The origin and evolution of life on earth

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The origin and evolution of life on earth

  1. 1. The Origin and Evolution of Life on Earth ―There is no fundamental difference between a living organism and lifeless matter. The complex combination of manifestations and properties so characteristic of life must have arisen in the process of the evolution of matter.‖ Oparin, A.I. The Origin of Life, Foreign Languages Publishing House (1924) Dover publication (1938).
  2. 2. Spontaneous Generation?A 17th century recipe for thespontaneous generation of mice:• place sweaty underwear and husks of wheat in an open- mouthed jar• Wait 21 days while sweat from the underwear penetrates the husks of wheat and changes them into mice!Although such a concept may seem laughable today, it was consistentwith other widely held cultural and religious beliefs of the time andpeople believed it up through the 1900‘s!
  3. 3. Pasteur Refutes Spontaneous GenerationSpontaneous generation was laid to rest in 1859 by Louis Pasteur.Pasteur boiled meat broth in a flask, heated the neck of the flask in a flame until itbecame pliable, and bent it into the shape of an S.Air could enter the flask, but airborne microorganisms could not - they would settle bygravity in the neck—no microorganisms grew.When Pasteur tilted the flask so that the broth reached the lowest point in theneck, where any airborne particles would have settled, the broth rapidly became cloudywith life. Pasteur had both refuted the theory of spontaneous generation andconvincingly demonstrated that microorganisms are everywhere - even in the air.
  4. 4. Spontaneous Origin if Life Probable or Improbable?Suppose the spontaneous generation of life is a reasonably improbableevent—say it has a chance of one in a million (10-6 / yr) of happening in anyparticular year.During a human lifetime, roughly 100 years, the chances of it happeningwould be quite small—about 10-6 per year x 102 years = 10-4.But during the Earth‘s lifetime, about 4.5 billion years, the chance that it willhappen at least once is very close to one. How close? The chance per yearthat it doesn‘t happen is 999,999/1,000,000. The chance that it doesn‘t happenin 4.5 billion years is 0.999,999 4,500,000,000 = 0 to nine decimal places.In other words, this incredibly long time has transformed the event from onewhich is highly improbable to an event which is now almost inevitable!Suppose, instead, that the probability of the spontaneous origin of life in anygiven year on Earth were one in a trillion, or 10-12 / yr.The probability that life would arise on Earth at any time during its existencewould now be about 10-12 per year x 4.5 x 109 years = 4.5 x 10-3, a very smallnumber. If this were the case, we are here only through an extraordinarystroke of luck!
  5. 5. Tornedo in the JunkyardMany scientists have argued that the latter scenario is more likely. Theybase this supposition on a specious argument, such as calculating theprobability of assembling a protein out of amino acids in a purely randomfashion.Consider a protein consisting of 100 amino acids. Any ‗slot‘ in the chaincan be occupied by one of twenty possibilities. The random assembly ofa particular amino acid sequence to form the protein has a probability of(1/20)100, or about 10 -130. The probability of randomly assembling all theother organic molecules that make up life boggles the mind!The distinguished astrophysicist, Fred Hoyle, estimated that the odds ofcellular life arising were about one in 1040000! He commented…“The chance that higher life forms might have emerged in this way iscomparable to the chance that a tornado sweeping through a junkyardmight assemble a Boeing 747 from the materials therein.…Life as we know it is, among other things, dependent on at least 2000different enzymes. How could the blind forces of the primal sea manageto put together the correct chemical elements to build enzymes?”
  6. 6. What‘s Wrong?Spontaneous generation of life was not a purely random process!Proteins, RNA, DNA and the other molecules of life did not suddenly formThe evolutionarymolecular building Dawkins saysformation is a functionrandomly out of biologist, Richard blocks. Their in ―The Selfish Gene,‖— the laws of chemistry and biochemistry. The process is decidedly notof―At someThe formation wasremarkable molecule was formed by accident.random. point a particular a multi-step process in which ever moreWe will call itassemblages emergednot necessarily have been the biggestcomplicated the Replicator. It may from slightly simpler ones thator most complicated molecule a much less it had the‗first molecule‘ whichpreceded them, but there was around, but complex extraordinaryproperty of being able to make copiesthough its formation might be verymight be characterized as alive, even of itself. This may seem like aunlikely‗an accident.‘termed sort of accident to happen. So it was. It was exceedinglyimprobable. In the lifetime of a man, things that are improbable may betreated for practical purposes as impossible. That is why you will neverwin the big prize on the football pools. But in our human estimates ofwhat is improbable and what is not, we are not used to dealing inhundreds of millions of years. If you filled in pools coupons every weekfor a hundred million years, you would very likely win several jackpots.‖Once a Replicator formed, mutations would occur and natural selectionwould begin—having a strong say in the way subsequent organicmolecules used by living organisms would emerge and proliferate.Modern proteins, DNA and RNA were still a long way away.
