PCS 182 Life In The Milky Way Galaxy PDF

2024-10-03 PCS 182 Life in the Milky Way Galaxy Week 5 – L13-L14. The Nature of Life on Earth Ch. 5, 6.1(, 6.2-6.6) Characteristics of life on Earth, to inform us about… where to look for life el...

2024-10-03 PCS 182 Life in the Milky Way Galaxy Week 5 – L13-L14. The Nature of Life on Earth Ch. 5, 6.1(, 6.2-6.6) Characteristics of life on Earth, to inform us about… where to look for life elsewhere, and what to look for 1 1 Attributes of Life ◆A definition of “Life” is difficult. ◆Instead, let’s try finding a definitive set of attributes of Life (qualities, general properties). ◆"Necessary" versus "Sufficient"… “Order is necessary for life” - Every life-form is ordered But order doesn't always mean something is living (e.g. crystals) 2 1  2024-10-03 Necessary vs. Sufficient ◆ Breathing is a... (A) necessary (B) sufficient (C) necessary and sufficient... condition for human survival during sleep. 3 Necessary vs. Sufficient ◆ Living in Toronto is a... (A) necessary (B) sufficient (C) necessary and sufficient... condition for living in Canada. 4 2  2024-10-03 Necessary vs. Sufficient ◆ Meeting all requirements for a college degree is a... (A) necessary (B) sufficient (C) necessary and sufficient... condition for receiving a college degree. 5 Attributes of Life ◆Order Structure to divide inside/outside Coherent activity of organism ◆Reproduction Guards against death of individuals Allows Hereditary mechanism to operate But note the example: horse + donkey = mule (i.e.- result may not be able to reproduce itself) 6 3  2024-10-03 Attributes of Life ◆Growth increase in size, and usually also complexity and ability ◆Energy utilization e.g.- consumes food! can come from unlikely places... (e.g.- tube worms living off microbes that utilize H2S from deep-ocean volcanic vents) 7 Attributes of Life ◆Response to environment Avoid getting killed Go where Life is better ◆Evolutionary adaption Species adapt as well as possible to their current environment Species adapt to changing environments 8 4  2024-10-03 Evolutionary adaption ◆This is generally considered the most fundamental property of life Explains diversity of life on Earth Explains the relations between different organisms (closely related species; fossil record; genetic (dis-)similarities) Early whale (Dorudon) Remnant tiny limbs 9 Theory of evolution: History ◆Anaximander (610-547 BC): life arose in water and evolved from simple to complex. ◆Aristotle (384-322 BC): species are fixed and don’t change. (This view got entrenched into Christianity) ◆Jean Baptiste Lamarck (1744 – 1829) noticed similarities of fossils to some living organisms and suggested link. Suggested heredity (transfer of characteristics from parents to offspring) ◆Gregor Mendel (1822 – 1884) experimentally studied heredity; foundations of genetics. His ideas were slow to be adopted. ◆Charles Darwin (1809-1882) ((…and also Alfred Russel Wallace!)) created a consistent theory for evolutionary changes. The concept of natural selection. (The book “The Origin of Species” in 1859, was based mainly on his observations of biological forms during his voyages.) 10 5  2024-10-03 Evolution: Fact and Theory ◆Geological record (fossils), relations among living species Evolution does happen (Evolution is a fact) Astonishing wealth of observations since Darwin fossil record, genetics, biochemistry (e.g.- cell membranes) cultivation, drug resistance,... ◆Charles Darwin: Scientific Theory of how evolution occurs. Makes predictions that have been verified. Many observations unknown to Darwin make perfect sense within the Theory of Evolution (but not otherwise). So many confirmations that “Law” is the most appropriate term Recall: (Hypothesis - Model - Theory - Law)... But the name “Theory of Evolution” has stuck. 