Origin of Cells

Learning Objective A2.1.1 - Conditions on Early Earth and the Pre-Biotic Formation of Carbon Compounds

Early Earth: Setting the Scene

• Around 4.5 billion years ago, Earth formed from collisions of dust and rock orbiting the young Sun.

• Early Earth’s surface was molten rock with intense volcanic activity, frequent meteor impacts, and no life.

• This period is called the Hadean eon (“Hades” = underworld) - symbolizing hellish, hostile conditions.

• As the planet cooled, volcanic gases were released and formed the first atmosphere.

Composition of the Early Atmosphere

• The early atmosphere was very different from today’s oxygen-rich air. •

• Contained mainly:

  • Water vapour (H2O)

  • Carbon dioxide (CO2) - much higher than today

  • Methane (CH4) - a potent greenhouse gas

  • Ammonia (NH3)

  • Nitrogen (N2)

  • Hydrogen sulfide (H2S) and other volcanic gases

• Free oxygen (O2) was absent, so no ozone (O3) layer existed.

Consequences of No Oxygen or Ozone

• Without ozone, ultraviolet (UV) radiation from the Sun reached the surface unchecked.

• UV radiation levels were ~100 times higher than today.

• UV radiation can damage DNA, but it also provided the energy needed for early chemical reactions.

• No oxygen also meant reducing conditions - ideal for the formation of simple organic molecules.

Greenhouse Effect and Surface Conditions

• High CO2 and CH4 concentrations trapped heat (greenhouse effect), making Earth very hot.

• The greenhouse effect:• Greenhouse gases absorb infrared radiation emitted by Earth. • They re-emit this energy, warming the surface.

• Result: early Earth was a steamy, high-temperature world with violent storms and lightning.

Energy Sources for Chemical Reactions

• Lightning, UV radiation, volcanic heat, and solar energy provided the energy for chemical synthesis.

• These energy inputs drove reactions between gases in the early atmosphere, producing organic molecules.

The Pre-biotic Formation of Carbon Compounds

• Under early Earth’s conditions, simple carbon compounds formed spontaneously - processes not seen today.

• Examples of possible compounds formed:•

  • Amino acids → building blocks of proteins

  • Fatty acids → components of membranes

  • Nucleobases → parts of DNA/RNA

  • Simple sugars (e.g., ribose)

• These reactions likely occurred in oceans, volcanic pools, or near hydrothermal vents - forming the so-called “primordial soup.”

The Primordial (Pre-biotic) Soup Hypothesis

• Proposed independently by Alexander Oparin (1924) and J.B.S. Haldane (1929).

• Suggested that life began in a “soup” of organic molecules formed in warm, shallow seas.

• Over time, these molecules accumulated, interacted, and gave rise to more complex biomolecules.

• Later experiments (e.g., the Miller–Urey experiment) supported this idea by synthesizing amino acids under simulated early-Earth conditions.

Dual Role of UV Radiation

• While UV light could destroy fragile molecules, it also drove chemical evolution:

  • When UV broke down molecules, reactive fragments recombined into new

compounds.

• For instance, pyruvic acid, when exposed to UV in water, can form larger organic molecules.

• RNA and nucleotides may have had an advantage:

  • Nucleotide bases absorb UV efficiently, offering protection and stability.

  • This could explain why RNA molecules were favored early in Earth’s history (link to the RNA world hypothesis).

A2.1.2 - Cells as the Smallest Units of Self-Sustaining Life

1. The Concept of Life

• Life is hard to define, but scientists agree that all living things share certain key features.

• According to cell theory, cells are the smallest units of life - all living organisms are made

  of one or more cells, and all cells come from pre-existing cells.

• Every cell performs the essential functions of life, either alone (unicellular organisms) or as part of a multicellular system.

2. Characteristics of Living Organisms

All living things perform these seven functions of life:

3. The Cell: Smallest Self-Sustaining Unit

• A cell can independently carry out all the above life functions.

• It is the basic structural and functional unit of all organisms.

• Every cell has:

  • Plasma membrane → separates it from the environment

  • DNA → stores hereditary information

  • Metabolic machinery → enzymes, ribosomes, organelles for energy use and synthesis

• Cells are self-sustaining chemical systems - they metabolize, respond, and reproduce without needing another living cell to function.

4. Defining Life Scientifically

• NASA’s working definition:

• This definition emphasizes three ideas:

1. Self-sustaining - performs metabolism and maintains homeostasis.

2. Chemical system - made of interacting molecules.

3. Darwinian evolution - reproduces with variation and can evolve by natural selection.

• This helps guide the search for extraterrestrial life — e.g., possible microbial life on Mars or icy moons like Europa.

Ask: How might life on another planet look different from life on Earth?

5. A Systems View of Life

• Living organisms are not just collections of molecules - they are integrated systems that:

  • Process and respond to information from their surroundings.

  • Regulate internal balance (feedback loops).

  • Evolve over time to improve survival.

• Systems biology studies how interactions between parts create emergent properties - like consciousness or metabolism - that don’t exist in individual molecules.

6. Viruses: The Boundary Between Living and Non-Living

• Viruses blur the line between life and non-life.

