Revision: Genetics and Evolution >> Origin of Life Biology (Theory) ISC (Science) ISC Class 12 CISCE

Abiogenesis is the concept that living organisms can arise naturally from non-living substances.

Autogenesis is another term for spontaneous generation, meaning self-origin of life from non-living matter.

Biogeny is the process by which living organisms arise from pre-existing living systems, leading to the formation of true cells from protobionts.

Chemogeny refers to chemical evolution, where simple inorganic molecules gradually formed complex organic compounds that led to the origin of the first cells.

Coacervates are aggregates of complex organic molecules capable of growth and division and are considered precursors of the first cell-like structures.

Cognogeny is the evolutionary stage involving development of complex life processes such as metabolism, communication, and differentiation, leading to multicellular organisms.

Liposomes are spherical vesicles formed by a lipid bilayer enclosing an aqueous compartment.

Protobionts are primitive cell-like structures formed from abiotically produced organic molecules, capable of basic life-like functions such as metabolism and reproduction.

• The universe originated about 10–20 billion years ago according to the Big Bang theory, which led to the formation of hydrogen, helium, galaxies, stars, and cosmic structures.
• Our galaxy, the Milky Way, is one among billions of galaxies, and the solar system formed about 4.5 billion years ago from a rotating cloud of gas and dust (nebula).
• The Earth initially formed as a hot, gaseous mass and later differentiated into a metallic core, mantle, and crust; gradual cooling led to the present stable surface.
• Earth supports life because it lies at the right distance from the Sun, allowing liquid water, regulation of CO₂, oxygen formation through photosynthesis, and a stable climate unlike Venus or Mars.

• The theory of special creation states that life originated on Earth through a supernatural event beyond scientific explanation.
• It proposes that all plants, animals, and humans were created in their present form by a divine power.
• This theory is supported by religious texts and beliefs, such as the Bible and Hindu mythology.
• Since it is based on faith and not observable or testable, it lies outside the scope of scientific study.

• The Earth is believed to have originated about 5–6 billion years ago from cosmic dust or as a part separated from the Sun during the formation of the solar system.
• Early Earth was a hot, molten, and gaseous mass with no atmosphere, gradually cooling over millions of years.
• During cooling, heavy elements formed the core, medium-weight elements formed the mantle and crust, while light gases accumulated to form the primitive atmosphere.

• Due to extremely high temperatures on early Earth, elements like hydrogen, carbon, oxygen, and nitrogen combined to form simple inorganic compounds such as oxides, carbides, and nitrides.
• As the Earth cooled, water vapour condensed into rain, leading to the formation of oceans containing dissolved gases like methane, ammonia, minerals, and salts.
• The primitive Earth had a reducing atmosphere rich in hydrogen, methane, ammonia, carbon dioxide, and water vapour, with no free oxygen present.

• As the Earth cooled to about 1000°C, simple hydrocarbons formed and later reacted with steam to produce organic compounds such as aldehydes, ketones, and organic acids.
• These compounds further polymerised to form complex organic molecules like sugars, amino acids, fatty acids, purines, and pyrimidines.
• In the absence of oxygen, these molecules accumulated in the oceans as a “hot dilute soup” (Haldane), leading to the formation of macromolecules essential for life.

• After the development of self-replicating nucleoproteins, coacervates evolved into the first cellular forms of life under favourable conditions in the primitive oceans.
• These primordial cellular organisms were prokaryotic, simple in structure, and comparable to present-day bacteria, as proposed by Oparin.

• Early prokaryotes were anaerobic heterotrophs that consumed the organic nutrients present in the primitive ocean, leading to competition for survival.
• Some prokaryotes evolved enzymes to synthesize carbohydrates from inorganic substances, giving rise to chemoautotrophism, which was favored by natural selection.
• Later, photosynthetic autotrophs evolved with chlorophyll-like pigments, eventually leading to chlorophyll-bearing prokaryotes similar to present-day blue-green algae (cyanobacteria).

• The release of free oxygen (O₂) by blue-green algae transformed the primitive reducing atmosphere into an oxidising atmosphere and led to the formation of an ozone layer, protecting life from harmful UV radiation.
• The presence of oxygen enabled the evolution of aerobic respiration, making energy production more efficient and allowing greater cellular complexity.
• Under these conditions, some prokaryotes evolved true nuclei, mitochondria, and other organelles, leading to the origin of eukaryotic cells in the primitive oceans.

• Evolution explains the gradual change in living organisms over time, leading from simple life forms to complex ones.
• It provides a framework to understand the diversity and relationships among all living organisms.
• Evolution is a natural, slow, continuous, and irreversible process driven by genetic variation and natural selection.
• All present-day organisms, including humans, have evolved from earlier ancestral forms over millions of years.