Chapter 18 Life’s Origin and Early Evolution (Sections 18.1 - 18.7)

1. Chapter 18Life’s Origin and Early Evolution (Sections 18.1 - 18.7)

2. 18.1 Looking for Life • Astrobiologists study properties of the ancient Earth that allowed life to arise, survive, and diversify • Presence of cells in deserts and deep below Earth’s surface suggests life may exist in similar settings on other planets • astrobiology • The scientific study of life’s origin and distribution in the universe

3. Chile’s Atacama Desert • Astrobiologists study Earth’s extreme habitats to determine the range of conditions that living things can tolerate

4. 18.2 Earth’s Origin and Early Conditions • Physical and geological forces produced Earth, its seas, and its atmosphere • Earth and other planets formed more than 4 billion years ago • Early in Earth’s history, there was little oxygen in the air, volcanic eruptions were common, and there was a constant hail of meteorites

5. From the Big Bang to the Early Earth • According to the big bang theory, the universe formed in an instant 13 to 15 billion years ago • Over millions of years, gravity drew the gases together and they condensed to form giant stars • big bang theory • Model describing formation of the universe as a nearly instant distribution of matter through space

6. An Early Sun • What the cloud of dust, gases, rocks, and ice around the early sun may have looked like

7. Conditions on the Early Earth • An oxygen-free atmosphere allowed assembly of organic compounds necessary for life (oxygen would destroy the compounds as fast as they formed) • As Earth’s surface cooled, rocks formed — rains washed mineral salts into early seas where life began

8. Early Earth • When volcanic activity and meteor strikes were common

9. Key Concepts • Setting the Stage for Life • Earth formed about 4 billion years ago from matter distributed in space by the big bang (the origin of the universe) • The early Earth was an inhospitable place, where meteorite impacts and volcanic eruptions were common and the atmosphere held little or no oxygen

10. 18.3 The Source of Life’s Building Blocks • All living things are made from organic subunits: simple sugars, amino acids, fatty acids, and nucleotides • Where did the subunits of the first life come from? There are several possibilities: • Lightning-fueled atmospheric reactions • Reactions at deep-sea hydrothermal vents • Meteorites from space

11. Lightning-Fueled Atmospheric Reactions • In 1953, Stanley Miller and Harold Urey showed that reactions in Earth’s early atmosphere could have produced building blocks for the first life • Provideed indirect evidence that organic compounds self-assemble spontaneously under conditions like those in Earth’s early atmosphere

12. Miller-Urey Experiment • Mix of water, hydrogen (H2), methane (CH4), and ammonia (NH3) • Sparks simulated lightning • Amino acids formed

13. Miller-Urey Experiment electrodes to vacuum pump spark discharge CH4 NH3 H2O H2 gases water out condenser water in water droplets water containing organic compounds boiling water liquid water in trap Fig. 18.4, p. 285

14. ANIMATION: Miller's reaction chamber experiment To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

15. Reactions at Hydrothermal Vents • Reactions in the hot, mineral-rich water near deep-sea hydrothermal vents also produce organic building blocks • Experiments combining hot water with carbon monoxide (CO) potassium cyanide (KCN) and metal ions formed amino acids • hydrothermal vent • Rocky, underwater opening where mineral-rich water heated by geothermal energy streams out

16. A Hydrothermal Vent • Mineral-rich water heated by geothermal energy streams out of the vent • Precipitation causes minerals to form a chimney-like structure around the vent

17. Delivery From Space • The presence of amino acids, sugars, and nucleotide bases in meteorites that fell to Earth suggests that such molecules may have formed in interstellar clouds of ice, dust, and gases and been delivered to Earth by meteorites

18. Key Concepts • Building Blocks of Life • All life is composed of the same organic subunits • Simulations of conditions on the early Earth show that these molecules could have formed by reactions in the atmosphere or sea • Organic subunits also form in space and could have been delivered to Earth by meteorites

19. ANIMATION: Building blocks of life To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

20. 18.4 From Polymers to Cells • Similarities in structure, metabolism, and replication among all life indicate descent from a common cellular ancestor • Experiments demonstrate how traits and processes seen in all living cells could have begun with physical and chemical reactions among nonliving collections of molecules

21. Steps on the Road to Life

22. Inorganic molecules self-assemble on Earth and in space Organic monomers self-assemble in aquatic environments on Earth Organic polymers interact in early metabolism self-assemble as vesicles become the first genome Protocells in an RNA world Are subject to selection that favors a DNA genome DNA-based cells Steps on the Road to Life Stepped Art Fig. 18.6, p. 286

23. Origin of Metabolism • Proteins that speed metabolic reactions might have first formed when amino acids stuck to clay, then bonded under the heat of the sun • Or, metabolism may have begun in rocks near deep-sea hydrothermal vents when iron sulfide in the rocks donated electrons to dissolved carbon monoxide

24. Iron Sulfide-Rich Rocks • Cell-sized chambers formed by simulations of conditions near hydrothermal vents • Could have served as environments for first metabolic reactions

25. Origin of the Cell Membrane • Membrane-like structures and vesicles form when proteins or lipids are mixed with water • They serve as a model for protocells, which may have preceded cells • protocell • Membranous sacs that contain interacting organic molecules; hypothesized to have formed prior to the earliest life forms