• They are non-cellular particles made of:

  • Genetic material (DNA or RNA)

  • A protein coat (capsid)

  • Sometimes a lipid envelope (in enveloped viruses like influenza).

Why viruses are considered non-living:

• No cells or organelles - cannot perform metabolism or generate ATP.

• No independent reproduction - must hijack a host cell’s machinery to make copies.

• No homeostasis - cannot regulate internal environment.

• No response to stimuli in a self-sustaining way.

Why some argue they are “borderline”:

• Contain genetic material and can mutate → allows evolution.

• Undergo natural selection, e.g., evolving drug resistance.

• Can adapt to new hosts or environments (e.g., COVID-19 variants).

Ask: If viruses evolve and adapt, should we redefine what it means to be “alive”?

Summary - Living vs Non-Living

A2.1.3 - Challenge of Explaining the Spontaneous Origin of Cells

The Central Question

All living cells today arise from pre-existing cells through cell division - this is the third principle of cell theory.

• However, the first cells on Earth had to come from non-living materials (abiogenesis).

• This creates one of biology’s biggest questions:

  • How did life begin from non-living matter?

• Scientists can’t observe this process directly, so they reconstruct plausible steps through chemistry and experimentation.

Ask: Why is it difficult to prove how the first cells formed?

What Makes a Cell a “Self-Sustaining Systemˮ?

All modern cells share three essential features:

1. Membrane - a stable boundary separating internal and external environments.

2. Genetic material (DNA/RNA) - to store, copy, and transmit information.

3. Metabolic processes - to generate and use energy for maintenance, growth, and reproduction.

For the first cell to evolve, all three features (or precursors of them) had to emerge spontaneously.

Ask: Which of these features do you think evolved first - membrane, metabolism, or genes?

Four Stages Required for the Origin of Cells

Scientists hypothesize that four major steps were necessary for life to arise from non-life:

1. Synthesis of simple organic molecules

   • Abiotic production of amino acids, sugars, and lipids.

   • Supported by the Miller–Urey experiment, which simulated early Earth’s atmosphere and produced amino acids from methane, ammonia, hydrogen, and water vapour.

2. Assembly of simple molecules into polymers

   • Small carbon compounds joined to form larger macromolecules (proteins, RNA,

     polysaccharides).

   • Could occur on hot sand, clay, or volcanic rock surfaces, even without enzymes.

3. Formation of membranes / compartmentalization

   • Lipids can self-assemble into spherical bilayers (vesicles) in water, forming protocells.

   • This created an internal environment distinct from surroundings - allowing chemical reactions to occur efficiently.

4. Development of self-replicating molecules

   • Molecules capable of copying themselves (like RNA) made inheritance possible.

   • This allowed Darwinian evolution - variation, selection, and adaptation.

Ask: Why was compartmentalization such a key step in the origin of life?

The “Protocellˮ - The First Living Systems

• A protocell is a hypothetical primitive cell-like structure that formed from organic molecules enclosed in membranes.

• Protocells likely:

  • Maintained internal chemistry different from their surroundings.

  • Absorbed simple molecules and grew.

  • Divided spontaneously when they became too large.

• Over time, protocells may have developed genetic systems and metabolic networks, leading to true cells.

Competing Hypotheses for the Origin of Life

Protocell-First Hypothesis

• Cell-like structures with primitive metabolism appeared before genetic material.

• These protocells grew and divided; later they developed RNA or DNA to store information.

• Prediction: Scientists might observe protocells forming spontaneously in lab conditions.

Gene-First (RNA World) Hypothesis

• Life began with self-replicating RNA molecules.

• RNA acted both as genetic material and catalyst (ribozyme activity).

• Prediction: Complex RNA molecules could form spontaneously and evolve collectively.

Metabolism-First Hypothesis

• A network of simple chemical reactions arose first, possibly on mineral surfaces (e.g., iron–sulfur minerals).

• These reactions produced energy and organic molecules, eventually leading to compartments and genetic systems.

• Prediction: Chemical mixtures under constant nutrient and energy flow could show autocatalysis and adaptive evolution.

Ask: Which theory do you think best explains how life could begin, and why?

Testing the Hypotheses: A Scientific Challenge

• Exact conditions on early Earth are unknown, so scientists can’t fully replicate them.

• The first protocells did not fossilize, leaving no direct evidence.

• Experiments (like Miller–Urey) give clues but not complete proof.

• Therefore, scientists take two approaches:

  • Bottom-up (chemical approach): Recreate pre-biotic chemistry (e.g., amino acid synthesis).

  • Top-down (biological approach): Study modern cells for ancient molecular relics (e.g., RNA catalysts).

• Both approaches build indirect evidence for how life may have originated.

Ask: Why can’t scientists ever “prove” how the first cell formed — only support a theory?

Nature of Science (NOS) Connection

• A scientific hypothesis must be testable, but not all hypotheses are easily testable.

• The origin of the first cells is a historical event, not directly observable.

• Scientists can test parts of the process (e.g., chemical synthesis of amino acids) but not the entire event.

• This highlights an important idea in science:

• Theories gain strength when multiple lines of evidence point in the same direction, even if direct proof is impossible.