Principal Trends in the Evolution of Life Dr. Alastair Lyndon

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Principal Trends in the Evolution of LifeDr.
Alastair Lyndon

• Geology for Life Sciences
• A2.2GZ2

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Lecture Structure 1

• Life and Evolution
• Origins
• Evolution of complexity
• Fallacy of progress
• Operation of natural selection
• Major transitions in biological complexity
• Biological diversity - radiations extinctions

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Lecture Structure 2

• Effects of the physical environment
• early conditions on earth
• first life
• oxygen
• environmental change
• plate tectonics
• the fossil record
• natural disasters

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Origins of Life

• Age of the Earth 4.6 billion years (4600 Ma)
• First signs of life 3.9 billion years
• Multicellular organisms 0.5 billion years
• Humans 0.000035 billion years (35000 y)

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The Precambrian

• How did life arise?
• What happened over the first 3 billion years?
• How did this lead to more complex organisms in
  the late Precambrian Cambrian?

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Early Precambrian Environment

• Hadean eon
• Earth condensed
• hot surface bombarded by meteorites
• reducing atmosphere (nitrogen, CO2, water vapour)
• Life not yet possible
• prebiotic compounds - hydrocarbons, amino acids,
  nitrogen base derivatives

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Archean Eon

• Atmosphere still reducing in character
• Formation of persistent crustal features
• Appearance of life (prokaryotes)
• Evolution of some major metabolic strategies
• chemoautotrophy
• fermentation
• photosynthesis

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How did life start?

• Complex molecules can arise abiotically
• Experiments of Miller Urey (1950s) produced
  amino acids and nucleic acid bases
• Similar syntheses could have occurred by action
  of lightning, u-v radiation heat
• Absence of oxygen essential

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What about cells?

• Mixtures of these organic compounds can form more
  complex molecules with appropriate treatment
  (heat surfaces)
• Around 50 compounds commonly arise, many familiar
  components of metabolism (e.g.acetate, oxalate,
  succinate, glutamate)
• Lipid membranes pose more of a problem

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Prokaryotic cells

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The Archaea

• The Archaea are anaerobic extremophiles
• Environments
• hot springs
• hydrothermal vents
• anoxic muds
• Thought to be most similar to ancient forms of
  life

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Evolution of complexity

• Natural selection
• proliferation of those forms able to survive in
  the environment
• Occurs at POPULATION level
• Symbiogenesis - new species formed when symbioses
  become permanent

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Examples of symbiogenesis

• Cellular organelles
• mitochondria
• chloroplasts
• flagella
• Lichens

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The fallacy of evolutionary progress

• There is no reason why natural selection should
  lead to increased complexity
• Evolution results from increased survival of
  offspring under given environmental conditions
• This could arise both from a decrease or an
  increase in complexity
• Examples of both can be found

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The Proterozoic Oxygen

• Photoautotrophs had been present sin the Archean
  eon (cyanobacteria)
• Generally split a hydrogen containing compund
  e.g. H2S
• Some switched to H20
• Very prolific -
• stromatolites

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Oxygen

• Oxygen is highly reactive
• Poisonous to obligate anaerobes
• Respiratory metabolism (converts oxygen to water)
  could have originated as a detoxifying system

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Major transitions in biological complexiyt

• replication of information - RNA world
• first cells
• oxygenic photoautotrophy
• symbiogenesis (origin of eukaryotes)

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Major transitions (contd.)

• Multicellularity
• required complex cell-cell junctions for
  communication
• resulted in division of labour amongst cells
• differentiation and specialisation of tissues

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Break?
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Biological Diversity

• Sometimes referred to as biodiversity
• The variety of organisms at all levels from
  molecular and genetic through to populations and
  species
• Arose as a result of adaptive radiation or
  diversification

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Diversification

• Diversification has been a feature of the
  evolution of life
• Likely to have resulted from the stochastic
  (chance) nature of natural selection
• Events must have occurred in Precambrian, but no
  remaining evidence
• Major event at the start of the Phanerozoic - the
  Cambrian Explosion
• Occurs to a lesser extent when evironments alter

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Extinction

• A major process in evolution
• About 90 of all species in earths history are
  extinct
• Extinction happens constantly
• Occasional mass extinctions (loss of gt75 of
  species)

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Big Five Extinctions

• Precambrian-Cambrian
• Lower Cambrian
• End-Ordovician
• Late Devonian
• End Permian
• End Triassic
• End Cretaceous (K-T)
• Modern day??

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Evidence

• Fossil record
• only good through Phanerozoic
• Geological features
• e.g. iridium layer associated with K-T event

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Causes?

• Asteroid impact?
• Sea-level change?
• Climatic change (snowball earth)?
• Oceanic anoxia?
• Supernova explosion?

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Plate tectonic effects

• Movements of continents have affected environment
  and isolation of biotas
• Species can arise through isolation
• Ancient species may persist as a result of
  isolation e.g. New Zealand bird fauna

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Summary

• The origins of life were intimately associated
  with geological conditions on the early Earth
• Life remained unicellular and prokaryotic for
  approx. 3 billion years
• Eukaryotes arose from symbiogenesis
• Increase oxygen in atmosphere appears to have
  been an important turning point
• Geological events have had an important input
  into the evolution of todays biosphere