11 Theory of Evolution ◆Fact 1: Over-reproduction (too many individuals) More offspring than the environment can support => Competition for survival. ◆Fact 2: Individual variations Individuals vary in inheritable traits passed from parents to offspring. ◆As a result: Unequal reproductive success. Those individuals whose traits enable them to survive better will leave a larger number of offspring. The heritable traits for success become more common and control further evolution. 12 6  2024-10-03 Evidence for adaption (natural selection) Darwin's famous example: 13 species of Galapagos finches 13 Artificial selection Bred by humans within a few thousand years 14 7  2024-10-03 Groupings of life Animals and Plants encompass only a tiny fraction of variations among all life 15 Groupings of life ◆Before 1960, just two kingdoms, based on appearance: Animals, Plants ◆In 1960’s, three additional kingdoms Fungi, & two kingdoms for microorganisms ◆Since then: biochemistry and genetics replace appearance Degree of similarity of DNA Degree of similarity of key biochemical processes (e.g. structure of cell-membranes) ◆Gives rise to “Tree of Life” 16 8  2024-10-03 Tree of Life ◆Lines represent evolutionary lineages ◆Three Domains Bacteria, Archaea, Eukarya ◆Branches can be subdivided down to individual species 17 Tree of Life ◆All Animals and Plants just a tiny corner of Tree of Life ◆Tremendously larger chemical and genetic variety in single-celled organisms. Single-cell ◆Eukarya more closely Single-cell or multi-cell related to Archaea than Bacteria, so now argued evolutionary branching should be shifted to this mapping (...but still very uncertain!) → 18 9  2024-10-03 Eukarya have cell nuclei ◆Prokaryotes No cell nucleus. Only single- cell organisms. The cells are small. The DNA is distributed in the central parts of the cell. ◆Eukaryotes (eukaryotic cell) Have a well defined nucleus containing the DNA material. Appears necessary for complex, multi-cell organisms, including all plants and animals. 19 Cells Chemistry, from simple to complicated 20 10  2024-10-03 The Cell The basic structure of life on Earth. Act as small chemical "factories" processing materials and energy. Operate on biochemical processes based on water and carbon compounds. bacteria amoeba plant cells animal cells 21 Chemical elements in living cells The most common element (by mass) in living organisms is oxygen (O). Common because its a part of water The most important element for life is carbon (C). 22 11  2024-10-03 CNO elements CNO peak Carbon, nitrogen and oxygen are produced in stars. They are expelled into the interstellar medium to enrich (form) new generations of stars (planets). CNO elements are relatively common. 23 Chemical bonds ◆Chemical bonds How electrons bind atoms in a molecule. ◆Ionic bonds Atoms bound by opposite (transferred) charges: NaCl (Na+ & Cl-) ◆Covalent bonds When atoms share electrons. Carbon can have up to 4 such bonds, with a possibility of double bonds. ◆Hydrogen bonds When molecules are electrically asymmetric, like molecules of water. 24 12  2024-10-03 Chemical bonds Full or empty shells energetically preferable. Leads to rules of how well atoms form bonds, and how many bonds they can make. 25 Ionic & covalent bonds Ionic bonds: electron moves to other atom. Resulting ions bound by electrostatic force Covalent bonds: shared electron-pair 26 13  2024-10-03 Molecules (representations) 27 Carbon Carbon can have up to 4 covalent bonds, with a possibility of double bonds. Linear molecules, as well as ring molecules, etc. Carbon based molecules = organic molecules. 28 14  2024-10-03 Carbon, the amazing Carbon molecules can form very soft materials (graphite) and unusually hard (diamond) and strong (graphene) materials. Among many allotropes, interesting fullerens: little spheres of 60, 70 and 540 atoms; and nanotubes. 29 Silicon based life? Silicon (28Si) has very similar chemical properties compared to carbon (12C). It can also form up to 4 bonds. However, several arguments indicate Si-based life to be unlikely: The Si chemical bonds are weaker than Carbon chemical bonds so Si-based molecules cannot exist long (in water). Si does not normally form double bonds. SiO2 (silicon dioxide) is solid (quartz, sand), not a gas like CO2, and therefore not easily mobile in the environment. Si is about 1,000× more abundant than C in Earth's crust. Yet life is C-based, not Si-based. So above noted deficiencies appear to be valid. 