, as oxygen is usually harmful to anaerobes. On the other hand, aerobic organisms evolved, developing the current cycle of oxygen circulation.

The amount of oxygen in the atmosphere was not always the same. Over the past few billion years, the concentration of oxygen has changed several times, from a low of 12% to a high of 35%. Plants and animals had to adapt to the fluctuating concentrations of oxygen. In the early days of the Paleozoic Era, when oxygen level was relatively low, creatures such as trilobites had wide gills to better take in oxygen.

From the mid-to-late Paleozoic Era, oxygen levels started to rise, which drove underwater animals to the land, contributing to an increase in the number of terrestrial species. In the second half of the Paleozoic Era, insects flourished. Monstrously large insects were flying in the sky. For instance, meganeura was a dragonfly-like insect with a wingspan of over 70 cm. Arthropleura, millipede-like arthropods, could grow to over 2 meters. Those insects were unbelievably gigantic.

These arthropods could become huge thanks to the oxygen concentration exceeding 30%. It is difficult for them to grow beyond a certain size because of how they oxygenate their bodies. In ancient times, however, the high oxygen concentration made them grow to a huge size. These animals became extinct at the end of the Paleozoic Era when oxygen levels dropped below 20%.

High oxygen levels are not always good for living things. All creatures have adapted to the environment in which they live. So have humans. If the oxygen concentration rises, more free radicals are produced, damaging cells and harming the body. Since anything can catch fire more easily, more serious natural disasters like wildfires will happen more often.

On the other hand, if the oxygen concentration decreases, it becomes harder for plants and animals to perform activity due to the lack of oxygen.

If you go up to a high altitude where oxygen is insufficient, you may get a headache. We have often seen soccer players native to sea level suffer from lower performance when playing at high altitudes.

As mentioned before, it is worrisome if the oxygen concentration falls due to the decrease of plants on the ground. However, scientists estimate that about 70% of oxygen production comes from phytoplankton in the oceans and descendants of cyanobacteria that first introduced oxygen on the Earth. The rest are made by plants on the ground. It is a relief to know that the sea is creating a lot of oxygen regardless of the decreasing number of plants. Nonetheless, we should still keep the Earth green to live longer on this planet.