1. reading: Chapter 5 Lecture 12. The Origin of Life, Earliest Evidence for Life on Earth, Impact Frustration of Life announcement: Exam on Friday availability for office hours: Tuesday & Thursday

2. Earliest Evidence for Life on Earth HadeanArchean Proterozoic Phanerozoic 4.56 3.8 2.5 0.55 present billions of years ago: origin of the solar system oldest rocks on Earth - end of heavy bombardment rise in oxygen first multi- cellular fossils Cambrian Explosion plate tectonics? Start to see some evidence in the early Archean. Evidence becomes stronger and more familiar with time.

3. Stromatolites Revisited First stromatolites seen at 3.5 Ga. Not very common until the end of the Archean, ~ 2.7 Ga. 3.5 Ga stromatolite, North Pole, Australia Biogenicity is unknown. May have a geochemical origin - minerals precipitating abiotically in dome shape. May have a biological origin.

4. CyanobacteriaCO 2 + H 2 O ---> “CH 2 O” + O 2 Sulfate reducers2”CH 2 O” + SO 4 2- ---> 2HCO 3 - + H 2 S Modern Stromatolites Modern stromatolites built up by the action of microorganisms. Cyanobacterium, Schizothrix. Generates oxygen, has a filamentous morphology, makes a slimy sheath. When they get buried, they move up towards the light. Structure builds up new layers, also traps sand grains. Other microbes (sulfate reducers) cause carbonate to precipitate. Glues structure together.

5. Earliest Microfossils Pioneering work of Bill Schopf, UCLA Found microfossils in Warrawoona, Australia. The Apex Chert, 3.45 Ga. Made of carbonaceous material. Shapes interpreted as filamentous cyanobacteria. Size consistent with cyanobacteria (are larger than most bacteria).

6. Carbon Isotopes 14 C, 13 C, and 12 C. Organisms prefer to use 12 C - most abundant, most reactive, preferred by enzymes in the cell. Biomass enriched in 12 C, carbonate enriched in 13 C. Ratio of 12 C: 13 C provides carbon isotopic fractionation value. Measured as a  13 C value:  13 C = [ 13 C/ 12 C] sample [ 13 C/ 12 C] standard ‰, per mil. Each organism produces organic material with a certain range of  13 C. Finding ancient organic carbon, can estimate what organisms produced it. - 1

7. Carbon Isotopes, cont. Organic material enriched in 12 C goes back to the earliest rocks. Suggests an active biosphere with photosynthesis dating back to 3.5 Ga.

8. Earliest Microfossils Pioneering work of Bill Schopf, UCLA Found microfossils in Warrawoona, Australia. The Apex Chert, 3.45 Ga. Made of carbonaceous material. Shapes interpreted as filamentous cyanobacteria. Size consistent with cyanobacteria (are larger than most bacteria). Organic C has isotopic values typically found in microbial biomass. -25 to -30 ‰ Consistent with: - biological source - cyanobacteria

9. Recent Controversy In 2002, paper in Nature by Martin Brasier. Took more pictures of the microfossils (found they were weird). Mapped out geology of the area very carefully. Found microfossil location was a hydrothermal vent system - 250-350˚C. Carbon isotopic fractionation could be from abiotic reactions.

10. State of the Earliest Evidence of Life ….sketchy! Carbon isotopes suggest life, but may be confused with abiotic processes. Stromatolites become abundant at ~ 2.7 Ga. Microfossils of microbial cells do not become abundant or familiar until the Proterozoic. We have to be really careful because minerals have shapes that look like bacteria:

11. Origin of Life Null hypothesis : life originated on Earth. Alternative hypothesis : life arose elsewhere and was transported to the Earth ( Panspermia ). Good evidence life had a common ancestor: biochemistry is very similar made of the same stuff (RNA, DNA, proteins, lipid membranes) L- amino acids D- sugars all use ATP, make ATP the same way. Life is very sophisticated - the last common ancestor (LCA) also likely sophisticated.

