What is Astrobiology? “Astrobiology is the study of life in the
ASTR-3040:Astrobiology Day 12 The Origin & Evolution of Life on Earth Chapter 6.
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Transcript of ASTR-3040:Astrobiology Day 12 The Origin & Evolution of Life on Earth Chapter 6.
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ASTR-3040:Astrobiology
Day 12
The Origin & Evolution of Life on Earth
Chapter 6
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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
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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
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Stromatolites
Photosynthetic at least in top microbes.
Modern ones resemble old fossils.
Date to 3.5 Gyr
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Microfossils
Biological?
photosynthetic?
Australia – 3.5 Gyr
Africa – 3.2-3.5 Gyr
2.7-3.0 Gyr - conclusive
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Isotopic Evidence Carbon-13 evidence of 3.85 Gyr life
But, no microfossils in the rocks
Sedimentary – so fossils might be destroyed.
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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.
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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
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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?
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How Did Life Begin?
Miller-Urey Experiment 1950s
H2O and CH
4, NH
3
Add electric spark
Pass condensates back to water flask.
Amino acids and many organics.
But, what was 1st atmo?
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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.
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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
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Then what?
Assuming self-replicating RNA is formed Rapid modification –
natural selection
Mutations
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Then what?
Pre-cells Keep molecules
concentrated – increase reaction rates
Protect from the outside world
Primitive structures form naturally and easily.
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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
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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.
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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
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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?
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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?
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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
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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.
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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.
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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
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Carboniferous Period By 360 Myr ago – vast forests, insects
Flooded land masses – so little decay These deposits formed coal.
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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
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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
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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.