  7. 7. More of What‘s Wrong The spontaneous generation of life did not depend on the random assembly of a specific replicator, a specific molecule or any specific process. It depended on the emergence of some replicator—or an assembly of some set of simple molecules that eventually led to replication via some process that worked. In other words, calculations of probability must take into account that there are many possible pathways to life that might work—not just one that is unique. A simple exampleHis friend, perhaps possessing a deeper understanding of statisticsHerein help— fallacy behind the simplistic probability calculation carried might lays thesays,by Hoylegolfer many others). If wegolf. to turn back his ball to 4.5 billionout “Yes, a (and playing a round of had He tees up the clock play the Consider John, but the damned ball could end up on some blade ofgrass!hole. starttakes out hisbillionwould exactly the same yardsof life basedyears AfterHe things out again, of and hits his ball 220 kind straight first and all—there are a “driver” them out here so unless you smackedthe ballthe fairway. He then walks down the molecule capable ofhole andupon exactly the same information-storing fairway towards the one of down into a sand trap, it’s 100% probable that it would land onthem!”his ball lying in the grass. He exclaims something different? Bill, willreplication emerge on Earth—or would it be finds to his friend, “Wow! Putanother way, if we find life elsewhere in the universe—or even our ownsolarlook at that? What are the chances that my storage basis and this you system—will it have the same information ball would land onreplicate in the sameprobability must be a billion to one— that’sare blade of grass? The way as ours? Not likely—many pathways almostavailable just like there are many ‗blades of grass‘ available to our golfer. impossible.What we are hinting at here is that the probability of emergence of somekind of life—given the right conditions such as exist on Earth—couldindeed be 100%.
  8. 8. Spontaneous Emergence of Life—Yes!A current school of thought suggests that life is aninevitable consequence of ―cosmic evolution,‖ shaped bythe fundamental laws of physics. All around us we seeevidence of self-organization that has been taking place on acosmic scale since the Universe began.Superclusters, clusters and galaxies have formed out ofstars and giant clouds of gas and dust—pulled together bythe action of gravity. Stars and their solar systems formed inthe same way. This large scale organization of matter wasdriven by the energy expended during gravitationalcontraction. the Sun and other stars, gravitationalIn the case ofcontraction heated them enough for nuclear fusion to start intheir cores—unleashing another source of energy that nowbathes all the planets in any solar systems that accompanythem. This energy source ultimately drove the emergence ofcomplexity that we now see on Earth—including life, itself.
