Origin of Life on Earth (The Chemical Evolution), Abiogenetic Theory.
# Life Origin from life-less chemicals when there was no oxygen on the primitive Earth! A story from 4.6 billion years ago.
#Reduced environment contributed to the starting Life!
Introduction to Start the Life!
The origin of life on Earth is still a subject of scientific investigation, and the exact mechanisms by which pre-biotic organisms emerged are not fully understood. There are many theories to reveal the secret of the origin of Life on Earth such as the Special Creation theory (all living organisms were created by God), Spontaneous generation (directly from decaying organic matter), Cosmozoic theory (life was descended from other planets where life existed previously), Abiogenesis (chemical evolution), Biogenesis theory (organisms come from organisms only but not from non-living entities), and more. Interestingly, among them, the most widely accepted hypothesis is that life arose from simple inorganic and organic molecules (non-living), spontaneously in the reduced alkali oceanic environment through the process called “Abiogenesis”.The alkali vents would have been rich in metals such as iron and sulphide etc., which can catalyse complex organic chemical reactions. Over time, these molecules became more complex from and added organic chemical products such as amino acids, nucleotides, lipid molecules etc., eventually leading to the formation of self-replicating entities, the RNA, and later the DNA, which are considered the precursors of modern-day cells. Origin of Life on the Earth is explored by chemical evolution (inorganic evolution), whereas the Origin of Species is explained by Biological evolution (organic evolution).
Abiotic Conditions in Primordial Oceans as Chemical Laboratory on the Earth:
Chemical Evolution (Inorganic Evolution)
On the earth, approximately 4.6 billion years ago oceans evolved as “chemical laboratories” and became the site for “primordial soup” with a rich amount of chemical inorganics, providing a universal medium for numerous chemical reactions. Early Earth’s condition was a “Reducing Atmosphere” composed of metallic substances such as iron, nickel, sulfur, copper, manganese and zinc etc. along with non-metals such as methane (CH4), ammonia (NH3), water vapour (H2O), and hydrogen (H2), but with “no oxygen (O2)”. Unambiguously, this “Reducing Atmospheric Environment” facilitated to elevate of numerous chemical reactions for the synthesis of “organic molecules” such as amino acids, lipid molecules and simple sugars from inorganic molecules referred to as “Prebiotic Soup (Prebiotic soup)”. The energy for these chemical reactions is contributed by the sunlight UV radiation, lightning, and geothermal heat of volcanic activity. The absence of Oxygen and other oxidizing agents favoured the conditions to prevent the degradation of newly emerged organic molecules.
Proof- Stanley Miller’s experiment (1953) supports the Abiogenetic theory of Evolution!
Objective:
The Miller-Urey experiment was to investigate the hypothesis that the basic building blocks of life, such as amino acids, could have formed under artificial which resemble the primordial soup of primitive oceanic conditions believed to be present in early Earth’s environment, providing experimental support for the concept of chemical evolution, the “Abiogenetic Theory” (Life origin from Lifeless chemicals).
Experiment:
The gases of inorganic Ingredients filled the apparatus with a mixture of gases that they believed to be representative of the early Earth’s atmosphere. This mixture included water vapour (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2). Heat and Energy are applied to simulated lightning or electrical discharges by introducing electrical sparks into the mixture to simulate the energy from lightning strikes. The apparatus had a cooling system that allowed the water vapour to condense and collect in a separate flask. Amino Acids, the building blocks of proteins were formed over a period of one week, Miller and Urey observed that the electrical discharges in the mixture of gases produced a variety of organic molecules.
Formation of Organic or Biomolecules (Sequential chemical reactions)
Simple organic molecules, the building blocks like amino acids, nucleotides, sugars, and fatty acids polymerized, condensed and materialized macromolecules, the proteins, DNA & RNA, Polysaccharides and Lipids, respectively in the primordial oceanic environments. Later, the chemical evolution prolonged for many years and the macromolecules would later play a crucial role in the emergence of life, “The self-replicating RNA”.
Emergence Self-Replicating RNA World
The first step involves the formation of ribonucleotides, the nucleotide building blocks of RNA. These include adenine (A), cytosine (C), guanine (G), and uracil (U). Various prebiotic chemistry scenarios suggest that these nucleotides could have formed through reactions involving simple molecules present on the early Earth, such as hydrogen cyanide (HCN), ammonia (NH3), and other compounds. When the RNA was first made in the water it could have acted as
a “catalyst for its own RNA replication (as enzyme-ribozyme)” as well as for “the synthesis of proteins“.
Error- Replication in RNA and Natural selection (Errors made the Correct Life)
Short RNA chains could serve as templates for the assembly of complementary strands. This process involves the binding of ribonucleotides to the template RNA strand, guided by base pairing rules (A with U and G with C). This creates complementary RNA strands. Contrastingly, the replication was erroneous and imperfect leading to the “creation of a variety of many RNA sequences”, randomly. Then, “Natural Selection” came into the scene and elected some RNA sequences that had better stability, higher replication rates, or other advantageous properties that increased their chances of replication and survival.
The emergence of DNA from RNA
DNA is more stable than RNA in preserving genetic material. Removal of Oxygen from ribose molecules converted the ribose sugar into deoxyribose sugars, thus stable DNA is formed, and complementary pairing enabled dsDNA (double-stranded DNA) and avoided being broken down.
Oxygen is removed by a reduction process mediated by excited UV radiation or metal ions such as iron and manganese etc.
Example 1: Non-enzymatic conversion of ribose sugar to de-oxy ribose sugar was elucidated as the removal of oxygen (de-oxygenation) from ribose sugar (reduction reaction) by metal ions, such as iron or manganese. Iron reduces the ribose sugar by removing an electron.
