Origin of Life on Earth

							                          Georgia Tech School of Biology



                    Origin of Life on Earth
  • Spontaneous generation
        – Greek philosophers to 19th Century Europe
  •    Steps in the origin of life
  •    Critical role of RNA
  •    Evolutionary milestones
  •    Three domains of life



Biology 1510/1511                                          Fall 2008
                                    Georgia Tech School of Biology



       Spontaneous Generation – Early Views
• Anaximander (ca. 611-547 BCE)
   – Living creatures form in water when
     acted on by sunlight.
   – The original living organisms differed
     from those now extant, and humans
     originated from some kind of fish.


• Anaximenes (588-524 BCE)
   – The sun’s action on a primordial
     terrestrial slime made of earth and
     water led to the formation of plants and
     animals

                      Anaximander image: http://www.mlahanas.de/Greeks/Bios/Anaximander.html
  Biology 1510/1511   Anaximenes image: http://en.wikipedia.org/wiki/Image:Anaximenes.jpg      Fall 2008
                             Georgia Tech School of Biology



          Spontaneous Generation – Aristotle

• Aristotle (384-322 BCE) was the most
  prominent Greek naturalist.
   – Four terrestrial elements: earth, air, fire, water
   – Organisms derive their properties from the four
     elements and a vital force (“pneuma”).
   – Described various types of spontaneous generation:
         • Insects generated from dew falling on leaves
         • Bivalves form spontaneously in mud or sand
         • Some fish form like bivalves (i.e., from mud or sand)


 Biology 1510/1511        Image: http://www.philosophypages.com/ph/aris.htm   Fall 2008
                                                  Georgia Tech School of Biology


     Spontaneous Generation – 19th Century
• Lamarck (1724 - 1829)
    – Each species arose from an independent event
      of spontaneous generation.
    – Spontaneous generation continues today.




                                          • Pasteur (1822 - 1895)
                                                    – Demonstrated experimentally that microbes
                                                      would not appear in flasks protected from
                                                      dust and other small particles.
                                                    – Spontaneous generation does not happen
                                                      today on experimental time scales.
                     Lamarck image: http://bio.research.ucsc.edu/people/bernardi/Marina/public_html/Bio175/Bio175Website/Index.html
 Biology 1510/1511   Pasteur image: http://www.uab.edu/reynolds/MajMedFigs/Pasteur.htm
                                                                                                                                      Fall 2008
                           Georgia Tech School of Biology



                    Steps in the Origin of Life
  • Formation of organic molecules
        – Can occur spontaneously.
  • Polymerization to form macromolecules
        – Minerals may have acted as a template
  • Protobiont formation
        – Can form spontaneously under laboratory
          conditions.
  • Development of a hereditary mechanism
        – RNA as both enzyme and genetic material.

Biology 1510/1511                                           Fall 2008
                           Georgia Tech School of Biology


                    Earth 3 Billion Years Ago




Biology 1510/1511        Campbell & Reece 7th Edition, Fig. 26.01   Fall 2008
                           Georgia Tech School of Biology


                    Miller & Urey Experiment
                                              • Simulated early earth with a
                                                strongly reducing atmosphere, an
                                                ocean, and a hydrologic cycle.
                                                     – Energy inputs via heating & an
                                                       electrical discharge.
                                                     – Inorganic reactants
                                                       CH4, NH3, and H2
                                              • Amino acids and other organic
                                                molecules formed spontaneously
                                                under these conditions.
                                              • The building blocks of living
                                                organisms can form spontaneously
                                                on short time scales.



Biology 1510/1511        Campbell & Reece 7th Edition, Fig. 26.02               Fall 2008
                          Georgia Tech School of Biology


          What of a Less Reducing Atmosphere?
• Simple organic molecules are
  much harder to generate in a
  less reducing atmosphere.
   – Marine environments out of
     direct contact with the
     atmosphere may have provided
     a critical site for organic
     material synthesis.
   – Hydrothermal vents are an
     excellent environment for
     synthesis of organic molecules.
   – Genomic studies imply a high-
     temperature origin of life.

  Biology 1510/1511         Campbell & Reece, Fig. 25.02   Fall 2008
                      Georgia Tech School of Biology



                     Protobionts
• Liposomes
   – Membrane-bound droplets

   – Form spontaneously from lipids
     and organic molecules in
     aqueous solution.

   – Primitive “metabolism” may
     occur when molecules with
     enzymatic activity are included
     in the mix.
Biology 1510/1511       Campbell & Reece, Fig. 25.03   Fall 2008
                           Georgia Tech School of Biology



                       RNA and DNA
• Genetic information is now
  encoded by DNA, but DNA
  synthesis requires proteins, and
  proteins in turn are encoded by
  DNA…
• In contrast to DNA, RNA can
  show enzymatic activity as well
  as encode genetic information.
   – Ribonucleotides are more easily
     formed than deoxyribonucleotides
   – Ribozymes are capable of self-
     splicing and copying



 Biology 1510/1511       Campbell & Reece, 7th Edition, Fig. 26.05   Fall 2008
                                  Georgia Tech School of Biology


                                   Microfossils




                                                    Bitter Springs Chert, 850 Maa
Gunflint Chert, ca. 2 Ga
 Biology 1510/1511         Campbell & Reece, Fig. 25.04 Supplementary Images        Fall 2008
                            Georgia Tech School of Biology


  Three Domains of Life                                                                            4 Symbiosis of
                                                                                                     chloroplast
                                                                                                     ancestor with
• All organisms we know of                                                                           ancestor of
                                                                                                     green plants
  on Earth today are
  descended from a common                                                                          3 Symbiosis of
                                                                                                     mitochondrial
  ancestor that lived about 4                                                                        ancestor with
                                                                                                     ancestor of
  billion years ago.                                                                                 eukaryotes


                                                                                                   2 Possible fusion of
                                                                                                     bacterium and
                                                                                                     archaean, yielding
                                                                                                     ancestor of
                                                                                                     eukaryotic cells



                                                                                                   1 Last common
                                                                                                     ancestor of all
                                                                                                     living things
                                                                                                     (LUCA)
                                                        Campbell & Reece 7th Edition, Fig. 25.18




                                  Campbell & Reece, Fig. 26.21


  Biology 1510/1511                                                                                    Fall 2008
                         Georgia Tech School of Biology


                    Evolutionary Milestones
  • Life arose from nonlife
  • The first organisms were single cells
  • Speciation has generated the diversity of life
  • Eukaryotes are "cells within cells"
  • Photosynthesis and sex changed the course of
    evolution - and the planet
  • Multicellular organisms developed relatively late
    in Earth history.



Biology 1510/1511                                         Fall 2008