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

Biogenesis is the idea that life arises only from pre-existing life.

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

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

Abiotically produced aggregates of organic molecules surrounded by a membrane-like structure, serving as direct precursors to the first living cells and exhibiting basic life properties such as metabolism and replication.

The origin of life from non-living organic matter without the intervention of pre-existing living organisms.

The biological phase of evolution where primitive living systems (protobionts) developed from complex organic structures like coacervates, eventually forming the first cellular life.

The gradual synthesis of complex organic macromolecules (such as proteins and nucleotides) from simple inorganic and organic molecules under the conditions of the primitive Earth.

The evolutionary phase involving the diversification of primitive cells into varied nutritional types (like chemoheterotrophs and photoautotrophs) and the subsequent development of multicellular organisms.

Spherical vesicles formed by a lipid bilayer enclosing an aqueous compartment, serving as models for early primitive cell membranes.

Colloidal aggregates of complex organic macromolecules capable of growth and replication are considered as early precursors to cell-like structures.

• The universe is an ever-expanding expanse encompassing all matter, energy, time, and celestial bodies.
• It is primarily structured by billions of galaxies, which are massive, gravitationally bound collections of stars and planetary systems.
• Our solar system is located within the Milky Way, which forms part of the Local Group of galaxies alongside its closest major neighbour, the Andromeda galaxy.
• Scientific consensus suggests the universe originated from a massive explosion known as the Big Bang approximately 13.7 to 15 billion years ago.
• Earth was formed around 4.5 billion years ago from a collapsing gaseous cloud, with the first life emerging roughly 500 million years later.

• The universe expanded from a dense, hot mass during the Big Bang 10–20 billion years ago, eventually forming galaxies like our Milky Way.
• The solar system originated 4.5–4.6 billion years ago from a condensing cloud of gas and dust known as the solar nebula.
• Early Earth gradually cooled and separated into distinct internal layers based on material density: the crust, mantle, and core.
• Earth's habitability is maintained by its optimal distance from the Sun, the presence of liquid water, and a regulated, oxygen-rich atmosphere.
• Unlike the extreme atmospheric and temperature conditions found on Venus and Mars, Earth uniquely sustains the stable climate required to support life.

• The origin of life is a unique event; Earth formed about 4.5 billion years ago, and life appeared about 4 billion years ago.
• Special Creation Theory: all organisms were created by a supernatural power; religion-based, with no scientific proof.
• Cosmozoic Theory (Panspermia): Life came from other planets as spores; not accepted, as it doesn't explain life's origin elsewhere.
• Spontaneous Generation (Abiogenesis): life arose from non-living matter; later disproven.
• Biogenesis: life arises only from pre-existing life; it explains continuity but not the first origin.
• Redi (1668): maggots appeared only in the uncovered meat jar, showing they came from fly eggs.
• Spallanzani (1767): sealed boiled broth showed no growth, proving microbes come from the air.
• Pasteur (1861): The swan-neck flask trapped airborne microbes, conclusively disproving spontaneous generation.

• Life originated through a gradual sequence of physicochemical transformations rather than an abrupt event.
• This process involved the progressive evolution of inorganic substances into organic compounds, driven by changing environmental conditions.
• The concept, also known as the abiogenic theory, was first scientifically formulated by A.I. Oparin in 1924 and J.B.S. Haldane in 1929.
• A.I. Oparin further detailed and consolidated these perspectives in his 1936 publication, The Origin of Life.

• 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 formed sugars, amino acids, fatty acids, purines, and pyrimidines.
• Organic molecules accumulated in the primitive ocean as a hot dilute soup.
• Further reactions produced macromolecules such as proteins, fats, and nucleic acids.
• Miller–Urey experimentally supported abiotic synthesis of organic molecules.

• Coacervates originated in primitive oceans as colloidal droplets formed by the aggregation of large organic macromolecules.
• They eventually developed a fatty acid limiting membrane, which established structural individuality and a distinct internal environment.
• Acting as anaerobic heterotrophs, they grew by absorbing ready-made organic substances from the surrounding ocean.
• Upon reaching an optimal size, these droplets multiplied by breaking apart into smaller units.
• They are considered essential prebiotic precursors that bridge the gap between chemical evolution and the first living cells.

• Nucleoproteins formed in primitive oceans became autocatalytic, i.e., self-duplicating systems.
• These systems showed the first hereditary role of nucleic acids.
• Short nucleotide chains evolved as primitive genes, storing and transmitting information.
• Primitive genes aggregated into protroviruses, similar to simple virus-like structures.
• Life originated in the Precambrian oceans around 3.7 billion years ago.
• Earliest organisms were heterotrophic and anaerobic, relying on existing organic molecules and living without free oxygen.

• In the primitive oceans, chemical evolution produced self‑duplicating nucleoprotein systems capable of heredity
• Complex organic molecules aggregated into colloidal droplets called coacervates, suspended in sea water.
• Coacervates showed basic life‑like properties such as metabolism, growth and simple division, so they acted as protocells/eobionts (protobionts).
• Some protocells containing nucleoproteins became more stable and self‑sustaining, forming the first true cellular living systems.
• These earliest cells were structurally simple, lacked a true nucleus and organelles, and are considered primordial prokaryotic cells, comparable to present‑day bacteria in the ancient oceans.

• 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 synthesise carbohydrates from inorganic substances, giving rise to chemoautotrophism, which was favoured 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).

• Blue-green algae-like prokaryotes initiated oxygenic photosynthesis, releasing free oxygen into the primitive environment.
• Earth's atmosphere shifted from reducing to oxidising, which spontaneously destroyed organic molecules and halted further abiotic origin of life.
• Free oxygen in the upper atmosphere formed the ozone layer, blocking harmful UV radiation and allowing life to eventually migrate to land.
• The availability of oxygen triggered the evolution of highly energy-efficient aerobic respiration, supporting greater cellular complexity.
• First eukaryotic cells with true organelles evolved in the primitive oceans approximately 1.5 billion years ago.

• 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.