THE MYSTERY BEHIND ORIGIN OF LIFE

THE ORIGIN OF LIFE

The riddle of the “origin of life” is considered to be one of the most challenging questions in science. How life arose is a question that is fundamental to both philosophy and science. It’s unexplainable topic and has always been very difficult to comprehend. Now, here we are in the 21st Century, in the most technologically advanced age to date. Yet, we are no closer to a definitive conclusion than the philosophers who questioned life centuries ago. However, many new scientific theories have been born since then.

Life formation on the earth may have been taken place due to supernatural or divine forces. There are different kinds of accreditations by different religions:

CHRISTIAN & ISLAM CONCEPT: God created the universe, human beings, plants, oceans and rivers in six days. All the plants and animals were created at once. All the living organisms were created in the same form as they exist today.

What is life? Life is when a living matter, or any matter that shows certain attributes which include responsiveness, growth, metabolism, energy transformation, and also reproduction and it comprises living beings.

In biology, abiogenesis or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds.

What is abiogenesis? Abiogenesis is a theory that explains the origin of life on Earth through natural processes. The main problem that abiogenesis addresses is defining the precise conditions under which the first cells on Earth arose and whether proteins or nucleic acids were the molecules that initially formed the basis of living matter.

History of the Abiogenesis Theory

In 1871, Charles Darwin addressed the question of how life arose on the planet, suggesting that the "original spark of life" was generated in a pond of hot water which contained a variety of chemical compounds, electricity, light and other conditions that produced the first proteins capable of organizing themselves and producing life. Since then, several scientists have proposed different hypotheses in reference to the environmental conditions of the primitive Earth that were probably the most conducive to the formation of the first biological molecules and the subsequent appearance of the first cells.

IT STARTED WITH AN ELECTRIC SPARK…

In 1927, Belgain priest and astronomer George Lemaitre think the universe began with a cosmic explosion called Big Bang Theory about roughly 13.8 b.y.a ( billion years ago) when the first clouds of the elements hydrogen and helium were formed. Over a long time period, gravitational forces collapsed these clouds to create stars that converted hydrogen and helium into heavier elements, including carbon, nitrogen and oxygen, which are the atomic building blocks of life on earth. These elements were returned to interstellar space by exploding stars called “supernovas” which created clouds in which simple molecules such as water, carbon monoxide, and hydrocarbons formed. The clouds then collapsed to make a new generation of stars and solar systems.

Our solar system began about 4.6 bya after one or more local supernovas explosions. According to one widely accepted scenario, hundreds of planetesimals consisting of rocky or icy bodies such as asteroids and comets occupied the region where Venus, Earth and Mars are now found. The Earth, which is estimated to be 4.55 billion years ago, grew from the aggregation of such planetesimals over a period of 100 or 200 million years ago. For the first half billion years or so after its formation, the Earth was too hot to allow liquid water to accumulate on its surface.

By 4 b.y.a, the Earth had cooled enough for the outer layers of the planet to solidify and for oceans to begin to form.

The period between 4.0 and 3.5 bya marked the emergence of life that we know about produced microscopic fossils, the preserved remains of organisms that existed in the past. These fossils, estimated to be about 3.5 billion years old, resemble modern cyanobacteria, which are photosynthetic bacteria.

THE FIRST STAGE OF THE ORIGIN OF LIFE – by pondering how nucleotides and amino acids may have been prior to the existence of living cells.

Oparin-Haldane hypothesis called "A warm little pond": primordial soup

In the 1920s, the Russian biochemist Alexander Oparin and later supported by the British evolutionary biologist J. B. S. Haldane in 1928. The Oparin and Haldane theory is known as biochemical theory for the origin of life. These independently proposed that organic molecules such as, nucleotides and amino acids, arose spontaneously under the conditions that occurred on early Earth. According to this hypothesis, the spontaneous appearance of inorganic molecules produced what they called a “primordial soup” hypothesis which eventually gave rise to living cells, it proposes that early Earth had very little oxygen (O2), but instead was made up of water vapour (H2O), hydrogen gas(H2), methane (CH4), and ammonia (NH3).

