Biochemical Evolution and Theories of Life

Biochemical Evolution

• Life on Earth originated from non-living matter approximately 3 billion years ago, a process known as protobiogenesis.
• Several theories attempt to explain the origin of life including:
  • Theory of Special Creation: posits that life was created by a supernatural power.
  • Cosmozoic Theory: suggests that life came to Earth from other planets in the form of spores or microorganisms carried within meteorites.
  • Theory of Spontaneous Generation/Abiogenesis: proposes that life arose spontaneously from non-living matter, a theory disproven by Louis Pasteur.
  • Theory of Biogenesis: explains the continuity of life, stating that all living organisms originate from pre-existing living forms through reproduction.
  • Modern Self Assembly Theories: these theories attribute protobiogenesis to the inherent properties of chemical molecules to self-organize and assemble, starting with the conversion of small molecules into larger ones, leading to cell-like microsystems.

Oparin Theory of Chemical Evolution

• Developed by Russian scientist Alexander Ivanovich Oparin in 1924 and known as the "Primeval Soup Theory," this theory proposes three stages for the chemical evolution of life:
  • Step 1: Origin of Earth and Primitive Atmosphere: The Earth initially formed from a hot cloud of gas (nebulous) containing various elements. As it cooled, heavier elements sank to form the core, while lighter elements like Hydrogen, Oxygen, Nitrogen, and Carbon formed the atmosphere.
  • Step 2: Formation of Ammonia, Water, and Methane: The highly reactive Hydrogen in the early atmosphere combined with other elements to form molecules like ammonia (NH3), water (H2O), and methane (CH4).
  • Step 3: Synthesis of Simple Organic Compounds: As the Earth cooled further, free radicals like CH and CH2 combined to form various hydrocarbons, leading to the formation of essential organic compounds like amino acids and sugars.

Miller-Urey Experiment

• Conducted in 1953 by Stanley Miller and Harold Urey, this experiment simulated conditions of the early Earth to test the hypothesis that organic molecules could form from inorganic matter.
• The experiment involved exposing a mixture of water, methane, ammonia, and hydrogen (simulating the primitive atmosphere) to electric sparks (representing lightning).
• The experiment successfully produced amino acids, building blocks of proteins, confirming that organic compounds can form from inorganic matter under the right conditions.

Prebiotic Earth

• The early Earth differed significantly from today's environment, with a different atmosphere and absence of life.
• Abiogenesis, the origin of life from non-living matter, is a complex process that is unlikely to occur spontaneously today due to the different conditions on Earth.

From Monomers to Polymers (RNA World)

• Early life likely involved the formation of complex molecules (polymers) from simple molecules (monomers).
• Clay minerals, like Cocaervates, may have acted as catalysts in this process by concentrating monomers on their surfaces and facilitating condensation reactions.
• RNA is considered the first hereditary molecule, capable of self-replication through complementary base pairing.
• RNA also exhibits catalytic properties, acting as ribozymes, further supporting its role as the central molecule in early life.

First DNA

• DNA, a more stable molecule than RNA, likely evolved later.
• The process of information transfer from RNA to DNA is possible, demonstrated by the reverse transcriptase enzyme found in retroviruses.

First Membranes and Cells

• Membranes, likely formed by phospholipids, enclose RNA molecules, creating an advantage for these molecules by allowing them to retain their protein products.
• These membrane-bound structures represent the first cells, leading to diversification in metabolism.

Diversity of Life

• Photosynthetic bacteria, responsible for the introduction of free oxygen into the atmosphere, played a significant role in the later evolution of life on Earth.