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Origin of Life WHEN Did Life Evolve? • Earth was molten during Hadean • cooled somewhat by 3.9 billion years ago • water could remain liquid at the surface Origin of the Oceans • outgassing and photochemical dissociaton both produce abundant water vapor • water vapor condenses and falls as rain • oceans accumulated quickly at first • decreased in growth as volcanic activity slowed • salts added over 3.9 billion years as material eroded from continents • • • • oxygen breaks down organic molecules life must have evolved before oxygen abundant Early Archean sediment dark-colored = unoxidized oxidized (rusted) sediment appears about 3.5 billion years ago • life must have evolved before 3.5 bya WHEN Did Life Evolve? • oldest (possible) life-forms ~3.5 billion years old • life likely evolved between 3.9 and 3.5 billion years ago Monomers • building blocks of life • must form for life to evolve • how did monomers first form? Amino Acids Monomer Synthesis • Miller and Urey - created “Early Earth Apparatus” • early Earth atmosphere in the top • electrodes to produce “lightning” • primordial pond in the bottom Results: • after a week, primordial pond became primordial soup • 12 of 20 most common amino acids synthesized + other stuff • next step: polymerization Polymers • • • • • chains of molecules proteins carbohydrates lipids nucleic acids Requirements for Polymerization • energy source: to drive reactions • protection: from too much energy • concentration: to bring materials together so they can react together • catalysts: to make reactions happen faster and more efficiently Energy Source Energy induces chemical reactions • volcanoes • lightening • cosmic rays • UV radiation Protection • too much energy can be a bad thing! • early organic material protected by: rock ledges, under ice, under thin film of sediment, just under surface of water Concentration • when chemicals are dispersed in water, reactions are less likely to happen • concentration brings reactants together • evaporation • freezing • scums • droplets, bubbles • clay Concentration - clay • clay forms from sedimentary particles called platelets • platelets are: – very small – flat – with negative charge on surface Clay • organic molecules are attracted to clay surface • concentrate and align • Examples: bentonite (kitty litter, mud masks), kaolinite (Kaopectate) Catalysts • decrease amount of energy needed for chemical reactions • so increase the rate of chemical reactions • catalysts in the body = enzymes • Inorganic catalysts often metal ions Making Cells • what is required to make a cell? • DNA: passes on genetic code • cell membrane: made of phospholipids Phospholipids • each molecule has a hydrophobic end and a hydrophilic end • when surrounded by water, fatty acids form shapes where the hydrophobic ends protected on the inside Protobionts • • • • Phospholipid spheres that form naturally polymers and enzymes concentrated inside reactions occur inside protobionts: – maintain their structure – increase in size over time – divide when too large – selectively absorb and release compounds – metabolize starch – store and release energy Are Protobionts Alive? • No • they can’t replicate themselves Replication • process by which organisms make genetic copies of themselves • asexual reproduction • sexual reproduction Origin of Heredity • many different types of protobionts • those best able to accumulate organic molecules, grow, and divide become most common • but “competition” is useless unless traits can be passed on/inherited • polymers that can replicate themselves: DNA and RNA Origin of Heredity • short strands (50 nucleotides) of RNA assemble naturally • replicate themselves if more monomers available • zinc, copper act as catalysts WHERE Did Life First Evolve? Desirable features: – energy source – protection – chance for concentration – warm (chemical reactions work faster) – wet (life requires liquid water) – good supply of chemicals/nutrients energy source? protection? concentration? nutrients? warm? wet? Archaebacteria sun, earth water small pools minerals hot you betcha Hot Springs Mud Flats and Tide Pools energy source? sun protection? water, mud concentration? Clay, evaporation nutrients? fewer warm? can be wet? yes alternate between wet and dry Deep Ocean • boiling water from vents through crust • very stable environment • very protected • lots of clay for concentration • lots of minerals • warm -> hot • Archaebacteria Aliens? QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. • some meteorites up to 6% organic molecules • comets up to 20% organic sludge • Halley’s Comet contains: C, O, H, N, CO2, ice, nucleic acid bases, amino acids and formaldehyde • 74 amino acids identified so far from meteorites Did Life as we Know it Arrive from Space? • it’s possible • small, simple organisms in a dormant state may be able to survive a ride from another planet in space What we do know is • up to 15% of organic material in early oceans may have come from space Earliest Life Forms Likely: • prokaryotic • Obligate anaerobes • chemoheterotrophs: – use organic molecules for energy – fermentation 3.4 byo, South Africa • source of organic molecules primordial soup modern The First Energy Crunch • organic molecules become depleted • competitive advantage goes to - Autotrophs • Cyclic psyn first autotrophs – did not make • O2 or sugars • Then later Non-cyclic psyn….2 types Anaerobic Non-cyclic Photosynthesis • light-absorbing pigments (like chlorophyll) probably already present • light provides energy to remove e- from H2S • S released as a by-product energy from sunlight 6CO2 carbon dioxide + 6H2S hydrogen sulfide C6H12O6 + 6S sugar sulphur Aerobic Non-cyclic Photosynthesis • some cyanobacteria evolved to use H2O instead of H2S as e- source • O2 released as a by-product • problem: O2 breaks bonds of organic molecules • (i.e. - it’s toxic) energy from sunlight 6CO2 carbon dioxide + 6H2O water C6H12O6 + 3O2 sugar oxygen The Oxygen Revolution (Pollution) Indirect (non-fossil) evidence for the presence of cyanobacteria ~ 3.5 bya Oxygen • the modern atmosphere contains about 21% oxygen • where did the oxygen come from? • Oxygen Revolution • Cyanobacteria - the first major global polluters Oxygen Crisis • eventually, oxygen built up in oceans • then outgassed from oceans into the atmosphere • corrosive (toxic) atmosphere led to first major mass extinction • anaerobic bacteria restricted to refuges without abundant oxygen (stagnant water, deep soils, etc.) Aerobic Bacteria • some bacteria evolved antioxidant mechanisms • allowed those bacteria to tolerate rising O2 levels • some bacteria even evolved to use O2 How Far Back Does the Fossil Record Extend? 3.5 byo - Australia Warrawoona Group, Australia Maybe? Modern cyanobacteria for comparison Likely 3.1 - 3.2 byo - South Africa Fig Tree Group Earliest (Undisputed) Evidence of Life Stromatolites 2.2 byo Michigan Stromatolites • • • • • • • • dome-shaped, layered structures Up to 3.5 byo became very abundant by 2.2 bya consist of layers of bacteria upper layers aerobic, photosynthetic lower layers anaerobic produce abundant oxygen how do we know? They are still alive today in special environments, notably Shark Bay, Australia Tide In Tide Out Formation of Stromatolites Cyanobacteria form a mat on top of sediment 1 cm A new layer of sediment is deposited on top Bacteria grow up through new layer Stromatolites provide evidence for the occurrence of cyanobacteria in the fossil record. Modern Ancient • if we use stromatolites to infer presence of cyanobacteria • we might expect to find fossils bacteria-like organisms in them... the of And we often find them . . . Gunflint Chert (~2.0 billion years old), Canada O2 in Atmos. O2 added The rise of cyanobacteria and the building up of oxygen in the Earth’s atmosphere had three significant effects: 1. Aerobic photoautotrophs became producers that fuel the food chains of surface world 2. Earth experienced the first mass extinction 3. an oxygen-rich atmosphere set the stage for the appearance of complex life (aerobic respiration) Eukaryotic Cells • First well established fossils 1.7 bya • Mid-Proterozoic eon • Early protists • unicellular • with nucleus • • 100x larger than bacteria • 10 – 100 μm Origin of Nucleus & E. R. • In-folding of cell membrane • Similar to bacterial psyn membrane • Surrounded DNA • Formed ER Origin of Chloroplasts & Mitochondria • Endosymbiotic Theory – symbiotic bacteria became dependent on host eukaryote cell • Evidence: • 1) separate DNA • 2) bacteria-like ribosomes • 3) inner membrane = bacterial membrane • 4) replicate independently