  9. 9. Life—An Inevitable Consequence of Cosmic Evolution Life is an emergent property of cosmic evolution that“… Laws of complexity spontaneously generate much of the order of inevitably arises in those locations where (i) suitablethe natural world… materials are exposed to (ii) a suitable energy source in (iii)Life is a natural property of complex systems. over a the number of of a suitable environment that is stable When long perioddifferent molecules in a chemical soup passes a certain characterized time. The theoretician, Stuart Kauffman, has threshold, a self-sustaining network of reactions—an autocatalytic metabolism—will life thusly—suddenly appear. Autocatalytic metabolisms arose in the primal watersspontaneously, built from a random conglomeration of whateverhappened to be around. …The collective system does possess astunning property not possessed by any of its parts. It is able toreproduce and evolve. The collective system is alive. Its parts are justchemicals.…If all this is true, life is vastly more probable than we havesupposed. Not only are we at home in the Universe, but weare far more likely to share it with unknown companions.‖
  10. 10. Stepping Towards Complexity1. Synthesis of simple organic molecular building blocks2. Polymerization—assembly of simple building blocks into long chains.3. Origin of the first replicator—the RNA world4. The emergence of genetically encoded protein synthesis5. The emergence of DNA as the vehicle of information storage6. Last Universal Common Ancestor (LUCA)—the first cell7. Origin of the eukaryotic cell8. Origin of multicellular life— specialization9. The Cambrian Explosion10.Origin of humans
  11. 11. Cosmic Dust Cosmic dust grains, like this one < 0.1 mm across, are Some researchers have arguedfound in Gianthave taken a that it would Molecular billion years for complex organic molecules to form onof Clouds. Most are made Earth graphite and silicate ―others say they could form in less than 100 million years. compounds. The evidence for life‘s early appearance on Earth is overwhelming so either those arguing for slow formation are wrong… Ices made ofor the organics formed slowly in GMC‘s and then fell to H2O, CO2, CO, CH4, formaldehEarth, like manna from heaven…and the time required for The icy mantles facilitate yde (H2CO) (which might playformation would be irrelevant! the formation of other more an role in the formation of the complex molecules. Over simple sugar, ribose), and 130 have been identified methanol (CH3OH) form and 65 of them are organic! mantles around dust grains.
  12. 12. Comets Organic molecules have been found in comets Spacecraft visits to comets 1P/Halley (in 1986) and 81P/Wild 2 (in 2004) found even more complex organic compounds than have been found in interstellar space by remote sensing. They remain in frozen form Giotto images Halley‘s nucleus in dust grains ejected from cometary nuclei in 1986 as the comet approaches the inner Solar system.Prebiotic molecules contained on these dust grains—ifcaptured by Earth‘s gravity—would float in the atmospherefor many years―ultimately falling to the surface—and themolecules would remain intact! Thus the delivery of cometarymaterial containing prebiotic molecules that could ‗jumpstart‘the origin of life on Earth would be a virtual certainty!
  13. 13. Meteorites Over 70 different amino acids have been found in the Murchison meteorite! Nucleotide bases, sugars and other organic compounds such as alcohols, carboxylic acids, amines and amides have also been found—in other meteorites as well!Murchisonmeteorite, which Complex organic molecules formed infell near the town space are able to survive passageof through Earth‘s atmosphere as well asMurchison, Austral ground impact.ia in 1969Slight excess of L over D-amino acids → processing on dustgrains by circularly polarized light → explains chirality of aminoacids and other biotic compounds!
  14. 14. This 4.5 billion-year-old rock, labeled meteorite ALH84001, is believed to haveonce been a part of Mars and to contain fossil evidence that primitive life may haveexisted on Mars more than 3.6 billion years ago. The rock is a portion of ameteorite that was dislodged from Mars by a huge impact about 16 million yearsago and that fell to Earth in Antarctica 13,000 years ago. The meteorite was foundin Allan Hills ice field, Antarctica, by an annual expedition of the National ScienceFoundations Antarctic Meteorite Program in 1984. It is preserved at the JohnsonSpace Centers Meteorite Processing Laboratory in Houston.
