History of Life – Overview, Chemogeny, Biogeny, RNA World, Major Events in History of Life

Sourav Bio

Historical Overview

Throughout human history, the origin of life has captivated our curiosity and inspired various creation myths involving supernatural forces. However, ancient Greek thinkers were the first to approach the topic from a scientific standpoint, proposing the concept of spontaneous generation as a mechanism for complex organic forms to arise from simpler inorganic matter. As European science flourished after the Middle Ages, biologists began to refute this hypothesis, highlighting the apparent divide between living organisms and lifeless matter through the principle “omne vivum ex vivo,” meaning that life only comes from preexisting life. Concurrently, chemists were making strides in demonstrating the applicability of chemical principles to both living and non-living entities. This progress eventually led to the synthesis of organic compounds from inorganic substances (e.g., urea) and the establishment of organic chemistry and biochemistry as scientific disciplines.

While it is theoretically conceivable that life may have originated on a distant planet with a chemistry distinct from Earth’s, there is presently no evidence to support the idea that life on Earth emerged from non-living matter. The first significant challenge to spontaneous generation came in 1668 when Francesco Redi conducted experiments demonstrating that maggots did not appear in meat when flies were prevented from laying eggs on it. Prior to Redi’s experiments, it was commonly believed that life could spontaneously arise. According to the prevailing popular belief at the time, maggots were thought to be generated spontaneously from decaying meat and flies were believed to arise from the chemicals released by decomposing flesh. In 1861, Louis Pasteur further validated this notion through a series of experiments, showing that organisms such as bacteria and fungi do not spontaneously emerge in sterile, nutrient-rich environments.

In his groundbreaking work, Charles Darwin proposed the idea that life might have originated through chemical processes in a small, warm pond teeming with various substances like ammonia, phosphoric salts, light, heat, and electricity. Throughout the 20th century, scientists focused on Darwin’s hypothesis to explore how the interaction of simple molecules in the lakes or oceans of the prebiotic world, without any supernatural intervention, could have given rise to the common ancestor of all life forms.

In the 1920s, Alexander I. Oparin in Russia and J.B.S. Haldane in England revived the concept of spontaneous generation and suggested that the presence of atmospheric oxygen hindered the chain of events necessary for the evolution of life. Haldane, as early as 1929, highlighted experiments showing how ultraviolet radiation could promote the formation of organic compounds from a mixture of water, carbon dioxide, and ammonia. Oparin proposed the idea of a “primeval soup,” where organic molecules could be synthesized in an oxygen-free atmosphere through the influence of sunlight.

Oparin and Haldane postulated that the Earth’s early atmosphere, similar to the outer planets like *******, lacked oxygen (O2) or had minimal amounts of it, while being rich in hydrogen (H2) and other compounds such as methane (CH4) and ammonia (NH3). Inspired by the concepts put forth by Darwin, Oparin, and Haldane, Stanley Miller and Harold Urey conducted groundbreaking experiments in 1953 (Fig. 1), simulating conditions believed to resemble those shortly after the Earth formed from the primordial solar nebula. This marked the beginning of experimental prebiotic chemistry, where they demonstrated that amino acids and other vital molecules for life could be generated from simple compounds that likely existed on the early Earth.

Miller and Urey conducted a groundbreaking experiment within a closed apparatus where they created a reducing atmosphere devoid of oxygen. The atmosphere consisted of water vapor, methane, ammonia, and hydrogen, which were situated above a simulated ocean of water. To simulate lightning, they passed electrical discharges through the gases in the apparatus. After a mere two days, they analyzed the contents of the simulated ocean.

Miller made an intriguing observation: approximately 10-15% of the carbon in the system had transformed into a limited number of identifiable organic compounds. Additionally, up to 2% of the carbon had contributed to the formation of amino acids, the fundamental building blocks of proteins. This discovery was particularly significant, suggesting that amino acids, crucial for life’s existence, would have been abundant on the early Earth. Miller’s experiments yielded a mixture of 22 amino acids, as well as other molecules like purines, pyrimidines, sugars, and lipids associated with living cells. According to Miller and Urey, these substances would have been washed into the early oceans of the Earth, where they played a role in the development of the first living cells. Notably, glycine (NH2CH2COOH) emerged as the most abundant amino acid in these experiments and subsequent ones.

