Life emerged at some point in Earth s history but how?

Transcription

1 Chapter 13: Colonizing the Surface The Early Earth and the Origin of Life 1 Life emerged at some point in Earth s history but how? 1

2 自然發生設 Wheat + dirty clothes + water Creates Mice in less than two weeks Experimentally verified! 4 2

3 The Modern Perspective on Life s Origin The origin of life is a cosmic process, as an aspect of planetary functioning when the appropriate conditions are present. Above all, the origin and function of life is a planetary process, not something isolated that occurs in a test tube. So we can begin with some of the basic chemical aspects of life. What is life made of? What is life made of? 6 3

4 Like planets, life reflects the basic chemical composition of the solar system Refractory elemen 7 Water is central Life on earth evolved in water, and all life still depends on water. At least 80% of the mass of living organisms is water and almost all chemical reactions of life take place in aqueous solution. 4

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6 Life is based on carbon why? Advantages of Carbon (1) Carbon can bond to itself as well as to other elements carbon-carbon bonds are central. (2) Organic molecules can bend and fold, creating large, complex three dimensional structures like proteins. (3) Carbon-bearing molecules can be in solid, liquid and gas phases at the same temperature and pressure, permitting transport and exchange among solid, liquid and gaseous planetary reservoirs. 12 6

7 Advantages of Carbon (4) Some carbon molecules are soluble in water and some insoluble, permitting within organisms the coexistence of and exchange between solid and liquid. (5) Carbon s multiple valence states (e.g. 4+ in CO 2 and 4- in CH 4 ) enable electron transfer reactions permitting energy flow and storage. 13 Simplest Organic Molecules Hydrocarbons made up of carbon backbone to which hydrogen atoms are attached. Simplest is CH 4, or methane 14 7

8 15 Organic molecules can also be highly complex (e.g. hemoglobin) C 3032 H 4816 O 872 N 780 S 8 Fe

9 Four Main Types of Organic Molecules Involved in Life Carbohydrates-- molecules in which hydrogen and oxygen atoms form whole numbers of water molecules (e.g. sugars and starches) Lipids-- much less oxygen than carbohydrates, and higher energy content per gram; fats in animals and oils in plants 17 Four Types of Organic Molecules Proteins-- long chains of amino acids, of which about 20 are the major building blocks of life Nucleic Acids---information carrying and replicating functions, DNA and RNA 18 9

10 What is the evidence that life has a common origin? Life has a remarkable unified set of characteristics. 19 Life is cellular 20 10

11 21 All of life uses common energy pathways 22 11

12 Of the ~ 500 known amino acids, only 20 are used to make proteins in living organisms, and all organisms use the same 20. Furthermore all amino acids come in both right and left-handed forms, and proteins use only left-handed varieties. This selection of handedness is called CHIRALITY 12

13 Amino acids can be either right or left-handed. Proteins are made up of only left handed varieties, a property called chirality 25 13

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15 To track the origin of life we need to work backwards to the most ancient and simple organisms 29 No multicellular life before about 650Ma But earlier uni-cellular life was abundant and long lasting 30 15

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17 What did early life look like? Bacterial slime The common ancestor of all of modern life was a primitive microbe

18 35 Life evolved from an ancient common ancestor. When did this ancestor come into being, and by what processes? 36 18

19 Earth s Oldest Rocks The Archean Eon is 4.0 to 2.5 Ga It is in the rocks that we need to look for first evidence of life Acasta Gneiss from the Slave Province of NW Canada: ~4.0 Ga Fig Locations of Archean rock terrains What is the evidence from the rocks? 38 19

20 Preservation of rocks on Earth s surface begins at about the time of the late heavy bombardment, after which surface conditions were likely much more stable. 39 It is a striking fact that many Archean rocks are sediments of deep water origin. Cherts are fine-grained quartz sediments that have been precipitated from water

21 Water was early, how about life? 850-Myr-old microfossil with much supporting evidence (Bitter Springs Chert, Australia) Chemical fossils contain molecules of biological origin Cyanobacteria: aquatic, photosynthetic bacteria 42 21