  15. 15. Miller—Urey Experiment Simulate primitive atmosphere H2O, CH4, NH3 and H2 Subject it to lightning Organic molecules produced—including the sugar, ribose, and all 20 amino acids! Early atmosphere was a secondary— CO2, N2, H2O, some CO and H2 Still obtain organics
  16. 16. Black Smokers Where oceanic plates separate, hot mantle oozes up to build new crust—sea water is heated up to temperatures of 350 oC and it dissolves and exchanges minerals with the rock Near black smokers, this heated water enriched in gasses, minerals (such as H2, H2S, CO, CO2, HCN and NH3) and ions is ejected back into surrounding cooler sea water where it interacts with catalytic clays Sudden drop in temperature of this enriched water from 350 oC down to 2 oC facilitates chemical reactions that produce simple organic molecules and polymers—including amino acids
  17. 17. Chicken and Egg Paradox Which came first—the chicken or the egg? Proteins can‘t make more proteins without DNA and RNA, but DNA and RNA can‘t be made (and its information store accessed) without proteins …or can it?There would be no ‗chicken and egg‘ paradox if the first livingorganisms did not require proteins at all. Could some simple single-stranded RNA-like polymer have spontaneously formed that couldcatalyze its own replication without the aid of proteins? Eventually,such a structure would have evolved the ability to make proteins thatwould greatly accelerate the replication process. Later on DNAappeared as a more robust form of information storage, thanks to itssuperior chemical stability.This idea is known as the ―RNA world hypothesis.‖
  18. 18. Evidence for RNA FirstIt received support with the discovery of ribozymes, which are types ofRNA that act as catalysts. RNA enzymes are ‗living fossils‘, still found intodays DNA-based life. In 2001, the 3-d structure of the ribosome wasdeciphered—they consist of RNA and proteins—but the key catalyticsites of ribosomes were revealed to be composed of RNA. The proteinsare of peripheral functional importance.The formation of the peptide bond that binds amino acids together intoproteins, is now known to be catalyzed by an adenine residue in therRNA of the ribosome: thus, the ribosome is a ribozyme. This findingsuggests that RNA molecules were most likely capable of generatingthe first proteins, i.e., they came first.The existence of the ribosome supports the hypothesis that a simpleRNA replicator appeared before DNA because the RNA in the ribosomecontains in its structure ‗fossil‘ evidence of the existence of an earlier‗RNA world.‘The hypothesis received further support from experiments in whichribozymes were produced in the laboratory that catalyze their ownsynthesis—such as the RNA polymerase ribozyme.
  19. 19. Polymerization of RNAA dilemma—the sugar of one nucleotide must form a bondspontaneously with the phosphate of the next…and so on.However, this process of polymerization in water is notthermodynamically favored since it involves the release ofwater into surrounding water and thus the process will nothappen spontaneously—in fact, the reaction goes theother way—polymers in water will undergo hydrolysis andbreak down eventually into individual monomers!However, if there is some external energy source to drivethe binding of monomers—along with the presence ofsome catalyst to facilitate the process—thenpolymerization in a water solution can occur.
  20. 20. Clay as a Catalyst Polymerization in a water solution can occur when clay minerals are present in the mix. Lab experiments have shown that they catalyze the polymerization of RNA chains up to 50 nucleotides long!Montmorillonite consists of 3 "layers"—a layercontaining aluminum sandwiched between twosilicate layers. They concentrate nucleotides andprovide metal ions to catalyze their polymerization.
  21. 21. Thermodynamic Argument for the Emergence of RNA and DNALife is ‗an irreversible thermodynamic process‘ which arises and persiststo produce entropy. The production of entropy is not merely incidental tothe process of life, but in fact it is the very reason for its existence. Theabsorption and transformation of sunlight into heat is the most importantirreversible process generating entropy in the biosphere. Thus, life mostprobably began as a catalyst for the absorption and dissipation ofsunlight on the surface of Archaean seas. The resulting heat could thenbe used to drive other irreversible processes such as the water cycle,hurricanes, and ocean and wind currents.RNA and DNA are the most efficient of all known molecules forabsorbing the intense UV that penetrated the early atmosphere and areremarkably rapid in transforming this light into heat in the presence ofliquid water. Thus, the origin and evolution of life were inseparablefrom water and the water cycle and would have resulted from thenatural thermodynamic imperative of increasing the entropy productionof the Earth in its interaction with its solar environment.
  22. 22. Entropy—an AsideThe Second Law of Thermodynamics tells us whichconfiguration came first—Nature tends from order todisorder in isolated systems—a statement that paraphrasesthe Second Law of Thermodynamics—or more technically—the entropy of isolated systems always increases.
  23. 23. Entropy and LifeAn infusion of energy is needed to generate and support any system thatis more complex than its surroundings—order has to be maintained.Earth a system morelong wavelength photons back into spaceLife is emits more complex than its surroundings. Its order needs tothan short wavelengthenergy—fromreceives from the Sun.sink‘.be maintained by a flow of photons it a ‗hot source‘ to a ‗coolerThe Sun, the Earth and surrounding of Thermodynamics,an isolatedAccording to the Second Law space—or universe— is thisprocess increasesthe entropy of the Universe.system—more entropy must be returned into space from the Earth than itreceives from the Sun—in other words, the organization of complexity inEarth‘s biosphere requires the removal of entropy.
