ASTR-3040:Astrobiology
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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

ASTR-3040:Astrobiology

The Origin & Evolution of Life on Earth

Chapter 6

Day 12


Astr 3040 astrobiology

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.


Astr 3040 astrobiology

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.


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