22 Microfossil from Gunflint Chert, Western Ontario (north shore of Lake Superior) ~1.9 Gyrs old. The structure has many fine features indicative of cells. When the visual evidence has this complexity, it is taken as definitive 43 Oldest morphological fossils (or just plain rocks???) Morphological evidence only. No telltale cellular features. No biomolecules 44 22

23 Other Morphological Evidence: Stromatolites Stromatolites are mounds of carbonate sediment trapped by microscopic algae. They were very common in some PreCambrian settings, but became less common after the Ordovician. Advent of burrowing and grazing invertebrates may have restricted stromatolites to marginal habitats,such as hypersaline environments. 45 Modern stromatolites Shark Bay in western Australia is a modern stromatolite locality 46 23

24 Ancient Stromatolite? Features such as these are interpreted by some as ancient stromatolites as old as 3.5Ga. But older micro-fossils are not so clear. Was this alive?? How do we pass beyond equivocal visual evidence? 48 24

25 Carbon isotopes as early evidence for life Photosynthetic organisms take CO 2 from the air and fix it into organic matter The two stable carbon isotopes, 12 C and 13 C isotopes are chemically almost identical, but living organisms distinguish between them on the basis of their mass difference All biochemistry is picky about which carbon isotope it uses: 12 C is preferred over 13 C (biochemical reactions using lighter isotopes go faster ) 49 Carbon isotopes and the earliest evidence for life Organic matter has a higher 13 C/ 12 C than atmospheric CO 2 This imprint can be left in rocks: The 3.8-Gyr-old sediments in Greenland have an lower ratio of 13 C/ 12 C --- the carbon is light Does this mean there was life 3.8 billion years ago? Maybe. These rocks have a long history, and there has been life on the surface for most of it

26 Biomarkers are resistant organic molecules that are also measured to look for evidence of life. Biomarkers were found in rocks as old as 3.5Ga. Does this mean there was life 3.8 billion years ago? No. These rocks have a long history, and there has been life on the surface for most of it. Subsequent work showed that the biomarkers resulted from later movement of organic molecule bearing fluids. But biomarkers are still being actively explored to see what evidence they might provide. 51 Earliest Life There is some evidence (and much discussion) that the very oldest rocks contain evidence of life No question that life was present by 3 billion years, as there is evidence for some oxygen production by that time. Maybe life is as old as 3.5 billion years In any case: THERE IS A LONG INTERVAL BETWEEN WATER (as old as 4.4Ga) AND EVIDENCE OF LIFE. A billion years to accomplish the origin of life is a very long time. 26

27 LIFE IS PRESENT 53 So how did the universal common ancestor come to be? 27

28 Pre-Biotic Conditions on Earth The dominant cations (Na +, Mg 2+, and Ca 2+ ) and the dominant anion (Cl - ) were probably the same as in the present ocean. The Earth s early atmosphere likely had a moderately reducing mix of gases: N 2, CO 2, and H 2 O, with only traces of H 2, CH 4 and NH 4. The early sun had a luminosity 30% lower than present. Higher levels of CO 2 in the primitive atmosphere kept the Earth warm (above 0 C at least), and maybe very warm (60 C?). Liquid water was present. 55 Given stable climate and water, for which we have direct evidence, what else is needed? 1. Organic molecules have to be made abiotically--the basic ingredients of all the groups 2. Those molecules, monomers, need to form into groups, polymers 3. The polymers have to have the correct handedness (chirality) 4. There needs to be a cellular container 5. There needs to be reproduction and the possibility of natural selection 28

29 All of these are likely simplistic. The origin of life was a planetary process, taking multiple steps probably in multiple interacting environments over hundreds of millions of years Organic Molecules: Dozens of organic molecular species have been discovered in galactic clouds 58 29

30 Conditions on the Early Earth High temperatures H 2 O, CO 2, N 2 H 2 S, CH 4, NH 3, H 2 No O 2 THESE ARE REDUCING CONDTIONS-- ESSENTIAL FOR LIFE S ORIGIN 59 Miller-Urey Experiment In 1950, a student of Harold Urey, Stanley Miller, designed an experiment in which he discharged an electric spark into a mixture thought to resemble the primordial composition of the atmosphere. From the water receptacle, designed to model an ancient ocean, Miller recovered some amino acids