Ribose sugar (C5H10O5) ——–> Ribose-Fe2+
Ribose-Fe2++ Energy——–> Ribose radical (.C5H9O5) + Fe3+
Ribose is radical, so it is highly reactive after the removal of electron forms De-oxy-ribose (C5H10O5).
Example 2: UV radiation excites reducing the ribose sugar by removing electrons.
UV light
Ribose sugar ———————————————-> De-oxy ribose sugar
@Normally, now, the ribonucleotide reductases enzyme present in the organisms catalyses this reaction. Whether this enzyme was formed or not in the primitive environment is still ambiguous and research is active on it.
Natural selection chosen DNA for a stable life
Once the stable dsDNA is blessed to evolve on the earth, natural selection conserves the DNA structure for further evolution. DNA replications and on the DNA template RNA molecules transcription continued for many years in the oceanic water and huge varieties of nucleic acids (DNA & RNA) occupied the earth’s environment.
DNA and Protein-based Life
Over time, RNA-based life forms could have given rise to more complex organisms with DNA as their genetic material and proteins for various cellular functions. DNA’s stability and protein’s versatility could have provided advantages enabling more complex chemical reactions within protocells
Protocells and Liposomes/ lipid membranes
Lipid molecules in water can form structures called liposomes or protocells, which have properties similar to cell membranes, providing a degree of “compartmentalization” and separation from the external environment. These structures could have provided a confined environment for chemical reactions to occur. Complexes of self-replicating RNA molecules or evolved DNA and other molecules could have formed within lipid-rich environments, leading to the emergence of protocells or vesicles with primitive lipid membranes.
Primitive Organisms
The earliest organisms would have been extremely simple, consisting of basic cellular structures and minimal metabolic processes that resembled modern prokaryotes such as bacteria and archaea without any cell organelles. These organisms would not resemble modern life forms but would represent the starting point of biological evolution.
Early Genetic Code and Protein Synthesis
As proteins and DNA evolved, a more programmable genetic code emerged. This code allowed for the precise assembly of amino acids into proteins, enhancing cellular functionality.
Simple Reproduction
Protocells might have developed mechanisms for asexual reproduction, such as budding or fission, allowing for the creation of daughter protocells.
Diversification and Speciation of Protocells into Emergence of Prokaryotic Cells
As protocells continued to evolve, variations in structures and functions would have arisen. Different lineages of protocells could have specialized for specific environments or roles, leading to the diversification of primitive cellular life forms. More advanced cellular structures, such as the development of a stable cell wall and improved internal organization, could have led to the emergence of prokaryotic cells (bacteria and archaea). These cells represented a major step in cellular complexity.
Origin of Eukaryotes (Endosymbiosis): The true organisms were taking birth on Earth.
The origin of Eukaryotes was a complex process and included many hierarchical steps and stages, some of the important events were:
- The origin of a flexible cell surface
- The origin of a nuclear envelope, which encloses a genome organized into chromosomes
- The origin of a cytoskeleton
- The appearance of digestive vacuoles (food vacuoles)
- Acquisition of certain organelles like mitochondria and chloroplasts by the “endosymbiosis” process.
Endosymbiosis is a biological process that involves one organism living inside another organism. It has played a significant role in the evolution of eukaryotic life on Earth by the addition of some more important cellular organelles such as mitochondria and chloroplasts etc.
Primitive eukaryotic cells engulfed a free-living prokaryotic cell, such as a bacterium, but instead of digesting it, the host cell formed a symbiotic relationship with the engulfed cell. The small organisms engulfed in the cell get protection and nutrition. The eukaryotic cell is enriched with the different organelles for more effective metabolic activities. Eukaryotic cells became the building blocks for the evolution of complex organisms like plants, animals, and fungi.
Evidence: The multicellular eukaryotes, origin or evolved from the prokaryotes, 1.5 billion years ago. These are protozoa to all higher chordates and single-celled algae to higher angiosperms that share many common attributes (similar characteristics) with Bacteria and Archaea etc.
- Getting energy by conducting glycolysis, primarily.
- The cells are surrounded by plasma membranes and ribosomes in abundance.
- DNA is the genetic material that encodes proteins.
- Ribosomes are universal protein preparation factories in all organisms.
- Transcription and translation synthesize those proteins.
- The same genetic code is used for the translation of proteins.
- Replicate the DNA, semi-conservatively.
Biological Evolution:
The origin of species is the gifted product of organic evolution. Once the biological systems were birth on this planet, they would have been undergoing many rapid changes with their dynamic RNA & DNA mutations. The physicochemical and biological environment is not stable always, dynamic over the period. Consequently, the organisms living in that particular ecosystem adapted to whatever changes the organisms required (structure, metabolic activities etc.) to live successfully. This may be visible as microevolution which does not show major recognizable traits, but the cumulative effect of these adaptations may lead to Macroevolution and leave new species on the earth (Origin of New Species).
At the beginning era of evolution under certain environmental stresses, for example, severe heat conditions of volcanic eruptions, the anaerobic and aerobic prokaryotic cells autotrophic, heterotrophic microbes of Kingdom Monera started with the symbiotic relationship and evolved into eukaryotic plant and animal cells to form Kingdom Protista. During further evolution, the Kingdom Protista evolved into three Kingdoms–Plantae(Metaphyta), Fungi and Animalia (Metazoa) by adaptive radiation or cladogenesis (phylogenetic development).
References:
- Life, The Science of Biology, ninth edition, David Sadava et al., 2011.
- P.S. VERMA & V.K. AGARWAL, Evolution, new edition.