Table of Contents
What is Chemogeny?
- Chemogeny, also known as the chemical evolution of life, refers to the process by which complex organic molecules are formed from simpler inorganic molecules through chemical reactions that took place in the early oceans of Earth. This is considered the initial step in the development of life on our planet. The period of chemical evolution is estimated to have lasted for less than a billion years.
- During this time, various chemical reactions occurred in the primordial oceans, leading to the synthesis of complex organic compounds. These reactions involved the combination and transformation of simple inorganic molecules such as carbon dioxide, methane, ammonia, water, and others. The energy required for these reactions came from sources like volcanic activity, ultraviolet radiation, and lightning strikes.
- The conditions on early Earth, including a reducing atmosphere and the presence of key elements, favored the formation of organic molecules. As the Earth cooled down and solidified, the oceans formed, providing a conducive environment for chemical reactions to take place. The interaction of inorganic molecules with the water in the oceans facilitated the dissolution of minerals and the accumulation of organic compounds.
- Through processes such as condensation, polymerization, and oxidation-reduction reactions, the complex organic molecules began to form. These molecules included amino acids, sugars, nucleotides, fatty acids, and others. Over time, these compounds accumulated in the primitive oceans, resulting in what has been described as a “primordial soup” or “broth.”
- Chemogeny provided the foundation for the subsequent stages of the origin of life, namely biogeny and cognogeny. The complex organic molecules formed during chemogeny played a crucial role in the emergence of nucleic acids, proteins, and other biomolecules, which eventually led to the development of primitive life forms.
- Understanding the process of chemogeny is essential for unraveling the origins of life on Earth. By studying the chemical reactions and conditions that facilitated the formation of complex organic molecules, scientists can gain insights into the early stages of evolution and the potential pathways that led to the emergence of life.
1. Origin of Earth’s Primitive Atmosphere
- The origin of Earth’s primitive atmosphere can be traced back to the early stages of the planet’s formation, around 4 billion years ago. At that time, the Earth was a hot and volatile place, with gases and vapors of various elements surrounding it. These gases gradually condensed and formed the primitive atmosphere.
- During the process of Earth’s formation, the heavier metallic elements, such as sodium, potassium, silicon, aluminum, magnesium, and sulfur, sank towards the core of the planet due to their higher densities. This led to the formation of the Earth’s solid core. Meanwhile, the lighter elements, including hydrogen, oxygen, argon, carbon, and nitrogen, flowed towards the surface and became part of the primitive atmosphere.
- The composition of the primitive atmosphere included nitrogen (in the form of ammonia, NH3), carbon (in the form of methane, CH4), and oxygen (in the form of water vapor, H2O). However, free oxygen was not present in the atmosphere during this time. The absence of free oxygen made the primitive atmosphere “reducing” in nature, meaning that it had a higher proportion of reducing compounds compared to oxidizing compounds.
- The Earth’s primitive atmosphere was primarily composed of elements like nitrogen (N), hydrogen (H), oxygen (O), and carbon (C). It was a result of the Earth being a hot, fiery ball of gases. Various chemical reactions occurring in this environment produced compounds like dicarbon, cyanogen, and metal carbides. Oxygen, although not present in its free state, existed as oxides of elements like aluminum, boron, and hydrogen.
- The primitive atmosphere and its composition played a crucial role in the subsequent chemical evolution and the origin of life on Earth. The presence of different elements and compounds provided the necessary building blocks for the formation of complex organic molecules during chemogeny, paving the way for the development of life in the following stages of Earth’s history.
2. Formation of Simple Inorganic Molecules (Water, Ammonia and Methane)
The formation of simple inorganic molecules, such as water, ammonia, and methane, played a significant role in the chemical evolution of the primitive Earth. During the early stages, when the surface temperature of the Earth was less than 100°C, the conditions were conducive to the formation of these molecules.
The most abundant element during this period was hydrogen. Hydrogen atoms combined with available oxygen atoms to form water molecules through a chemical reaction:
2H + O → H2O (water)
Similarly, hydrogen atoms combined with nitrogen and carbon atoms to form ammonia and methane, respectively:
3H + N → NH3 (ammonia) 4H + C → CH4 (methane)
These reactions occurred in the atmosphere of the primitive Earth, where nitrogen existed as ammonia, carbon existed as methane, and oxygen existed as water vapor. It’s important to note that during this time, there was no free oxygen in the atmosphere.
The primitive atmosphere of the Earth was reducing, meaning it had a higher proportion of reducing compounds like ammonia and methane compared to oxidizing compounds. These substances were referred to as protoplasmic compounds.
As the Earth cooled down, the surface temperature decreased, and the water vapor in the atmosphere condensed, leading to the formation of rain. The rainwater accumulated in depressions on the Earth’s surface, gradually forming large water bodies known as oceans. These oceans contained dissolved minerals like chloride and phosphates, which further enriched the chemical composition of the water.
