6' The Origin and Evolution of Life on Earth

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6. The Origin and Evolution of Life on Earth
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Figure 4.10 Geological time scale
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6.1 Searching for Lifes Origin

• Life from non-life Until the 19th century, most
  people believed in spontaneous generation, i.e.,
  the formation of small organisms from inanimate
  matter.
• For example, maggots and flies materialized
  from rotting meat. However, in 1862, L. Pasteur
  showed that this was a contamination effect
  nutrients remained sterile when isolated from
  microorganisms.
• Life does not arise spontaneously from nonliving
  matter.

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Figure 6.1 Stromatolites...

• Rock beds layered structures
• microorganisms sediment
• Contemporary ? 3.5 billion years old
• Photosynthesis today 3.5 billion years ago?
• Life started earlier.

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Gunflint Chert, 1.8 billion years old(the rock
looked black)
50 µm
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Figure 6.2 Ancient microfossils...?

• Remnants of the biosphere?
• Abiotic origins?
• Shallow sea vs. a hydrothermal vent (deep ocean)

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Figure 6.3 Isua Formation, Akilia Island,
Greenland

• These rocks possess controversial evidence for
  life based on ratio of 13C/12C in oldest rocks
  life prefers 12C and these rocks are enriched in
  12C (depleted in 13C).

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Figure 5.11 The tree of life(Mapping
evolutionary relationships)
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Where did life begin?

• Surface environments where water meets land seems
  ideal, e.g., evaporating ponds, tide pools.
  Water is needed to combine chemicals to form
  molecules, but dehydration needed to get them to
  join (polymerization) geologic evidence.
• Hydrothermal environments?
• hydrothermal vents (hot springs) are good sources
  of reduced chemicals (even today).
• hydrothermal vents (hot springs) are shielded
  from surface impacts and UV radiation.
• most of the bacteria closest to last common
  ancestor of all life are thermophilic, thriving
  in high-temperature environments.

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Searching for Lifes Origins Darwin

• If we could conceive in some warm little pond,
  with all sorts of ammonia and phosphoric salts,
  light, heat, electricity present, that a protein
  compound was chemically formed, ready to undergo
  still more complex changes. At the present day,
  such matter would be instantly devoured or
  absorbed, which would not have been the case
  before living creatures were formed.
• Charles Darwin, The Origin of Species (1871)
• Note, conditions no longer exist on Earth today
  for life to arise from non-life.

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Searching for Lifes Origin - Chemistry

• A. Oparin (1924) and J.B.S. Haldane (1929) were
  the first to make an explicit connection between
  the origin of life and the conditions on early
  Earth.
• At the time, Earths early atmosphere was
  considered to be highly reducing mostly methane,
  ammonia, hydrogen, and water vapor, but no
  oxygen.
• With energy input from lightning, solar
  uv-radiation, volcanism, and radioactivity the
  chemical components of all living things could be
  formed in seawater.
• Haldane called this primordial mixture a hot
  dilute soup, or primordial soup for short.
• Liquid water on the Earths surface by 4.4 bya.
• The theory life may have arisen by chemical
  processes over a (geologically) long period of
  time.

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Figure 6.4 The Miller-Urey experiment ? the
primordial soup
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Primordial Soup Lab results

• After the experiment ran for a week, Miller
  analyzed the brownish goo that had collected in
  the hydrosphere and found that it contained
  significant amounts of complex organic molecules
  among them, the amino acids glycine and alanine,
  two of the building blocks of proteins.
• 85 of the goo was unidentified.
• Subsequent experiments by Miller and others
  showed that even under weakly reducing
  conditions, i.e., with less hydrogen amino
  acids, sugars, and nucleotide bases can be
  produced.
• Experiments under oxidizing conditions (similar
  to Earth today) failed to produce similar
  substances.
• Anoxic prebiotic organic chemistry could have
  produced the monomers of life.

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An extraterrestrial alternative

• Input of organic carbon from meteorites, e.g.,
  the Tagish Lake or Murchison meteorites.
• Water is a great solvent therefore, some of
  these organic compounds would have dissolved in
  the primitive ponds, lakes or oceans.
• Impact craters formed by larger, earlier impacts
  would have made good ponds or lakes, formed by
  the organic material brought in by comets or
  meteorites.
• The net result is the same, a primordial soup.

