1. 1.D.1 Hypotheses of Life’s Origin There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence natural origin of life on Earth, each with supporting scientific evidence.

2. Scientific evidence supports the various models of the origin of life on earth.

3. Louis Pasteur Experimentally found that the growth of bacteria in nutrient broths was due to biogenesis, not spontaneous generation.

4. So, if life only comes from life, how did life on Earth begin?

5. In the “Organic Soup Model,” life first formed in the early oceans, which were a mixture of water and dissolved organic compounds: the building blocks of life.

6. Miller-Urey Experiment In 1952, Stanley Miller and Harold Urey showed that organic compounds could be synthesized from inorganic precursors.

7. Alternate hypothesis: Did meteors seed Earth with organic molecules from space? (Panspermia Model)

8. The Murchison meteorite found in Australia is rich in organic compounds, including 70 amino acids and nucleotide bases.

9. So…primitive Earth provided inorganic precursors from which organic molecules could have been synthesized due to the presence of available free energy and the absence of a significant quantity of oxygen.

10. In turn, these molecules served as monomers or building blocks for the formation of more complex molecules, including amino acids and nucleotides..

11. The joining of these monomers produced polymers with the ability to replicate, store and transfer information.

12. These complex reaction sets could have occurred in solution (organic soup model) or as reactions on solid reactive surfaces, such as clay.

13. These polymers were packaged into spontaneously forming protobionts: droplets with fatty membranes.

14. Liposomes: membrane-bound droplets that form when lipids are added to water. spontaneously organize into a lipid bilayer selectively permeable

15. The RNA World Hypothesis proposes that RNA could have been the earliest genetic material. RNA is important in protein synthesis and can also have enzyme-like functions.

16. RNA catalysts are called ribozymes. Ribozymes probably were protected inside protobionts. Natural selection would act on such RNA- carrying protobionts; the most successful would increase in number.

17. RNA would later provide the template for the first DNA to be assembled.

18. Conditions on early Earth made the origin of life possible.

19. Prokaryotic Evolution Stomatolites – oldest known fossils (3.5 bya). Formed from many layers of bacteria and sediment.

20. The First Prokaryotes Autotrophic, some photosynthetic. Heterotrophs emerged later. The only life form on earth for about 1.5 billion years. Autotrophic, some photosynthetic. Heterotrophs emerged later. The only life form on earth for about 1.5 billion years.

21. Electron Transport Systems Electron transport systems and chemiosmotic mechanisms (ATP production) common to all life; evolved before the three domains diverged.

22. Photosynthesis Early photosynthetic bacteria did not split water and liberate oxygen. The only photosynthetic bacteria are cyanobacteria. Early photosynthetic bacteria did not split water and liberate oxygen. The only photosynthetic bacteria are cyanobacteria.

23. When oxygen was first produced, it saturated the oceans, then rusted iron (the first “red rock” appeared). Finally, oxygen gassed out of the oceans and entered the atmosphere for the first time. The new oxygen-rich atmosphere doomed many of the ancient prokaryotic groups to extinction. When oxygen was first produced, it saturated the oceans, then rusted iron (the first “red rock” appeared). Finally, oxygen gassed out of the oceans and entered the atmosphere for the first time. The new oxygen-rich atmosphere doomed many of the ancient prokaryotic groups to extinction.

24. Eukaryotic cells arose around 2.1- 2.7 bya.

25. Endosymbiotic Theory The ancestor of mitochondria was an aerobic heterotroph. The ancestor of plastids (such as a chloroplast) was a photosynthetic prokaryote. An ancestral prokaryote came to live in a host cell.

26. Serial endosymbiosis – sequence of endosymbiotic events that led to the first ancestral eukaryote. Mitochondria came about first; plastids later.

27. Evidence in support of endosymbiosis: Mitochondria and plastids: have membranes that contain transport systems homologous to membrane of prokaryotes. replicate independently of cell by process resembling binary fission. contain circular DNA not associated with histones. have own tRNA, ribosomes and other molecules needed to make their own proteins. have prokaryotic ribosomes (70 S). mitochondria have rRNA sequence akin to modern proteobactera; plastids have rRNA sequence akin to modern cyanobacteria. Mitochondria and plastids: have membranes that contain transport systems homologous to membrane of prokaryotes. replicate independently of cell by process resembling binary fission. contain circular DNA not associated with histones. have own tRNA, ribosomes and other molecules needed to make their own proteins. have prokaryotic ribosomes (70 S). mitochondria have rRNA sequence akin to modern proteobactera; plastids have rRNA sequence akin to modern cyanobacteria.

28. Eukaryotic cells as genetic chimeras Eukaryotic cells are chimeras of prokaryotic parts: mitochondria from one type of bacteria, plastids from another, DNA a combination of many types of bacteria, possibly even Archaea.

29. The genome of eukaryotes is a product of genetic annealing – horizontal gene transfer between different bacteria and archaea.

30. The Golgi and the endomembrane system resulted from infolding of the prokaryotic plasma membrane.

31. Multicellularity in Eukaryotes Earliest multicellular eukaryotes evolved 1.5 bya. First came colonies – collections of autonomously replicating cells. Earliest multicellular eukaryotes evolved 1.5 bya. First came colonies – collections of autonomously replicating cells.

32. The first animals were plentiful in the Cambrian period, a time known as the “Cambrian explosion”.

33. Colonizing Land Plants evolved from green algae. Colonization of land could not have have happened before ozone, Earth’s UV shield. Plants evolved from green algae. Colonization of land could not have have happened before ozone, Earth’s UV shield.

34. Adaptations of Plants for Land Waterproof wax coating that prevents desiccation. Plant roots associated with mycorrhizal fungi that aid in absorption (symbiotic relationship). Spores with tough walls to prevent drying out and protect from UV. Stomata, or holes, in the bottom of leaves to allow gas exchange. Vascular systems to transport water from the ground up. Tough, lignified cell walls for structural support on land. Waterproof wax coating that prevents desiccation. Plant roots associated with mycorrhizal fungi that aid in absorption (symbiotic relationship). Spores with tough walls to prevent drying out and protect from UV. Stomata, or holes, in the bottom of leaves to allow gas exchange. Vascular systems to transport water from the ground up. Tough, lignified cell walls for structural support on land.

35. Tetrapods – terrestrial vertebrates.

36. Continental Drift Tectonic plates drift over earth’s surface.

37. Pangaea – all landmasses collected together as one continent. As continents drifted apart, separate evolutionary events happened on each one.

38. Example: Eutherian mammals did not evolve in Australia; only marsupials.

40. Learning Objectives LO 1.27 The student is able to describe a scientific hypothesis about the origin of life on Earth. [See SP 1.2] LO 1.28 The student is able to evaluate scientific questions based on hypotheses about the origin of life on Earth. [See SP 3.3] LO 1.29 The student is able to describe the reasons for revisions of scientific hypotheses of the origin of life on Earth. [See SP 6.3] LO 1.30 The student is able to evaluate scientific hypotheses about the origin of life on Earth. [See SP 6.5] LO 1.31 The student is able to evaluate the accuracy and legitimacy of data to answer scientific questions about the origin of life on Earth. [See SP 4.4]