The primitive Earth had significant quantities of hydrogen, nitrogen, carbon dioxide, methane, ammonia, and water vapor in its atmosphere. However, free oxygen was absent during this stage of the planet’s history. This unique atmospheric composition, coupled with the availability of inorganic compounds, set the stage for further chemical reactions and the subsequent emergence of more complex organic molecules, leading to the development of life.
3. Formation of Simple Organic Compounds
As the Earth’s surface cooled, a solid crust formed, creating depressions and elevations on the planet. At the same time, atmospheric water vapor condensed and fell as rain onto the surface. The accumulated water dissolved minerals and eventually gave rise to large bodies of water known as oceans.
When the Earth’s surface temperature cooled to around 50-60°C, the inorganic molecules present in the water bodies combined and recombined in various ways, leading to the formation of simple organic compounds. Some of these compounds include acetylene, ethylene, ethane, and methane.
The formation of these organic compounds involved different chemical reactions:
a. Condensation Reactions: HC≡ CH + H2O → CH3CHO (acetaldehyde) CH3CHO + CH3CHO → CH3CHOHCH2CHO (aldol)
b. Oxidoreduction: CH3CHO + H2O → CH3COOH + C2H5OH
c. Polymerization: CH3COOH + C2H5OH → CH3COOCH3CH2 + H2O CH2OHCOOH + NH3 → CH2NH2COOH + H2O
In the absence of consumers, enzyme catalysts, or oxygen, these organic molecules accumulated in the water bodies. In the present oxidizing environment, such a transition would be unlikely as oxygen or micro-consumers would degrade or destroy any living particles that might arise by chance.
The energy required for these photochemical reactions could have been provided by factors such as volcanic eruptions (intense dry heat of the Earth), solar radiation (UV-rays), electrical energy during lightning, or the decay of radioactive elements.
The complex organic compounds formed in the oceans gradually settled, forming a “hot thin soup” or “pre-biotic soup,” which set the stage for further chemical reactions and the potential emergence of life.
4. Formation of Complex Organic Compounds (Carbohydrates, Proteins, and Fat)
The formation of complex organic compounds such as carbohydrates, proteins, and fats involves a series of chemical reactions and polymerization processes. In the primordial ocean, simple organic compounds underwent random chemical reactions and polymerization, eventually giving rise to complex organic compounds.
The primary energy sources for these chemical reactions and polymer synthesis were electrical discharge, lightning, solar energy, ATP (adenosine triphosphate), and pyrophosphates. These energy sources provided the necessary energy to drive the formation of complex molecules. Additionally, the concentration of monomers increased through water evaporation, which facilitated polymerization reactions.
The water bodies were predominantly filled with these polymers because they were more stable compared to the simple monomers. As the concentration of polymers increased, the chemical equilibrium shifted towards the synthesis of stable polymers from unstable monomers. This process further favored the formation of complex organic compounds.
The synthesis of these complex organic compounds, including polysaccharides (carbohydrates), lipids (fats), nucleotides, nucleic acids, and polypeptides (proteins), played a crucial role in the genesis of life in the primordial ocean. These complex chemicals are essential components of protoplasm, the living matter in cells.
The result of these chemical reactions led to the formation of a rich blend of organic molecules in marine water, often referred to as the “hot dilute soup” or “primordial soup” by Haldane. This mixture of organic compounds provided a fertile environment for the emergence of life.
In summary, the formation of complex organic compounds involved the condensation, polymerization, and oxidoreduction of simple organic compounds in the presence of energy sources such as ultraviolet rays, volcanic eruptions, and electrical energy from lightning. Through these processes, simple monomers underwent chemical reactions and polymerization, leading to the formation of complex organic compounds like carbohydrates, proteins, and fats, which are vital for the origin and development of life.
- Condensation reactions:
- HC≡ CH + H2O → CH3CHO (acetaldehyde)
- CH3CHO + CH3CHO → CH3CHOHCH2CHO (aldol)
- CH3CHO + H2O → CH3COOH + C2H5OH
- CH3COOH + C2H5OH → CH3COOCH3CH2 + H2O
- CH2OHCOOH + NH3 → CH2NH2COOH + H2O
5. Formation of Complex Aggregates
- The formation of complex aggregates, known as coacervates or microspheres, is a significant step in the process of the origin of life. These aggregates were first described by Oparin as minute, spherical, stable, and motile structures, while Sidney Fox referred to them as microspheres.
- Coacervates are formed when a protein and a polysaccharide are mixed together and shaken. The core of these coacervates primarily consists of proteins, polysaccharides, and water. These aggregates tend to extract these components from the surrounding aqueous solution, which contains a lower concentration of proteins and polysaccharides. However, since coacervates lack a lipid-based outer membrane, they are unable to replicate on their own.
- To address this limitation, coacervates were later enveloped by a limiting membrane composed of fatty acids such as lecithin and cephalin. This membrane formation was crucial for further development and replication. Once the limiting membrane was established, various substances started accumulating inside the coacervates.