While these experiments demonstrated the spontaneous formation of some basic organic compounds, such as amino acids, which serve as the building blocks of life, it should be noted that the transition from simple organic molecules to fully functional self-replicating life forms is a complex and intricate process. Additionally, the conditions and atmosphere in Miller and Urey’s experiments may not precisely mirror those of early Earth, as scientists now believe the early Earth’s atmosphere differed.

Despite the discrepancies in atmospheric conditions, subsequent experiments conducted over the years have shown that organic building blocks, especially amino acids, can form from inorganic precursors under a wide range of conditions. These findings lead scientists to envision that at least some of life’s building blocks could have formed abiotically on the early Earth. However, the exact mechanisms and specific conditions under which these processes occurred remain open questions in the scientific community.

Origin of Earth: hypothesis

The origin of Earth is estimated to have occurred approximately 5-6 billion years ago. Two primary hypotheses have been proposed to explain the origin of our planet.

  1. Planetesimal Hypothesis: The Planetesimal hypothesis suggests that Earth originated as a fragment that broke off from the molten mass of the sun. In this hypothesis, a portion of the sun’s material separated and formed our planet.
  2. Nebular Hypothesis: The Nebular hypothesis is the most widely accepted explanation for the origin of Earth. It proposes that our planet formed through the gradual condensation of interstellar dust or cosmic dust known as a nebula. Around 10,000-20,000 million years ago, a highly compacted mass of cosmic material called ylem existed. This mass consisted of particles similar to neutrons, protons, and electrons.

An explosion occurred within this cosmic material, resulting in the formation of numerous pieces known as nebulae. This cosmic explosion is commonly referred to as the Big Bang, which is part of the Big Bang theory. According to the nebular hypothesis, initially, Earth was a spinning ball of hot gases and vapors made up of various elements.

Over time, as the Earth gradually cooled down, the gases began to condense into solid forms, leading to stratification based on their density. Heavier elements such as nickel, iron, zinc, etc., sank towards the center, forming the core of the Earth. Lighter elements like aluminum, silicon, sulfur, etc., comprised the mantle and crust of the planet.

The lightest and gaseous substances, including hydrogen, helium, carbon, nitrogen, and oxygen, formed the Earth’s atmosphere. Through this process of condensation and differentiation, Earth’s distinct layers and atmospheric composition were established. The Nebular hypothesis provides a comprehensive explanation for the formation and composition of our planet.

Theories of origin of life

1. Theory of special creation

  • The Theory of Special Creation proposes that the entire universe was brought into existence by a supernatural power, namely God. This theory, which was proposed by various Hebrew scholars and supported by Father Suarez, suggests that God created the universe over a period of six days.
  • According to the Christian interpretation of this theory, God created different aspects of the universe on each of the six days. On the first day, the Earth and the heavens were formed. The sky was created on the second day, followed by the formation of land and plants on the third day. The sun, moon, and stars were brought into existence on the fourth day, while fish and birds were created on the fifth day. Finally, on the sixth day, animals, including human beings, were created. Adam was considered the first man, and Eve was the first woman according to this belief.
  • Similarly, in Hindu mythology, the Theory of Special Creation is associated with the concept of God Brahma. It is believed that Brahma created life in a single stroke, without the gradual progression seen in the Christian interpretation. The first man in Hindu mythology is referred to as Manu, and the first woman is believed to be Shradha.
  • However, due to the lack of scientific explanations and empirical evidence supporting this theory, it has been largely rejected by the scientific community. In the quest for understanding the origins of the universe and life, scientific theories such as the Big Bang Theory and the theory of evolution have provided more comprehensive and evidence-based explanations.

