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Lifes Origin and Early Evolution

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					Life’s Origin and Early Evolution

           Chapter 20
Impacts, Issues
Looking for Life in All the Odd Places

 Life can adapt to nearly any environment with
  sources of carbon and energy – including
  extreme temperatures, pH, salinity, or pressure
Origin of the Universe
and Our Solar System


 Big bang model
  • The universe began in an instant, with a big bang
    about 13 to 15 billion years ago
  • The universe is still expanding

 Earth formed from dust and debris orbiting the
  sun, about 4.6 billion years ago
Conditions on the Early Earth



 Earth’s early atmosphere came from gas
  released by volcanoes, and was low in oxygen

 Rain washed minerals and salts out of rocks to
  form early seas
Early Earth
Origin of the Building Blocks of Life



 Small organic molecules that serve as building
  blocks of life can be formed by nonliving
  mechanisms

 Stanley Miller’s experiments showed that amino
  acids form in reaction chambers designed to
  simulate early Earth
Stanley Miller’s Experiments
20.1 Key Concepts
Origin of Organic Compounds


 When Earth formed more than 4 billion years
  ago, conditions were too harsh to support life

 Over time, its crust cooled and seas formed

 Organic compounds of the sort now found in
  living cells may have self assembled in the seas
  or arrived in meteorites
20.2 How Did Cells Emerge?




 We will never know for sure how the first cells
  came to be, but we can investigate the possible
  steps on the road to life
Origins of Proteins and Metabolism



 All living cells carry out metabolic reactions, are
  enclosed within a plasma membrane, and can
  replicate themselves

 Concentration of molecules on clay particles or
  in tiny rock chambers near hydrothermal vents
  may have helped start metabolic reactions
Possible Origins of
Complex Organic Compounds

 Clay templates on tidal flats; iron-sulfide rocks at
  hydrothermal vents on the deep ocean floor
Origin of the Plasma Membrane


 Vesicle-like structures with outer membranes
  form spontaneously when some organic
  molecules are mixed with water

 Protocell
  • Any membrane-enclosed sac of molecules that
    captures energy, concentrates materials,
    engages in metabolism, and replicates itself
Models for Protocells

 Selectively permeable vesicles and RNA-coated
  clay
Origin of Genetic Material


 An RNA-based system of inheritance may have
  preceded DNA-based systems

 RNA world
  • A time when RNA both stored genetic information
    and functioned like an enzyme in synthesis

 Ribozymes
  • Synthetic, self-replicating RNAs
Hypothesis: Chemicals to Living Cells
20.2 Key Concepts
Origin of Cells



 In all living cells, proteins catalyze metabolic
  reactions, a plasma membrane encloses the
  cell, and DNA is the molecule of inheritance

 Laboratory experiments provide insight into how
  cellular components and processes could have
  evolved
20.3 Life’s Early Evolution




 Fossils and molecular comparisons among
  modern organisms inform us about the early
  history of life
The Golden Age of Prokaryotes


 Life that arose 3-4 billion years ago was
  probably anaerobic and prokaryotic

 Early divergence separated ancestors of
  bacteria, archaeans and eukaryotes

 The first photosynthetic cells were bacteria that
  used the cyclic pathway (does not produce O2)
The Proterozoic Era


 2.5 billion years ago, the oxygen-producing,
  non-cyclic pathway of photosynthesis began
  evolving in cyanobacteria (stromatolites)

 Oxygen accumulation in the air and seas halted
  spontaneous formation of molecules of life,
  formed a protective ozone layer, and spurred
  evolution of organisms using aerobic respiration
Fossil Prokaryotic Cells
The Rise of Eukaryotes

 Oldest eukaryotic fossils: 2.1 billion years
20.4 Where Did Organelles Come From?



 Eukaryotic cells have a composite ancestry, with
  different components derived from different
  prokaryotic ancestors

 A nucleus and other organelles are defining
  features of eukaryotic cells
Origins of Nucleus, ER, and Golgi Body

 The nucleus and ER may have arisen through
  modification of infoldings of plasma membrane
Evolution of
Mitochondria and Chloroplasts


 Mitochondria and chloroplasts descended from
  bacteria that were prey or parasites of early
  eukaryotic cells

 Endosymbiosis
  • The symbiont lives out its life inside a host
  • Eventually, host and symbiont become incapable
    of living independently
Evidence of Endosymbiosis
20.5 Time Line for
Life’s Origin and Evolution
20.6 About Astrobiology


 Astrobiology
  • Study of origins, evolution, and persistence of life
    on Earth as it relates to life in the universe

 Studying conditions on other planets provides
  clues to how life arose on Earth
  • Examples: Unmanned missions to Mars and
    Europa; telescopic studies
Lessons of Chile’s Atacama Desert

 What we learn about life on Earth informs our
  ideas about the possibility of life elsewhere
20.3-20.5 Key Concepts
Early Evolution


 The first cells were prokaryotic

 Eukaryotes arose after evolution of the noncylic
  pathway of photosynthesis in some prokaryotes
  put oxygen into the air

 Mitochondria and chloroplasts are descendants
  of bacteria that lived in other cells
Major Events in the History of Life

				
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