30 15  2024-10-03 The water molecule Water is the essential ingredient for − − life. Water, H2O, forms a simple, asymmetric molecule. Hydrogen can have only one bond, oxygen can have two bonds. + + The water molecule is very special as it has (slightly) negatively and positively charged ends. It is a polar molecule. The charge excess is very small, yet permits the hydrogen bonds to occur. 31 The water molecule The attraction of partial electric charges keeps water molecules together => Comparatively high freezing and boiling temperatures. Below 0C, water forms a regular crystal structure, ice. In ice, water molecules are spaced relatively far apart (→ lower density; so ice floats in water). Large heat storage and high heat of vaporisation due to the large amount of energy that can be stored in the hydrogen bonds. At a water surface, unbalanced hydrogen bonds (=> strong surface tention, etc.). 32 16  2024-10-03 Molecular components of cell ◆Carbohydrates Provide energy and cellular structure (eg. sugar, starch, cellulose). ◆Lipids Store energy in cells (fats). They are the major ingredients of cell membranes. Can form membranes on water and may have been the earliest cell components. ◆Proteins Main constituents of cells serving many functions. Enzymes serve as catalysts facilitating or accelerating chemical reactions. All proteins are made of amino acids through polymerization. ◆Nucleid acids (DNA+RNA) crucial for transferring genetic information. 33 Organic carbon compound components 34 17  2024-10-03 Amino acids Amino acids contain an amino group NH2 and a carboxyl group COOH. Several hundred amino acids have been identified, but only 20 occur in living organisms. All amino acids (except one) naturally appear in two forms of "handedness" Simplest example: alanine. (chirality). In living organisms, only left-handed forms appear. Complex organic molecules (proteins) are built of amino acids. 35 Proteins Proteins consist of long chains (>50) of amino acids linked together: polypeptides. Have distinct end points (N, C). Amino acids polymerize (form long chains) by loss of water (condensation) or break into single components (hydrolysis). carboxyl carboxyl carboxyl amino amino amino 36 18  2024-10-03 Proteins Primary structure: Order of amino acids in a polypeptide. Secondary structure: 3-D links between strands of a protein molecule. 37 Proteins, the stuff of life Proteins can be very complex. With 20 amino acids, with length n: a total of 20n different polypeptides. Incredible variety possible. 38 19  2024-10-03 Enzymes Enzymes have catalytic properties. They either help reactions due to long, complex molecules and/or are recycled. 39 Metabolism Chemistry inside cells 40 20  2024-10-03 Metabolism ◆Metabolism Chemical reactions occurring in living organisms and providing energy or nutrients to cells. The reactions are accelerated within the cell (breaking down of the nutrient molecules and building new ones). ◆4 major categories based on... Source of raw material (carbon compounds) Heterotrophs: “eating others”, get material by consuming; Autotrophs: “self feeding”, from atmosphere or dissolved in water. source of energy Photo- using the light Chemo- using organic or non-organic compounds. Photoheterotophs, photoautotophs, chemoheterotophs, chemoautotrophs. 41 Metabolism Life elsewhere will need energy, and a carbon source, so same divisions will likely apply 42 21  2024-10-03 Metabolism and Water ◆Liquid water is essential for metabolism: Dissolves organic compounds within the cell making it available for reactions. Provides medium for transportation of materials into the cell and waste out of the cell. Required for storage and release of energy in ATP-cycle (adenosine tri-phosphate) ◆No organism on Earth can survive without liquid water. 