2. Theory of spontaneous Generation (Abiogenesis)

  • The Theory of Spontaneous Generation, also known as Abiogenesis, suggests that living organisms can arise spontaneously from non-living materials such as dung, mud, or earth. This theory was proposed by VonHelmont and found support from philosophers like Anaximenes and Aristotle.
  • According to the Theory of Spontaneous Generation, various forms of life were believed to originate from specific sources. For example, insects were thought to arise from dew, frogs and toads from the muddy bottom of ponds, maggots from decaying meat, tapeworms from the excreta of animals, and microorganisms from the air or water.
  • However, through a series of experiments, prominent scientists like Francisco Redi (1668), Louis Pasteur (1864), and Spallanzani (1765) provided evidence that refuted the concept of abiogenesis. These scientists conducted controlled experiments to demonstrate that the generation of life forms does not occur spontaneously from non-living matter.
  • Francisco Redi’s experiments in 1668, involving the prevention of flies from laying eggs on meat, showed that maggots did not spontaneously appear in the absence of flies. This experiment challenged the belief that maggots were generated from decaying meat itself.
  • Louis Pasteur’s experiments in 1864 involved the use of swan-necked flasks to prevent microorganisms from entering the sterile broth. He demonstrated that the broth remained free from microbial growth unless the flask was tilted or the neck was broken, allowing contamination from the outside. This experiment supported the idea that microorganisms do not spontaneously generate in sterilized environments.
  • Similarly, Spallanzani conducted experiments in 1765 that involved boiling broth in sealed containers to eliminate airborne microorganisms. His experiments showed that no microbial growth occurred in the sterilized broth unless the seal was broken and the external microorganisms were allowed to enter.
  • These experimental findings effectively debunked the concept of spontaneous generation and provided evidence in favor of the idea that life only arises from pre-existing life. This marked a significant shift in scientific understanding and laid the foundation for the modern understanding of biogenesis, which states that all living organisms are derived from other living organisms.

3. Biogenesis theory

  • The Biogenesis Theory proposes that life can only arise from pre-existing life, in contrast to the earlier theory of spontaneous generation. This concept is supported by several key experiments conducted by notable scientists.
  • One of the significant experiments supporting the Biogenesis Theory was conducted by Francisco Redi in 1668. Redi placed pieces of boiled meat in three separate jars. One jar was left open, one covered with parchment paper, and the other covered with muslin cloth. He observed that only the open jar showed the growth of maggots. The flies had access to the open jar and laid eggs, which developed into maggots. Based on this experiment, Redi concluded that life can only come from pre-existing life and not from non-living substances.
  • Lazzaro Spallanzani performed experiments that further supported the concept of biogenesis. He placed hay infusion in eight bottles, four of which were airtight and four were loosely corked. After a few days, he observed dense growth of microorganisms in the loosely corked bottles, while no organisms developed in the airtight bottles. This led to the conclusion that the air contained microorganisms that served as a source of contamination, supporting the idea that life can only originate from pre-existing life.
  • Louis Pasteur’s experiments in 1864 provided additional evidence for biogenesis. He conducted an experiment using a flask with a bent neck (known as the Swan-neck flask) filled with nutrient solution. The flask was then boiled to kill all existing microorganisms. Pasteur sealed the flask and left it undisturbed for several days, observing no signs of life inside. However, when the neck of the flask was broken, microorganisms appeared. This experiment demonstrated that life arises only from pre-existing life and not from spontaneous generation.
  • These experiments by Redi, Spallanzani, and Pasteur collectively supported the concept of biogenesis and refuted the notion of spontaneous generation. They provided strong evidence that living organisms can only originate from pre-existing living organisms, contributing to our modern understanding of the origin and propagation of life.

4. Cosmozoic theory

  • The Cosmozoic Theory proposes that life on Earth originated from spores or seeds called panspermia, which arrived from another planet. This idea was suggested by Richter in 1865 and supported by Arrhenius. According to this hypothesis, life was transported through space and reached Earth from a different celestial body.
  • However, one of the challenges with the Cosmozoic Theory is explaining how the panspermia, which may have been exposed to high temperatures and harmful radiations during its journey through space, could survive and thrive under the adverse conditions present on Earth at that time. The theory fails to provide a satisfactory explanation for how these spores or seeds could withstand such extreme conditions and still give rise to life on Earth.
  • While the concept of panspermia suggests the possibility of life originating from other celestial bodies, the specifics of how this process occurred and the viability of the transported life forms under Earth’s conditions remain uncertain. The Cosmozoic Theory raises intriguing questions about the potential interstellar origins of life, but further research is needed to better understand the mechanisms and feasibility of panspermia in explaining the origin of life on Earth.

