One of the fundamental questions in all sciences is how humanity, or more generally, life came to be. Although much data has shown it is likely that life developed here on earth as a direct consequence of molecules colliding with each other, the process has not been described in detail. After all, the amount of planets in the universe is so vast that this process could have taken place on any number of them. Yet, earth is the only habitable planet as far as we are aware, so how is this possible?
To start at the beginning, one needs to understand a major law of biology: evolution (Trevors & Saier, 2010). According to Darwin’s theory of evolution, which he published in 1859, individuals produce more offspring than can survive in the environment. In his book, Darwin wrote that only the offspring that is adapted to the environment the best, and therefore has the highest fitness, is able to survive and to procreate (Darwin & Kebbler Wallace, 1859).
Though Darwin did not know it at the time, the molecule that is responsible for evolution is DNA. The offspring of any individual belonging to any species has intrinsic variations in its DNA which causes variability. This is exactly why often, brothers and sisters do not look exactly like each other. This way, some offspring have a random chance of being better adapted to the environment and to be more likely to survive. Consequently, this individual’s genes will be passed on to the following generations. This process is known as natural selection. Darwin's book, the On the Origin of Species, was the first published work to contain a description on this process (Darwin & Kebbler Wallace, 1859).
However, researchers have not identified DNA as the first step towards the origin of life. Instead, it is thought that life started out based on RNA, the cousin of the former (Alberts et al., 2017). Structurally, RNA is much less complex than DNA (Vlachakis et al., 2021). The origin of RNA, however, remains unclear. Yet, in 2021, a study was performed by three researchers in Athens, who showed, through complex mathematical models, that when carbon atoms collide often enough they eventually bind to each other in rings of 5 and 6 atoms (Vlachakis et al., 2021). By the attachment of a few nitrogen, oxygen and hydrogen atoms, carbon rings form RNA. The study showed many steps would have needed to be taken before RNA could come into existence.
According to Kahana et al. (2019), the current hypothesis is that earth was full of atoms and simple molecules that were not able to create living things, called the primordial soup. After enough time, protons, neutrons and electrons had collided so much that carbon and other atoms had formed. These had collided and formed molecules, which in turn had collided to form RNA (Vlachakis et al., 2021).
One of the abilities of living things is that they can replicate. If a bacterium, a human or anything else would not be able to replicate, no living creature could make progeny, or evolve. This is why even the first living things on earth should have been able to replicate themselves (Joyce & Szostak, 2018). RNA is able to replicate itself, as single stranded RNA can simply bind to complementary bases as herein explained. Although at this point in time, no proteins were present to accelerate the process, given enough time this would have been a possibility, as shown by the simulations of Vlachakis and his group.
In modern life, RNA fulfils two functions. First of all, it is able to store genetic information. Some viruses, for example, have an RNA-based genome rather than one that consists of DNA (Alberts et al., 2017). Additionally, RNA is translated into protein. The cells that make up life consist of complex mechanisms and a large variety of proteins, or tiny machines, that accelerate biochemical processes such as the synthesis of proteins. Such machines required for protein synthesis are very complex, and were not around when the first RNA molecules formed. In other words, the basic concept of RNA being translated into a sequence of amino acids, also known as a protein, would not have been able to occur in this prebiotic era. Instead, it is thought that RNA was translated into very simple proteins without the aid of such machineries. These evolved later. The fact that this still happens in fungi creating antibiotics, for example, attests to the viability of this hypothesis as Vlachakis et al. state (2021).
One question that leaves people confused about the scenario of an RNA-based world is why modern biology is DNA-based. This is a simple consequence of natural selection. DNA is much more stable than RNA, and therefore increases the fitness of living things with respect to RNA-based organisms (Alberts et al., 2017). RNA is however, still a big player in molecular biology, and is involved in every single process that occurs in any cell. It can be seen as a less valuable version of DNA. For the synthesis of proteins, the genetic code is required, which is given in an intermediate, RNA, so that the DNA can stay safely in the well-protected nucleus of the cell (Alberts et al., 2017).
The question why life developed exclusively on earth, as far as is known, remains unanswered. Simply put, earth is likely to check all the boxes of requirements for life. One of these requirements is the localisation in the habitable zone. The habitable zone is the area around a star in which liquid water may exist (Ramirez, 2018). The planets closer than earth to the Sun are too warm for liquid water to exist. The planets further away do also not contain liquid water, as H2O would freeze directly (Lovett, 2012).
The definition of the habitable zone is somewhat short-sighted. The distance between the sun and the planet in question indeed suggests whether liquid water can be found. Yet, the habitability of a planet severely depends on multiple factors of both the star it orbits, and the planet itself. The magnetic field of the earth, for example, protects its surface from bombardments of charged particles and cosmic radiation, which are severely carcinogenic to living things (Khodachenko et al., 2008). Similarly, the ozone layer, which is part of the earth’s atmosphere, protects the surface from the harmful ultraviolet-radiation (Björn, 2007). The impact of a meteorite has been linked to the most recent mass extinction, in which the dinosaurs died (Siraj & Loeb, 2021). However, such impacts are not solely devastating to life. In the early years of the earth, they are thought to be responsible for the deliverance of certain chemicals that eventually played the role in the evolution of life (Osinski et al., 2020). All in all, there are many other factors that play a role in the habitability of a planet.
The fact that no other forms of life have been found makes it difficult to pinpoint what is so truly unique about earth and what enables it to sustain life. Perhaps the earth developed through the exactly right steps required for the development of life. Much remains unknown, and one of the questions that humans have asked themselves for thousands of years remains unanswered. Where we come from, no one really knows.
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Cover image: Polarpedia. (n.d). The Origins of LIFE on Earth. [image]. https://polarpedia.eu/en/the-origins-of-life-on-earth/
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