43 Role of ATP Most living cells on Earth use ATP to store and release energy External energy used to produce ATP ATP then used for cellular reactions ATP completely recyclable “energy currency” of cells Attach/remove 3rd phosphate group Adenosine-Di-Phosphate Adenosine-Tri-Phosphate 44 22  2024-10-03 DNA The mechanism behind evolution 45 Nucleic acids: RNA & DNA ◆RNA: Ribonucleic acid | DNA: Deoxyribonucleic acid ◆Consist of monomers called nucleotides Each nucleotide: (1) phosphate group, (2) sugar, (3) nitrogenous base. (similar to the discussion about amino acids and proteins) cytosine thymine guanine adenine Ribose Deoxyribose 46 23  2024-10-03 DNA molecule A G C T The bases are complex molecules themselves. They are linked to sugars and phosphate groups. 47 DNA molecule ◆Genetic information is stored through the order of nucleic acid bases (A, G, T, C) in the double- helix DNA strand. A attaches only to T C attaches only to G ◆RNA is very similar, but simpler: single helix Bases: A, G, U, C 48 24  2024-10-03 DNA molecule ◆Every living cell on Earth uses DNA to pass hereditary information. Only some viruses use RNA. ◆Evidence of common origin of all life on Earth ◆Differences in DNA sequence are used to determine evolutionary relations between different organisms Tree of life 49 DNA molecule replication 50 25  2024-10-03 DNA molecule replication 51 Base-Gene-Chromosome-Genome ◆Bases A-C-G-T ◆Gene Sequence of bases coding one protein ◆Chromosome Many genes combined ◆Genome The entire set of genes of an organism ◆Human 46 chromosomes 20-25K genes 3 x 109 bases 95% noncoding DNA 52 26  2024-10-03 Genetic code ◆Three letter “words” represent amino-acids CTG-CAG-AAG-GTA-... ◆Same coding for almost every cell on Earth Evidence for common ancestor ◆Many amino-acids fixed by first two letters Indicates earlier alphabet with two-letter-words, which later evolved into three-letter-words 53 Mutations ◆DNA copying extremely accurate Less than one error per billion bases Errors occur by chance, enhanced by UV, X-ray, particle radiation or by chemicals (carcinogens) ◆Most mutations harmless More chances that it will occur in non-coding DNA Additional protection because often different encodings are allowed for same protein (e.g. ATT vs ATC) ✦ Some mutations not harmless replacement of base, changing one amino-acid insertion/deletion of base, changing the entire rest of the gene the fat cat ate the rat the afa tca tat eth era 54 27  2024-10-03 Genetic mutations ◆Mutations can be: advantageous disadvantageous ◆from the point of survival of the next generations and for the evolution of the species ◆The “sickled” red blood cell A decease killing some 100,000 people each year But also a genetic mutation that gives resistance to malaria 55 DNA & Evolution ◆DNA accomplishes hereditary passing of information Hereditary mechanism near certainty in non-Earth Life Chemical representation could vary (other molecules instead of DNA) ◆Genetic mutations molecular origin of individual variation Mechanism of Darwinian Theory of Evolution ◆Evolution is blind Only immediately advantageous mutations will be kept No “foresight” to accomplish complicated changes “all at once” 56 28  2024-10-03 Gene transfer ◆Genetic information can be passed between different organisms. ◆Vertical (normal) gene transfer parent to offspring ◆Lateral (horizontal) gene transfer Copying of genes to other organisms. Discovered in 1959: Transfer of antibiotic resistance between different types of bacteria. Common in prokaryotes, not clear if it occurs (naturally) in eukaryotes. ◆Artificial gene transfer Genetic engineering. 57 Implications for extraterrestrial life ◆Life needs structure Likely something similar to cells ◆Carbon versatile building element, widely available No reasonable alternative known. ◆Life elsewhere needs energy and carbon source division photo/chemo -- hetero/auto -- trophs will likely apply ◆Water required for cells to function Almost certainly need water or some other useable liquid ◆Need hereditary mechanism perhaps similar to DNA, perhaps not 58 29  2024-10-03 Surprising discoveries? 