With the consumption of organic molecules naturally produced in the primitive ocean, some cells gradually evolved new metabolic pathways, which can use other energy sources and inorganic molecules to synthesize the required organic molecules, thus becoming autotrophs. Autotrophic organisms include chemoautotrophic organisms that use chemical energy and CO2, H2 and H2S to synthesize organic substances. At the same time, there may be the earliest photoautotrophs that use H2S instead of H2O as the hydrogen source for photosynthesis, so they do not produce O2. Fossil evidence shows that the above autotrophic cells may have appeared about 3.5 billion years ago. 3. Photoautotrophic cells that produce oxygen appear.
The oldest and most reliable cyanobacteria fossils found were formed about 2.8 billion years ago. It is inferred that photosynthetic cells that can produce oxygen began to release oxygen to the environment about 3 billion years ago. Since then, life activities have had a great impact on the evolution of the earth. 4. Oxygen-tolerant and aerobic cells appear
With the increase of oxygen content in the atmosphere, some cells have gradually evolved a metabolic pathway that can utilize oxygen, that is, aerobic respiration. The energy metabolic efficiency of aerobic respiration is 18 times higher than that of anaerobic respiration. 5. Eukaryotic cells appeared.
About10.5 billion years ago, eukaryotic cell fossils began to appear in rock formations. The widely accepted theory about the origin of eukaryotic cells is the endosymbiont hypothesis.
This hypothesis holds that a kind of prokaryotic cells that live on phagocytosis often invaginate the outer membrane, and then the invaginated outer membrane wraps the exposed DNA in the original cell to protect its genetic material from foreign substances, thus gradually forming primitive eukaryotic cells. After that, the primitive eukaryotic cells swallowed the aerobic prokaryotic cells and formed a symbiotic relationship with them for the energy metabolism of aerobic respiratory pathway, and finally the symbiotic aerobic cells evolved into mitochondria. Chloroplasts in plant cells have similar origins. They evolved into chloroplasts through endosymbionts through the endocytosis of photosynthetic prokaryotes such as cyanobacteria.
Mitochondria and chloroplasts of modern organisms have their own independent small circular DNA genomes and gene expression systems similar to prokaryotes. Mitochondrial genomes mainly use genetic codons slightly different from nuclear genomes. These are all evidences of the endosymbiosis hypothesis. (b) Earth evolution and biological evolution
The evolution of the earth and the evolution of living things are two closely related and interactive processes. The geological changes of the primitive earth led to the origin of life, and life activities caused special changes in the earth's environment. Any change in the earth's environment would affect the evolution of life.
The long-term activities of early unicellular photosynthetic organisms (mainly cyanobacteria) have brought the most remarkable permanent change to the earth's environment, which is the accumulation of oxygen in the atmosphere. The increase of oxygen in the atmosphere in turn affects biological evolution, such as the emergence of aerobic respiratory metabolic pathways. Another change brought by the increase of oxygen content in the atmosphere is that sunlight converts some oxygen molecules (O2) into ozone molecules (O3), and then forms the ozone layer in the upper atmosphere, which can absorb most of the ultraviolet rays in the sunlight. With the protection of the ozone layer, life may appear everywhere on the earth's surface.
Another important influence of single-celled photosynthetic organisms on the earth's environment is that they use a large amount of CO2 in the atmosphere to synthesize organic carbon compounds, and then directly or indirectly form carbonates (such as calcium carbonate). ), deposited in the lithosphere. One of the direct consequences is that the concentration of CO2 in the atmosphere and the greenhouse effect of the atmosphere are greatly reduced, thus basically offsetting the continuous and slow increase of surface temperature caused by solar radiation for billions of years. Today, such a modern atmosphere with low CO2 content and high O2 content and a protective layer of ozone is the result of the long-term co-evolution of the earth and living things. (Network information)