AST 205. Lecture 18. November 19, 2003 Microbes and the Origin of Life Context Definition of life Cells, the atoms of life Major classes & families of cells Origin/evolution of biochemistry of life Origin/evolution of unicellular life Extremophiles Implications for extraterrestrial life Panspermia? Precept assignment for week of Dec 1 A class debate/discussion in the same format as this week s Bring a one page brief to class outlining your argument/views & be prepared to discuss them. TOPIC: Assume the existence of an extrasolar planet similar to Earth with physical conditions equally hospitable to life and of the same age. Will life exist there, and if so, how closely will it resemble terrestrial life? No precept assignment for the week of Nov 24. 1
N = f(p)n(e)f(l)f(i)f(c)r * L Radial velocity techniques have provided first direct clues about f(p), 5-10% No direct information on n(e) yet R * measured by astronomical observations f(l) depends on biochemistry & cell biology Detailed knowledge of one & only one case But/thus no first principles general theory Life as we know it is the practical option Convergence or Divergence of Cosmic and Biological Evolution? (How similar to here?) Large/coarse scales -> convergence But on some small/fine scales -> divergence Divergence might begin on the scale of planetary systems since known extrasolar systems are unlike the Solar System However it might not occur until far finer levels of detail <- assumption! 2
Definition of Life (many possibilities) Metabolism (chemical activity) Growth/development Energy utilization Preservation of information/identity Procreation Mutation Spatial boundaries Functional in abiotic environment Cells Cells are alive, satisfy all definitions of life All normal life forms are cellular Most terrestrial life is unicellular Cells are enclosed by a membrane Within cells the processes of molecular biology occur in an aqueous solution Cells organize/utilize a large number of biomolecules & their interactions -> life 3
Two fundamental classes of cells Prokaryotes: no nucleus & relatively little internal structure Eukaryotes: nucleus containing cell s DNA, defined by an inner membrane, & complex internal structures Quite different in many ways Major clue to the evolution of life on Earth Properties of prokaryotes No nuclear membrane Single circular strand of DNA mrna generated from start to stop codons No internal organelles & little structure Relatively small (0.1-10mm diameters) Ancient,oldest life forms (3.9 Gyr ago?) Two evolutionary branches (split 3.5 Gyr?) 4
Two typical prokaryotes Properties of eukaryotes DNA segregated into nucleus by membrane Multiple linear stands of DNA An intermediary mrna is edited into exon and intron segments -> final mrna Complex internal structure/many organelles Relatively large (10-100mm diameters) Relatively recent (appeared 2-3 Gyr ago) Unicellular and all multicellular life forms 5
Exon/intron editing during transcription Internal structures of a typical eukaryote 6
Major eukaryote organelles Nucleus Cytoskeleton Flagellum Lysosome Mitochondrium* Peroxysome Endoplasmic reticulum Golgi apparatus Plastids DNA, DNA->mRNA Internal transport/support Movement Digestion/waste removal Food+oxygen -> ATP Far metabolism Protein & lipid synthesis Protein & lipid storage photosynthesis Mitochondria and Lysosomes *Mitochondria have their own internal DNA (loop) and reproduce separately from the cell! Note internal complexity of these organelles, likely endosymbionts. 7
General Characteristics of the Molecular Biology of Terrestrial Life Extraordinarily complex & inter-connected chemical processes, vastly richer than any other known chemical systems Basic biochemistry shared by all known terrestrial organisms as well as many of its details Carbon based and water dependent Hierarchically structured (using much simpler subcomponents), polymerized macromolecules Few (4) general classes of compounds but many individual ones with highly specialized and specific biological functions Origin of biochemistry First produce the macromolecule building blocks Happened very fast, 4 Gyr ago (Earth just cooled) Possible locations/environment Shallow tidal pools or lagoons (Darwin) Deep sea hydrothermal vents On wet clay surfaces Deep underground? Proteins or nucleic acids first?? (chicken & egg issue) RNA only biology first (no DNA or proteins)? 8
Urey-Miller Experiment (1953!!) water (H2O) methane (CH4) ammonia (NH3) hydrogen (H2) no oxygen + sparks YIELDS amino acids! (nucleic acid bases) Urey-Miller experiment issues & developments Seminal influence on origin of life studies Many variations on details work also No progress in assembling building blocks into useful macromolecules by similar techniques Now believed that Earth s primordial atmosphere was CO2 dominated & had little CH4 which very much reduces the amino acid yields U-M conditions resemble oceanic hydrothermal vents Clay surfaces may facilitate macromolecule assembly 9
Stanley Miller & his apparatus Origin of cellular life Also very very fast (3.7-3.9 Gyr) Requires formation of enclosing lipid membranes Simple protein membranes have been formed spontaneously in lab experiments Membranes need to effectively isolate important macromolecules & their reactions but not seal off environment completely (complex function) Speculative possibility of noncellular ancestors?? 10
Spontaneously formed protein spheroidal membranes (Fox 1970s) Evolution of cellular life Last Common Ancestor = prokaryote, anaerobic heterotrophe, maybe 250 genes, resembling present day mycoplasmas Even simpler RNA-only cells a possibility Split into Archaea and Bacteria classes (3.5 Gyr?) Anaerobic autotrophs/chemoautrophs next Photoautotrophs, cyanobacteria (2.7-2.5 Gyr) O 2 respiration by 2.2 Gyr (high octane biology!) Eukaryotes w ~6000 genes, evolved via endosymbiont colonization? (3-2 Gyr) Multicellular life consisting of eukaryotes (1 Gyr) 11
Prokaryote microfossil dated at 3.7 Gyr Masses of cyanobacteria at Shark s Bay, Australia 12
The Tree of Life from Genomic Sequencing Life in deep sea hydrothermal vents (Did life originate in such extreme environment or adapt to them?) 13
Implications for extraterrestrial life Multiple hurdles: biochemistry (proteins & nucleic acids), cells, autotrophism/food, organelles, oxygen, multicellular cooperation Speed of appearance time is often interpreted to imply ease/probability of each development Extremophiles -> life can survive exotic environments & perhaps can originate in them Panspermia - speculative possibility that unicellular life colonized Earth, originated elsewhere A possible panspermic organism: Deinococcus radiodurans (Conan the Bacterium) Resistant to radiation (UV to g-rays), cold, vacuum, dormancy, oxidative damage, extreme desiccation, wide ranges of ph etc. Uses multiple, redundant copies of its DNA and enzymes which carry out error checking and repair of its genetic material. 14
Precept assignment for week of Dec 1 A class debate/discussion in the same format as this week s Bring a one page brief to class outlining your argument/views & be prepared to discuss them. TOPIC: Assume the existence of an extrasolar planet similar to Earth with physical conditions equally hospitable to life and of the same age. Will life exist there, and if so, how closely will it resemble terrestrial life? No precept assignment for the week of Nov 24. 15