Lecture 15: Origins and Early History of Life: How, When, Where, and What?

 

1. Definitions of Life

    1. 2 key processes that characterize all living systems:
      1. metabolism
      2. reproduction with variation → adaptation (multiplication, variation, heredity)
    2. Carl Sagan: "Life is any system capable of reproduction, mutation, and reproduction of its mutations."

 

  1. When did life first occur on Earth?
    1. Universe ~ 10-20 billion years old; Earth ~ 4.5 — 5 billion years old
    2. Fossil imprints show that microbes were well established by 3.5 billion years ago; best estimates that life existed 3.8 — 4 billion years ago
    3. Heavy bombardment of the earth by meteors 4.5 — 4.8 billion years ago
      1. Not calm conditions!
      2. Possibly killed off early life several times before it ultimately survived
    4. Early sun was weak; what kept the Earth from a deep freeze?
      1. "methane blanket" → warmth

- abrupt rise to 10-15% oxygen

 

3. Hypotheses: How did life originate on Earth?

  1. Darwin: "Warm Little Pond" Hypothesis (1871)
      1. Warm waters + organic molecules + long time → simple organisms
  2. Stanley Miller experiment (1953) to simulate supposed conditions on primitive earth
      1. Water; methane; ammonia and nitrogen (atmosphere); sparks (as lightening)
      2. One week later → goop of organic chemicals, including amino acids
      3. But now it is believed that CO2 was the major component of the early Earth’s atmosphere, and what about genetic molecules?
  3. The "Hot World" Hypothesis
      1. Nearly constant bombardment by meteors during earth’s 1st billion years
      2. Where could life have evolved and been safe from most asteroids?
        1. deep sea hydrothermal vents were common on early earth
        2. RNA from modern sulfur-eating bacteria indicate that they are very primitive
        3. at high temperatures, organisms can get extra energy from nutrients

 

 

 

4. How did organic molecules come together to form polymers, polypeptides, and amino acids?
  1. delivered in comets, meteors, interplanetary dust = panspermia hypothesis

b. chemical evolution
    1. abiotic synthesis and accumulation of monomers (Fig. 26.6) = building blocks
    2. join monomers into polymers (such as proteins or nucleic acids)
    3. formation of protobionts = aggregates of abiotically produced molecules able to maintain an internal environment different from their surroundings and exhibit some life properties such as metabolism and storing energy (Fig. 26.5)
    4. origin of heredity during or before protobiont appearance
    5. clay may have been an important substrate for abiotic synthesis of polymers (concentrate monomers on clay surface; metal ions in clay act as catalysts)

      6.  

      5. The earliest important organic molecule was probably RNA, not protein or DNA

      • RNA molecules functioned both as genetic material and as enzyme-like catalysts
      • Primitive life: amino acids aligned along RNA (= the first genes) (Fig. 26.7); replication errors → mutation

       

      6. Membrane-bound protobionts _ Life
          • Had genetic information
          • Selectively accumulated monomers from the surroundings
          • Used enzymes programmed by genes to make polymers and carry out other chemical reactions
          • Grew and split, distributing copies of its genes to offspring
          • Variation among related protobionts (due to mutation) would be subject to natural selection