1 ◆ Inlight of the current understanding of life on Earth, would the following discoveries be (A) Not surprising (B) Unexpected (C) Very surprising, indeed ◆ A multicellular organism that can grow and reproduce even in the absence of water 59 Surprising discoveries? 2 ◆ Inlight of the current understanding of life on Earth, would the following discoveries be (A) Not surprising (B) Unexpected (C) Very surprising, indeed ◆ A single-celledorganism that lives deep in peat bogs, where no oxygen is available 60 30  2024-10-03 Surprising discoveries? 3 ◆ Inlight of the current understanding of life on Earth, would the following discoveries be (A) Not surprising (B) Unexpected (C) Very surprising, indeed ◆ A bacterium with cells that lack the molecule ATP 61 Surprising discoveries? 4 ◆ Inlight of the current understanding of life on Earth, would the following discoveries be (A) Not surprising (B) Unexpected (C) Very surprising, indeed ◆ An organism that does not belong into any of the three Domains (Archaea, Bacteria, Eukarya) 62 31  2024-10-03 Life in unexpected places - Extremophiles ◆Volcanic vents in the mid-oceanic ridges temperature +350 C, water pressure of 400 atm. ◆Organisms live nearby living at +121C and surviving at +130 C... “thermophiles” Chemo-autotrophs: consume dissolved CO2 and energy from inorganic reactions in hot water. Some thermophiles actually die at more normal temperatures ◆Most extremophiles are archaea and anaerobic (do not need oxygen) 63 Dry & Cold; Radioactivity 1cm ◆Desert in Antarctica Only 500 hours per year above freezing Organisms live inside rocks Can live as long as traces of liquid water exist “Psychrophiles” ◆Deinococcus radiodurans Survives radiation at 1000x the lethal dose for humans 64 32  2024-10-03 Underground; in Space ◆In rocks, up to several km underground Chemoautotrophs Energy from reactions of percolating water within rocks nutrients from rock-chemicals and percolating CO2 “Endoliths” --- also on Mars?? ◆Endospores dormant “resting cells” of bacteria Can survive complete lack of water, extreme heat/cold, even vaccum of space Maybe able to survive a meteorite journey from Mars to Earth ◆Must have open mind when searching for life elsewhere 65 When did life on Earth begin? Life started quickly… within ~600Myr of Earth’s formation (i.e.- by the end of the Late Heavy Bombardment). But perhaps much more quickly. 66 33  2024-10-03 Stromatolites Colonies of single-cell microbes, sediment layers intermixed with microbes Structures similar to today’s stromatolites, dated 3.5Gyr old Already had photosynthesis by then? 67 Microfossils May be fossilized cells, 3.5Gyr old Controversial Other microfossils dated at 2.7-3Gyr present more conclusive evidence 68 34  2024-10-03 Isotopic evidence ◆13C and 12C Carbon isotopes both stable. in non-organic matter: 1 out of 89 Carbon atoms is 13C ◆Living organisms prefer 12C 13C depleted relative to 12C in organic matter ◆Greenland rocks (3.85Gyr old) show lower 13C ratio. Life? Controversial and indirect evidence But other rocks (3.8Gyr old) have now been found elsewhere also with isotope-ratios indicative of life 69 Genetic hints toward early life ◆Treeof life grouped by amount of differences in DNA small differences among species that diverged recently large differences: branched long ago ◆Organisms close to “root” tend to be extremophiles. 70 35  2024-10-03 Conclusions: Life started early ◆The earliest records of Life: 3.85 – 3.5 Gyr ago (and maybe even earlier). ◆Late heavy bombardment ended at ~3.9 Gyr ago. => It took at most 0.5 Gyr for the life to start, but possibly as short as 100 Myr (0.1 Gyr). ◆The earliest organisms were likely similar to the present single-cell extremophiles: Only such could survive the conditions of the Hadean & Archean eons. 71 36