5. Modern or Chemosynthetic Theory of Origin of life (Scientific hypothesis)

  • The Modern or Chemosynthetic Theory of the Origin of Life is a scientific hypothesis that rejects the possibility of spontaneous generation and proposes specific requirements for the appearance of life. The concept that inorganic chemicals could give rise to life was initially suggested by T.H. Huxley and John Tyndall; however, due to the limited knowledge of biochemistry at that time, their ideas lacked clarity.
  • The comprehensive formulation of this theory was presented by the Russian biochemist A.I. Oparin in 1923, with assistance from J.B.S. Haldane. In their book “The Origin of Life on Earth” published in 1936, they provided a detailed explanation of how life could have originated through chemical evolution. This theory is commonly known as the Oparin and Haldane theory or the biochemical theory of the origin of life.
  • According to this hypothesis, approximately 4.2 billion years ago, life emerged in water on the primitive Earth through a series of chemical reactions. It is often referred to as the modern synthetic theory of the origin of life. The theory can be described in three main stages: Chemogeny, Biogenesis, and Cognogeny.
  • Chemogeny refers to the process of chemical evolution, where complex organic compounds and biomolecules gradually formed from simple inorganic molecules in the early Earth’s environment. Biogenesis involves the development of self-replicating molecules and the emergence of the first living organisms from these prebiotic chemical reactions. Cognogeny describes the subsequent evolution and diversification of life forms through genetic variation and natural selection.
  • The Modern or Chemosynthetic Theory of the Origin of Life provides a scientific framework for understanding how life may have emerged from non-living matter through chemical processes. While it offers valuable insights into the early stages of life’s development, ongoing research and investigation are still necessary to fully comprehend the intricacies of the origin of life on Earth.

i. Chemogeny

Chemogeny is the process of chemical evolution in the context of the origin of life. It involves the creation of complex organic molecules from basic chemicals, such as polysaccharides, fats, polypeptides, nucleic acids, and more. The different phases of chemogeny can be outlined as follows:

  1. Primitive atmosphere formation: During the early stages of Earth’s formation, the primitive atmosphere consisted of elements like nitrogen (N), hydrogen (H), oxygen (O), carbon (C), and others. The Earth was a hot and fiery spinning ball of gases, resulting in the production of various compounds such as dicarbon, cyanogen, and metal carbides. Oxygen was not present in its free state but existed as oxides of other elements like aluminium, boron, and hydrogen.
  2. Formation of inorganic compounds: As the temperature of the primitive Earth gradually decreased, chemical evolution became more favorable. The prevalent atoms were hydrogen, which combined with available oxygen to form water. Hydrogen also reacted with nitrogen and carbon atoms to produce ammonia and methane, respectively: 2H + O → H2O (water) 3H + N → NH3 (ammonia) 4H + C → CH4 (methane)
  3. Formation of simple organic compounds: As the Earth cooled down and developed a solid crust, atmospheric water vapor accumulated and eventually fell as rain. The water collected in depressions and formed large bodies of water known as oceans. When the Earth’s surface temperature reached around 50°C-60°C, the inorganic molecules combined to form simple organic compounds. Examples include acetylene, ethylene, ethane, methane, and others, in various forms.
  4. Formation of complex organic compounds: The previously formed saturated and unsaturated hydrocarbons mixed and recombined to give rise to complex organic compounds. Through processes such as condensation, polymerization, and oxide reduction, compounds like acetaldehyde, aldol, ethyl acetate, acetic acid, ethyl alcohol, amino acids, glycol, and more were formed. The sources of energy for these reactions were ultraviolet rays, volcanic eruptions, and electric energy produced during lightning.
  5. Formation of carbohydrates, proteins, and fats: In the primitive ocean, the complex organic compounds polymerized further, leading to the synthesis of massive macromolecules such as proteins, carbohydrates, and fats. These compounds, being key constituents of protoplasm in living cells, played a crucial role in establishing the possibilities for the origin of life in the primitive ocean. Additionally, hydrocarbons reacted to form nitrogen bases like purines and pyrimidines when exposed to hot water vapor containing hydrocyanic acid and ammonia.

The accumulation of these complex organic compounds in the primitive ocean resulted in a rich blend of organic matter known as hot dilute soup, primordial soup, or broths, as described by J.B.S. Haldane. These chemical reactions and the synthesis of organic compounds set the stage for the potential emergence of life on Earth.

ii. Biogeny

Biogeny refers to the formation of primitive life following chemogeny, the chemical evolution process. It encompasses several events that led to the emergence of early organisms. The key stages of biogeny are as follows:

