Early Earth

1

• Early Earth
• Earth 4.6 billion years old
• First water 3.9 bya
• First evidence of life 3.8 bya (controversial)
• First fossils (bacteria-like) 3.5 bya
• First eukaryotes 2.2 bya
• First animal fossils 700 mya
• Conditions
• High energy environment
• Violent thunderstorms
• Widespread volcanic activity
• Intense radiation (including uv) from early sun
• No ozone layer and sun more intense then vs. now
• Bombardment by meteors and other objects from
  space

2

• Origin of Life on Earth
• Requirements for Origin of Life
• Little or no free oxygen (O2)
• O2 highly reactive would have destroyed reduced
  compounds (chemical building blocks)
• Source of energy
• Intense radiation, volcanism, bombardment
• Availability of chemical building blocks
• Elements C, H, N, O, P, S
• Time
• End of bombardment to first evidence of life gt100
  million years
• First Cells May Have Arisen through Four Stages
• Abiotic synthesis of small organic molecules
  (e.g. amino acids, nucleotides)
• Assembly of small organic molecules into polymers
  (e.g. proteins, nucleic acids)
• Packaging of macromolecules into protobionts
• Membranes
• Internal environment different from external
  environment
• Development of self-replicating molecules

3

• Origin of Life on Earth
• Pre-Biotic Chemistry
• A.I. Oparin J.B.S. Haldane (1920s) Organic
  molecules necessary to life on earth might have
  arisen spontaneously from inorganic raw materials
• First empirical studies in 1950s by Harold Urey
  Stanley Miller (U. of Chicago)

4
(No Transcript)
5

• Origin of Life on Earth
• Pre-Biotic Chemistry
• Urey-Miller Apparatus
• Formation of amino acids, complex hydrocarbons,
  sugars, lipids, nucleotides
• Questions Enough NH3 and CH4 in earths early
  atmosphere? Reducing atmosphere?
• Alternative Origin of life at hydrothermal vents
• Sources of hot water and minerals, including S
  Fe compounds
• Alternative Panspermia
• Amino acids found in meteorites

6

• Origin of Life on Earth
• Pre-Biotic Chemistry
• Abiotic Synthesis of Polymers
• Clays may have helped
• Negatively charged (bind positively charged
  organic molecules)
• Contain minerals that might have catalyzed
  polymerization
• Amino acid solution hot clay ? AA polymers
• Protobionts
• Abiotically produced molecules surrounded by
  bilayered lipid membrane
• Highly organized
• Some forms reproduce when large enough
• Internal environment chemically distinct from
  outside
• Some forms carry out catalytic activity
• Some forms produce electrochemical gradients

7
Fig. 25.3
8

• Origin of Life on Earth
• Characteristics of Early Life
• Self-Replication
• Proteins arent self-replicating
• RNA may carry out catalytic functions
• Ribozyme Autocatalytic RNA
• RNA may have been first informational molecule
• DNA more stable may have arisen from RNA
• Nutrition
• First cells likely heterotrophic
• No free oxygen in atmosphere of early earth
• First heterotrophs probably used anaerobic
  fermentation (less efficient than aerobic
  metabolism)
• First autotrophs may have used hydrogen sulfide
  (H2S) as hydrogen source (modern purple green
  sulfur bacteria still get H from H2S)
• First autotrophs to split water for H probably
  ancestors of modern cyanobacteria (3.1 3.5 bya)
• Production of O2 had profound effects

9

• Origin of Life on Earth
• Characteristics of Early Life
• Aerobes
• O2 abundant by 2.5 bya
• Replaced obligate anaerobes in most areas
• Aerobic metabolism much more efficient than
  anaerobic metabolism
• Stabilized concentrations of O2 and CO2 in
  atmosphere
• Development of ozone (O3) layer
• Eukaryotes
• Appeared 2.1-2.2 bya
• How might eukaryotes have arisen from
  prokaryotes?
• Organelles (mitochondria, chloroplasts) may have
  originated from symbiotic relationships between
  prokaryote species
• Chloroplasts closely related to cyanobacteria
• Mitochondria closely related to alpha
  proteobacteria
• Serial endosymbiosis

10
Fig. 25.9
11

• Origin of Life on Earth
• Characteristics of Early Life
• Eukaryotes
• Evidence for serial endosymbiosis
• Inner membranes of plastids mitochondria have
  enzymes and transport systems similar to those of
  plasma membranes in modern bacteria
• Plastids mitochondria replicate by binary
  fission process similar to that of bacteria
• Plastids mitochondria each contain single,
  circular DNA molecule without histones or other
  proteins (similar to bacteria)
• Plastids mitochondria have ribosomes that
  resemble prokaryotic more than cytoplasmic
  ribosomes (size, sequence, sensitivity to
  antibiotics)