Dr. Ikehara, getting a quick overview of the current state of the research the origin of life, describes the following.
“The riddle of the origin of life remains unsolved despite the strenuous efforts by many researchers. What is the major difficulty,- which has become the obstacle against solving the origin of life? The reason would be as follows.- : all kinds of extant organisms on the present Earth are living under the core life system composed of three biopolymers: DNA/RNA that carry genetic information, tRNA that realizes the genetic code and proteins with catalytic function. Nowadays, all the biopolymers with ordered sequence are synthesized under the genetic system,-. As DNA is synthesized by self-replication, both tRNA and protein are produced by expression of the respective genes. However, all the first three biopolymers, as a matter of course, must be synthesized through random processes, because the sequence of every biopolymer with an ordered sequence never was designed in advance. Therefore, the greatest obstacle,- which makes it difficult to solve the riddle of the origin of life,- would be to explain the way how synthesis of mere polymers with random sequence could be converted to the production of biopolymers with ordered sequence during a repeated random process.”
Source: Towards Revealing the Origin of Life, P.195, Fig. 9.2
＊Life (red box) emerged on the primitive Earth as a result of the establishment of a fundamental life system, which is composed of proteins, cell structure, metabolism, tRNA, the genetic code and genes. Outline of the steps from chemical evolution for syntheses of organic compounds to the emergence of life, which are deduced from [GADV]-protein world hypothesis or GADV hypothesis, is shown in the figure. It is observed that all the steps to the emergence of life are supported by [GADV]-amino acids and the related organic compounds as [GADV]-peptides, GNC genetic code encoding [GADV]-amino acids and a (GNC)n RNA gene encoding a [GADV]-protein and so on.
Source: Towards Revealing the Origin of Life, P.209, Fig. 9.5
＊According to the GADV hypothesis, the steps to the emergence of life can be reasonably explained as follows. (1) Immature [GADV]-proteins, actually aggregates of [GADV]-peptides, were produced by random joining of [GADV]-amino acids under protein 0th-order structure. (2) Cell structure or [GADV]-microsphere was constructed by aggregation of the [GADV]-proteins. In parallel, [GADV]-protein world was formed in the [GADV]-microsphere. (3) In the [GADV]-protein world, the initial metabolic system was formed. (4) AntiC-SL tRNA was produced by catalytic reactions with immature [GADV]-proteins in the initial metabolism. (5) The GNC primeval genetic code was established accompanied by the formation of four specific AntiC-SL tRNAs. (6) Double-stranded (ds)(GNC)n gene was formed through complementary strand synthesis of ss-(GNC)n RNA, which was produced by random joining of anticodons carried by the AntiC-SL tRNAs. (7) The first life eventually emerged upon the formation of ds-(GNC)n genes.
Source: Towards Revealing the Origin of Life, P.217, Fig. 9.10
＊(A) Three biopolymers, proteins, tRNAs and RNAs, were produced on the primitive Earth from [GADV]-proteins. Yellow curved thick arrows indicate that the first and second keys contributed to the formation of the subsequent second and third keys, respectively. Formation of the third key was assisted not only by the second key shown in the yellow box but also the first key described in the pale blue box. Downward blue arrows indicate proteins synthesized under the respective systems. The first imm-[GADV]-protein* (the most left, bottom box) was actually an aggregate of [GADV]-peptides containing organic compounds as simple alkyl amines and alkyl carboxylic acids. In contrast, immature (imm)-[GADV]-protein means that the protein was composed of only [GADV]-peptides with random (rand) amino acid sequence.
(B) A mature (mat)-protein like a precision polymer machine with ordered sequence (OS) could be first produced under the ds-(GNC)n gene. White bold arrow indicates ds-(GNC)n RNA formation by complementary strand synthesis of ss-(GNC)n RNA. Red bold arrow indicates maturation process from ds-(GNC)n RNA encoding an immature protein with random amino acid sequence to ds-(GNC)n RNA gene encoding a mature protein with ordered amino acid sequence.
Dr. Ikehara describes the circumstances that led to the GADV hypothesis as follows.
“I was fortunate to reach the GADV hypothesis. It would be important to consider the reasons why I could reach the GADV hypothesis. The reasons, are as follows.”
1. I have studied theoretically the origin of life by analyzing databases of microbial genes, proteins and so on. I feel now that the analyses of the databases led me to the GADV hypothesis.
2. I have explored the study on the origin of life through a top-down approach, as I started with my studies with a question of how and from where an entirely new gene could be created on the present Earth.
3. Consequently, I could reach a novel establishment process for the fundamental life systems in order of the origins of protein, the genetic code and gene, owing to the top-down approach.
4. Furthermore, I could reach a novel idea,- which can explain the six origins of protein, cell structure, metabolism, tRNA, genetic code and gene comprehensively. Nobody would doubt that extant organisms are living under the fundamental life system composed of the six members. In addition, many researchers would agree with the idea that the first life emerged on the primitive Earth upon acquisition of the six members.
5. During my studies, I could fortunately notice a protein 0th-order structure composed of GADV-amino acids, in which water-soluble globular protein could be produced by random joining of GADV-amino acids. Inversely, it is difficult to explore the origin of life without the concept of protein 0th-order structure.
6. This meant that I could reach the GADV hypothesis as a trigger of acquisition of GC-NSF(a) with an entirely new gene creation in present microorganisms. Another reason is because, as a result, my research has accidentally but fortunately progressed up through the core part of the fundamental life system (gene, the genetic code and protein).
“Thus, I could fortunately find the protein 0th-order structure as a clue of the GC-NSF(a) hypothesis during my research process. Inversely, I have to state that all my ideas on the origin of life based on the six members or the fundamental life system are wrong,- If the protein 0th-order structure is wrong. Therefore, I consider that it would become an important problem in the future whether or not the protein 0th-order structure is correct or wrong.”
Reference: Ikehara K(2021) Towards Revealing the Origin of Life. Springer Nature