Methane and ammonia are reducing agents because they readily donate their electrons. In the absence of oxygen, their reducing capability is powerful. Thus, the early earth had a reducing atmosphere in which oxidation-reduction (redox) reactions could have driven the chemical evolution, or abiotic synthesis, of organic monomers from inorganic molecules in the presence of strong energy sources.

According to the Oparin-Haldane model, life could have arisen through a series of organic chemical reactions that produced ever more complex biochemical structures. They proposed that common gases in the early Earth atmosphere combined to form simple organic chemicals, and that these in turn combined to form more complex molecules. Then, the complex molecules became separated from the surrounding medium, and acquired some of the characters of living organisms. They became able to absorb nutrients, to grow, to divide (reproduce), and so on.

The conditions on early Earth, which were much different from today, may have been more conductive to the spontaneous formation of organic molecules and eventually macromolecules, formed spontaneously. This is termed prebiotic (before life) or abiotic (without life) synthesis. These slowly forming organic molecules accumulated because there was little free oxygen gas, so they were not spontaneously oxidized, and there were as yet no living organisms, so they were also not metabolized. The slow accumulation of the molecules in the earlier ocean over a long period of time formed what is now called “PREBIOTIC SOUP” . The formation of this medium was a key event that preceded the origin of life.

Since the Oparin-Haldane hypothesis was proposed, there has been a debate regarding what were the most favourable environmental conditions on the primitive Earth that gave rise to biological molecules and the first cells. For instance, in the 1940s, Professor J.D. Bernal proposed that clays, organic lipids and molecules from comets and meteorites were important in contributing to abiogenic processes. In the 1950s, M. Calvin proposed a different composition of the primitive atmosphere with respect to the one that Oparin considered responsible for the origin of biological molecules. On the other hand, some researchers consider heat as a key condition of the primitive Earth for the synthesis of organic molecules. Still others hypothesize that low temperatures close to the freezing point could be decisive in the synthesis of several compounds.

Fossil Evidence of Abiogenesis

There is geochemical evidence of the environmental conditions of the primitive Earth in rock deposits from 3.4 billion years ago. Likewise, the first fossil records of living beings have been found in rocks from 3 billion years ago and have been interpreted as bacteria.

Abiogenesis Experiments

For more than a thousand years, naturalists believed that various forms of life could arise from inert matter. This idea was known as the Theory of Spontaneous Generation.

One ancient view of the origin of life, from Aristotle until the 19th century, is of spontaneous generation. This theory held that "lower" animals were generated by decaying organic substances, and that life arose by chance. This was questioned from the 17th century, in works like Thomas Browne's Pseudodoxia Epidemica. In 1665, Robert Hooke published the first drawings of a microorganism. In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria. Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s convinced himself, using experiments ranging from sealed and open meat incubation and the close study of insect reproduction, that the theory was incorrect. In 1668 Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. By the middle of the 19th century, spontaneous generation was considered disproven.

Pasteur's Refutation of Abiogenesis

In the 19th century, Louis Pasteur carried out an experiment in which a "broth" of nutrients was heated in a container to kill all the microorganisms within it and then isolated it to prevent the entry of air. Pasteur showed that microorganisms did not appear spontaneously in this broth and that the only way they could be found inside the container was if they came from the external environment. This finding provided enough evidence to disprove the Theory of Spontaneous Generation, thus helping to consolidate Theory of Biogenesis, which states that living things can only come from other living things.