  • Formation of nucleic acids and nucleoproteins: In the primordial water or organic soup, the organic compounds present underwent reactions and combined to form new molecules of greater size and complexity. Nitrogen bases, sugars, and phosphates reacted at high temperatures in primitive soil, resulting in the formation of nucleotides. Nucleotides are the building blocks of nucleic acids. Through the joining of numerous nucleotides in various combinations, highly complex molecules known as nucleic acids were formed. Nucleic acids have the ability to replicate, allowing for the transmission of genetic information.
  • Formation of coacervates: Within the primordial soup, the complex organic compounds exhibited intermolecular attraction and began to aggregate, giving rise to large colloidal cells called coacervates or microspheres. Coacervates formed through a process called coacervation. These coacervates had the ability to grow and divide, demonstrating some of the characteristics associated with living organisms.
  • Formation of primary organisms: Around 3.8 billion years ago, according to Oparin’s hypothesis, coacervates acquired nucleoproteins either from the seawater or through self-synthesis. These coacervates developed into the first cellular organisms known as eobionts, pre-cells, or protobionts. The outer limiting membranes of these entities were formed by certain fatty acids that had a strong affinity for water. Within the eobionts, some proteins began to act as enzymes, facilitating both constructive and destructive reactions. The eobionts resembled protoviruses, which are similar to viruses found in the present-day. The early organisms were anaerobic and prokaryotic chemoheterotrophs, meaning they obtained energy from chemical sources in their environment.

These stages of biogeny mark significant milestones in the emergence of early life forms, setting the foundation for the subsequent evolution and diversification of life on Earth.

iii. Cognogeny

Cognogeny refers to the diversification of primary species into various modes of life, including the origin of autotrophs and eukaryotes. The key events involved in cognogeny are as follows:

a. Origin of autotrophs: As the population of chemoheterotrophs, which obtained nutrients from organic sources, increased, they started depleting the natural food resources in the oceans. In response, primitive organisms began to synthesize organic compounds from inorganic molecules abundantly present in the sea. These organisms relied on the anaerobic breakdown of chemicals in the absence of chlorophyll to provide the energy necessary for synthesizing organic food. These early autotrophs were known as chemoautotrophs, including nitrifying bacteria, sulfur bacteria, iron bacteria, and others.

Later, some autotrophic prokaryotes developed a green substance called bacteriochlorophyll, derived from the magnesium porphyrin in the sea. This enabled them to engage in photosynthesis, marking the emergence of photoautotrophs. These organisms, such as marine planktonic bacteria, were anoxygenic, meaning they did not produce oxygen as a byproduct of photosynthesis. They utilized carbon dioxide and hydrogen sulfide as raw materials for the synthesis of organic compounds using solar energy.

Over time, certain changes occurred in the bacterial chlorophyll, leading to the formation of true chlorophyll. This development gave rise to true photoautotrophs, which synthesized their food through photosynthesis using water as a raw material. Cyanobacteria, the first oxygenic and aerobic photoautotrophs, evolved approximately 2.7 billion years ago. Through photosynthesis, they released oxygen into the atmosphere, gradually increasing the availability of free oxygen.

b. Origin of eukaryotes: With the shift to aerobic respiration among true photosynthetic prokaryotes, cyano bacteria underwent significant changes, including the development of a true nucleus. This transformation marked the transition from prokaryotes to eukaryotes. These early eukaryotes existed as unicellular species resembling those seen today. The evolution of multicellular organisms from unicellular organisms occurred through a process known as colonization, where individual cells formed specialized roles and collaborated to create complex, multicellular organisms.

The events of cognogeny highlight the development of autotrophs, which played a crucial role in shaping the Earth’s atmosphere by releasing oxygen. Additionally, the emergence of eukaryotes paved the way for the diversification of life and the eventual evolution of complex organisms.

The RNA World Hypothesis

The RNA World Hypothesis, proposed in the 1960s by Carl Woese, Francis Crick, and Leslie Orgel, suggests that early life forms may have relied on RNA alone as the primary genetic material. This hypothesis gained further attention when Walter Gilbert, a molecular biologist from Harvard, coined the term “RNA World” in 1986. According to this hypothesis, RNA played a crucial role in the emergence of life, and DNA later replaced RNA as the genetic material through the process of evolution.

The RNA World Hypothesis proposes that approximately 4 billion years ago, RNA served as the fundamental building block of life. One of the key reasons for this proposition is the dual functionality of RNA as both genetic material and catalyst (enzymes). Unlike DNA, which is relatively stable, RNA has the ability to store genetic information and catalyze chemical reactions.

The central idea behind the RNA World Hypothesis is that RNA could self-replicate, allowing it to carry and transmit genetic information from one generation to the next independently. This characteristic suggests that RNA molecules could have acted as primitive genes, giving rise to early life forms. Over the past 50 years, this hypothesis has been a subject of intense scientific debate and scrutiny.