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Meteoritic Bombardment of Early Earth

• Influx high early in Earth history
• Lots of water organics delivered to Earth
• State of early atmosphere determines how much
  endogenous organic material available for
  pre-biotic chemistry, i.e., reducing vs. oxidising

Chyba and Sagan (1992)
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Polymerization

• How do we build polymers like proteins and RNA
  from smaller molecules, e.g., amino acids and
  nucleotide bases?
• Note, no one has yet produced functional polymers
  from scratch, despite the fact that we know
  their structure exceedingly well, e.g., the map
  of entire genomes.
• For nucleic acids, we need both a dehydrated
  environment and a template to tell the molecules
  how to replicate.
• Dehydration hydrologic cycle and evaporative
  ponds or craters.
• It is possible that the templates for
  polymerization were minerals.

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An RNA world?

• RNA first instead of DNA?
• RNA can form a variety of 3-D shapes depending on
  the nucleotide sequence. DNA, however, occurs
  mainly in double helix form. While its
  extremely stable and error-resistant, its
  stability hinders its ability to behave like an
  enzyme.
• RNA, not DNA, is involved in protein synthesis.
• In 1983, this RNA world idea was given a boost
  when T. Cech S. Altman discovered the first
  ribozymes, i.e., enzymes made of RNA, which
  catalyze reactions involving other RNA molecules.

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Figure 6.5 Self-replicating RNA...
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From polymers to protocells

• What about cells (cell body)? Cells are
  needed today to keep materials for metabolism
  together within a relatively impermeable membrane
• Early protocells may have been more like water
  droplets, with semi-permeable membranes to allow
  exchange of nutrients. They would have existed
  alongside self-replicating molecules, which may
  have occasionally become trapped inside a
  droplet.
• Permeable membranes could have allowed monomers
  to move in and out (allowing replication), but
  kept polymers inside.

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Micelles ? Plasma Membrane
Brock, 1986
David Malin
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Figure 6.6 Proteinoid microspheres (S.A. Fox)
lipid vesicles
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Figure 6.7 RNA lipid, pre-cells clay
minerals
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Figure 6.8 The origin of life
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Prokaryotic cell structure

• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA)
• Protein
• Carbohydrate
• Phospholipids

0.5 µm
Could life have migrated to Earth?
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6.3 The Evolution of life

• Prokaryotic evolution (an autotrophic progenote?)
• Chemoautotrophs
• Chemical energy (FeS H2S FeS2 H2)
• CO2 fixation
• Anoxygenic photosynthesis
• Light H2S
• CO2 fixation
• Oxygenic photosynthesis
• Light H2O
• CO2 fixation

Stromatolites
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Chemoheterotrophs first?

• Chemoheterotrophs (anaerobes)
• Chemical energy organic carbon SO42- ? H2S
• Organic carbon (the organic soup)
• Oxygenic photosynthesis (from the stromatolites?)
• Light H2O ? O2
• CO2 fixation
• Chemoheterotrophs (aerobes lots of energy!)
• Chemical energy organic carbon O2 ? CO2 H2O
• Organic carbon

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Figure 6.12 - Cyanobacteria
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Prior to the Vendian
Photoautotroph / oxygenic photosynthesis
(microbial mats) 6 H2O 6 CO2 sunlight (ATP) ?
C6H12O6 6 O2
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The origin of atmospheric oxygen.

• 6 H2O 6 CO2 sunlight (ATP) ? C6H12O6 6 O2
• Between gt 2.6 to 1.8 bya, cyanobacteria were
  releasing large amounts of O2 into the
  atmosphere, which could no longer be absorbed by
  the following inorganic processes
• Fe2 ¼ O2 H ? Fe3 ½ H2O
• Fe3 3 H2O ? 3 Fe(OH)3(s) 3 H
• The reaction between iron and oxygen is actually
  considered to be a detoxification mechanism that
  would have protected early life on Earth.

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Figure 6.13 Banded Iron Formations.
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Oxygen as a poison

• Aerobic respiration
• C6H12O6 6 O2 ? 6 CO2 6 H2O lots of energy
• superoxide anion O2-
• hydrogen peroxide H2O2
• hydroxyl radicals OH?
• singlet oxygen 'O2
• Aerobic heterotrophs have enzymes (e.g., catalase
  and peroxidase), that detoxify these reactive
  oxygen compounds.

