12-1 How Did Life Begin?

I. The Age of Earth

• Earth is approximately 4.5 billion years old, having formed about the same time as the solar system

A. Measuring Earth's Age

• radiometric dating is used to calculate the age of an object using the proportion of radioisotopes

• half-life is the time in which one-half of a radioisotope decays to its breakdown products

II. Formation of the Basic Chemicals of Life

• energy in the environment combines inorganic molecules into organic compounds, which are carbon-containing molecules, often large and complex

A. The primordial soup model

• primordial soup model advanced by Oparin and Haldane is based on the presumed conditions of the early Earth
• Miller/Urey experiment tested the hypothesis that organic molecules could spontaneously form from inorganic molecules

     1. a closed set of glassware simulated the early Earth's conditions
          a. sealed to isolate the experiment from the surrounding atmosphere
          b. water in the bottom to simulate the oceans
     2. heat to circulate the water vapor, as a simulated water cycle
          a. atmospheric gases included nitrogen, methane, hydrogen and ammonia, but not oxygen, which is thought
          to have been absent from the Earth's early atmosphere
          b. electrical sparks simulate lightning
          c. a condenser cooled the gases, simulating rain
     3. after a period of time, samples were collected, which contained a variety of organic compounds
          a. amino acids
          b. fatty acids
          c. hydrocarbons

B. reevaluating the Miller-Urey model

     1. early Earth lacked an ozone layer, so UV radiation would have destroyed any ammonia or methane
     2. without methane and ammonia, experiment does not produce key organic molecules

C. Lerman bubble model proposes oceanic hydrothermal vents played a key role in producing organic molecules

• Step 1: deep oceanic hydrothermal vents produce ammonia, methane, other gases
• Step 2: bubbles concentrate gases, allowing reactions, and protect them from UV radiation
• Step 3: bubbles rose to ocean surface, releasing organic molecules into atmosphere
• Step 4: UV, lightning in atmosphere provides energy for further reactions producing more complex organic molecules
• Step 5: rain returns complex organic molecules to ocean, beginning next cycle of reactions

II. Precursors of the First Cells

A. RNA was probably the 1st genetic material

• the relationship between RNA and protein is more direct than it is for DNA and protein
• RNA is able to self-assemble from individual nucleotides into macromolecules
• RNA is able to self-replicate
• RNA is able to catalyze protein synthesis
• therefore, RNA was likely the 1st information carrying, self-replicating, protein building molecule

B. Microspheres and coacervates

• microsphere are proteins that form vesicles that have some properties of membranes
• coacervates are combinations of proteins, lipids and carbohydrates that have some properties of membranes

C. Origin of heredity

• the first cells required membranes and genetic material, probably RNA
• DNA replaced RNA as the genetic material some time later

12.2 Lecture Notes