The Archean era (the era of ancient life) began 3.5 billion years ago.
Living organisms that appeared at the beginning of the Archean era were anaerobic heterotrophs and fed on ready-made organic matter contained in the ocean. Most likely, the first living organisms were primitive prokaryotes.
Gradually, the reserves of abiogenic organic substances decreased. This led to an intensification of competition between organisms, the complication of their structure and the emergence of new methods of metabolism.
The emergence of autotrophic organisms
About 3 billion years ago, the most important aromorphosis took place – the emergence of autotrophic nutrition (chemosynthesis and photosynthesis), i.e., the ability to synthesize organic substances from inorganic ones.
Autotrophic photosynthetic prokaryotes (cyanobacteria) evolved free oxygen. Its accumulation changed the atmosphere and had a decisive influence on the entire further evolution of living things. The presence of oxygen led to the emergence of a more favorable oxygen type of metabolism and the emergence of aerobic organisms.
The emergence of eukaryotes
The next most important stage of evolution is the emergence of eukaryotic unicellular organisms about 1.5 billion years ago. Most scientists believe that the first eukaryotic cells arose from prokaryotic cells.
The authors of the autogenous hypothesis suggest that cellular organelles arose by invagination and separation of sections of the outer membrane with parts of the cytoplasm containing DNA.
The authors of the symbiotic hypothesis argue that the eukaryotic cell arose as a result of several symbiosis. The starting point was a prokaryotic cell capable of amoeboid movement. She swallowed smaller cells that were not digested, but gradually turned into various organelles.
The onset of the sexual process
Further evolution of unicellular eukaryotes led to the emergence of the sexual process, which significantly increased the diversity of living organisms and accelerated their further evolutionary development.
The emergence of multicellularity
Some scientists suggest that the first multicellular organisms evolved from colonial flagellates (such as volvox). Multicellularity led to a further complication of life: cell differentiation and tissue formation, the emergence of organs and organ systems. Multicellular organisms have a number of advantages over unicellular organisms. They were capable of a longer existence and active movement, had more opportunities for obtaining and digesting food.