1. Chapter 25 The History of Life on Earth

2. Question u How have events in the Earth’s history contributed to life as we know it?

3. How did Life get started? u One Idea - Chemical Evolution: the evolution of life by abiogenesis (non-living).

4. Steps 1. Monomer Formation 2. Polymer Formation (macromolecules) 3. Protocells (Protobiont) Formation 4. Origin of self-replicating molecules (heredity)

5. Primitive Earth Conditions u Reducing atmosphere present. u Simple molecules existed u Ex: H 2 O, CH 4, H 2, NH 3

6. Complex Molecule Formation u Requires energy: u UV radiation u Radioactivity u Heat u Lightning

7. Oparin and Haldane 1920s u Hypothesized steps of chemical evolution from primitive earth conditions.

8. Miller and Urey, 1953 u Tested O and H’s hypothesis. u Experiment - to duplicate primitive earth conditions in the lab.

10. Results u Found organic monomers including Amino Acids. u Suggested that under the primitive Earth’s conditions, monomers could form.

11. Other Investigator's Results u All 20 Amino Acids u Glucose, Ribose, Deoxyribose etc. u Glycerol and fatty acids u Nucleotides u ATP u Monomers for all macromolecules have been found.

12. Polymer Synthesis u Problem: u Monomers dilute in concentration. u No enzymes for bond formation.

13. Possible Solutions 1. Clay 2. Iron Pyrite u Both could have served as a substrate to position monomers close together. u Have metals that can act as catalysts.

14. Another possibility u Waves could have placed monomers on beaches. u Alternating wet/dry conditions were favorable to life

15. Protocells- (Protobionts) u Combination of abiotically produced molecules. u Exhibit some properties of life. u Ex: Osmosis, Electrical Charge, Fission

16. Protobionts

17. Results u Protobionts have membrane-like properties and are very similar to primitive cells. u Protect polymers from depolymerization u Start for natural selection that lead to cells?

18. Question ? u Where did the energy come from to run these early cells?

19. Answers u ATP. u Reduction of sulfur compounds. u Fermentation. u Rs and Ps developed much later.

20. Self-replicating molecules (Heredity) u DNA  RNA  Protein u Too complex for early life. u Other forms of genetic information?

21. RNA Hypothesis u RNA as early genetic information: Why? u RNA polymerizes easily. u RNA can replicate itself. u RNA can catalyze reactions including protein synthesis.

23. Ribozymes u RNA catalysts found in modern cells. u e.g. ribosomes u Possible relic from early evolution?

24. Molecular Cooperation u Interaction between RNA and the proteins it made. u Proteins formed may serve as RNA replication enzymes.

25. Molecular Cooperation u Works best inside a membrane. u RNA benefits from the proteins it made.

27. Selection favored: u RNA/protein complexes inside membranes as they were the most likely to survive and reproduce.

28. DNA u Developed later as the genetic information. u Why? More stable than RNA

29. Alternate View 1. Life developed in Volcanic Vents.

30. Volcanic Vents u Could easily supply the energy and chemical precursors for chemical evolution. u Most primitive life forms today are the prokaryotes found in or near these vents.

31. Another Alternative 2. Panspermia: The idea that life on Earth came from space u Molecules for life came on comets to Earth u Some comets have been found to carry glycine (amino acid)

32. Modern Earth u Oxidizing atmosphere. u Life present. u Prevents new abiotic formation of life. u However – some suggest that “alien” life is still present.

33. Hypothesis u Life as a natural outcome of chemical evolution. u Life possible on many planets in the universe (?).

34. Key Events in History of Life u Origin of Life and single-celled organisms u Ps and Oxygen revolution u First Eukaryotes u Origin of Multicellularity

36. 1. Origin of Life u Dates to about 3.5 billion. u Stromatolites are the fossil evidence. u Prokaryotic cells – asexual reproduction only.

38. Stromatolites u Successive mats of photosynthetic cyanobacteria u Were responsible for most of the oxygen present u Oldest known fossils

39. 2. Ps and Oxygen Revolution u Early atmosphere was a reducing atmosphere. u Oxygen produced by Ps. u Oxygen caused Fe precipitation (rusting). u Oxygen levels up to 30+%.

41. Result u Favored aerobic Rs. u Loss of many anaerobic life forms.

42. 3. First Eukaryotes u Appeared about 2.1 billion years ago. u Endosymbiosis theory suggests how.

43. Endosymbiosis Theory u Folding of cell membrane to create nuclear and endomembrane system. u Engulfing of aerobic heterotrophic prokaryote. u Engulfing of photosynthetic prokaryote.

45. Evidence u Circular DNA without nucleosomes in mitochondria and chloroplast. u Small ribosomes in mitochondria & chloroplasts.

46. 4. Origin of Multicellularity u Started about 1.2 billion years ago. u Time of “snowball” Earth. u Diversification started about 565 million years ago when Earth thawed.