The History of Life

1. 17 The History of Life 0

2. Chapter 17 At a Glance • 17.1 How Did Life Begin? • 17.2 What Were the Earliest Organisms Like? • 17.3 What Were the Earliest Multicellular Organisms Like? • 17.4 How Did Life Invade the Land? • 17.5 What Role Has Extinction Played in the History of Life? • 17.6 How Did Humans Evolve?

3. 17.1 How Did Life Begin? • Pre-Darwinian thought held that all species were simultaneously created by God a few thousand years ago • Until the 19th century, most people thought that new members of species sprang up all the time through spontaneous generation both from nonliving matter and other, unrelated forms of life

4. 17.1 How Did Life Begin? • Medieval beliefs reflected the concept of spontaneous generation • Maggots were thought to arise from meat • Microbes were thought to arise from broth • Mice were thought to arise from mixtures of sweaty shirts and wheat • The maggots-from-meat idea was disproved by Francesco Redi in 1668 • No maggots developed when he kept flies away from uncontaminated meat

5. 17.1 How Did Life Begin? • The broth-to-microorganism idea was disproved by Louis Pasteur and John Tyndall in the mid-1800s • Microorganisms did not appear in sterile broth unless the broth was first exposed to existing microorganisms in the surrounding environment • Pasteur and Tyndall’s work demolished the notion of spontaneous generation

6. 17.1 How Did Life Begin? • The broth-to-microorganism idea was disproved by Louis Pasteur and John Tyndall in the mid-1800s (continued) • Pasteur and Tyndall’s work did not address the question of how life on Earth originated in the first place • Stanley Miller stated that “Pasteur never proved it didn’t happen once, he only showed that it doesn’t happen all the time”

7. Figure 17-1 Spontaneous generation refuted no growth growth The long, S-shaped neck allows air, but not microorganisms to enter the flask The broth in a flask is boiled to kill preexisting microorganisms If the neck is later broken off, outside air can carry microorganisms into the broth

8. 17.1 How Did Life Begin? • The first living things arose from nonliving ones • Modern scientific ideas about the origin of life began to emerge in the 1920s • Alexander Oparin in Russia and John B. S. Haldane in England showed that spontaneous formation of the complex organic molecules necessary for life would not be permitted in today’s oxygen-rich atmosphere

9. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Modern scientific ideas about the origin of life began to emerge in the 1920s (continued) • Oxygen reacts readily with other molecules, disrupting chemical bonds • An oxygen-rich environment tends to keep molecules simple

10. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Oparine and Haldane speculated that the atmosphere of early Earth contained little oxygen because an oxygen-rich atmosphere would not have permitted the spontaneous formation of complex organic molecules • Oxygen’s high reactivity with chemical bonds would have prevented large molecules from forming

11. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Some kinds of molecules could persist in the lifeless environment of early Earth better than others and would therefore become more common over time • The chemical version of the “survival of the fittest” is called prebiotic (meaning “before life”) evolution

12. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions • In 1953, Stanley Miller and Harold Urey set out to simulate the first stage of prebiotic evolution in the laboratory • They noted that the atmosphere of early Earth probably contained methane (CH4), ammonia (NH3), hydrogen (H2), and water vapor (H2O), but no oxygen

13. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions (continued) • In 1953, Stanley Miller and Harold Urey set out to simulate the first stage of prebiotic evolution in the laboratory (continued) • They simulated early Earth’s atmosphere by mixing the gases in a flask and adding an electrical discharge to simulate lightning • Simple organic molecules appeared after a few days

14. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions (continued) • In 1953, Stanley Miller and Harold Urey set out to simulate the first stage of prebiotic evolution in the laboratory (continued) • The experiment showed that small molecules likely present in the early atmosphere can combine to form larger organic molecules if electrical energy is present

15. Figure 17-2 The experimental apparatus of Stanley Miller and Harold Urey An electric spark simulates a lightning storm electric spark chamber Energy from the spark powers reactions among molecules thought to be present in Earth’s early atmosphere CH4 NH3 H2 H2O Boiling water adds water vapor to the artificial atmosphere cool water flow When the hot gases in the spark chamber are cooled, water vapor condenses and any soluble molecules present are dissolved condenser boiling chamber water Organic molecules appear after a few days

16. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions (continued) • Similar experiments by Miller and others have produced amino acids, short proteins, nucleotides, ATP, and other molecules characteristic of living things

17. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions (continued) • Modern geochemists believe the early atmosphere was somewhat different from that modeled in Miller and Urey’s experiments • Additional experiments with more realistic (but still oxygen-free) simulated atmospheres have also yielded organic molecules

18. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions (continued) • The experiments proved that electricity is not the only suitable energy source • Other sources include heat and ultraviolet (UV) light, which have been shown to drive the formation of organic molecules in experimental simulations of prebiotic conditions

19. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions (continued) • Additional organic molecules probably arrived from space when meteorites and comets crashed into the Earth’s surface • Analysis of present-day meteorites recovered from impact craters on Earth has revealed that some meteorites contain relatively high concentrations of amino acids and other simple organic molecules

20. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can form spontaneously under prebiotic conditions (continued) • Additional organic molecules probably arrived from space when meteorites and comets crashed into the Earth’s surface (continued) • When small molecules known to be present in space were placed under space-like conditions of very low temperature and pressure and bombarded with UV light, larger organic molecules were produced

21. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can accumulate under prebiotic conditions (continued) • Prebiotic synthesis was neither very efficient nor very fast • Prebiotic molecules must have been threatened by the sun’s high-energy UV radiation, because early Earth lacked an ozone layer

22. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can accumulate under prebiotic conditions (continued) • The ozone layer is a region high in today’s atmosphere that is enriched with ozone molecules • The ozone molecules form when incoming solar energy splits some O2 molecules in the outer atmosphere into individual oxygen (O) atoms

23. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can accumulate under prebiotic conditions (continued) • The ozone layer is a region high in today’s atmosphere that is enriched with ozone molecules (continued) • The oxygen (O) atoms react with O2 to form O3 (ozone)

24. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can accumulate under prebiotic conditions (continued) • Before the formation of the ozone layer, UV bombardment must have been fierce • UV radiation can provide energy for the formation of organic molecules, but can also break them apart

25. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Organic molecules can accumulate under prebiotic conditions (continued) • Sites beneath rock ledges or at bottoms of even fairly shallow seas, would have been protected from UV radiation • In these locations, organic molecules may have accumulated

26. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Clay may have catalyzed the formation of larger organic molecules • In the next stage of prebiotic evolution, simple molecules combined to larger molecules • One possibility is that small molecules accumulated on surfaces of clay particles may have a small electrical charge that attracts dissolved molecules of the opposite charge

27. 17.1 How Did Life Begin? • The first living things arose from nonliving ones (continued) • Clay may have catalyzed the formation of larger organic molecules (continued) • Initially, these molecules might had formed on clay at the bottom of early Earth’s oceans or lakes, but have now become the building blocks of the first living organisms

28. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule • DNA was probably not the earliest informational molecule • DNA replication requires large complex protein enzymes • The instructions for building these enzymes are coded in DNA

29. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • DNA was probably not the earliest informational molecule (continued) • This chicken-and-egg interdependency makes DNA an unlikely candidate for self-replication • The current DNA-based system of information storage likely evolved from an earlier system

30. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • RNA can act as a catalyst • RNA is a prime candidate for the first self-replicating informational molecule

31. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • Thomas Cech and Sidney Altman (1980s), working with the single-celled organism called Tetrahymena, discovered a cellular reaction that was catalyzed by a protein, by a small RNA molecule • Cech and Altman named their catalytic RNA molecule ribozyme

32. Figure 17-3 A ribozyme

33. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • RNA can act as a catalyst • Since Cech and Altman’s initial discovery, dozens of naturally occurring ribozymes have been found that catalyze reactions, including • Cutting other RNA molecules • Splicing together different RNA fragments • Attaching amino acids to growing proteins

34. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • RNA can act as a catalyst (continued) • The most effective replication of ribozyme so far synthesized can copy RNA sequences up to 95 nucleotides long

35. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • Earth may once have been an RNA world • Discovery of ribozymes led to the hypothesis that RNA preceded the origin of DNA, in an “RNA world” • According to this view, the current era of DNA-based life was preceded by one in which RNA served as the information-carrying genetic molecule and the catalyst for its own replication

36. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • Earth may once have been an RNA world (continued) • This first self-reproducing ribozyme probably wasn’t very good at its job and produced copies with lots of errors • Natural selection acted on these errors to improve the function of these early ribozymes

37. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • Earth may once have been an RNA world (continued) • This first self-reproducing ribozyme probably wasn’t very good at its job and produced copies with lots of errors (continued) • With increased speed and accuracy of replication, these variant ribozymes reproduced, copying themselves and displacing less efficient molecules

38. 17.1 How Did Life Begin? • RNA may have been the first self-reproducing molecule (continued) • Earth may once have been an RNA world (continued) • Molecular evolution continued • By some unknown chain of events, RNA gradually receded into its present role as intermediary between DNA and protein enzymes

39. 17.1 How Did Life Begin? • Membrane-like vesicles may have enclosed ribozymes • Self-replicating molecules on their own do not constitute life • In all living cells such molecules are contained within some kind of enclosing membrane • Chemists have shown that if water containing proteins and lipids is agitated to simulate waves beating against ancient shores, the proteins and lipids combine to form hollow vesicles

40. 17.1 How Did Life Begin? • Membrane-like vesicles may have enclosed ribozymes (continued) • Vesicles resemble living cells • Vesicles have a well-defined outer boundary that separates internal and external environments • Depending on composition, their membranes may be remarkably similar to that of a real cell • Under certain conditions, they may absorb material from the external solution, grow, and divide

41. 17.1 How Did Life Begin? • Membrane-like vesicles may have enclosed ribozymes (continued) • Certain vesicles (protocells) may have contained organic molecules, including ribozymes, and would have been the precursors of living cells • The membranes would have served to sequester the molecules of life and to protect them from extraneous ribozymes

42. 17.1 How Did Life Begin? • Membrane-like vesicles may have enclosed ribozymes (continued) • Certain vesicles (protocells) may have contained organic molecules, including ribozymes, and would have been the precursors of living cells (continued) • After sufficient time, these protocells may have developed the ability to divide and pass on copies of their enclosed ribozymes to daughter protocells

43. 17.1 How Did Life Begin? • Membrane-like vesicles may have enclosed ribozymes (continued) • Certain vesicles (protocells) may have contained organic molecules, including ribozymes, and would have been the precursors of living cells (continued) • The transition from protocell to living cell was a continuous process, with no sharp boundary between one state and the next

44. 17.1 How Did Life Begin? • But did all this really happen? • Despite a great diversity of assumptions, experiments, and contradictory hypotheses in origin-of-life research, certain facts support the central tenets • The experiments of Miller and others show that amino acids, nucleotides, and organic molecules, along with simple membrane-like structures, would have formed on early Earth

45. 17.1 How Did Life Begin? • But did all this really happen? (continued) • Despite a great diversity of assumptions, experiments, and contradictory hypotheses in origin-of-life research, certain facts support the central tenets (continued) • Given enough time and a sufficiently large pool of reactant molecules, even extremely rare events can occur many times • There was ample time and space for these rare events to lead to the development of life

46. 17.2 What Were the Earliest Organisms Like? • Earth formed about 4.5 billion years ago and was hot • Meteorites smashed into the forming planet, and the kinetic energy of these extraterrestrial rocks was converted into heat on impact • Geological evidence suggests that Earth cooled enough for water to exist in liquid form 4.3 billion years ago • The oldest fossil organisms found so far are in rocks that are approximately 3.4 billion years old

47. 17.2 What Were the Earliest Organisms Like? • Life arose 3.9 billion years ago in what is called the Precambrian era • Geologists and paleontologists have devised a hierarchical naming system of eras, periods, and epochs to delineate the immense span of geological time

48. Figure 17-4 Early Earth

49. Table 17-1, 1 of 4

50. Table 17-1, 2 of 4