Origin of life and prokaryotes

• Evolution of Life on Earth: How living organisms might have originated from non-living precursors
• Carbon isotopes from Greenland estimated 3.8 billion years old
• Earth formed ~4.6 billion years ago (bya)
• Viruses could be considered “alive” because they contain nucleic acid, replicate, and evolve rapidly. They could be considered “not alive” because they don’t make their own proteins and can’t complete their life cycle without invading living cells
• Main hurdle in defining life: a universal definition
• Conditions on early Earth made the origin of life possible through a sequence of stages: Abiotic synthesis of small organic molecules, joining of these small molecules into macromolecules, packaging of molecules into “protocells”, origin of self-replicating molecules
• What defines a cell: Metabolism (process through which organisms produce and consume energy to sustain themselves), Reproduction (all cells arise from previous cells), Composition (complex organic molecules: protein, carbohydrate, lipids, nucleic acids - DNA & RNA)
• Questions to consider about life: Why is it so hard to define life? How do viruses complicate our idea of life? What are the seven characteristics shared by all cellular life forms?
• In order for living cells to emerge: Formation and aggregation of organic molecules, a means of reproduction. Many hypotheses exist
• There is no universally accepted definition of life due to the discovery of new species and a lack of evidence of life on other planets making a strict definition difficult. Organisms on Earth possess a key list of attributes
• Conditions on early Earth

  • More frequent & intense lightning
  • Volcanic activity
  • Primitive atmosphere with CH4, NH3, CO2, N2, H2, & H2O
  • No O3 layer à intense UV radiation
  • “Reducing atmosphere”
• Origin of life: 4 key steps - Step 1: Abiotic synthesis of monomers. Oparin/Haldane hypothesis suggests a “Reducing” atmosphere, higher UV radiation, lightning could lead to abiotic synthesis of amino acids, sugars, lipids, nucleotide bases. Miller-Urey experiment supports this
• NASA’s exobiology advisory board suggested a working definition of life: “a self-sustaining system capable of Darwinian evolution”
• Stromatolites are dome-shaped layered rocks formed when microorganisms bind particles of sediment together, dated to as early as 3.4 billion years ago in Western Australia, southern Africa, northern Canada
• Abiotically produced organic molecules surrounded by a membrane or membranelike structure may have been precursors of cells
• Miller-Urey experiment supported the Oparin-Haldane hypothesis by showing that the abiotic synthesis of organic molecules in a reducing atmosphere is possible
• Polymerization of organic molecules can be catalyzed by certain types of clay or other mineral surfaces
• RNA monomers produced spontaneously from simple molecules on Earth
• Origin of life: 4 key steps - Step 1

  Abiotic synthesis of monomers
• Aggregates of abiotically produced vesicles exhibit simple reproduction and metabolism and maintain an internal chemical environment
• Experiment showed that vesicles form faster in the presence of a type of volcanic clay thought to be common 4 billion years ago
• RNA molecules called ribozymes have been found to catalyze various reactions, including self-splicing and making complementary copies of RNA sequences
• Evolution of mechanisms of heredity involves DNA to RNA to protein in modern cells
• Oparin/Haldane hypothesis

  Proposes a "reducing" atmosphere, higher UV radiation, and lightning leading to the potential for abiotic synthesis of amino acids, sugars, lipids, nucleotide bases
• Polymers of organic molecules can be produced by combining amino acids/nucleotides on hot sand, clay, or rock
• Experiment showed that polymers of organic molecules can also be formed in interstellar space
• Origin of life: 4 key steps - Step 2

  Formation of polymers
• Origin of life: 4 key steps - Step 3

  Packaging of molecules into "protocells"
• Liposomes, small membrane-bound droplets, can form when lipids/other organic molecules are added to water
• Vesicles can divide on their own
• First genes were probably RNA molecules that polymerized abiotically and were able to self-replicate
• Early genetic material might have formed an "RNA world": RNA to DNA to protein