The Miller-Urey Experiment

In 1953, S.L.(Stanley) Miller and H.C (Harold Urey). Urey performed the famous Miller experiment in order to demonstrate that it was possible to synthesize simple organic molecules under the conditions of the primitive Earth proposed by Oparin-Haldane. In this experiment, Miller and Urey combined simple substances such as methane (CH4), ammonia (NH3), water (H2 O), carbon dioxide (CO2) and others. Electrical discharges simulated the electrical storms in the primitive atmosphere and helped to catalyze chemical reactions. Under these conditions, Miller and Urey succeeded in synthesizing amino acids, the building blocks of proteins and living organisms. This experiment represents the beginning of a series of other experiments and theoretical approaches trying to determine the exact moment in which organic molecules organized and helped to create the emergence of the first true cells.

THE SECOND STAGE OF ORIGIN OF LIFE: cellular characteristics may have evolved via chemical selections beginning with RNA world.

What came first: DNA or RNA?

Nowadays, the importance of DNA is recognized as the molecule that stores the instructions to synthesize proteins and build living beings. However, the importance of DNA in modern organisms represents a problem for abiogenesis researchers. DNA replication requires pre-existing enzymes that in turn need the existence of DNA to be synthesized. This leads to the question: what came first? DNA or enzymes? A possible solution to this dilemma could be found in RNA.

Structure of RNA

RNA is similar in structure to DNA. RNA nucleotides contain adenine, uracil, cytosine or guanine attached to ribose molecules and these attached to phosphates. These nucleotides are arranged in a single strand.

RNA Can Spontaneously Form

In the 1960s, researchers proposed that RNA could have arose before DNA, therefore playing an extremely important role in the emergence of life on Earth.

The reasons for this are the following:

• There is evidence that RNA strands can be spontaneously synthesized from monomers under abiotic conditions such as catalysis in clay minerals.

• RNA can store genetic information in their own sequence of purine and pyrimidine nitrogenous bases.

• RNA is capable of catalyzing different types of RNA and is crucial for the formation of proteins.

RNA has the ability to store some information on its nucleotide sequence. Due to base pairing its nucleotide sequence has the capacity for self-replication RNA can perform a variety of catalytic functions. The results of many experiments have shown that some RNA molecules function as ribozymes RNA molecules that catalyse chemical reactions. By comparison, DNA and proteins are not as versatile as RNA, DNA has very limited catalytic activity, and proteins are not known to undergo self-replication. RNA can perform functions that are characteristics of proteins, and at the same time, can serve as genetic materials with replicative and informative functions. How did RNA molecules that were first made prebiotically evolve into more complex molecules that produced cell-like characteristics? Researchers propose that a chemical process called a chemical selection was responsible. CHEMICAL SELECTION occurs when a chemical within a mixture has special properties or advantages that causes it to increase in number relative to other chemicals in the mixture. Chemical selection results in chemical evolution, in which a population of molecules change overtime to become a new population with a different chemical composition. The RNA world is hypothetical period on Earth when both the information needed for life and the catalytic activity of living cells were contained solely in RNA molecules. In this scenario, lipid membranes enclosing RNA exhibited the properties of life due to RNA genomes that were occupied and maintained through the catalytic functions of RNA molecules. Scientists envision that, over time, mutations occurred in this RNA molecules, occasionally introducing new functional possibilities, chemical selection would have easily produced and increase in complexity in these cells, with RNA molecules accruing activities such as the ability to link amino acids together into proteins and other catalytic functions. But is RNA world a plausible scenario? Remarkably, scientist have been able to perform experiments in the laboratory that can select for RNA molecules with a particular function. American biologist David Bartel and Jack Szostak conducted the first study of this type in 1993 in which they selected for RNA molecules with the catalytic ability to link nucleotides together. After 10 rounds of chemical selection, they obtained a collection of RNA molecules that had catalytic activity.

Deep sea hydrothermal vents

The earliest known life forms are putative fossilized microorganisms, found in white smoker hydrothermal vent precipitates. They may have lived as early as 4.28 Gya (billion years ago), relatively soon after the formation of the oceans 4.41 Gya, not long after the formation of the Earth 4.54 Gya.

