A1.1

Formation of Earth

  • Approximately 4.5 billion years ago, the force of gravity caused scattered rock in the Solar System to coalesce into Earth.

  • As Earth cooled, water vapour condensing created the oceans, referred to as ‘primordial soup’, composed of simple carbon-based compounds.

  • This primordial soup likely yielded the first primitive life forms between 3.5 and 3.9 billion years ago.

    • Figure 1: A primitive cell structure likely consisting of a lipid membrane and RNA as genetic material.

Early Earth Conditions

  • Early Earth had a hostile environment characterized by:

    • Frequent bombardment by asteroids.

    • High degrees of volcanic activity.

    • Surface temperatures of approximately 75°C to 95°C.

    • A single global ocean with no solid land masses.

  • The atmosphere lacked free oxygen, had no ozone layer, and contained:

    • Methane (CH₄)

    • Ammonia (NH₃)

    • High concentrations of carbon dioxide (CO₂)

    • Water vapour (H₂O)

  • The inner core of Earth was predominantly liquid and hotter than today, leading to a smaller protective magnetic field, exposing Earth to higher cosmic and solar radiation levels.

    • These early conditions influenced extreme weather events, including frequent electrical storms.

Pre-Biotic Formation of Carbon Compounds

  • Key precursor for life: formation of simple, carbon-based compounds.

  • The early Earth had a reducing atmosphere rich in methane and ammonia, promoting electron donation to enable chemical reactions.

  • Resulted in complex carbon compounds including:

    • Simple amino acids.

    • Hydrocarbons.

  • These compounds served as building blocks for cellular components such as:

    • Proteins.

    • Lipids.

    • Nucleic acids.

  • Some complex molecules developed the ability to self-replicate, eventually enclosed within membranes, forming cells.

  • Modern conditions differ greatly, hindering the spontaneous formation of biological molecules today.

Life Processes and Cell Characteristics

  • Cells as the smallest unit of life, capable of performing the eight essential processes of life.

    1. Metabolism:

    • Definition: Chemical reactions within cells driven by molecules like enzymes.

    1. Response to Stimuli:

    • Definition: Ability to react to external environmental changes.

    1. Homeostasis:

    • Definition: Maintenance of stable internal conditions despite external fluctuations.

    1. Movement:

    • Definition: Control over position changes; specialized structures assist.

    1. Growth:

    • Definition: Increase in size or cell number over time.

    1. Reproduction:

    • Definition: Production of offspring using genetic material.

    1. Excretion:

    • Definition: Removal of metabolic waste from cells.

    1. Nutrition:

    • Definition: Intake or production of nutrients; autotrophs synthesize their own.

  • Learn the acronym MR HM GREN to remember these processes.

  • Viruses are classified as non-living since they cannot reproduce without a host cell.

Origin of Cells Steps

  • Theoretical steps to the emergence of the first cells include:

    1. Formation of simple organic molecules such as amino acids.

    2. Acceleration of chemical reactions via catalysis.

    3. Assembly of larger organic molecules, including RNA and phospholipids.

    4. Self-replication potential in RNA.

    5. Creation of a membrane-bound compartment to differentiate internal chemistry.

  • Uncertainties in cell origins arise from the challenges in testing hypotheses related to early Earth conditions.

Evidence Supporting Organic Compound Formation

  • Miller-Urey Experiment (1952):

    • Objective: Simulate Earth's pre-biotic conditions to create organic molecules.

    • Methodology: Closed system including:

    • Water to represent oceans.

    • Gaseous inlet for methane, ammonia, and hydrogen.

    • Electrical sparks to mimic early electrical storms.

    • Findings: After a week, the mixture contained basic organic monomers, including amino acids, validating non-living synthesis possibilities under early Earth conditions.

  • Caveat: The experiment demonstrated possible mechanisms, not definitive origins of carbon compounds.

Membrane Formation and Cell Evolution

  • Formation of vesicles: Fatty acids coalescing into spherical bilayers is crucial for the cell's evolution.

  • Separation rightfully allows chemical environments to differ between inside and outside of these vesicles aiding in cellular functioning.

RNA as the First Genetic Material

  • The ‘RNA first’ hypothesis suggests:

    1. RNA could synthesize from inorganic sources.

    2. RNA could replicate using ribozymes.

    3. RNA catalyzes protein production.

    4. Compartmentalization occurs via a membrane.

    5. RNA produced proteins and later DNA evolved as the more stable genetic material.

    6. Proteins became the main catalytic molecules (enzymes).

    • Supporting Evidence:

    • Observations of RNA self-replication and catalytic activity within ribosomes (peptide bond formation).

Alternative Theories for Cell Origins

  • Other hypotheses include:

    • Miller-Urey hypothesis: Suggests pre-biotic spontaneous generation under suitable conditions.

    • Metabolism-first hypothesis: Life origins tied to primordial metabolic processes.

    • Sulfur world hypothesis: Early life forms centered around iron-sulfur chemistry.

    • Lipid world hypothesis: Evolution of protective lipid bilayers predating RNA.

  • Ongoing research aims to substantiate or challenge these theories.

Last Universal Common Ancestor (LUCA)

  • The hypothetical ancestor, LUCA, likely:

    • Existed between 2.5 and 3.5 billion years ago.

    • A simple, single-celled autotrophic microbe possibly possessing an RNA genome.

    • Led to extinction or competition with other early life forms.

  • Evidence includes:

    • Universal genetic code shared by all life forms, indicating a common ancestor.

    • CARBON based life forms rely on similar nucleotide triplet coding, exemplifying the shared characteristics among different taxa.

  • Recent findings indicate a refined view of evolution:

    • Two domains of life arose directly from LUCA: bacteria and archaea.

    • The third domain, eukaryotes, evolved more recently via endosymbiosis.

  • Efforts include analyzing 11,000 genes common to bacteria and archaea.

Stromatolites and Evidence of LUCA

  • Stromatolites provide fossil evidence of past microbial communities:

    • Found in rocks, offering insights into ancient organisms and their habitats.

    • Support hypotheses regarding LUCA’s characteristics:

    • Existence in alkaline hydrothermal vents, rich in hydrogen, allowing cellular assembly and protection.

  • Findings indicate that LUCA was:

    • Autotrophic, using inorganic carbon for energy and growth.

    • Anaerobic, correlating with early Earth's oxygen-depleted atmosphere.

    Hydrothermal Vents and Early Life Research

  • Hydrothermal vents likely provided:

    • Physical protection from ocean extremes.

    • Energy source for complex organic molecule formation required for cellular structures.

Additional Resources

  • Interactive animations and activities are available to solidify understanding of:

    • Comparisons of ancient vs. modern Earth conditions.

    • Experimental setups similar to Miller-Urey's methodology.

    • The evolutionary tree of life in relation to LUCA.