31 Subsequent modifications of the atmosphere of the experiment have led to production of representatives or precursors of all four organic macromolecular classes. Including the molecules below: 61 Formation of organic molecules can thus occur by several means 1. In interstellar clouds seen by astronomers and also found in carbonaceous chondrites so we know these molecules can be delivered to Earth. 2. By processes in Earth s early atmosphere, provided the atmosphere was reducing 3. Under conditions similar to hydrothermal vents in the deep ocean 31

32 2. Abiotic synthesis of polymers For life to arise, polymers must be able to form without enzymes or cells Monomers splashed onto hot rock/clay/sand lose water and polymerize Amino acids can join to form Polypeptides this way, by a series of replenishment and evaporation steps. 63 Individual Amino Acids, the building blocks of proteins Figure

33 Individual amino acids can be joined by peptide bonds which are dehydration reactions that might occur in evaporative environments Figure Chirality The problem of chirality-- left and right handedness Amino acids made in experiments are both. Amino acids in proteins in living organisms are all left-handed. How could this happen? 66 33

34 Figure Some common mineral faces are also chiral 68 34

35 How to form a cellular container? Outside of layer Inside of Layer 69 The Cell Membrane 70 35

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37 4. Abiotic replication of molecules RNA seems likely to have been probably the 1st hereditary material: called THE RNA WORLD RNA molecules can be synthesized abiotically RNA can also have catalytic activity (ribozymes) In solution with ribonucleotides, a new strand is formed using basepairing rules; early-heredity? 73 A possible scenario that includes an RNA world 74 37

38 75 Uncertainties about the origin of terrestrial life abound What were the actual conditions of the early Earth? How did the first replicating systems evolve and how did they function? The leap from an aggregate of molecules that reproduces to even the simplest prokaryotic cell is immense. Even prokaryotic cells are very complex machines. The changes must have occurred in many smaller evolutionary steps. Where on Earth life originated is also under debate. It may have involved coupled steps at many different geological settings 76 38

39 Chicken/Egg problems Life as an energetic imperative? Given disequilibrium, or an external energy source, what is the most rapid path to equilibrium? e.g. enzymes facilitate this path 39

40 Could life also originate elsewhere? As our understanding of our own solar system has increased, the hypothesis that life is not restricted to Earth has received more attention. The presence of ice on Europa, a moon of Jupiter, has led to hypotheses that liquid water lies beneath the surface and may support life While Mars is cold, dry, and lifeless today, it was probably relatively warmer, wetter, and had a CO 2 -rich atmosphere billions of years ago Some think Mars subsurface may still be capable of having life Many scientists see Mars as an ideal place to test hypotheses about Earth s prebiotic chemistry Why not outside our solar system? Conditions found on Earth might be common in the galaxy. 79 Might hydrothermal vents have played a role? Hydrothermal Vents Provide a New Perspective on Planetary Li Courtesy of John Delaney, 80 U. Washington 40

41 Earthquake Swarm Jun-July 1993 Juan de Fuca Ridge 81 Plume of microbial particulates from a recently erupted sea-floor volcano 82 41

42 83 The Origin of Life is a Planetary Process Multiple environments over long periods of time and changing conditions are likely Many of these environments of the ancient Earth likely have no modern analogues Many of the processes may have occurred before the existence of the rock record, leaving large uncertainties and potential speculation It may be that the energetic imperatives and characteristics of molecules permit many avenues to life

43 Is Life likely to be a frequent planetary process?: C, H, O and N are four of the most abundant elements in the universe Organic molecules are everywhere-in the interstellar clouds from which solar systems are formed, in comets On successful replication in millions of years wins and makes life possible. Time is on the side of life. Life increases entropy production, therefore is energetically preferred Discovery of any one avenue to life will show that origin of life is a natural and likely common planetary process. Whether that particular avenue is the path taken by Earth will be more difficult to establish 85 43