ASTR-3040:Astrobiology

1. ASTR-3040:Astrobiology The Origin & Evolution of Life on Earth Chapter 6 Day 12

2. 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

3. 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

4. Stromatolites • Photosynthetic at least in top microbes. • Modern ones resemble old fossils. • Date to 3.5 Gyr

5. Microfossils • Biological? • photosynthetic? • Australia – 3.5 Gyr • Africa – 3.2-3.5 Gyr • 2.7-3.0 Gyr - conclusive

6. Isotopic Evidence • Carbon-13 evidence of 3.85 Gyr life • But, no microfossils in the rocks • Sedimentary – so fossils might be destroyed.

7. 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.

8. 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

9. 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?

10. 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?

11. 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.

12. 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

13. Then what? • Assuming self-replicating RNA is formed • Rapid modification – natural selection • Mutations

14. Then what? • Pre-cells • Keep molecules concentrated – increase reaction rates • Protect from the outside world • Primitive structures form naturally and easily.

15. 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

16. 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.

18. 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

19. 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?

20. 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?

21. 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

23. 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.

24. 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.

25. 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

26. Carboniferous Period • By 360 Myr ago – vast forests, insects • Flooded land masses – so little decay • These deposits formed coal.

27. 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

28. 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

29. 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.