The first cells


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The first cells

  1. 1. The prokaryotes The first cells Image Credits: Thermophilic archaebacteria Image Credits Gary Gaugler Bacillus anthracis
  2. 2. Origins <ul><li>Evidence for prokaryotic cells is found as early as 3.9 billion years ago </li></ul><ul><li>The prokaryotes had the Earth to themselves for another 2.4 billion years </li></ul><ul><li>Prokaryotes show an extraordinary diversity of biochemistry </li></ul><ul><li>Structurally prokaryotes are quite small and simple (1-10µm in diameter). </li></ul>© 2010 Paul Billiet ODWS
  3. 3. Where? <ul><li>“ (a) warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes” Charles Darwin (1871) </li></ul><ul><li>organic compounds would accumulate in the Earth's oceans until they &quot;reached the consistency of hot, dilute soup.&quot; JBS Haldane (1929) </li></ul><ul><li>Deep sea hydrothermal vents </li></ul><ul><li>Volcanoes </li></ul><ul><li>Outer space. </li></ul>© 2010 Paul Billiet ODWS
  4. 4. Panspermia (aka cosmozoan) <ul><li>Life came from somewhere else and seeded Earth </li></ul><ul><li>The support for this depends up on evidence that life exists elsewhere than on Earth </li></ul><ul><li>and the evidence that it may travel through open space. </li></ul>© 2010 Paul Billiet ODWS
  5. 5. Exobiology <ul><li>Mars Viking Probe (1976) revealed conflicting evidence of life </li></ul><ul><li>Mariner Probe (1997) did not reveal any more evidence </li></ul><ul><li>Tantalising evidence of water on Mars </li></ul><ul><li>Venus has inhospitable conditions (surface temperatures of over +400°C). </li></ul>Viking Image Credit NASA
  6. 6. Water and geothermal energy is the key <ul><li>The moons of Jupiter and Saturn could provide the right conditions </li></ul><ul><li>Europa appears to be covered in ice </li></ul><ul><li>Io shows volcanic activity </li></ul><ul><li>Titan has organic molecules present. </li></ul>Europa Image Credit NASA
  7. 7. Life can survive in outer space <ul><li>Bacteria, inadvertently left on a lunar probe </li></ul><ul><li>Collected and cultured successfully after nearly 2 years in space </li></ul><ul><li>Meteorites of Martian origin show that its early atmosphere would have been similar to Earth’s early atmosphere </li></ul><ul><li>They also showed (debatable) evidence of bacteria transported by meteorites. </li></ul>Surveyor 3 Image Credit NASA
  8. 8. Alien Earth <ul><li>Life has been shown to exist on Earth in very inhospitable conditions that could exist on other planets </li></ul><ul><li>Antarctic dry valleys </li></ul><ul><li>Mid-ocean ridges </li></ul>Image Credit: Earth Science Australia Image Credit: Antarctica
  9. 9. Earliest evidence of life <ul><li>The origin of the Earth itself is estimated as 4.5 billion years </li></ul><ul><li>Earliest evidence of life processes 3.9 billion years ago </li></ul><ul><li>This leaves little time for biochemical evolution. </li></ul>© 2010 Paul Billiet ODWS
  10. 10. The eternal question <ul><li>If it can be shown that life came from an extraterrestrial source the question still remains… </li></ul><ul><li>How did life evolve in the first place? </li></ul>© 2010 Paul Billiet ODWS
  11. 11. The poisonous gas …Oxygen <ul><li>The amount of free oxygen in the atmosphere increased after the evolution of photosynthesis </li></ul><ul><li>O 2 appeared from about 2.4 billion years ago and reached about 21% 1 billion years ago </li></ul><ul><li>Obligate anaerobes either became extinct or found niches where oxygen is absent. </li></ul>Image Credit: Stromatolites, Shark Bay Australia
  12. 12. The Great Oxidation Event © 2010 Paul Billiet ODWS
  13. 13. Evidence <ul><li>Iron is soluble in water where there is no oxygen </li></ul><ul><li>Iron rich water is produced by volcanic activity </li></ul><ul><li>Iron rich water entered shallow sunlight seas </li></ul><ul><li>Sunlight permitted cyanobacteria to photosynthesize and produce O 2 </li></ul><ul><li>O 2 combined with the iron and formed rust (iron III oxide) </li></ul><ul><li>Removing the O 2 from the water </li></ul><ul><li>Methane produced by methanogen bacteria also reduced the oxygen levels </li></ul><ul><li>This carried on until the rate of photosynthesis was so great, free O 2 appeared in the atmosphere (about 2.4 bn years ago) </li></ul><ul><li>This may have been precipitated by a shortage of nickel (Ni) </li></ul><ul><li>Ni is used in the enzymes of methanogens </li></ul>© 2010 Paul Billiet ODWS Image Credit: P.Hoffman
  14. 14. The evolution of the eukaryotes <ul><li>A third possibility was open with the teaming up of microbes </li></ul><ul><li>Endosymbiosis – a large anaerobic cell teams up with an aerobic cell </li></ul><ul><li>The aerobic prokaryote became a mitochondrion </li></ul><ul><li>Eukaryotic cells were formed, bigger and more complex, eventually forming multicellular organisms . </li></ul>© 2010 Paul Billiet ODWS
  15. 15. The evidence for endosymbiosis <ul><li>Certain eukaryotic organelles have their own DNA </li></ul><ul><li>S ingle naked loop of DNA , like the prokaryotes </li></ul><ul><li>The amount of hereditary information is a lot less than free-living prokaryotes </li></ul><ul><li>These organelles have their own ribosomes </li></ul><ul><li>Smaller ( 70S ) than those in the cytoplasm (80S) </li></ul><ul><li>The ribosomes of mitochondria and chloroplasts are the same size as those in prokaryotes </li></ul><ul><li>The protein synthesis of these organelles is semi-independent of that taking place in the cytoplasm </li></ul><ul><li>It is inhibited by the same antibiotic that affects prokaryotes (chloramphenicol). </li></ul>© 2010 Paul Billiet ODWS
  16. 16. The structural evidence <ul><li>These organelles are found in membrane envelopes </li></ul><ul><li>As though they were captured in a vacuole or vesicle by a larger cell </li></ul><ul><li>These organelles are about the same size as a prokaryotic cell. </li></ul>© 2010 Paul Billiet ODWS Image Credit: Mitochondrion
  17. 17. Mitochondria <ul><li>These represent an aerobic prokaryote that took up residence in a larger cell </li></ul><ul><li>These are found in all the eukaryotic kingdoms (plants, animals, fungi and protoctista). </li></ul>© 2010 Paul Billiet ODWS Image Credit: Mitochondrion
  18. 18. Chloroplasts <ul><li>These represent a cyanobacterium type of prokaryote that was trapped in ancestral plants and some protoctista </li></ul>© 2010 Paul Billiet ODWS Image Credit: Cyanobacterium heterocyst