Why do organisms generally grow larger in an oxygen-rich environment?

The day before yesterday, Hanhai Langshan (Hun Langshan) mentioned that the earth's current atmospheric oxygen content is only 21%, but this is not the era with the highest atmospheric oxygen content in the earth's geological history. The era of the highest oxygen content in Earth's geological history is thought to account for 27% to 28% of the atmospheric content, which is more than 30% of the oxygen content at sea level today. It is thought that there may be two such peaks of oxygen enrichment. One appeared in the Carboniferous period between 360 million and 300 million years ago. Plants at this time grow very large because of the rich oxygen. Ferns are often tens of meters to hundreds of meters high. After the death of these giant plants, a large amount of accumulation was generated, which is the largest source of coal formations today, so this era is called the Carboniferous. Another peak of oxygen enrichment may have appeared in the Jurassic period 200 million to 140 million years ago, when the world was full of giant reptiles, becoming the most prosperous era of dinosaurs. Therefore, once the earth is enriched with oxygen, the size of both plants and animals will increase simultaneously on a large scale. This is already the basic consensus. But the problem is,

Once enriched with oxygen, why do organisms generally grow larger? This may be the result of natural selection. First of all, it is necessary to understand a major premise, that is, whether it is a plant or an animal, growing larger, the benefits to itself and the population must outweigh the disadvantages. Whether it is an animal or a plant, once it becomes relatively large, the first benefit is that it becomes more long-lived. For example, huge trees can live for hundreds or even thousands of years without dying. Smaller plants generally live throughout the year. The same is true for animals. The age of giant elephants and blue whales starts at 80 years old, and many can live to 120 years old. And most mice only have a lifespan of five or six years. According to the proportion of body size, the normal life expectancy of human beings is about 45 years. The average life expectancy of the ancients is precisely 45 to 50 years old. Modern humans generally reach the lifespan of elephants, which is the result of technological progress and the absolute dominance of human beings on the earth. And the larger the size and weight of the species, the fewer natural enemies. It can also heal itself better when encountering ordinary injuries. Therefore, organisms have a natural requirement to pursue greater volume and weight.

Earth creatures have a natural need to make themselves bigger, so if conditions allow, they should try to grow bigger. Except for some bacteria, animals and plants all need respiration to circulate the substances and energy in their bodies. And breathing itself is very energy-intensive. For example, when a person is sleeping, the largest energy-consuming part of the body is the respiratory muscles. In fact, the same is true for plants, and respiration requires high energy consumption. Therefore, whether it is an animal or a plant, under the condition of constant oxygen content, the larger the body mass and weight, the larger the respiratory system must be. And the respiratory system is difficult to enlarge quickly, which is also one of the most retarding factors that animals and plants cannot suddenly enlarge. In an oxygen-enriched environment, the respiratory system does not require much energy to meet the oxygen demand of the body, and the most direct manifestation is in insects. Insects are special. In addition to breathing through the respiratory organs, that is, the original lungs, insects can also breathe directly through the stomata on the body. Plants also have stomata. Of course, in humans, only the nostrils have no stomata. In the case of high oxygen-rich atmosphere, insects and most plants can meet most of the respiration requirements only by the natural penetration of oxygen surface.

On the contrary, it does not use the respiratory organs that consume a lot of energy. Therefore, the plants and insects of the Peat period were terrifyingly huge. The general enlargement of plants in turn leads to stronger photosynthesis and more oxygen release. Plants and insects have grown larger, and more animals on the food chain will have unfinished food, and will naturally grow larger. At this time, the earth ushered in the most prosperous era in prehistory. And if the Earth's oxygen levels have always been this high. Plants are overwhelming, and there are giant pythons and insects everywhere. Humans have no room for evolution at all. It is precisely because the earth was hit that the oxygen content dropped sharply, and the dinosaurs either died out or became little birds. Human beings have the premise of development and evolution. So it looks like everything is arranged. It's really just a matter of probability.