A2.1 origin of cells

Early Earth was inhospitable, with changing conditions over billions of years.
Development of building blocks of life was facilitated through gradual changes.
Over time, the complexity of life led to approximately 8.7 million species today.
Science seeks to explain the origin of life through hypotheses and models.

Early Earth lacked free oxygen and had high levels of carbon dioxide and methane.
Higher temperatures and UV light may have allowed spontaneous formation of carbon compounds.
Earth formed approximately 4.5 billion years ago from small particles colliding into larger masses.
Initially, Earth lacked an atmosphere, allowing space objects to impact and heat the surface.

Around 4 billion years ago, Earth's atmosphere was dense with water vapor from volcanic eruptions.
Gases present: methane (CH₄), ammonia (NH₃), carbon dioxide (CO₂), hydrogen sulfide (H₂S).
Coupling of these gases with lightning led to spontaneous formation of carbon compounds.

All living organisms (unicellular or multicellular) perform essential functions: metabolism, growth, reproduction, response, homeostasis, nutrition, and excretion.
Virus distinction: nonliving as they cannot perform life functions independently.
Cell Theory: 1) All organisms comprise one or more cells, 2) Cells are the smallest units of life, 3) All cells arise from pre-existing cells.
Contribution of scientists (Robert Hooke, Matthias Schleiden, Louis Pasteur, etc.) to Cell Theory.

Cells can only create by division of pre-existing cells; thus, determining the origin is complex.
Required steps for cell evolution: synthesis of small carbon compounds, formation of polymers, encapsulation by membranes, emergence of self-replicating molecules.
Challenges exist due to inability to replicate prebiotic conditions in experiments.

Miller-Urey Experiment (1953): Simulated early Earth conditions to explore organic molecule formation.
- Used compounds (CH₄, NH₃, and H₂) and produced amino acids and hydrocarbons.
- Suggested life originated in a primordial soup, but assumptions regarding early conditions are debated.
- Alternative hypotheses suggest life originated near hydrothermal vents due to their rich mineral environment.

Fatty acids, when in water, can form vesicles with hydrophilic and hydrophobic properties.
Vesicles may have acted as protocells, encapsulating biological processes needed for life.
Compartmentalization within these vesicles became crucial for growing cellular complexity.

RNA likely acted as both genetic material and catalyst in early cells.
RNA's properties: spontaneous assembly, self-replication, and enzymatic function.
Overview of cell formation stages: early Earth → biotic compounds → organic molecules → polymers → protocells → cells.

Evidence includes universal genetic code, shared genes across organisms, and similar molecular processes.
Biochemical similarities point to a common ancestor existing approximately 3.5 billion years ago.

Fossil evidence provides insights into the evolution timeline.
Techniques: Radiometric dating using isotopes (e.g., Uranium-238, Carbon-14).
Evolution occurred over billions of years, with LUCA speculated to exist ~3.5 billion years ago.

Hydrothermal vents offer a potential origin location for life due to abundant minerals and energy-rich environments.
Genetic evidence suggests a possible connection between modern organisms and ancient forms living near these vents.