26. Laboratory-Produced Protocells • One type consists of a bilayer membrane of fatty acids that holds strands of RNA • Ribonucleotides diffuse into the protocell and become incorporated into complementary strands of RNA • Vesicle enlarges by incorporating additional fatty acids • Another type consists of RNA-coated claysurrounded by fatty acids and alcohols

27. Laboratory-Produced Protocells • Fatty acids and RNA (left); RNA and clay (right)

28. Field-Testing a Hypothesis • No vesicle-like structures formed when David Deamer poured a mix of small organic molecules and phosphates into a hot acidic pool in Russia

29. Origin of the Genome • Protein synthesis depends on DNA, which is built by proteins; how did this cycle begin? • An RNA world, a time in which RNA was the genetic material, may have preceded DNA-based systems • RNA world • Hypothetical early interval when RNA served as the genetic information

30. RNA World • RNA is part of ribosomes that carry out protein synthesis • Discovery of ribozymes (RNAs that function as enzymes) supports the RNA world hypothesis • A later switch from RNA to DNA would have made the genome more stable

31. Key Concepts • The First Cells Form • All cells have enzymes that carry out reactions, a plasma membrane, and a genome of DNA • Experiments provide insight into how cells arose through physical and chemical processes, such as the tendency of lipids to form membrane-like structures when mixed with water

32. 18.5 Life’s Early Evolution • Fossils and molecular comparisons among living species inform us about the history of life on Earth • The first cells evolved when oxygen levels in the atmosphere and seas were low, so they probably were anaerobic

33. Origin of Bacteria and Archaea • Early divergence separated bacteria from ancestors of archaeans and eukaryotes • An oxygen-releasing, noncyclic pathway of photosynthesis evolved in one bacterial lineage (cyanobacteria) that, over generations, formed stromatolites • Over time, oxygen released by cyanobacteria changed Earth’s atmosphere

34. Stromatolites • stromatolite • Dome-shaped structures composed of layers of bacterial cells and sediments • Each layer formed when a mat of living cells trapped sediments • Descendant cells grew over the sediment layer, then trapped more sediment, forming the next layer

35. Stromatolites • Artist’s depiction: stromatolites in an ancient sea • Cross-section of fossilized stromatolite

36. Fossils of Early Life • Possible bacterial cells 3.5 billion years old, and fossils of two types of cyanobacteria approximately 850 million years old

37. Effects of Increasing Oxygen 1. Oxygen interferes with self-assembly of complex organic compounds – prevented evolution of new life from nonliving molecules 2. Presence of oxygen gave organisms that thrived in aerobic conditions an advantage 3. Formation of an ozone layer in the upper atmosphere protected Earth’s surface from high levels of solar ultraviolet (UV) radiation

38. The Rise of Eukaryotes • Lipids (biomarkers for eukaryotes) in 2.7-billion-year-old rocks suggest when eukaryotic cells may have branched off from the archaean lineage • biomarker • Molecule produced only by a specific type of cell; a molecular signature

39. The Rise of Eukaryotes (cont.) • Fossils with sexual spores may also be evidence of early eukaryotes (only eukaryotes reproduce sexually) • Protists were the first eukaryotic cells, and their fossils date back a little more than 2 billion years • Diversification of protists gave rise to ancestors of plants, fungi, animals

40. Fossil History of Eukaryotes • Possible oldest eukaryote (2.1 billion years old); an early alga; and fossils of red alga (1.2 billion years old)

41. Fossil History of Eukaryotes Fig. 18.10a, p. 289

42. Fossil History of Eukaryotes Fig. 18.10b, p. 289

43. Fossil History of Eukaryotes Fig. 18.10c, p. 289

44. Key Concepts • Life’s Early Evolution • The first cells were probably anaerobic • An early divergence separated bacteria from archaeans and ancestors of eukaryotic cells • Evolution of oxygen-producing photosynthesis in bacteria altered Earth’s atmosphere, creating conditions that favored aerobic organisms

45. 18.6 Evolution of Organelles • Scientists study modern cells to test hypotheses about how organelles evolved in the past • By one hypothesis, internal membranes typical of eukaryotic cells may have evolved through infoldings of plasma membrane of prokaryotic ancestors • Existence of some bacteria with internal membranes supports this hypothesis

46. Origin of the Nucleus • In eukaryotes, DNA resides in a nucleus that protects the genome from physical or biological threats • The nuclear envelope consists of a double layer of membrane with protein-lined pores that control flow of material into and out of the nucleus • The nucleus and endomembrane system probably evolved when the plasma membrane of an ancestral cell folded inward

47. Model: Origin of Nuclear Envelope and Endoplasmic Reticulum

48. Model: Origin of Nuclear Envelope and Endoplasmic Reticulum infolding of plasma membrane in prokaryotic ancestor ER nuclear envelope of early eukaryote Fig. 18.11, p. 290

49. Bacteria with Internal Membranes

50. Bacteria with Internal Membranes A Marine bacterium (Nitrosococcus oceani) with highly folded internal membranes visible across its midline. Fig. 18.12a, p. 290