Current scientific consensus acknowledges that the direct transition from non-living chemicals to bacterial cells in a single step is highly unlikely. Therefore, it is believed that intermediate pre-cellular life forms must have existed. Among the various proposed models for pre-cellular life, the RNA World Hypothesis has gained significant popularity. It suggests that the RNA World represents a plausible stage in the origin of life, where RNA molecules played a crucial role in early biochemical processes.

While the RNA World Hypothesis provides a compelling explanation for the emergence of life, it remains an active area of research and investigation. Scientists continue to study the properties and capabilities of RNA molecules, seeking to understand how they could have led to the development of more complex life forms and the eventual transition to the DNA-based genetic systems observed in present-day organisms.

Ribozymes and the RNA world

Ribozymes, discovered by Sidney Altman, Thomas Cech, and their colleagues, have played a significant role in supporting the RNA World Hypothesis. Previously, it was believed that only proteins could catalyze essential chemical reactions within cells. However, the discovery of ribozymes revealed that certain RNA molecules also possess catalytic activity. For their groundbreaking work, Altman and Cech were awarded the Nobel Prize in Chemistry in 1989.

The existence of ribozymes provided compelling evidence for the RNA World Hypothesis. One of the strongest arguments supporting this hypothesis is the fact that the ribosome, the cellular machinery responsible for protein synthesis, is itself a ribozyme. Although the ribosome consists of both RNA and protein components, the catalytic processes involved in translation are carried out by RNA, not proteins. This suggests that early life forms might have relied on RNA to catalyze chemical reactions before the emergence of proteins.

The discovery of ribozymes challenged the notion that proteins were the sole catalysts in cellular processes. It expanded our understanding of the diverse functional capabilities of RNA and reinforced the idea that RNA played a central role in the early stages of life on Earth. The existence of ribozymes, along with the ribosome’s ribozyme activity, lends support to the concept that the RNA World was a crucial phase in the origin of life, where RNA served as both genetic material and catalytic agent.

Further research on ribozymes continues to uncover new insights into their structure, function, and potential applications. Studying these catalytic RNA molecules enhances our understanding of the chemical processes that might have occurred during the early stages of life. Ribozymes serve as a bridge between the RNA World Hypothesis and the role of RNA in modern biological systems, shedding light on the evolutionary transition from RNA-based catalysis to the complex protein-based enzymatic machinery observed in contemporary organisms.

Evidences in support of origin of life: Miller-urey experiment

The Miller-Urey experiment, conducted in 1953 by Stanley Miller and Harold Urey, provided evidence in support of the biochemical origin of life hypothesis proposed by Oparin and Haldane. The experiment aimed to simulate the conditions of primitive Earth and investigate the formation of organic molecules from inorganic substances. Here are the details of the experiment and its observations:

  1. Experimental Setup: Miller constructed an apparatus known as the spark discharge apparatus, consisting of a glass tube and flask. The apparatus simulated the conditions of early Earth, including a reducing atmosphere and an ocean. A mixture of gases, methane, ammonia, and hydrogen, was introduced into the gas chamber in a ratio of 2:2:1, while water was present in another chamber.
  2. Energy Supply: The gas mixture was circulated through the apparatus, and energy was supplied using electrodes in the gas chamber. This energy was provided by boiling water and creating electric sparks, simulating the effects of lightning and volcanic activity.
  3. Duration of the Experiment: The experiment was conducted for a week, with the electric source switched on and water continuously boiling.
  4. Observation: During the experiment, Miller and Urey observed the formation of a condensed liquid with a dark color. This liquid was collected and subjected to chromatographic analysis. The analysis revealed that the liquid consisted of a mixture of sugars, amino acids (such as glycine and alanine), and fatty acids.
  5. Conclusion: Based on the experimental results, Miller and Urey concluded that organic molecules, including sugars, amino acids, and fatty acids, could be generated from inorganic molecules under conditions resembling those of early Earth. These findings provided support for the Oparin-Haldane theory of the origin of life, which suggests that the complex organic compounds necessary for life can arise from simpler inorganic compounds through natural chemical processes.

The Miller-Urey experiment demonstrated the plausibility of chemical reactions leading to the formation of organic molecules essential for life. It contributed to our understanding of the potential mechanisms involved in the origin of life on Earth and provided evidence for the idea that life could arise from non-living matter through natural processes.

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