RNA World

Introduction

The origin of life is a topic that has been widely explored in academia, with various hypotheses being presented. To evaluate the evolution timeline of life, it is important to consider the origin of its precursors, which are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Examining how the two components of genetics evolved could provide a clear understanding of the origin of life. A leading hypothesis in scientific literature is the RNA world, which stipulates that RNA formed the basis of early life on earth (Higgs & Lehman 7). According to the theory, RNA emerged before DNA, that is the basis for modern life forms. In that regard, it is important to consider specific changes in the earth’s evolution that may have led to the formation of RNA and subsequently DNA.

Formation of the Earth

Whereas there is no concrete evidence on how the earth was formed or when, various geological studies have attempted to provide possible scenarios of its formation. It is estimated that the earth was formed approximately 4.5 billion years ago and during the process various chemical reactions took place that influenced the composition of the earth surface (Joyce 214). The impacts during the bombardment phase, involving comets and asteroids, caused a significant change in the earth’s crust and mantle. Consequently, the process influenced the composition of the earth’s early atmosphere, like the presence of various chemical elements. As a result of such changes, the earth formation process is linked to the origin of life. However, the evolution of early life and the formation of the earth did not occur concurrently since the bombardment could have posed a significant challenge to the survival of any organism. 

As the earth surface stabilized, various conditions that favored the growth of life may have formed. A key element that must have been present was water to provide the medium for biochemical reactions to take place. The presence of oceans and seas may have provided the medium for early chemical processes. Additionally, there must have been a stable climate to allow for the accumulation of complex molecules. Other factors that may have provided the foundation for early life forms during the earth formation phase include a source of energy, like the sun and volcanic activity, to trigger chemical reactions and the synthesis of molecules. The presence of favorable conditions after the bombardment phase in the earth’s formation may have catalyzed specific chemical reactions that led to the emergence of early life forms. 

Prebiotic Chemistry

Before the emergence of life, there must have been favorable conditions on earth for the formation of some of the basic molecules that are the building blocks of life. In that regard, the prebiotic period may have occurred immediately after the formation of the earth and before the emergence of the first living things. Various scientists have attempted to simulate the conditions that existed during that time, leading to the emergence of the building blocks of life. In the Miller-Urey experiment in 1953, Stanley Miller and Harold Urey simulated the prebiotic conditions (Criado-Reyes 1). They exposed a mixture of water, methane, ammonia, and hydrogen to electrical sparks to mimic lightning (Criado-Reyes 1). The result of the experiment was the production of amino acids, which are the basic building blocks of proteins. Subsequent experiments over the years have shown that amino acids can be formed through various prebiotic pathways, including different gas mixtures, impact processes, and the energy sources (Criado-Reyes 1). Therefore, the prebiotic period may have provided the appropriate conditions for the formation of amino acids that are essential for the formation of proteins.

Nucleotides are another essential building block of RNA and DNA. Under controlled conditions, scientists have been able to synthesis different nitrogenous bases, like adenosine and guanosine (Suárez-Marina et al. 2). Most experiments have focused on the dehydration reaction of ribose with suitable purine bases and in the presence of a phosphate group, such as inorganic phosphate salts (Suárez-Marina et al. 2). The success of such experiments supports the theory that reactions on the earth surface during the prebiotic period may have led to the formation of simple molecules like amino acids and nucleotides that are the basis of different life forms. 

RNA World

The RNA world hypothesis posits that RNA was the primary carrier of genetic information, unlike in modern cellular life that uses DNA. RNA may have served a dual role that is as a storage and carrier of genetic information (Higgs & Lehman 7). Similar to DNA, RNA is composed of nucleotides that may have served as carriers of genetic information, providing the necessary genetic instructions for protein synthesis. Additionally, RNA is involved in the transcription process, with messenger RNA carrying the genetic code to ribosomes for protein synthesis. Studies have also shown that RNA has catalytic properties, such as self-splicing introns (Higgs & Lehman 7). Such findings suggest that RNA may have catalyzed early biochemical reactions in a similar manner as proteins. Therefore, the ability of the molecule to serve a dual role, simplified the complexity of early cellular systems. It provided the basis for later evolution into complex life forms.

The RNA world hypothesis also proposes that some RNA molecules were capable of self-replication. They could form complementary strands through non-enzymatic template-directed polymerization in the absence of a protein enzyme or a ribosome (Higgs & Lehman 9). Such as process is comparable to the synthesis of complementary DNA strands in modern life forms. Additionally, an RNA molecule can serve as a template for the synthesis of the complimentary strand through polymerization by a replicase. Considering that ribozymes can serve as catalysts as discussed earlier, they may have accelerated the self-replication process and the synthesis of RNA strands. Based on those observations, it is evident that RNA was essential in the perpetuation of early life forms. 

Transition to DNA-Based Life

The self-replication process of RNA may have had evolutionary implications, leading to the emergence of DNA-based life. Similar to the issues that have been observed in DNA replication, there may have been the occurrence of errors and mutations over time, which caused a variation in RNA sequences. Notably, natural selection may have favored the transition to DNA as it provides various advantages, such as increased genetic stability and a low likelihood of errors during replication. The link between RNA and DNA still exists in modern-life forms with RNA primers initiating the synthesis of DNA strands. Moreover, the ability of retroviruses to use reverse transcription to convert RNA to DNA provides evidence of the possibility of DNA being integrated into the genetic profile of early life forms through the effect of a retroviral agent. Therefore, existing evidence supports the RNA world hypothesis that DNA-based life was a transition for early RNA-based life forms. 

Conclusion

The evolution of life on earth has occurred over billions of years. Based on the analysis conducted, the formation of earth was crucial to establishing the appropriate atmospheric and surface conditions for the growth of early life forms. The presence of water, specific gases, and energy sources may have led to the formation of simple molecules, like amino acids and nucleotides, which are the building blocks of life. They provided the basis for the synthesis of RNA that served a dual role of storing genetic information and as a catalyst for its self-replication to allow for the continuity of life. Evolutionary pressures may have later triggered the transition to the DNA-based life that is observed today.

Works Cited

Criado-Reyes, Joaquín, et al. “The Role of Borosilicate Glass in Miller–Urey Experiment.” Scientific Reports, vol. 11, no. 1, 2021, pp. 1-8.

Higgs, Paul G., and Niles Lehman. “The RNA World: Molecular Cooperation at the Origins of Life.” Nature Reviews Genetics, vol. 16, no. 1, 2015, pp. 7-17.

Joyce, Gerald F. “The Antiquity of RNA-Based Evolution.” Nature, vol. 418, no. 6894, 2002, pp. 214-221.

Suárez-Marina, Irene, et al. “Integrated Synthesis of Nucleotide and Nucleosides Influenced by Amino Acids.” Communications Chemistry, vol. 2, no. 1, 2019, pp. 1-8.