Chemical Evolution and The First Cells

1. Chemical Evolution and The First Cells

2. Big Bang? • Earth is approx. 4.6 billion years old • Formed as a result of the Big Bang which resulted in formation of universe approx. 15 bya • NOTE: Currently the Big Bang theory is being challenged by physicists such as Stephen Hawkings

4. Early Earth • Atmosphere: CO2, H2O, CO, H2, N2 • UV radiation • – broke down NH3, H2S, & CH4 rapidly • Little to no free oxygen (O2)

5. 4 Requirements for Chemical Evolution of Life: • Little to no free O2 • Source of energy • Presence of chemical building blocks • Time

6. How did life 1st form? 2 theories regarding formation of organic molecules

7. Prebiotic Soup Hypothesis • Life began at earth’s surface • 1st proposed by Oparin and Haldane in 1920s • Organic molecules could form spontaneously • Oparin hypothesized that these molecules would accumulate in shallow seas

8. 1950s – Miller-Urey experiment • Evidence since Oparin’s time indicates organic polymers form on rock or clay surfaces

9. Iron-Sulfur World Hypothesis • Life began at hydrothermal vents • Hot water, CO, minerals • Better protected from meteorites at bottom of ocean • Testing is difficult

10. Evolution of first cells • Could polymers spontaneously organize into more complex structures? • Evidence: • Protobionts • Microspheres • How could “pre-cells” make the jump to become living cells?

11. Self-replicating Molecules • Currently information flows from DNA  RNA  protein • RNA world model states RNA was 1st info molecules • RNA has catalytic properties (ribozymes)

12. RNA arose 1st, catalyzed own replication • RNA also catalyzed protein synthesis • Perhaps RNA made double-stranded copies of itself • Natural Selection at molecular level: • DNA became info storage molecule • RNA remains involved in protein synthesis • Protein enzymes catalyze most cellular reactions

13. Evidence of Biological Evolution • Fingerprints of organic C in rock from 3.8 bya • Microfossils (?) from 3.5 bya • Stromalites

14. First Cells • Prokaryotes • Heterotrophs • Anaerobes • Ferment organic compounds to yield ATP without oxygen

15. Natural Selection in 1st cells • As organic molecules used up, only certain organisms could survive • Early photosynthesis probably split H2S instead of H2O, releasing S instead of O • 1st to split H2O in photosynthesis were cyanobacteria

16. Build up of O2 in atmosphere affected life profoundly • 2 bya • Many obligate anaerobes perished • Some survive in places O2 didn’t reach • Some anaerobes neutralize O2 • Aerobes evolved respiratory pathways to extract more energy from food with O2, selective advantage

18. Aerobic Respiration stabilized levels of CO2 and O2 in the atmosphere • A second consequence of photosynthesis: build up of the ozone layer • Once O2 builds up and ozone layer forms, conditions no longer support abiotic synthesis of molecules

20. Endosymbiont Theory • Explains the appearance of eukaryotic organisms 2.2 bya • Organelles such as mitochondria and chloroplasts originated from mutualistic relationships: • Mitochondria – aerobic bacteria living within anaerobic cells • Chloroplasts – photosynthetic bacteria living within heterotrophic bacteria

22. Evidence of Endosymbiosis • Have own circular DNA • Can grow and reproduce on their own • Size of prokaryotic cells • Double membrane • Limited protein synthesis • Affected by antibiotics that affect pro- but not eukaryotes