Charles Darwin is historically renowned for his theory of evolution as the basis of variation of biological life that exists around us. Similar views were also echoed by the contemporary biologist, Alfred Russel Wallace, who is credited with independently conceiving the theory of evolution through natural selection. His paper on the subject was jointly published with some of Charles Darwin's writings in 1858. Despite strong opposition of the contemporary Christian Church of the time, gradually the natural selection theory for the biological evolution was accepted and remains pivotal to explain both inter- and intraspecies variation. However, a small section of theologists, philosophers and biologists continue to hold different views for the origin of life and its developments into numerous species and variation. Both schools, however, believe and acknowledge the genetic basis of life within the realms of nucleic acids, the ribose nucleic acid (RNA) and the deoxyribose nucleic acid (DNA). The fundamental questions remain—what must have been the earliest form of life on Earth? What is the relationship to life beyond Earth, if it exists at all?
Molecular biologists agree that the earliest form of life must have been based on the RNA molecule, with DNA evolving later following successive chemical processes to facilitate the RNA functions. It is argued that the earliest life would have been heterotrophic, arising from and metabolically processing prebiotic organics as proposed in the Oparin-Haldane theory. In this context, it is important to consider the abiogenesis theory that life on Earth arose from nonlife more than 4 billion years ago. Abiogenesis proposes that the original life forms were very simple and gradually became increasingly complex. It probably preceded biogenesis, in which life is derived from the procreation of other life. Abiogenesis became impossible once the present composition of the Earth was achieved. The abiogenesis theory is neither proved nor disproved. The cosmic origins of life discussed in the many articles in this volume remains at the forefront of modern discussions.
In the 1920s, the famous British biometric scientist John Haldane and Aleksandr Oparin, the Russian biochemist independently set forth similar ideas for the origin of life on Earth. Both believed that organic molecules could be formed from abiogenic materials in the presence of an external energy source, for example the ultraviolet radiation, in combination with very low atmospheric oxygen, ammonia, water vapor, and probably other gases. Both agreed that the earliest form of life probably first appeared in the warm oceans of Earth and were heterotrophic, nurtured by preformed nutrients from chemicals in existence on early Earth, or brought in by comets compared to being autotrophic, generating food and nutrients from complex interactions of the sunlight, including cosmic radiations with inorganic materials. Oparin believed that the life developed from microscopic spontaneously formed spherical lipid molecules held together by electrostatic forces, probably the earliest form of cells. These molecules most likely functioned as enzymes, essential for the biochemical metabolic reactions necessary for life's evolution. Haldane, unfamiliar with Oparin's theory, believed that simple organic molecules formed first and in the presence of ultraviolet light became increasingly complex, ultimately forming cells. Haldane and Oparin's ideas formed the foundation for much of the research on the origin of life, specifically formation of cells.
The Haldane-Oparin theory on the origin of life was tested to a limited extent by two American chemists, Harold Urey and Stanley Miller. They successfully produced organic molecules from some of the inorganic components thought to have been necessary for the appearance of life, the prebiotic phase. The Miller-Urey experiment included combination of warm water with a mixture of water vapor, methane, ammonia, and molecular hydrogen exposed to the atmospheric electrical discharges in the form of lightening. Miller and Urey found that simple organic molecules, including amino acids (the building blocks of peptides), had formed under the simulated conditions of early Earth. The Miller-Urey experiment demonstrated that organic molecules could form from abiogenic materials under the constraints of Earth's prebiotic atmosphere. Later research showed that amino acids can spontaneously form small protein molecules (peptides). It is also shown that the RNA molecule can be artificially synthesized from nucleotides (nitrogen containing compounds or bases) linked to sugar and phosphate groups. In fact, in addition to carrying and translating genetic information, RNA acts like a catalyst, a molecule that increases the rate of a reaction without itself being consumed. It is, thus, logical to accept that multiple forms of RNA existed in the prebiotic phase of abiogenesis that led to the formation of life on Earth or in any wider context.
