Apart from humans, the plants and those inconspicuous algae may have the greatest impact on the global environment. Today, the oxygen content in the atmosphere is very high, reaching 20.94%, while the carbon dioxide content is very low, only accounting for 0.03%. However, the early atmosphere of the Earth was completely different. At that time, the main components of the Earth's atmosphere were hydrogen, water vapor, carbon monoxide, carbon dioxide, methane, and ammonia, which were more similar to the atmospheres of other planets in the solar system and the gases emitted by volcanic eruptions on Earth. This kind of atmosphere would be toxic to today's animals, but it was suitable for the growth and reproduction of certain bacteria and cyanobacteria. Cyanobacteria contain chlorophyll, which can absorb carbon dioxide in the atmosphere and carry out photosynthesis with the participation of sunlight to produce organic matter and release oxygen. Although cyanobacteria are very small and can only be seen with a microscope, they reproduce very quickly, increasing from 1 to 2, 2 to 4, and 4 to 8 in 20 minutes, so their number is quite astonishing. The oxygen released by cyanobacteria (and later algae) gradually increased the oxygen content in the atmosphere. Oxygen combines with carbon monoxide to form carbon dioxide, thereby gradually reducing the carbon monoxide in the atmosphere; oxygen can also combine with hydrogen to form water, thus gradually increasing the amount of water on Earth; and oxygen combines with methane to form carbon dioxide and water, and with ammonia to form nitrogen and water. Because oxygen consumption is very high, the amount of oxygen in the early Earth's atmosphere increased very slowly. By 3 billion years ago, it was only 0.1% of the current free oxygen level on Earth, and by 2 billion years ago, this figure had increased to 1%, and by 1 billion years ago, it had increased to 10%. On the contrary, the hydrogen, carbon monoxide, ammonia, and methane in the atmosphere were constantly decreasing, so the composition of the atmosphere became increasingly similar to the current level.
The increase in oxygen led to the formation of the ozone layer in the upper atmosphere. When the oxygen content in the atmosphere reached 1% of the current level, oxygen could appear in the atmosphere at an altitude of 10-50 kilometers, especially at an altitude of 25 kilometers above the Earth's surface. These free oxygen in the upper atmosphere, under the action of solar high-energy particles and cosmic rays, form the ozone layer. The ozone layer acts as a barrier to solar ultraviolet rays and cosmic rays, thus protecting the surface organisms from harm. The appearance of the ozone layer enables a large number of algae in the ocean to rise to the ocean surface to reproduce and grow, thereby releasing more oxygen.
The early Earth lacked oxygen, but volcanic activity emitted a large amount of toxic gases. Plants and microorganisms changed the composition of seawater. At 1.9 billion years ago, there was a large amount of divalent iron dissolved in seawater. When the oxygen content in the seawater increased greatly, these iron were oxidized to insoluble red trivalent iron, so at this time, many places of seawater sedimentary rocks contained sandstone with a small amount of iron oxide due to its red color. When the iron oxide content was higher, huge sedimentary iron deposits were formed, such as the huge Anshan Iron Mine in northeastern China. There are also large iron deposits formed during this period in Australia and Brazil. The increase in oxygen also increased the carbon dioxide content in the water, making the seawater contain a high level of bicarbonate, so its magnesium carbonate and calcium carbonate could precipitate, eventually forming a large amount of dolomite and some limestone. Between 2.6 billion and 540 million years ago, the seawater also contained a large amount of soluble phosphorus elements, which formed huge insoluble sedimentary phosphorus deposits under the action of microorganisms in the seawater, such as those found in Guizhou and Hubei, China.
Plants also covered the Earth with green, thereby improving the climate environment. The land area of the Earth increased rapidly in the Silurian period that began 4.43 billion years ago. But at that time, the land was bare and there were no plants, let alone animals. By the late Silurian period of 430 million years ago, the primitive naked ferns (with pseudo-roots and stems, and roots and leaves not yet differentiated) began to appear.Things began to land and gradually occupied the land, and then animals also began to develop on land, making the land gradually thrive. Especially in the Carboniferous period, due to the humid climate, most of the land was covered by forests, becoming the period with the most extensive plant coverage and the highest oxygen content in geological history. It can be seen from this that the impact of plants on the global environment is enormous.