  24. 24. The First ReplicatorSimple nucleotides could have ‗polymerized‘ forming short strands ofRNA in a variety of possible ways. Many combinations that formedwould break apart because the strength of their bonds wasn‘t very greatand the energy required to break the nucleotide chain wascorrespondingly small. However, certain base pair sequences havecatalytic properties that strengthen the bonds of a forming chain,allowing them to stay together for longer periods of time. Such chainsThe idea of emergence of RNA ribozymes has recently been givenwould grow longer Lincoln andmore matching nucleotides faster untilcredence by Tracy and attract Gerald Joyce of the Scripps Researchthey eventually formed two RNA ribozymes in the lab from individual RNAInstitute who ‗evolved‘ at a faster rate than they broke down. Thus, theywould begin catalyzed the on early Earth. each other. The evolution ofstrands that to proliferate reproduction ofthese two RNA ribozymes capable of self-replication took about an hour.Theiremergence of an example of natural selection in action—it theThe emergence was such chains would mark the origin ofoccurred as a result of of life—the beginning of ancandidate enzymefirst primitive form molecular competition between ‗RNA world‘ inmixtureswhich different forms of RNA compete with each other forfree nucleotides and are subject to natural selection. Themost ‗fit‘ RNA molecules—ribozymes—the ones able toefficiently catalyze their own replication—would survive andevolve, ultimately forming modern RNA.
  25. 25. RNA Replication Without EnzymesThermodynamic arguments hypothesize that RNA became self-replicating when the temperature of the primitive seas had cooled tosomewhat below the denaturing temperature of RNA (around 70±15 °C).During the night, the surface water temperature would be below thedenaturing temperature and single-stranded RNA could act as atemplate for the formation of double-stranded RNA. During thedaylight, double-stranded RNA and DNA would absorb UV light andconvert this directly to heating of the ocean surface, raising the localtemperature enough to allow for denaturing of RNA (breakup into 2single strands). The copying process would be repeated during the coolperiod overnight.Such a temperature assisted mechanism of replication is similar topolymerase chain reaction (PCR), a routine laboratory procedure tomultiply DNA segments.Thus, RNA/DNA at the beginning of life did not require enzymes for self-replication—reproduction was instead promoted by the day/nightfluctuation of the sea-surface skin temperature about the denaturingtemperature of RNA/DNA!
  26. 26. Proteins EnterEventually, however the first replicator appeared in Earth‘sprimordial soup, RNA chains developed catalytic propertiesthat help amino acids bind together. These amino acidchains were primitive proteins that assisted the synthesis ofRNA, giving those RNA chains that had this catalyticproperty a highly selective evolutionary advantage.The ability to catalyze one step in protein synthesis(aminoacylation) of RNA has been demonstrated in the lab ina short (five-nucleotide) segment of RNA.Furthermore, competition between various RNA chains mayhave favored the emergence of chains that actedcooperatively—which could have opened a pathway to theformation of the first proto-cell!
  27. 27. The Emergence of the CellNo other ‗mechanism of life‘ has proved to be ‗more fit‘ than the cell. It isvirtually impossible to imagine any other mechanism that offers moreadvantages than those derived from the cell. For example—Molecules synthesized within a cell‘s membrane do not escape into thesurrounding environment and their concentrations are maintained atdesired levels by regulating their formation and break-up.Transport of molecules thru the cell is regulated by other cellularmolecules that act together in a coordinated way.Since the molecules that are produced by the cellular machinery arebased upon genetic information stored in the cell‘s DNA and since thesemolecules interact with the cellular membrane, those cells with the mostfavorable interaction are the ones that tend to proliferate—in other wordsgenes evolve based upon their products.This physical interaction with cell membranes allows for jointmaintenance of different genes within the cell‘s DNA and makes possibleco-evolution towards enhancing synergistic function. In essence, the cellmembrane makes it possible for all cellular components to function andevolve as one well-coordinated unit. So—how did cellular life emerge?