12. Origin of Life, cont. Geologic record tell us little about how life originated. Studying modern life also tells us little about how life originated. Origin of Life no doubt a complex series of events that occurred before the first cell appeared. Origin of life a product of the early Earth. One of the 25 greatest challenges to Science.

13. Where Did Life Begin? On the surface? - no ozone (O 3 ) - no trees or fungi - soil different - ponds or lakes - shallow seas - evaporating ponds - on mineral surfaces (clays, pyrite) - hot springs, vents Deep oceans? - hydrothermal systems Icy surface? - faint young sun

14. How Did Life Begin? Origin of Life a product of the early Earth environment. Life today is made of organic compounds. Are a number of sources of organic compounds on the early Earth: - comets - meteorites - interplanetary dust - lightning - abiotic synthesis in hydrothermal systems

15. Early Ideas Thales 611-547 BCE Believed spontaneous generation - in residue of mud and mist while the water evaporated by the sun. Many Greek philosophers struggled with the nature of the universe and the nature of life. Some mixture of The 4 Elements : earth, wind, air, fire created life. Aristotle influential Greek life arose directly from the elements and the “ pneuma ” of the Earth (Latin word for “soul”, animating force) Idea of spontaneous generation carried through the middle ages.

16. Early Ideas, cont. Erasmus Darwin (grandfather) believed in spontaneous generation. Wöhler 1828 synthesized urea - showing organic molecules were not “special” Cagniard-Latour, Kützing, and Schwann 1836-7 found that yeast made up of minute organisms causing fermentation cells grew as fermentation occurred “Hence without parents, by spontaneous birth, Rise the first specks of animated earth.” “Organic life beneath the shoreless waves Was born and nurs’d in oceans’s pearly caves;” The Temple of Nature (1802)

17. Rudolph Virchow 1855 pronounced Omnis cellula e cellula (all cells from cells) Louis Pasteur set out to disprove spontaneous generation 1860’s used pasteurization (boiling) to sterilize culture flasks flasks didn’t grow unless exposed to air (carried germs) Darwin described a warm little pond, but not a theory for the OOL Early Ideas, cont.

18. More Modern Ideas Oparin Russian astrobiologist uninfluential 1924 book The Origin of Life J.B.S. Haldane 1929 paper on the origin of life published in The Rationalist Annual precursors of life were like viruses In 1953: Watson and Crick’s Nature paper on the structure of DNA Miller and Urey’s Science paper on abiotic production of organics In 1961: Oparin’s book translated into English

19. Polymerization Problem amino acid 1 + amino acid 2 dipeptide (aa1-aa2) + H 2 O Is called a problem because: 1. reaction produces water. 2. reactions must occur in the presence of water 3. life requires liquid water 4. water drives the chemical equilibrium in the wrong direction! Any valid hypothesis of the origin of life has to explain The Polymerization Problem. Somehow organic molecules came together, began reacting. Key: understanding polymerization. Called “The Polymerization Problem”

20. Problem of Chirality Chiral compounds have multiple stereoisomers. Amino acids occur in D and L forms in the universe. Any valid hypothesis of the origin of life has to explain The Chirality Problem : - amino acids in proteins “chose” L-amino acids - sugars/carbohydrates “chose” D-sugars

21. When Did Life Originate? Don’t know when life originated. Knowing when gives us some clues about the environment. If we have cellular life at 3.8 Ga: - during late heavy bombardment. - origin of life some time before this (4.1-4.2 Ga) - only took a couple hundred million years - hot conditions If cellular life arose later: - after late heavy bombardment - may have taken much longer to originate - billion years

22. Impact Frustration of Life Likely several 150-190 km diameter impactors between 4.5 and 3.8 Ga. If life originated in the deep sea: could have originated 4.2 - 4.0 Ga If life originated at the surface: could have originated 4.0 - 3.7 Ga

23. reading: Chapter 5 Lecture 13. Panspermia

24. Cassini Spacecraft found older terrains and major fractures on moon Enceladus Course crystalline ice which will degrade over time. Must be < 1000 years old! Organic compounds found in the fractures. Must be heated - required T > 100K (-173˚C) Erupting jets of water observed. Cause of eruptions not known…. Mystery of Enceladus