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Life Out There…

Life Out There…

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Life Out There…

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  1. Life Out There…

  2. Extraterrestrial Life • Can/does life exist out there? • astrobiology/xenobiology • no evidence accepted by scientific community • most scientists hold that, if extraterrestrial life exists, it likely occurred independently in many places • (minority) panspermia hypothesis, where life originated in few points of origin and spread

  3. Life on Earth • Water • neutral pH can dissolve positive metallic ions & negative non-metallic ions with equal ability • medium for interactions • organic molecules can be hydrophobic (repelled) or hydrophilic (soluble) enables formation of water-enclosing membranes

  4. Life on Earth • Water • solid ice is less dense than liquid water • ice floats • oceans don’t freeze • latent heat of vaporization moderates climate • cools tropics • warms poles • helps maintain thermodynamic equilibrium

  5. Life on Earth • Carbon • immense flexibility in creating covalent chemical bonds with non-metallic elements (N, O, H) • CO2 & H2O together enable storage of solar energy in sugars • oxidation of glucose fuels all other biochemical reactions • able to form organic acids & amine bases • permits building DNA, ATP, other stuff

  6. Alternative biochemistries • Silicon • chemically similar to carbon • large mass & size • formation of double & triple covalent bonds difficult • long-chain silicone molecules are more unstable than carbon counterparts • SiO2 (analog to CO2) non-soluble in ranges where water is liquid

  7. Alternative biochemistries • Silicon • cosmic abundances of carbon & silicon roughly 10:1 • molecules identified in interstellar medium • 84 carbon-based • 8 silicon-based • terrestrial planets are exceptionally Si-rich, C-poor • yet carbon is used on Earth • some suggestion early life forms may have been Si-based

  8. Alternative biochemistries • Nitrogen & Phosphorus • Phosphorus can build long chain molecules like carbon • fairly reactive • nitrogen can allow more stable covalent bonds • Nitrogen is nearly inert and energetically expensive to “fix” due to triple bond

  9. Alternative biochemistries • Nitrogen & Phosphorus • NO2 atmosphere • NH3 atmosphere • lots of debate – several aspects of this biology would be energy deficient • nitrogen & phosphorus are unlikely to be formed in useful quantities & ratios • Carbon likely wins again

  10. Alternative biochemistries • Chlorine • much less abundant than oxygen • typically becomes bound in salts & other inert compounds • Sulfur • high reactivity problems of phosphorus & silanes • but there are strains of sulfur-reducing bacteria

  11. Alternative biochemistries • ammonia as solvent • hydrogen bonds weaker • low heat of vaporization • surface tension three times smaller • reduces ability to concentrate non-polar molecules through hydrophobic effects • seems unlikely to hold prebiotic molecules to allow emergence of self-replicating system • Highly combustible in an oxidizing environment

  12. Alternative biochemistries • other non-water solvents • methanol • hydrogen sulfide or hydrogen chloride • low abundances • hydrocarbon mixtures • methane/ethane on Titan • lack polarity

  13. Life on Other Worlds • The Nature of Life • The Origin of Life • Communication with Distant Civilizations

  14. Life on Other Worlds • The Nature of Life • The Physical Basis of Life • Information Storage and Duplication • Modifying the Information

  15. Life on Other Worlds • The Origin of Life • The Origin of Life on Earth • Geologic Time • Life in Our Solar System • Life in Other Planetary Systems

  16. Life on Other Worlds • Communication with Distant Civilizations • Travel Between the Stars • Radio Communication • How Many Inhabited Worlds?

  17. Life on Earth • Physical Basis of Life • All life forms on Earth, from viruses to complex mammals (including humans) are based on carbon chemistry • Carbon-based DNA and RNA molecule strands are the basic carriers of genetic information in all life forms on Earth

  18. Life on Earth • Physical Basis of Life • The Tobacco Mosaic Virus contains a single strand of RNA, about 0.1 mm long

  19. Life on Earth • Physical Basis of Life • The Tobacco Mosaic Virus (TMV) • first virus discovered • single strand of RNA, ~0.1 mm long • This complex mammal contains about 30 AU of DNA.

  20. Life on Earth • Information Storage & Duplication • All information guiding all processes of life stored in long spiral molecules of DNA (deoxyribonucleic dcid)

  21. Life on Earth • Information Storage & Duplication • basic building blocks are four amino acids • Adenine • Cytosine • Guanine • Thymine

  22. Life on Earth • Information Storage & Duplication • Information is encoded in the order in which those amino acids are integrated in the DNA molecule

  23. Life on Earth • RNA • ribonucleic acid • several roles in translating information from DNA into protein products • messenger between DNA and the protein synthesis complexes (ribosomes)

  24. Life on Earth • RNA • ribonucleic acid • several roles in translating information from DNA into protein products • forms vital portions of ribosomes

  25. Life on Earth • RNA • ribonucleic acid • several roles in translating information from DNA into protein products • acts as essential carrier molecule for amino acids used in protein synthesis

