Why should the oxygen content of the earth be maintained at about 21%? What are the consequences of high oxygen content?

This paper takes part in the series essay competition of “great science” of Recordunkown.

I believe many friends know that except for a few anaerobic organisms, life on earth basically needs oxygen to breathe and survive. However, the oxygen content on the earth does not always remain unchanged, but in the long life years, it has been increasing and decreasing, and finally maintained at about 21%.


In the early stage, 4 billion years ago, the oxygen content of the earth was very scarce. Later, life was born in the ocean. At this time, life needed very little oxygen, that is to say, these organisms were basically anaerobic. However, as time goes on, there is a particularly domineering creature in the ocean, that is cyanobacteria.

Cyanobacteria has a very powerful function of releasing oxygen. The emergence of a large number of cyanobacteria in the ocean also makes the oxygen content of the earth begin to increase sharply. In the early Earth environment, the highest oxygen content reached 40%. The environment with high oxygen content has a direct impact on the evolution of the earth’s organisms. The most obvious thing is that the organisms at that time were very huge, whether they were marine organisms or terrestrial organisms.

There was an age of giant insects in the early Earth, that is, when insects dominated the earth. In the eyes of modern people, insects can be said to be the life at the bottom of the biological chain. They are basically very small. However, in the era of giant insects hundreds of millions of years ago, insects were very big. Ordinary dragonflies could be as long as one meter, and centipedes could be as long as three meters.


The main reason why insects grow so big is that the oxygen content in the atmosphere was too high at that time, which created the time when insects dominated. Later, with the emergence of five mass extinctions, the oxygen content of the earth is also decreasing with each ecological upheaval.

With the fifth mass extinction, after the end of the age of dinosaurs, the age of giant creatures will come to an end. The organisms that reappeared later are also shrinking. The reason for this change is related to the decreasing oxygen content of the earth. Since the end of the fifth mass extinction, the oxygen content of the earth has been maintained at about 21%.


Stable oxygen content plays an important role in the earth’s ecology, especially in the evolution of life. Some people may say, why should the oxygen content of the earth be maintained at about 21%? What are the consequences of higher oxygen levels?

The stability of oxygen content is very important. It can be said that without this long period of stability, there will be basically no human birth. You know, in the earth’s long billions of years of life, there have been many overlord life periods. But those creatures have not evolved into intelligent creatures, why is it that only human beings have done it?

For human beings to become intelligent life, many people may come up with a variety of answers. But in Xiaobian’s opinion, one of the important reasons is that the oxygen content of the earth has dropped to the perfect position of 21%, and it has been stable until now. 21% oxygen content may be very important for mammals, which can promote faster and more perfect evolution of mammals.


This can be seen from the sensitivity of human beings to oxygen. For human beings, when the oxygen concentration is lower than 19.5%, it belongs to the state of hypoxia. When the oxygen concentration is higher than 23.6%, it belongs to the state of oxygen enrichment, which will have a certain destructive effect on the human body. Only in the small range of about 21%, the human body can maintain a healthy state.

It’s not just humans who are so sensitive to 21% oxygen, but most other mammals are. This shows that 65 million years ago, after the extinction of dinosaurs, mammals can rise rapidly, which is closely related to the 21% oxygen content. The 21% oxygen content may also play an important role in the evolution of human intelligence.


If the earth’s oxygen content increases, the impact on mammals will be huge, especially for humans. We all know that in addition to intelligence, human beings are relatively poor in physical quality, far from being able to compare with many mammals. In fact, oxygen is a toxic substance, 21% of the oxygen content just makes it impossible for oxygen to cause any harm to human beings, which is a balance between human body and oxygen.

If this balance is broken, oxygen will become a kind of poisonous gas, which will do harm to human body. We all know that oxygen poisoning is actually the harm of high oxygen content. Therefore, higher oxygen content is a disaster for human beings, and it is also not a good thing for other mammals.

We can even say that if the oxygen content of the earth rises sharply in a short period of time, the earth’s ecosystem will undergo drastic changes. At that time, it is possible that the sixth mass extinction will come. So, if the earth’s oxygen content is to spend hundreds of thousands of years or millions of years to achieve a slow rise, then it will not have a disastrous impact on human beings?


If it takes a long time for the oxygen content to rise, then there will be enough time for the earth’s organisms to adapt, and the ecosystem will have enough time to adjust. At this time, human beings will gradually adapt to the ever-increasing oxygen environment over a long period of time, but even so, the increase of the earth’s oxygen content will not be good news for human beings.

We should know that the slow increase of oxygen content may completely change the existing biosphere model. In the past, the organisms at the bottom may become the overlord again, such as very small insects. Once the earth’s oxygen content continues to increase, the size of insects will continue to grow. And in the earth’s biosphere, the largest number of insects, their reproductive capacity is beyond the reach of other organisms.


Now the main reason why insects can’t pose too much threat to other creatures is that they are too small. But if the size of insects increases by tens or even hundreds of times, it will be a disaster for other creatures, especially for mammals.

The environment with high oxygen content will not have a great impact on the size of mammals. How big is the Tiger now? In a higher oxygen environment, it will still be how big, and the size will not change much. But the influence of insects is very big. Imagine that when the Centipede’s size reaches about three meters, tigers and lions are just their food, just like other mammals.

At that time, although human beings were intelligent beings with the help of science and technology, we might not be able to gain a decisive advantage in the face of a terrible number of insects. The wild may become a dangerous area for human beings, and even some primitive forests may become a forbidden area for human beings.


Thus, it can be seen how important 21% of the oxygen environment is for human beings. Can this oxygen content be maintained in the future? It may not be easy to know that the earth’s ecological environment will not remain unchanged. Once the ecological environment changes dramatically, the oxygen content in the atmosphere may also change dramatically. In order to survive in the future ecological upheaval, we can only rely on more powerful scientific and technological forces. Therefore, the rapid development of science and technology is the hope that human beings can continue to survive.

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