  28. 28. The First Cell The current cell is the result of some three and one-half billion years of evolution. The first cell was much more primitive in its structure and make-up. Proteins, if the RNA-first hypothesis is correct, did not exist, so the phospholipid bilayer that make up current cell membranes could not have been bio-synthesized and the protein channels that are responsible for molecular transport through cell membranes could not have existed.The first primitive membranes were most likely fatty lipids, whichspontaneously formed vesicles in water. Recent experiments havedemonstrated that membranes formed out of amphiphilic-lipid chainsspontaneously assemble into bi-layers and then spontaneously formvesicles. Such vesicles could have enclosed double-stranded RNAsegments which then served as compartments for their replication.
  29. 29. Recent ExperimentsCertain membranes made of simple fatty acids, such as oleic acid, wereshown to be semipermeable—small molecules like nucleotides andamino acids pass through them, but large polymers do not.Thus, vesicles that form from such fatty acids would spontaneously takein these simple monomers but would keep the larger polymers thatformed inside from passing back out.The formation of vesicles that grow—and divide—has beendemonstrated in the lab.Experimenters have incorporated short segments of single-stranded DNAinto such vesicles immersed in an aqueous solution containing DNAnucleotides. The nucleotides passed through the vesicle membranespontaneously and once inside the ‗model protocell‘ lined up itsmatching nucleotides on the DNA template and then reacted with eachother to form a complementary DNA strand. The experimentdemonstrated that the first protocells that spontaneously formedcontained some RNA-like polymer carrying genetic information andreplicated, grew and divided without the aid of any enzymes. Proteinswere not necessary.
  30. 30. Protocell Growth & Division (1) Membrane forms around RNA and nucleotides If protocell formed pass through— near volcanic complementary vent—heat provided strand forms. by heated water and (2) RNA double cooling occurs when strand completed. convection carries (3) Heat breaks protocell into cooler double strand water. Continued circulation would lead (4)cell growth until it to Cell incorporates more fatty acids breaks in two. and membrane grows. (5) Protocell divides and daughters repeat cycle.
  31. 31. The First Cell (1) Evolution starts with a protocell(3) Metabolism begins: Other emerge as a mutated RNA sequence. The(2) RNA catalysts (ribozymes)ribozymes catalyze metabolism—chains ofchemical reactions that enable protocells to tap cell membrane.ribozymes accelerate replication and strengtheninto nutrients from theenvironment and produce energy.Replication no longer needs an external stimulus!
  32. 32. The First Cell—Continued(5) Proteins appear: Eventually, natural selection would produce the cell(6) DNA appears: over: Some proteins could a more robust molecule like(4) take the enzymes would produce form bridges acrossmembrane that made proteinsentry could carry outand nutrients neededDNA to store genetic information that thenucleotides RNA would be to act asribozymes allowing selective and of main task of a variety of tasks, suchtobridge between DNA and proteins. The RNA world would be taken over by aa support chemical reactions within the cell. In the cell membrane evenas assisting with replication and strengthening addition some proteinswould act as enzymes taking over the task of ribozymes.DNA worldmore.
  33. 33. LUCA Emerges Organisms resembling modern bacteria adapt to living virtually everywhere on earth and rule unopposed for billions of years, until some of them begin to evolve into more complex organisms. Life as we know it has begun with LUCA —the Last Universal Common Ancestor.
  34. 34. Origin of the Eukaryotic Cell Protists are unicellular and are the simplest of eukaryotes. Some carry out photosynthesis, such as diatoms—a major group of algae. Others move around and act like animals— such as amoebae. … and finally some act like fungi—they act as decomposers by releasing enzymes into dead organisms that break it down — releasing materials useful to other organisms into the surrounding environment—such as water molds... which during the period of 1845–1860, due to wet growingSome simple protist was most likely the first eukaryote toseasons, infested all of Ireland‘s potato crops and led to the death of 1/3emerge on Earth, approximately 2 Gya, which eventually gaveof Ireland‘s population!rise to the entire line of eukaryotes
  35. 35. Endosymbiosis
  36. 36. Early Evolution and Rise of O2First organisms had simple metabolismAtmosphere was O 2 free, must have been anaerobicProbably chemoheterotrophs• Obtained nutrients from organic material• Obtained nutrients from inorganic material • Modern archaea appear to be close to the root of the tree of life • Obtaining energy from chemical reactions involving hydrogen, sulfur and iron compounds (all abundant on early Earth)
  37. 37. Early EvolutionNatural selection probably resulted in rapid diversificationModern DNA has enzymes that reduce the rate of mutationsRNA is not so lucky, more likely to have copying errorsHigher mutation rate in early evolution than now
  38. 38. PhotosynthesisMost important new metabolic process evolvedgraduallyOrganisms that lived close to ocean surfaceprobably developed means of absorbing sunlight(UV in particular)Once absorbed, developed method of turning itinto energy• Modern organisms of purple sulfur bacteria and green sulfur bacteria much like early photosynthetic microbes, use H2S instead of H2O for photosynthesis
  39. 39. PhotosynthesisUsing water for photosynthesis developed later, perhaps 3.5billion years agoFirst appearing in cyanobacteria (blue-green algae)By product of O2, released into atmosphereChanged the world!
