A 2.1 Origins of cells (HL)

Origins of Cells (HL)

Conditions on Early Earth

  • Early Earth conditions differed significantly from present conditions:
    • Lack of Free Oxygen and Ozone: Allowed higher penetration of ultraviolet light.
    • Higher Concentrations of Carbon Dioxide and Methane: Resulted in higher temperatures due to the greenhouse effect.
  • These conditions facilitated the spontaneous formation of carbon compounds through chemical processes that no longer occur today.
Atmospheric Composition
  • Prebiotic Atmosphere:
    • Very low oxygen concentrations due to reactions with elements like iron.
    • High methane concentrations from volcanic activity and meteorite impacts.
    • High carbon dioxide concentrations from volcanic emissions.
  • Modern Atmosphere:
    • Oxygen concentration stabilized at approximately 20%.
Titan's Atmosphere vs. Earth's Atmosphere
  • Titan (Saturn's largest moon):
    • Nitrogen: 95%
    • Methane: 4.9%
    • Oxygen: 0.0001%
    • Thick orange smog due to ethane, propane, propene, and other hydrocarbons.
  • Earth:
    • Nitrogen: 78%
    • Methane: <0.001%
    • Oxygen: 21%

Cells as the Smallest Units of Self-Sustaining Life

Defining Life
  • MRHGREN Characteristics: Used to describe requirements to sustain life rather than life itself.
    • Metabolism
    • Response
    • Homeostasis
    • Growth
    • Reproduction
    • Excretion
    • Nutrition
  • Living things use energy to maintain a highly ordered state.
  • Cells are self-sustaining units; individual components alone are not considered alive.
Viruses
  • Viruses are pathogens but are generally considered non-living because they cannot self-sustain or reproduce without a host cell.

Challenge of Explaining the Spontaneous Origin of Cells

The Problem
  • Cells are complex structures that are currently produced by division of pre-existing cells.
Historical Context
  • Abiogenesis/Spontaneous Generation: The idea that life could arise from non-living matter (e.g., rats from grain, maggots from meat).
  • Francesco Redi (1668): Challenged spontaneous generation by showing maggots came from flies, not rotting meat.
  • Louis Pasteur (1864): Disproved spontaneous generation by preventing airborne particles from entering nutrient broth, thus stopping microbial growth.
Theories of Life's Origin
  • Abiogenesis (Spontaneous Generation):
    • Life developed independently from non-living substances.
    • Proposed by Stanley Miller & Harold Urey.
    • Not scientifically supported, though the Miller-Urey experiment attempted to provide evidence.
  • Biogenesis (Non-Spontaneous Generation):
    • Complex life arises only from pre-existing living things through reproduction.
    • Proposed by Theodore Schwann & Mathias Schleiden.
    • Scientifically proven by experiments (Pasteur, Redi).
    • Does not explain how the first cells arrived on Earth.
Necessary Requirements for the Evolution of First Cells
  • Catalysis
  • Self-replication of molecules
  • Self-assembly
  • Emergence of compartmentalization
Steps for Cells Arising from Non-Living Materials
  1. Non-living synthesis of simple organic molecules (from primordial inorganic molecules).
  2. Assembly of these simple organic molecules into more complex polymers.
  3. Packaging of molecules into membranes with internal chemistry different from surroundings (protobionts).
  4. Formation of polymers with the capacity to self-replicate (enabling inheritance).

Evidence for the Origin of Carbon Compounds

Miller-Urey Experiment (1953)
  • Aim: To simulate early Earth's atmosphere and test the possibility of creating organic molecules from inorganic chemicals.
  • Procedure:
    • Water was boiled to simulate high temperatures.
    • Vapor mixed with gases (H2, CH4, NH3) to create a reducing atmosphere (no O2).
    • Electrical discharge (“lightning”) provided energy for reactions.
    • Cooled mixture analyzed after a week.
  • Results: Traces of organic molecules, including amino acids, were found.

Spontaneous Formation of Vesicles

Role of Phospholipids
  • Phospholipids naturally assemble by coalescence of fatty acids into spherical bilayers.
  • These bilayers form vesicles, resembling plasma membranes, which allow for compartmentalization.
    • Compartmentalization: Formation of a membrane-bound compartment is needed to allow internal chemistry to become different from that outside the compartment.

RNA as a Presumed First Genetic Material

The RNA World Hypothesis
  • Challenge: DNA replication requires enzymes, but enzymes are encoded by genes (DNA).
  • Solution: RNA can store information like DNA, self-replicate, and act as an enzyme (ribozyme).
  • Ribozymes:
    • RNA molecules that can catalyze reactions.
    • Involved in cutting introns from mRNA and catalyzing changes in tRNA.
  • This theory suggests that RNA initially served as both the genetic material and enzymes in early cells.
Protocells
  • Protocells consist of self-replicating RNA molecules encapsulated by a membrane.
  • They grow by capturing new membrane material from the environment.

Evidence for a Last Universal Common Ancestor (LUCA)

Genetic Evidence
  • Comparing genetic sequences determines similarities and differences between species.
  • Species with similar genes are closely related; those with different DNA are distantly related.
  • All organisms share a common ancestor (LUCA) that existed approximately 4 billion years ago.
Universal Genetic Code
  • All living organisms store information in the genetic code, using the same 4-letter code that translates into codons specific to the same amino acids.
  • This shared feature traces all organisms back to LUCA.
Shared Genes
  • Researchers identified 355 genes coding for protein families found widely in bacteria and archaea.
  • These genes were inherited from LUCA and are essential for anaerobic metabolism and carbon dioxide fixation.
Hydrothermal Vents
  • Many of the genes found in LUCA are needed for anaerobic metabolism and carbon dioxide fixation – conditions preferably found around hydrothermal vents.
  • Hydrothermal vents are fissures in the oceanic crust that release hot water containing reduced inorganic chemicals (e.g., iron sulfide, hydrogen, methane, ammonia).
  • These chemicals provide energy for assembling simple carbon compounds into polymers.
Competition and Extinction
  • It is likely other forms of life evolved, but they became extinct due to competition from LUCA or its descendants through natural selection.

Approaches Used to Estimate Dates of the First Living Cells and LUCA

Challenges
  • Fossilization is a rare event requiring specific conditions and minimal soil erosion over millions of years.
Methods
  • Fossil Evidence: Numerous fossils have been found, contributing to the understanding of life's history.
  • Radiometric Dating: Techniques help determine the age of fossils.
  • Genomic Analysis: Data from sequenced microbial genomes provide insights.
Evidence from Ancient Rocks
  • Stalks of iron-rich minerals in northern Quebec, Canada, may be evidence of early life forms (4.28 billion years ago).
  • Hematite tubes produced by microbes around undersea hydrothermal vents suggest similar biochemistry to ancient organisms.
Characteristics of LUCA's Environment
  • LUCA likely lived near submarine hydrothermal vents, utilizing gases (H2, CO2, CO, N2) for chemosynthesis.
  • The bacteria found in these rock formations likely had similar biochemistry to ancient bacteria, and this environment might have been where opportunities for chemosynthetic pathways arose.