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Figure 6.10 The origin of eukaryotes
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Eukaryotic cell structure
10 µm
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Eukaryotes

• The use of oxygen as an electron acceptor allowed
  eukaryotes to become larger and more complex.
• Long before plants and animals came along, our
  single-celled ancestors (prokaryotes and
  eukaryotes) had perfected most of the biochemical
  apparatus needed for life.
• Multicellular life may have developed from
  cooperative colonies of eukaryotes 750 mya -
  Vendian.
• Regulatory genes arose, allowing different kinds
  of cells to be produced from same genetic info
  (tissue differentiation).
• Sexual reproduction.

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Figure 4.30 The CO2 cycle and Snowball Earth

• Two events 2.4 to 2.2 billion years ago, and 750
  to 580 million years ago.
• Temperatures as low as -50ºC, a km of ocean ice.

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What prompted the Cambrian Explosion?

• Period immediately before Cambrian, the Vendian,
  contained a few, organized, multicellular sea
  fauna, but no hint of the plethora of forms and
  functions found in the Burgess shale, the richest
  of the Cambrian fossil sediments. Possible
  triggers
• Increased oxygen levels evidence for a decrease
  in 12C vs.13C just prior to the Cambrian. This
  could have boosted levels of O2, meaning more
  energy for aerobic life.
• Genetic complexity Regulatory genes in
  multicellular organisms.
• Absence of predators Initially, diverse phyla
  all occupied their own ecological niche.
  Crowding didnt occur until later.

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Vendian
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The Cambrian Explosion

• Critical period in evolution of life occurred 545
  mya, when huge variety of new animals emerged.
• Animal/Fossil classification is closely related
  to phylum or basic body plan (versus the genetic
  code).
• There are 30 phyla in the animal kingdom today,
  that comprise some 10-40 million species.
• In the early Paleozoic era, the Cambrian period,
  something remarkable happened, never again
  repeated in the history of life in only 40
  million years, all but one of animal phyla
  existing today appeared in the fossil record.
  This was the Cambrian explosion.

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The Burgess Shale

• The diverse origins of multicellular organisms
  suggest that multi-cellularity appears to be
  probable.
• Walcott Quarry

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Andrew MacRae, 1995
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Burgess Shale environment
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Figure 6.11 - The Carboniferous PeriodThe
colonisation of land
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6.4 Impacts and Extinctions

• Meteor Crater, AZ 1 km wide pit, 200 m deep
• 50,000 years old
• Formed by a 50 m metallic impactor

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Figure 6.15 The K-T Boundary
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The Cretaceous-Tertiary (K/T) extinction

• 65 mya
• caused or aggravated by impact of an asteroid
  that created the Chicxulub crater now hidden on
  the Yucatan Peninsula and beneath the Gulf of
  Mexico.
• impactor estimated to be 10 km in size.
• Flood basalts (The Shivan Crater Deccan Traps,
  India)?
• Killed, 16 percent of marine families, 47 percent
  of marine genera and 18 percent of land
  vertebrate families, including the dinosaurs who
  had flourished for 150 million years.

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Figure 6.16 Mexicos Yucatán Peninsula
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Figure 6.17 The K-T Impact
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Figure 6.18 Mass extinctions
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Figure 6.19 Tunguska, Siberia 1908
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Comet Shoemaker-Levy 9
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Figure 6.21 Impact frequency
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6.5 Human Evolution

• Limber arms, the ability to hang on branches
  use tools
• Hand dexterity
• Eyes close together ? overlapping field of
  vision, producing enhanced depth perception.
• Parental care
• Homo erectus (approx. 2 million years ago...
  200,000 years ago?)
• Homo sapiens 100,000 years ago to present
• Homo neanderthalensis (Neanderthals) 300,000
  years ago to 30,000 years ago, when they died out.

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Figure 6.22 Major primate branches
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Figure 6.23 Apes ? humans almost entirely
wrong
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Figure 6.24 Sahelanthropus tchadensis
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Figure 6.25 Human evolution(the last 7 million
years)
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6.6 The process of science in action Artificial
life

• How might we create artificial life?
• Making life from raw ingredients
• A bottom up approach... Using raw ingredients...
• Engineering new species from existing organisms.
• A top down approach... Genetic engineering
• Should we create artificial life?