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ASTR-3040:Astrobiology. The Origin & Evolution of Life on Earth Chapter 6. Day 12. Homework. Due Tue. March 1 Chapter 6: 1, 4, 8, 13, 23, 28, 34, 35, 42, 46, 49, 52, 53, 56 Exam 1 – Tuesday March 1 Chapters 1 - 6. Searching for Life's Origins.

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ASTR-3040:Astrobiology

The Origin & Evolution of Life on Earth

Chapter 6

Day 12

slide2

Homework

  • Due Tue. March 1
  • Chapter 6:
    • 1, 4, 8, 13, 23, 28, 34, 35, 42, 46, 49, 52, 53, 56
  • Exam 1 – Tuesday March 1
    • Chapters 1 - 6
searching for life s origins
Searching for Life's Origins
  • Geologic record details much of life history.
  • Evolution theory tells us how life has changed.
  • But, how did life arise?
  • Three lines of fossil evidence.
    • Stromatolites – date to 3.5 Gyr. Photosynthesis
    • Microfossils – date to 3.5 Gyr.
    • Isotopic evidence
stromatolites
Stromatolites
  • Photosynthetic at least in top microbes.
  • Modern ones resemble old fossils.
  • Date to 3.5 Gyr
microfossils
Microfossils
  • Biological?
  • photosynthetic?
  • Australia – 3.5 Gyr
  • Africa – 3.2-3.5 Gyr
  • 2.7-3.0 Gyr - conclusive
isotopic evidence
Isotopic Evidence
  • Carbon-13 evidence of 3.85 Gyr life
  • But, no microfossils in the rocks
  • Sedimentary – so fossils might be destroyed.
implications
Implications?
  • The carbon dating – if it stands – puts life at 3.85 Gyr ago – at least. Rocks this old are scarce.
  • Life itself must be older than this.
  • Arose and colonized Earth in ~100 Myr?
  • Probably after the LHB period (4.2-3.9 Gyr)‏
  • Suggests life will arise and spread quickly.
what did early life look like
What did early life look like?
  • Evolutionary relationships.
    • Track changes through DNA sequence.
    • Large difference in genome between two life forms indicates a longer time since they shared common ancestor.
  • Extremophiles (hyperthermophiles) are probably the closest to first life.
    • Chemoheterotrophes?
  • Where? Deep-sea hot water vents – most likely
origin of life
Origin of Life
  • Experiments try to re-create chemical conditions on Earth indicate life may have started through natural, chemical processes.
  • Panspermia – could life have originated elsewhere and been transported to Earth?
how did life begin
How Did Life Begin?
  • Miller-Urey Experiment
    • 1950s
    • H2O and CH4, NH3
    • Add electric spark
    • Pass condensates back to water flask.
    • Amino acids and many organics.
    • But, what was 1st atmo?
other sources of organics
Other sources of organics
  • Chemical reactions near deep-sea vents
  • Material from space – meteorites, comets
    • Organics can form in space?
  • Protoplanet & solar nebula
  • When was chemical ==> biological transition?
    • DNA is a complex molecule.
slide12
RNA
  • Single strand rather than double
  • Easier to manufacture
  • Recent (early 1980s) work show RNA can self-catalyze using rybozymes
  • Experiments show “clay” can facilitate self-assembly of complex, organic molecules.
  • Abundant on Earth and in oceans
  • Laboratory experiments
then what
Then what?
  • Assuming self-replicating RNA is formed
    • Rapid modification – natural selection
    • Mutations
then what1
Then what?
  • Pre-cells
    • Keep molecules concentrated – increase reaction rates
    • Protect from the outside world
    • Primitive structures form naturally and easily.
pre cells
Pre-cells
  • Amino acids will form spherical structures when cooled.
    • Grow by adding chains
    • Split to form daughters
  • Lipids in water form membrane-like structures
put it together
Put it together
  • 1. Some combination of atmo. chemistry, deep-sea chemistry, molecules from space.
  • 2. More complex molecules -RNA- grew form the building blocks. Some become self-replicating.
  • 3. Membranes form spontaneously.
  • 4. Natural selection among RNA molecules .
    • Eventually these become true living organisms.
  • 5. Natural selection – diversity.
    • DNA becomes favored hereditary molecule.
migration of life to earth
Migration of Life to Earth
  • We've seen some organisms survive in space.
  • Could life arise on Venus or Mars first?
  • Possibility of migration
    • 20,000 meteorites cataloged
    • ~36 come from Mars.
    • 1. Large impacts.
    • 2. Survival during transit.
    • 3. Atmo. entry.

ALH8400

transit
Transit
  • Endoliths could survive both blast and entry.
  • Transit survival depends on time in space.
    • Most rocks millions or billions of years
    • A few ten years or less.
    • Probably no interstellar meteorites (none known).
  • Why migration?
    • Does life form easily on early Earth?
    • Does life form too easily on any planet?
implications of transit
Implications of Transit
  • Of the early solar system planets
    • Mercury and Moon are probably not favorable.
    • Early Venus and Mars might have been hospitable.
  • Migration from Earth?
  • Why migration?
evolution of life
Evolution of Life
  • Major events.
    • Early microbes – anaerobic (primitive atmosphere).
    • Chemoautotrophes – underwater probably
    • Photosynthesis – multiple steps to arise
      • ~3.5 Gyr ago (stromatolites)‏
    • Oxygen crisis ~2.4 Gyr ago?
    • Evolution of Eukarya – cell complexity
      • Symbiosis?
        • Mitochondria & Chloroplasts
cambrian explosion
Cambrian Explosion
  • Life started slowly (?)‏
  • Multi-cell organisms ~1.2 Gyr ago
    • Microbes had 2+ Gyr by themselves
    • Animals – little change from 1.2 – 0.7 Gyr ago
    • Then a huge diversification
      • 30 body plans
      • 40 Myr for all this to occur.
why cambrian explosion
Why Cambrian Explosion
  • Oxygen level reached a critical value
    • Survival of large, energy-intensive life forms
  • Genetic diversity of eukaryotes
  • Climate change – coming out of snowball
  • No efficient predators
    • May explain why no similar explosion since.
colonization of land
Colonization of Land
  • Oxygen level reached a critical value
    • Ozone could form UV protective layer.
  • Need to evolve a method to obtain oxygen and nutrients.
  • Plants first ~475 Myr ago
    • Probably evolved from alga.
    • Specialization in larger plants (leaves, roots)‏
  • Amphibians and insects within 75 Myr
carboniferous period
Carboniferous Period
  • By 360 Myr ago – vast forests, insects
  • Flooded land masses – so little decay
    • These deposits formed coal.
rise of oxygen
Rise of Oxygen
  • Critical to animal life
  • Molecular Oxygen – reactive gas.
    • Disappears quickly if not replenished
    • Early – oxidation reactions (rust, iron-oxides...)‏
    • Now – use by animals
  • Cyanobacteria
timing
Timing
  • Fossil and rock studies
    • 2-3 Gyrs – banded iron formations
    • < 1% of present level
    • Sulfur isotope studies ~2.35 Gyrs for oxygen.
  • Cyanobacteria started ~2.7 Gyrs (350 Myr gap)‏
    • Removal by non-biologicals – oxidation
    • Slow build-up – no “explosion”
    • 200 Myr ago – first charcoal
implications1
Implications
  • If Earth is typical – probably few planets with complex, oxygen using life (rqr ~4 Gyr to form)‏
  • If Earth was delayed – complex life might be flourishing elsewhere.