  26. Life on Earth • Processes of Life in Cells • Information stored in the DNA in the nucleus is copied over to RNA (ribonucleic acid) strands, which act as messengers to govern the chemical processes in the cell

  27. Life on Earth • Human DNA & descent • mitochondrial DNA • Mitochondrial Eve (mt-mrca) • matrilineal most recent common ancestor (MRCA) for all living humans • mitochondrial DNA (mtDNA) passed down from mothers to offspring for over 100,000 years • now found in all living humans: every mtDNA in every living person is derived from hers • believed to have lived ~140,000 years ago • in what is now Ethiopia, Kenya or Tanzania

  28. Life on Earth • Human DNA & descent • Y-chromosomal Adam (Y-mrca) • patrilineal human most recent common ancestor (mrca) • from whom all Y chromosomes in living men are descended • geneticist Spencer Wells has concluded all humans alive today are descended from single African man who lived ~60,000 - 90,000 years ago

  29. Life on Earth • Duplication & Division • In the course of cell division, DNA strands in nucleus (chromosomes) duplicate by splitting double-helix strand and replacing open bonds with corresponding amino acids

  30. Life on Earth • Duplication & Division • Process must be sufficiently accurate, but also capable of occasional minor mistakes to allow for evolution

  31. Life on Earth • Origin of Life on Earth • Life develops into more complex forms through gradual evolution, spanning many thousands of generations • Life began in the sea as single-celled creatures

  32. Life on Earth • Origin of Life on Earth • Those as well as early multi-celled creatures had no hard parts to leave fossils • Earliest, microscopic fossils date back ~ 4 billion years

  33. Life on Earth • Origin of Life on Earth • ~½ billion years ago, in the Cambrian Period, the diversity and complexity of life on Earth dramatically increased • “Cambrian Explosion” • trilobites from Cambrian period • all known fossils from Cambrian period sea creatures • no traces of life on land until ~400 million years ago

  34. Life on Earth • Miller Experiment (1952) • simulating conditions on Earth when life began ~4 billion years ago • water (oceans) • primitive atmosphere gases (hydrogen, ammonia, methane) • energy from electric discharges (lightning)

  35. Life on Earth • Miller Experiment (1952) • simulating conditions on Earth when life began ~4 billion years ago • produced some of the fundamental building blocks of life: amino acids, fatty acids, and urea

  36. Life on Earth • Miller Experiment (1952) • shows that basic building blocks of life form naturally • Amino acids and other organic compounds naturally tend to link up to form more complex structures • early oceans on Earth were probably filled with a rich mixture of organic compounds: the “Primordial Soup” • Chemical evolution leads to the formation and survival of the most stable of the more complex compounds

  37. Life on Earth • Miller Experiment (1952) • recent studies – amino acid composition of products of “old” areas in “old” genes • defined as those found to be common to organisms from several widely separated species • assumed to share only the last universal ancestor (LUA) of all extant species

  38. Life on Earth • Miller Experiment (1952) • recent studies – amino acid composition of products of “old” areas in “old” genes • found that the products of these areas are enriched in those amino acids that are also most readily produced in the Miller-Urey experiment • suggests that original genetic code based on smaller number of amino acids – only those available in prebiotic nature – than the current one

  39. Life on Earth • Extraterrestrial Origin of Life on Earth • Alternative theory: most primitive living entities transported to Earth in meteorites or comets • some meteorites do show traces of amino acids • theory of extraterrestrial origin of life is currently untestable

  40. Life on Earth • Age of Life on Earth • oldest fossils known are stromatolites • built up layer by layer from single-celled creatures, similar to bacteria • ~3.5 billion years ago • During the Cambrian period (~500 million years ago), life became complex

  41. Life on Earth • Age of Life on Earth • In geologic terms, higher life forms have evolved only very recently • mammals & humans in particular • humans have existed only ~3 million yrs

  42. Life on Other Worlds • Evolution of Life • Could life originate on another world if conditions were suitable? • Will life always evolve toward intelligence? • How common are suitable conditions for the beginning of life?

  43. Life on Other Worlds • Evolution of Life • Could life originate on another world if conditions were suitable? • Miller experiment etc. indicate: probably yes • Will life always evolve toward intelligence? • How common are suitable conditions for the beginning of life?

  44. Life on Other Worlds • Evolution of Life • Could life originate on another world if conditions were suitable? • Miller experiment etc. indicate: probably yes • Will life always evolve toward intelligence? • If intelligence favors one species over another: probably yes • How common are suitable conditions for the beginning of life?

  45. Life on Earth

  46. Life on Other Worlds • Evolution of Life • How common are suitable conditions for the beginning of life? • investigate conditions on other planets and statistics of stars in our Milky Way • Astronomy!

  47. Life on Other Worlds • Requirements • liquid water • atmosphere • moderate temperatures • time

  48. Life on Other Worlds • Requirements • liquid water • for chemical reactions and as transport medium