Early micro-fossils may have come from a hot world of gases such as methane, ammonia, carbon dioxide and hydrogen sulphide, toxic to much current life. Analysis of the tree of life places thermophilic and hyperthermophilic bacteria and archaea closest to the root, suggesting that life may have evolved in a hot environment. The deep sea or alkaline hydrothermal vent theory posits that life began at submarine hydrothermal vents. Martin and Russell have suggested "that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methyl sulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments".

These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-mafic olivine with seawater and a pH interface with carbon dioxide-rich ocean water. The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); see Iron–sulphur world theory. These are exothermic reactions.

Hot springs

Mulkidjanian and co-authors think that marine environments did not provide the ionic balance and composition universally found in cells, or the ions required by essential proteins and ribozymes, especially with respect to high K+/Na+ ratio, Mn2+, Zn2+ and phosphate concentrations. They argue that the only environments that mimic the needed conditions on Earth are hot springs similar to ones at Kamchatka. Mineral deposits in these environments under an anoxic atmosphere would have suitable pH (while current pools in an oxygenated atmosphere would not), contain precipitates of photocatalytic sulfide minerals that absorb harmful ultraviolet radiation, have wet-dry cycles that concentrate substrate solutions to concentrations amenable to spontaneous formation of biopolymers created both by chemical reactions in the hydrothermal environment, and by exposure to UV light during transport from vents to adjacent pools that would promote the formation of biomolecules. The hypothesized pre-biotic environments are similar to hydrothermal vents, with additional components that help explain peculiarities of the LUCA.

A phylogenomic and geochemical analysis of proteins plausibly traced to the LUCA shows that the ionic composition of its intracellular fluid is identical at hot springs. The LUCA likely was dependent upon synthesized organic matter for its growth. Experiments show that RNA-like polymers can be synthesized in multiple wet-dry cycles and exposure to UV light. These polymers became encapsulated in vesicles after condensation, which would not happen in saltwater conditions because of the high concentrations of ionic solutes. A potential source of biomolecules at hot springs was transport by interplanetary dust particles, extraterrestrial projectiles, or atmospheric or geochemical synthesis. Hot spring fields could have been abundant at volcanic landmasses during the Hadean.

Clay

The clay hypothesis was proposed by Graham Cairns-Smith in 1985. It postulates that complex organic molecules arose gradually on pre-existing, non-organic replication surfaces of silicate crystals in contact with an aqueous solution. The clay mineral montmorillonite has been shown to catalyse the polymerization of RNA in aqueous solution from nucleotide monomers, and the formation of membranes from lipids. In 1998, Hyman Hartman proposed that "the first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic acid and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hot spring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids."

Iron–sulfur world

Main article: Iron–sulphur world hypothesis

In the 1980s, Günter Wächtershäuser and Karl Popper postulated the Iron–sulphur world hypothesis for the evolution of pre-biotic chemical pathways. It traces today's biochemistry to primordial reactions which synthesize organic building blocks from gases. Wächtershäuser systems have a built-in source of energy: iron sulphides such as pyrite. The energy released by oxidising these metal sulphides can support synthesis of organic molecules. Such systems may have evolved into autocatalytic sets constituting self-replicating, metabolically active entities predating modern life forms. Experiments with sulphides in an aqueous environment at 100 °C produced a small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%). However, under the same conditions, dipeptides were quickly broken down.

Several models postulate a primitive metabolism, allowing RNA replication to emerge later. The centrality of the Krebs cycle (citric acid cycle) to energy production in aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, suggests that it was one of the first parts of the metabolism to evolve. Concordantly, geochemists Jack W. Szostak and Kate Adamala demonstrated that non-enzymatic RNA replication in primitive protocells is only possibly in the presence of weak cation chelators like citric acid. This provides further evidence for the central role of citric acid in primordial metabolism. Russell has proposed that "the purpose of life is to hydrogenate carbon dioxide" (as part of a "metabolism-first," rather than a "genetics-first," scenario). The physicist Jeremy England has argued from general thermodynamic considerations that life was inevitable. An early version of this idea was Oparin's 1924 proposal for self-replicating vesicles. In the 1980s and 1990s came Wächtershäuser's iron–sulphur world theory and Christian de Duve's thioester models. More abstract and theoretical arguments for metabolism without genes include Freeman Dyson's mathematical model and Stuart Kauffman's collectively autocatalytic sets in the 1980s. Kauffman's work has been criticized for ignoring the role of energy in driving biochemical reactions in cells.

