BIO372 : Evolution : Origin of Life : Lesson

I. Beginnings of Life on Earth History Fossilized bacteria from 3.55 billion years ago, like Cyanobacteria Remnants of carbon assimilation, older age of life on Earth

2. Hypotheses
   • Life originated on Earth spontaneously by natural processes
   • Germs of life came from outer space, originated elsewhere by natural processes
     • Planets outside our solar system
     • Search for Extra-Terrestrial Intelligence (SETI)
     • University of Washington initiates a program to hunt for life in outer space
   • Supernatural origin: where then did the supernatural originate?
   • Organic compounds possess a special "vital force"

3. Evidence
   • Chemistry
     • Synthesis of urea, an organic compound, by Friedrich Wöhler in 1828
     • Miller and Urey's simulation of primitive conditions on Earth
     • Organic chemistry is carbon chemistry: it is special because of the unique associative properties of the carbon atom, not because of a special "vital force"
   • Outer space
     • Interstellar dust contains precursors of organic compounds
     • Amino acids and other organic compounds in meteorites, on comets, on planets

4. Biological Molecules of the Cell
   • Proteins: made of amino acids
     • Structural
     • Catalytic: enzymes facilitate biochemical reactions in the cell, e.g. digestion and respiration
     • Regulatory: control cellular activities

5. Nucleic Acids
   • DNA
     • A linear string of nucleotides: A,C,T,G
     • Genetic code: sequence of nucleotides codes for 20 amino acids
     • Double helix of two complementary strands in eukaryotes: A is opposite T and C is opposite G
   • RNA
     • Linear string of nucleotides: A,C,U,G
     • Transcription
       • The sequence of nucleotide codes in DNA is transcribed to a complementary sequence of nucleotide codes in messenger RNA
       • Three successive nucleotides (a codon) specify one amino acid
     • Translation
       • At the ribosomal RNA, the code on messenger RNA is read
       • Transfer RNA brings the amino acid which matches a codon
       • Catalyst in ribosomal RNA attaches the amino acid to the growing polypeptide chain

6. RNA World
   • Photometabolism
     • Bilipid layers
   • Molecular selection
     • Complex chemical environment
     • Molecules which were more stable and replicated faster (i.e. RNA) became prevalent
     • RNA's which produced proteins enhancing their replication became prevalent
     • DNA: more stable than RNA to store genetic information

7. Thioesters
   • A likely presence on a prebiotic Earth: formed when a thiol joins with a carboxylic acid
   • Key role in present day metabolism: close association with ATP
   • Congruence between protometabolism and metabolism, thioesters present in both worlds

8. Extraterrestrial Life
   • Life was a natural consequence of early conditions on Earth
   • It is likely that life has developed numerous times in the universe

1. Prokaryote
   • Single celled of one compartment with an external cell wall

2. Eukaryote
   • Single cell or multicellular with specialized subcompartments and an internal cytoskeleton

3. Early Conditions on Earth
   • Atmosphere of methane and CO₂, no oxygen
   • Prokaryotes were the first organisms
     • Obligate anaerobes
       • Could not survive in oxygen
       • Metabolism based on sulfur and hydrogen sulfide (H₂S)

4. Evolution of Eurkaryotes
   • Photosynthesis
     • Primitive bacteria acquired the ability to photosynthesize
       • Chloroplast incorporated into cell through symbiosis
       • Used light energy to convert CO₂ into sugar: energy source
     • Increase in atmospheric oxygen
     • Small, aerobic organism engulfed: mitochondrion
     • Symbiosis benefited both host and aerobe

   • Cellular Skeleton
     • Prokaryote lost ability to use muramic acid to make its cell wall
       • Adaptation 1: Archaebacteria made a different cell wall
       • Adaptation 2: Internal proteinaceous skeleton (cytoskeleton) preserved membrane flexibility enhancing cell communication, subsequent association with additional symbionts, and the pathway to eukaryotes; endocytosis and exocytosis; aerobic metabolism came later

   • Giardia lamblia: a mixture of prokaryotic and eukaryotic characteristics which suggests a transition ("missing link") from prokaryotes to eukaryotes
     • Unicellular
     • No mitochondria or Golgi apparatus
     • Internal cytoskeleton, lysosomes, and four chromosomes in each of two equal nuclei
     • RNA close to prokaryotes

When you have completed this lesson, go on to Review Questions