Life is the most complex phenomenon on Earth. How did life originate? This is a scientific question that has not yet been fully resolved.
In 1924, Soviet scientist Oparin proposed the theory of aggregates. Aggregates are polymers of proteins and nucleic acids, which have a membrane-like structure and can undergo reactions such as starch or polymerized nucleotide synthesis under the action of enzymes. In 1953, American scientist Miller simulated ancient atmospheres by discharging electricity in a mixture of hydrogen, water vapor, methane, and ammonia, and found nitrogen-containing small molecules such as amino acids in the condensed liquid. This suggests that biological small molecules such as amino acids may have formed on the land or in the air of early Earth, and then entered the ocean with water or rain, polymerizing into proteins and nucleic acids. These aggregates interacted with organic matter in the ocean to form a membrane structure, eventually leading to the formation of primitive life.
The early Earth was very different from today. At that time, volcanoes were widespread, the atmosphere was thin, and due to the lack of oxygen and the absence of an ozone layer, the entire Earth was exposed to intense ultraviolet rays. Even if amino acids were accidentally produced by lightning, they would easily be destroyed by natural forces. Therefore, some scholars believe that life substances may have been brought to Earth from outer space by meteorite impacts. However, it is difficult to find convincing evidence, because meteorites face extremely high temperatures during their passage through the atmosphere, making it difficult to preserve life substances.
Another possibility is that life did not originate on the surface of the Earth, but in the deep sea. The "black smokers" in the deep sea emit methane and ammonia, which, under the action of high temperatures and sulfide catalysts, may also form organic substances such as purines and nucleotides, and eventually form aggregates or even life. Although the environment of the deep sea is extremely harsh for today's surface organisms, it is stable and has sufficient energy and material supply. If life originated there, it might not be destroyed and preserved. Currently, scientists have found some prokaryotic organisms in the deep sea and deep underground that are very similar to bacteria. They survive in extreme environments (such as temperatures above 100°C, sulfur-rich anoxic environments, high salinity, and strong acid or alkali conditions), and their molecular biological characteristics, such as the process of gene transcription, are significantly different from those of bacteria and closer to eukaryotes. In 1977, microbiologists established a new group of prokaryotes called Archaea. They may be closely related to the earliest life. However, research on the origin and evolution of Archaea is still in its early stages and cannot provide sufficient evidence for the origin of life.
More reliable evidence of the origin of life comes from fossil records. Fossil records can to some extent preserve the forms of the early stages of life origin, such as fossils of primitive algae and bacteria, as well as some research in the field of sedimentology.
The earliest algal fossils are mainly stromatolites, which can be traced back to 3.5 billion years ago. Paleontologists have also found fossils of filamentous organisms resembling bacteria in volcanic rocks, which are believed to be fossils of ancient bacteria, dating back to about 3.5 billion years ago. Theoretically, there should be even earlier fossils of early life buried in the strata, which still need to be further discovered.
Research in the field of sedimentology also provides some help in understanding the origin of life. One of the oldest sedimentary rocks on Earth - banded iron formations, formed mostly in the Proterozoic eon about 2 billion years ago. These iron layers are important resources for iron mining in North China and Russia today, mainly composed of iron oxides. The large-scale deposition of these iron layers requires a large amount of oxygen. The increase in oxygen content caused the ferrous ions, which were soluble in water, to be oxidized into ferric ions, resulting in the deposition of iron oxides. The early Earth had a low oxygen content, and the large-scale appearance of banded iron formations records the early stages of the Earth's history.It was the first large-scale increase in the concentration of oxygen in the atmosphere or seawater. And the earliest oxygen-enrichment event on Earth might have been accomplished by the earliest life forms.