Formation of Organic Monomers and the Emergence of Life

Organic Monomers Formation

Introduction

  • The origin of organic molecules is a fundamental question in understanding the emergence of life.
  • There are three main hypotheses regarding the formation of organic monomers:
    • Arrival on Earth from space
    • Formation on Earth through chemical reactions in shallow oceans
    • Formation on Earth through chemical reactions near hydrothermal vents

Hypothesis 1: Organic Monomers from Space

  • The early solar system's dust, from which Earth formed, was rich in organic chemicals.
  • Meteorites, containing organic monomers, struck early Earth.
  • Due to the thicker atmosphere, the velocity of impact was lower, allowing the monomers to survive.
  • Old meteorites discovered today still contain organic chemicals.
  • An estimated 5 million kg (approximately 11 million pounds) of organic chemicals fall to Earth each year as cosmic dust.

Hypothesis 2: Organic Monomers from Reactions in the Shallow Ocean

  • Proposed in the 1920s by Oparin and Haldane.
  • The hypothesis states that organic chemicals, the basis of all life, could have evolved in Earth's early atmosphere.
  • In 1953, Stanley Miller tested this hypothesis.
  • Miller's experiment involved an artificial mixture of inorganic molecules simulating conditions on primitive Earth to test Oparin and Haldane's idea.

Stanley Miller's Experiment

  • The experiment simulated lightning using sparks.
  • It included a flask of warmed water to represent the primeval sea.
  • The early atmosphere was composed of:
    • Water vapor
    • Hydrogen
    • Methane
    • Ammonia
  • Energy was provided by electrodes discharging sparks into the gas mixture.
  • A condenser cooled and collected water and dissolved chemicals.
  • Within days, the experiment produced some of the 20 organic monomers presently found in organisms, and other organic molecules.
  • Organic monomers were found.
  • Subsequent work identified some defects in Miller's work, but experiments, taking into account new information about early Earth, have produced most of the naturally occurring organic molecules, validating Miller's hypothesis. These molecules include:
    • Amino acids
    • Sugars
    • Lipids
    • Nucleotides
  • Miller's and similar experiments demonstrate that scientists can test hypotheses about events from the distant past.
  • They also show the iterative nature of science, where experiments are built upon and corrected.

Hypothesis 3: Organic Monomers from Reactions near Hydrothermal Vents

  • Hydrothermal vents are found in the deepest parts of the ocean (2,000-8,000 meters below the surface), where the crust is thin, and magma flows close to the ocean floor.
  • Hydrothermal vents spew hot chemicals from the Earth's crust into deep ocean waters.
  • These vents are essentially underwater volcanoes.
  • Chemicals released from vents include CH4 (methane), NH3 (ammonia), and metals that can act as catalysts.
  • Chemicals in the surrounding water include CO2 and H2.
  • The combination of these chemicals, catalysts, heat, and time could have supported the formation of organic molecules, such as amino acids.
  • Rich communities thrive around hydrothermal vents, depending on bacteria that use the vents as their energy source.
  • Recent data suggests that all living things may have originated from hydrothermal vent bacteria.

Emergence of Life near Hydrothermal Vents

  • The following steps may have occurred over 700 million years:
    1. Organic monomers formed spontaneously.
    2. Monomers combined to form organic polymers.
    3. Some polymers were RNA molecules, capable of self-replication.
    4. Polymers used energy and materials from the vents for metabolic reactions.
    5. Cells formed by enclosing RNA molecules and metabolic pathways within lipid membranes for processing information and energy.
  • All cells are believed to have descended from these first cells.

Real-World Exploration: Hydrothermal Vent Organisms

  • Organisms living around hydrothermal vents were unknown until 1977.
  • Their way of life is independent of the sun; their energy source is Earth's internal heat.