The active site of the acetyl-CoA synthase enzyme, part of the acetyl-CoA pathway, contains nickel-iron-sulfur clusters.

A multistep biochemical pathway like the Krebs cycle did not just self-organize on the surface of a mineral; it must have been preceded by simpler pathways. The Wood–Ljungdahl pathway is compatible with self-organization on a metal sulphide surface. Its key enzyme unit, carbon monoxide dehydrogenase/acetyl-CoA synthase, contains mixed nickel-iron-sulphur clusters in its reaction centres and catalyses the formation of acetyl-CoA. However, prebiotic thiolates and thioester compounds are thermodynamically and kinetically unlikely to accumulate in the presumed prebiotic conditions of hydrothermal vents. One possibility is that cysteine and homocysteine may have reacted with nitriles from the Stecker reaction, forming catalytic thiol-rich polypeptides.

Zinc-world

Armen Mulkidjanian's zinc world (Zn-world) hypothesis extends Wächtershäuser's pyrite hypothesis. The Zn-world theory proposes that hydrothermal fluids rich in H2S interacting with cold primordial ocean (or Darwin's "warm little pond") water precipitated metal sulphide particles. Oceanic hydrothermal systems have a zonal structure reflected in ancient volcanogenic massive sulphide ore deposits. They reach many kilometres in diameter and date back to the Archean. Most abundant are pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (ZnS), with additions of galena (PbS) and alabandite (MnS). ZnS and MnS have a unique ability to store radiation energy, e.g. from ultraviolet light. When replicating molecules were originating, the primordial atmospheric pressure was high enough (>100 bar) to precipitate near the Earth's surface, and ultraviolet irradiation was 10 to 100 times more intense than now; hence the photosynthetic properties mediated by ZnS provided the right energy conditions for the synthesis of informational and metabolic molecules and the selection of photo stable nucleobases.

The Zn-world theory has been filled out with evidence for the ionic constitution of the interior of the first proto-cells. In 1926, the Canadian biochemist Archibald Macallum noted the resemblance of body fluids such as blood and lymph to seawater; however, the inorganic composition of all cells differ from that of modern seawater, which led Mulkidjanian and colleagues to reconstruct the "hatcheries" of the first cells combining geochemical analysis with phylogenetic scrutiny of the inorganic ion requirements of modern cells. The authors conclude that ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K+, Zn2+, Mn2+, and [PO

4]3−

. Geochemical reconstruction shows that this ionic composition could not have existed in the ocean but is compatible with inland geothermal systems. In the oxygen-depleted, CO2-dominated primordial atmosphere, the chemistry of water condensates near geothermal fields would resemble the internal milieu of modern cells. Therefore, precellular evolution may have taken place in shallow "Darwin ponds" lined with porous silicate minerals mixed with metal sulphides and enriched in K+, Zn2+, and phosphorus compounds.

That was 3 million times higher than their original random collection of molecules. Like the work of Miller and Urey, Bartel and Szostak showed the feasibility of another phase of the prebiotic process that led to life. In this case, chemical selection resulted in chemical evolution. The results showed that chemical selection can change the functional characteristics of a group of RNA molecules over time by increasing the proportions of those molecules with enhanced function.

References

The origin of life edited by Ejalonibu Iseoluwa, student of Landmark University Omu Aran. https://www.visionlearning.com/en/library/Biology/2/Origins-of-Life-I/226, ttps://www.eniscuola.net/en/argomento/beginning-of-life,

Abiogenesis | Definition & Theory | Britannica.