- These coacervates acted as anaerobic heterotrophs, absorbing organic substances from the surrounding oceanic soup. As they absorbed more organic molecules, they grew in size and exhibited variable chemical compositions. To reproduce, the coacervates would undergo multiplication by breaking down into smaller droplets after reaching a certain size or attaining growth.
- The formation of these complex aggregates, coacervates or microspheres, represented a crucial step in the emergence of early life forms. They provided a protected environment for chemical reactions and the concentration of organic molecules necessary for the development of more complex structures and eventually the evolution of living organisms.
- In summary, the formation of complex aggregates called coacervates or microspheres occurred when proteins and polysaccharides were mixed together. Initially lacking a lipid-based outer membrane, these aggregates were unable to replicate. However, with the formation of a limiting membrane composed of fatty acids, coacervates could accumulate substances, absorb organic molecules from the environment, grow in size, and multiply by breaking down into smaller droplets. This process marked an important milestone in the origin of life, providing a foundation for further chemical reactions and the emergence of more complex structures.
6. Formation of Protobionts
- The formation of protobionts, which are considered the first non-cellular forms of life, played a crucial role in the origin of life on Earth. According to Oparin and Sidney Fox, complex organic compounds synthesized abiogenetically tended to aggregate and form large colloidal cell-like structures known as protobionts.
- Protobionts are believed to have emerged approximately 3 billion years ago. They consisted of giant molecules, including RNA, proteins, polysaccharides, and other biomolecules. These aggregates possessed the ability to separate specific combinations of molecules from their surroundings, creating a controlled internal environment.
- However, one significant limitation of protobionts was their inability to reproduce. The earlier microscopic, spherical, stable, and motile aggregates, known as coacervates or microspheres, lacked a lipid outer membrane and could not fulfill the requirements necessary for life’s reproduction.
- To address this limitation, microspheres emerged as a more successful candidate for protobionts. Microspheres had the capacity for growth and division through processes such as budding, fragmentation, and binary fission. These mechanisms allowed them to multiply and continue their existence.
- The formation of protobionts marked a critical step in the evolution of life. These early structures, with their complex molecular compositions and the ability to maintain internal environments, paved the way for the development of more advanced cellular life forms.
- In summary, protobionts were large colloidal cell-like aggregates formed from complex organic compounds synthesized abiogenetically. These structures had the ability to separate specific combinations of molecules and maintain a controlled internal environment. While earlier forms such as coacervates lacked the ability to reproduce, microspheres emerged as a more successful type of protobiont capable of growth and division. The formation of protobionts represented an important milestone in the origin and evolution of life on Earth.
Experimental Proof of Abiogenic Molecular Evolution of Life
Experimental evidence supporting the abiogenic molecular evolution of life was obtained through the famous Miller-Urey experiment conducted by Stanley L. Miller, a biochemist, and Harold C. Urey, an astronomer, in 1953. The purpose of this simulation experiment was to test the validity of Oparin and Haldane’s hypothesis regarding the formation of organic molecules in the early Earth.
In the Miller-Urey experiment, a mixture of methane, ammonia, hydrogen, and water (representing the primordial atmosphere) was exposed to an electric spark simulating lightning. The conditions were reducing, resembling the early Earth environment. The spark discharge apparatus, consisting of two tungsten electrodes, generated an electric spark of approximately 75,000 volts and provided a temperature of about 800°C. This setup allowed for the production of simple organic compounds from simpler compounds.
After one week of continuous exposure to the electric spark, the hot products were passed through a condenser to collect the aqueous end product, analogous to Haldane’s primordial soup. The researchers also conducted a control experiment, maintaining similar conditions but excluding the electric spark as the energy source.
After eighteen days, the chemical components of the products were investigated using chromatographic and colorimetric methods. The experimental results revealed the presence of peptides, purines, pyrimidines, organic acids, and amino acids such as lysine, alanine, aspartic acid, and glutamic acid. Purines and pyrimidines are precursors to nucleic acids, while proteins require amino acids for their formation. The intermediate products included aldehydes and hydrogen cyanide (HCN).
Comparing the experimental results with the control experiment, it was evident that the electric spark played a crucial role in the formation of organic compounds. Furthermore, similar organic compounds were found in meteorite material, indicating that similar processes are occurring elsewhere in space.
One notable observation was the condensed dark-colored liquid that was chromatographically analyzed. The liquid was found to be a mixture of sugars, amino acids (such as glycine and aniline), and fatty acids.
Based on these experimental findings, it can be concluded that the Oparin-Haldane theory of the origin of life, proposing the abiogenic formation of organic molecules from inorganic molecules, is supported. The Miller-Urey experiment provided important evidence for the possibility of the spontaneous generation of complex organic compounds under early Earth conditions, contributing to our understanding of the molecular evolution that led to the emergence of life.