  40. 40. Rise of O2O2 is highly reactiveAll initial O2 would react with rock and minerals inwaterO2 could not accumulate in atmosphere untilsurface rock was saturatedRocks 2-3 bill. Yr old called banded ironformations, show atmosphere had <1% of currentamount of O2Rock evidence suggests that O2 amounts inatmosphere began to rise about 2.0 bill. Yr agoClear evidence of O2 near current levels appearsonly 200 million yr ago• Find charcoal (fossil fuel)
  41. 41. Rise of O2Rise of O 2 would have created a crisis for lifeO2 reacts with bonds of organic materialsSurviving species avoided effects of O 2 because they lived or migrated tounderground locations• Many anaerobic microbes found in such locales today
  42. 42. Early EukaryotesFossil evidence dates to 2.1 bill. Yr agoDates to when O2 rising in atmosphereDNA evidence suggests that prokaryotes andeukaryotes separated from common ancestormuch earlierO2 played a key role in eukaryote evolution• Cells can produce energy more efficiently using aerobic metabolism than anaerobic metabolism• Adaptations of aerobic organisms could develop adaptations that required more energy that would be
  43. 43. The Cambrian ExplosionAnimal branch of the tree of lifeDifferent classifications based on body planAll known body plans made appearance in fossil record in a time span of 40million years• <1% of Earth’s age• Animal diversity began 545 mill. Yr ago
  44. 44. Colonization of LandLife flourished where liquid water existLife on land was more complicated• Had to develop means of collecting solar energy above ground and nutrients belowLife in shallow ponds or edges of lakes• Water evaporates• Natural selection favored that which could withstand periods of drought
  45. 45. Colonization of LandDNA evidence suggests that plants evolved froman algaeIt took only 75 mill. Yrs for animals to followplants out of water
  46. 46. Mass Extinctions
  47. 47. Mass ExtinctionsPossible Causes• Impacts • Impact sites found for K-T boundary • Suspected for Permian extinction 245 mill yr ago• Active volcanism • Climate change• External influence for copying errors • Increase in solar particles or radiation hitting surface • Local supernova
  48. 48. Primate EvolutionMonkeys, apes, lemurs and humans have common ancestor that lived intreesTree life• Limber arms for swinging between branches• Eyes in front of head for depth perception• Offspring would be born more helpless than other animals
  49. 49. Emergence of HumansDid NOT evolve from gorillas or monkeysShare a common ancestor that lived just a few million years ago98% of human genome is identical to genome of the chimpanzee2% difference in genome separates the success of humans verses chimps• Also indicates evolution of intelligence is complex
  50. 50. Emergence of Humans
  51. 51. Emergence of HumansAfter hominids diverged from chimps andgorillas, evolution has followed a complex pathNumerous hominids species existed, some duringthe same time period• All humans are the same speciesFirst skull fossils that are identical to modernhuman skulls dates to 100,000 yr oldOur ancestors shared the Earth withNeanderthals• Went extinct 35,000 years ago
  52. 52. Emergence of Humans
  53. 53. Cultural and Technological EvolutionHave not undergone biological evolution in40,000 years• Mutation rates are slowDramatic cultural changes• Transmission of knowledge between generations • Spoken to written word, thousands of years • agricultureTechnological evolution• Result of coupling between science and technology• About 100 years between industrial revolution to landing on the Moon and generating weapons of mass destruction

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