It has been suggested that peptide nucleic acid (PNA) might have been the first genetic material that preceded the RNA and DNA. PNA forms very stable double helical structures and even stable triple helices. However, the chemical nature of PNA does not allow it be a sustainable genetic material necessary for replication, transfer of genetic information or its reorganization. Activated PNA monomers tend to cyclize easily and thus formation of oligomers is very difficult under prebiotic conditions. Further, and PNA hydrolyses rather rapidly and thus restricts the chances of it ever being accumulated in sufficient quantity, for instance deep in the primitive oceans.
The concept of sunlight (ultraviolet radiation) and energy sources from other planets in the cosmos (cosmic radiations) influencing the origin of life on Earth, or indeed anywhere in the cosmos, falls within the field of Astrobiology. The main question remains on the likelihood of the existence of life beyond the confines of Earth or perhaps in the wider cosmos. Astrobiology deals with the processes that may lead to the origin and evolution of life on Earth. Scientists continue to focus first on sites for understanding the processes of chemical (prebiotic) evolution (Titan, asteroids, and comets) and second on targets for searching for life beyond Earth (Mars, Venus, Europa, Enceladus, and others). Mars is one of the most popular targets due to the fact that early Mars and early Earth had a similar history. The quest for life on Mars is one of the most important missions of the modern astrobiology. Several ethicists and philosophers have strong reservations in relation to ethical, social and legal concerns that fall within the new discipline of astrobioethics. Perhaps the main stumbling block of research in life on Mars is the anticipation of a close similarity to life on Earth.
Most astrobiologists are guided by the following broad requirements and principles on the origin of life:
-
(1)
The elements of life, carbon, hydrogen, oxygen, and nitrogen (CHON).
-
(2)
CHON + energy (e.g., UV from stars, atmospheric lightning discharge) = monomers (Miller-Urey micromolecules).
-
(3)
Monomers + aqueous environment (to protect prebiotic organics from destruction by UV) + concentration/assembly/ordering = polymers.
-
(4)
Some such polymers presumably incorporated both gene-like information and enzyme-like catalytic activity (ribozymes). The information content in the enzymes being of a super-astronomical magnitude opens the way to the concept of a cosmological origin of life and the ideas of panspermia discussed by Sir Fred Hoyle and Prof. Chandra Wickramasinghe.
-
(5)
Earliest life, wherever it started, would thus have been heterotrophic and thermophilic.
The concept of cosmic genetic evolution emerged from discussions with the guest editors of this thematic volume. It is an extension of astrobiology to explore the extent of ultraviolet radiation (sunlight) and range of complex cosmic radiation systems implicated in genetic or genomic variation and function. The cosmic radiations (cosmic rays) include high-energy protons and atomic particles (nuclei) which move through space at nearly the speed of light. They originate from the sun, from outside of the solar system, supernova explosions of star, and from distant galaxies. These pass through the ozone layer and upon impact with the Earth's atmosphere, cosmic rays can produce cascade of secondary particles capable of striking the Earth's surface.
The fundamental questions remain:
-
•
Whether cosmic rays are capable of inducing spontaneous benign or deleterious genetic changes (mutations and variants)?
-
•
How far the cosmic ray phenomenon along with the introduction of new cosmic viruses could account for successive evolution of life on Earth?
-
•
Could cosmic radiation be an important factor in the emergence of different forms of biological life and variations?
-
•
How far does the impact of cosmic rays on the Earth might account for the creation of new viral variants capable of causing human disease, for example the Corona/Covid-19/SARS-CoV-2 virus?
The guest editors and other contributing authors have produced this impressive volume on a very unusual subject full of past and present hypotheses. Information and supporting arguments are most relevant and will form the basis for further discussion and design of new experiments and investigations. They all deserve to be congratulated on this exciting achievement. Now authors, editors and publishers shall wait for acceptance and appreciation of this unique contribution to the genetics literature.
Disclaimer and acknowledgements
This article is based on author's own perceptions, understanding and amateur knowledge of cosmology and astrobiology. Some of the ideas and concepts are derived from author's informal discussions with Professor Chandra Wickramasinghe, the Co-Editor of this thematic volume. His thoughtful insights and guidance are greatly appreciated. The author prefers to avoid add literature references as the subject is widely discussed in diverse digital open access public domains.
The author would be delighted to deal with any questions or observations related to this article: d.kumar@qmul.ac.uk; genomicmedicineuk@gmail.com.