Life’s Origin and HistorygyThe age of the earth is estimated at about 4.5 billion yearsbased on: the age of meteorites, moon rocks, and the oldest k th (3 9 billi ld)rocks on earth (3.9 billion years old).The earliest sedimentary rocks are about 3.8 billion years old and these contain organic deposits.The first definite microfossils date from 3.5 to 3.6 billion yearsresemble modern stromatolites - mats of blue-green algaeggIt is clear that life got started very early in ve y e yEarth’s history.How did life begin?Lifhi hl i d h i l h i bl i iLife - a highly organized chemical system that is able to maintain itself and reproduce.One hypothesis is that life came here from elsewhereanotherOne hypothesis is that life came here from elsewhere -another planet either in our solar system or outside our solar system. This is the “Panspermia Hypothesis.” • life came here unaided as a spore from another solar system (several thousand solar systems are within 100 light years)•life came here from another solar system aided by anlife came here from another solar system aided by an intelligent life form (e.g. Voyager)• life came here from another planet in our solar system (e.g. Mars rocks)Mars rocks)This hypothesis begs the question of how life got started elsewhere d t thi i t ’t b i ti t d i tifi lland, at this point, can’t be investigated scientificallyWorking hypothesis: Life originated from chemical building blocks on earthblocks on earth.If life organized itself according to physical laws it can be investigated scientifically. If it can be shown that one of the properties of life, or one of the basic building blocks of living systems, could not have become organized on its own under conditions that existed in the past, gp,then we would have to reject the working hypothesis. It is clear that it would be difficult for amino acids, proteins, and nucleic acids to become organized today because of the high concentration of oxygen in the atmosphere and surface waters. Any high-energy compound will donate its electrons to any yggy p yready electron acceptor. Oxygen is a good electron acceptor. So, the environment of early earth must have been different than today. It must have not have had many oxidizing agents. The environment must have been a reducing environment.Is there evidence that early earth had a reducing environment?Other planets in our solar system (Venus, Mars, Jupiter, Saturn, and Uranus) all have reducing atmospheres - rich in CH4, CO2, CO, )gp42H2S, NH3Sediments that were laid down prior to 2.5 bya contain reduced chemical compounds. After 2.2 bya the chemicals deposited in sediments tend to be oxidized.Reduced Iron (Ferrous Iron-Fe++) is soluble in waterReduced Iron (Ferrous Iron Fe) is soluble in waterOxidized Iron (Ferric Iron - Fe+++) is insoluble in waterThe “red-beds” or “banded iron formations” were created as l bl i idi d d i it t d t f l tisoluble iron was oxidized and precipitated out of solution in the earth’s oceans.It appears that oxygen was rare in the earth’s atmosphere until about 2.2 bya.Can complex chemicals form in a reducing atmosphere?1953 - Miller and Urey created an apparatus to address the question. Used atmosphere of CH4, NH3, HdHOd dl ilH2, and H2O and used electrical discharges as a source of energy (simulated lightning).(gg)After a few days the mixture yielded some amino acids, HCN, yyH2CO (formaldehyde), and these subsequently reacted to produce sugars, purines, and pyrimidines.Later experiments showed that heat or UV radiation and other mixtures of gases lacking oxygen produced similar results.Current estimates of the fh damounts of methane and ammonia that were present in Earth’s early yatmosphere suggest that over time oceans would have been rich in organichave been rich in organic molecules with a concentration similar to khik bthThtweak chicken broth. That broth has been labeled the “prebiotic soup” or “primordial soup.”Macromolecules are necessary for life. Could polymers ( i l i id ) f h b ildi bl k h f d?(proteins, nucleic acids) of the building blocks have formed?This is a potential problem because polymers form through deh dration s ntheses In aq eo s en ironments h drol sis isdehydration syntheses. In aqueous environments hydrolysis is more likely.Fox found that dry mixtures of amino acids spontaneously polymerize at 130°CAmino acid adenylates (amino acids charged with ATP) formAmino acid adenylates (amino acids charged with ATP) form random polymers spontaneously at 60°CHuber found if CO was added to the mixture there was a preference for stable peptide bonds between amino acidspreference for stable peptide bonds between amino acids.The primordial soup may have been dried on hot rocks or in evaporating pools to become a“primordial pizza”whereevaporating pools to become a primordial pizza where spontaneous polymerization would have been favorable.CTP, GTP, TTP, UTP when placed together at 55°C spontaneously ,,, p gpyform polymers.However the polymers are 5’→3’and 5’→2’in orientationHowever, the polymers are 5→3 and 5→2 in orientation.The addition of zinc results in only 5’→3’ polymerization.(Today DNA polymerase and RNA polymerase require zinc as a f)cofactor.)DNA and RNA are autocatalytic with or without polymerases when ypyplaced in mixtures of triphosphate nucleotides. Some RNA molecules are catalytic (ribozymes) and can promote the formation of complimentary copies of themselvesthe formation of complimentary copies of themselves.So if nucleic acids were present they would spontaneously create li i f h lcomplimentary copies of themselves.Read: Experimental Evidence on the Origins of Natural SelectionpgSpiegelman and colleagues showed thatcolleagues showed that a test tube RNA replication systems evolves in response to different selective regimes.gA theoretical model for a self-replicating systemselfreplicating systemThe R3C ribozyme can catalyze the formation of copies of itself.Life requires isolation from the surrounding environment for the qgmaintenance of order. Can compartmentalized systems form spontaneously?Phospholipids spontaneously form lipid bilayers and micelles.Lipid bilayers exhibit ifllmany properties of cell membranes - selective permeability and pyaccumulation of some chemicals.Mixtures of polymers also form coacervates -polymer rich droplets po y e c d op e sthat can be stable in solution.Various models suggest how chemical systems