When Earth formed along with the Solar System about 4.6 billion years ago, it looked much different than today. Even after the formation, asteroids continuously crashed into Earth for another hundreds of million of years, which created friction heat that covered the entire Earth with magma resembling spurts from volcanic eruptions. How did life first appear in this situation? Since it is difficult to reproduce the experiment and as there are no relevant fossils, we must make assumptions on the emergence of life based on partial scientific findings.

First, experiments on the origin of life confirmed that basic elements presumed to have existed in the early atmosphere can create amino acids and nucleic acids such as DNA and RNA. These experiments provide grounds for the hypothesis that the first cell was developed through the chemical evolution of basic elements of life that were created over tens of millions of years. Beginning in the sea, then concentrated in standing water, these elements transformed into more sophisticated compounds.

A second hypothesis puts forth the idea that the organic compounds necessary for life were delivered to the Earth on meteors or asteroids from outer space. This is based on some 100 types of organic compounds in the interstellar clouds, leading some to suggest that key elements of life such as amino acids were created in the outer space. However, since asteroids and meteors enter the Earth’s atmosphere at an extremely high temperature, it is doubtful whether the compounds could have survived the journey through Earth’s atmosphere.

However, the recent discovery of life existing in extreme conditions on Earth has renewed interest in this idea that life could have started in such seemingly unfavorable conditions. This hypothesis suggests that life was created from deep sea geothermal springs on the ocean floor at depth of one kilometer or more. Despite an environment that reaches temperatures of 400°C, that is highly acidic, and does not receive a single ray of sunlight, organisms such as tube worms, crabs, shellfish, and shrimps have been found to live at these great depths. This discovery has led scientists to consider the possibility that first life on Earth was created with chemical energy of compounds such as hydrogen sulfide on the ocean floor.

Along with questioning the origin of life, scientists are also asking the questions of whether protein or DNA came first. If the amino acid sequence of protein depends on the base sequence of DNA, then DNA should come first. However, DNA replication requires the help of protein. In other words, it is a question of which came first, the chicken or the egg?

In the 1980s, RNA, a type of nucleic acid, was found to enable enzymatic reactions with the information of base sequences. Nowadays, it has been widely accepted that first life began with RNA which divided the function between DNA and protein. Even if protein was made from amino acids, and RNA and DNA were made from glucose, organic bases, and phosphoric acid, it cannot be considered a form of life without cells and some form of self-organization among these substances. Therefore, it is believed that various high molecular compounds came together in the last stage of chemical evolution to create a cell with the functions of metabolism and genetics.