English edit: Katyanne M. Shoemaker
Part II: Ideal conditions for the origin of life (as we know it)
Artist's conception of early Earth. Font
Earth's first 400 million years were hostile and desolate: temperatures of over 200 oC liquefied the crust, and volcanic gases, especially CO2, were released in large quantities into the forming atmosphere. As the Earth cooled, the crust solidified and the lower temperature allowed liquid water to remain on the surface. This cooling was a key factor in the emergence of life.
In addition, organic molecules, generated in the nebula that gave rise to our solar system, underwent chemical reactions. This resulted in more complex organic molecules, composed especially of Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus and Sulfur. These were the building blocks for the first biological molecules.
Another important event that allowed the development of life was our planet’s impact with a celestial body the size of Mars, which resulted in the formation of our Moon. It is curious to think that a collision with 100 million times more energy than the impact that killed the dinosaurs was pivotal in the establishment of life on our planet. The gravitational force of the newly formed Moon stabilized the incline of the Earth's axis. Without this stability, major climatic changes would occur, and complex life forms would likely not have developed.
Origin of the Moon: Artist's conception. Font
Other features of our planet were also fundamental to the emergence and maintenance of life, including the presence of a metallic nucleus, which generates a magnetic field and acts as a protective shield against cosmic radiation. Additionally, the presence of a mantle and its movement below the crust promotes tectonic activities such as volcanism and continental shift. Volcanism was very important in the emergence of life, since its gaseous emissions provided the compounds (CO2, H2S, etc.) that may have been used for energy by the first unicellular organisms. Volcanoes also help maintain the planet's climate and help recycle carbon back to living organisms.
Part III: Our chemical origins: the formation of biomolecules
An incredibly rare set of conditions (see Part II) allowed life to arise on our planet from organic molecules and chemical reactions. Today, all of Earth’s living organisms are composed of biomolecules such as proteins, nucleic acids, polysaccharides and lipids.
These biomolecules consist of small units interconnected with one another, called monomers. The biomonomers that form proteins, nucleic acids (DNA and RNA) and polysaccharides are respectively the amino acids, nucleotides and monosaccharides. We now know that most biomonomers can be produced spontaneously when given the necessary conditions.
One of the first attempts to produce biomolecules in the laboratory was done by Stanley Miller and Harold Urey in 1953. They were based on studies conducted by Alexander Oparin and J.B.S. Haldane who suggested that biomolecules and life would have emerged in a “primordial soup,” an atmosphere rich in methane, ammonia, hydrogen, and water vapor.
The Miller-Urey experiment attempted to simulate these primitive Earth conditions described by Oparin-Haldane. In a sealed system, gases were introduced to create the primitive atmosphere described above, a heat source and liquid water were added, as well as electric discharges. Under these conditions, a number of biomonomers, such as the amino acids glycine and alanine, and other organic compounds such as urea and formic acid were produced.
Although recent studies indicate that the composition of the primitive atmosphere was not exactly as Oparin and Haldane proposed, the importance of Miller-Urey's experimental results revolutionized our concept of the origin of life by solidifying the idea of a chemical origin for all living organisms.
Types of biomolecules. Font
The next step in the emergence of the first living cells was the polymerization of these small structural biomonomers. How did amino acids, monosaccharides and nucleotides form protein chains, polysaccharides, or the complex structure of DNA and RNA? Unfortunately we still do not have all of the answers to these questions, and the hypotheses that have been developed are difficult to test.
An important question when discussing the origin of life is how these biomolecules clustered together to form the first living cells capable of carrying genetic information and reproducing themselves. This is also a question that still challenges science, but many researchers are exploring new ideas that may explain the great leap from an essentially chemical world to a biological one.
Genetic information flux. Font
One of the first steps of this great leap is to understand how a nucleic acid molecule has the essential role of storing information that can be transmitted to subsequent generations. One of the most accepted hypotheses for the origin of genetic information is that of the RNA world, which suggests that RNA arose before the DNA molecule. However, in living organisms today, the flow of genetic information begins with DNA. Why then, would the first cells or proto-cells have RNA as the main source of genetic information?
DNA in today's cells require a complex machinery of proteins to be replicated. These proteins, in turn, require a DNA molecule that carries the information for later translation. Thus, the dichotomy of which originated first, DNA or protein, makes this question virtually unsolvable.
RNA world hypothesis. Font
For this reason, many scientists suggest that RNA was the first informational molecule to emerge, as it contains two essential properties for the maintenance of a primitive cell: a ribozyme activity, which makes it capable of catalyzing its own replication, and a catalytic activity capable of synthesizing some proteins. We still do not understand how mutations in the RNA molecule gave rise to DNA or how DNA was subsequently selected as the main source of genetic information of the cells.
Another important step for the formation of the first living cells is the emergence of compartmentalization. All cells have a plasma membrane composed essentially of phospholipids that guarantees the protection of the cytoplasmic content. Compartmentalization stores the molecules inside the membrane, facilitating chemical interactions. In addition, the selective permeability of the plasmatic membrane makes the chemical concentration inside of the cell different from the concentration of the surrounding environment, a characteristic fundamental for many cellular processes.
Lipid compartments are spontaneously formed due to their amphipathic nature - just mix a little oil into a glass with water and soap and watch. On primitive Earth, the compartments likely formed around biomolecules and some constituents that eventually gave rise to the first forms of metabolism and cellular functioning.