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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 the spontaneous 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 consistent with other widely held cultural and religious beliefs of the time and people believed it up through the 1900‘s!
  3. 3. Pasteur Refutes Spontaneous Generation Spontaneous 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 it became pliable, and bent it into the shape of an S. Air could enter the flask, but airborne microorganisms could not - they would settle by gravity in the neck—no microorganisms grew. When Pasteur tilted the flask so that the broth reached the lowest point in the neck, where any airborne particles would have settled, the broth rapidly became cloudy with life. Pasteur had both refuted the theory of spontaneous generation and convincingly 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 improbable event—say it has a chance of one in a million (10-6 / yr) of happening in any particular year. During a human lifetime, roughly 100 years, the chances of it happening would 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 will happen at least once is very close to one. How close? The chance per year that it doesn‘t happen is 999,999/1,000,000. The chance that it doesn‘t happen in 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 one which is highly improbable to an event which is now almost inevitable! Suppose, instead, that the probability of the spontaneous origin of life in any given 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 existence would now be about 10-12 per year x 4.5 x 109 years = 4.5 x 10-3, a very small number. If this were the case, we are here only through an extraordinary stroke of luck!
  5. 5. Tornedo in the Junkyard Many scientists have argued that the latter scenario is more likely. They base this supposition on a specious argument, such as calculating the probability of assembling a protein out of amino acids in a purely random fashion. Consider a protein consisting of 100 amino acids. Any ‗slot‘ in the chain can be occupied by one of twenty possibilities. The random assembly of a 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 the other organic molecules that make up life boggles the mind! The distinguished astrophysicist, Fred Hoyle, estimated that the odds of cellular life arising were about one in 1040000! He commented… “The chance that higher life forms might have emerged in this way is comparable to the chance that a tornado sweeping through a junkyard might assemble a Boeing 747 from the materials therein. …Life as we know it is, among other things, dependent on at least 2000 different enzymes. How could the blind forces of the primal sea manage to 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 form The evolutionarymolecular building Dawkins saysformation is a function randomly out of biologist, Richard blocks. Their in ―The Selfish Gene,‖ — the laws of chemistry and biochemistry. The process is decidedly not of ―At someThe formation wasremarkable molecule was formed by accident. random. point a particular a multi-step process in which ever more We will call itassemblages emergednot necessarily have been the biggest complicated the Replicator. It may from slightly simpler ones that or most complicated molecule a much less it had the‗first molecule‘ which preceded them, but there was around, but complex extraordinary property of being able to make copiesthough its formation might be very might be characterized as alive, even of itself. This may seem like a unlikely‗an accident.‘ termed sort of accident to happen. So it was. It was exceedingly improbable. In the lifetime of a man, things that are improbable may be treated for practical purposes as impossible. That is why you will never win the big prize on the football pools. But in our human estimates of what is improbable and what is not, we are not used to dealing in hundreds of millions of years. If you filled in pools coupons every week for a hundred million years, you would very likely win several jackpots.‖ Once a Replicator formed, mutations would occur and natural selection would begin—having a strong say in the way subsequent organic molecules 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 example His friend, perhaps possessing a deeper understanding of statistics Herein help— fallacy behind the simplistic probability calculation carried might lays the says,by Hoylegolfer many others). If wegolf. to turn back his ball to 4.5 billion out “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 of grass!hole. starttakes out hisbillionwould exactly the same yardsof life based years 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 smacked the ballthe fairway. He then walks down the molecule capable ofhole and upon exactly the same information-storing fairway towards the one of down into a sand trap, it’s 100% probable that it would land on them!”his ball lying in the grass. He exclaims something different? Bill, will replication emerge on Earth—or would it be finds to his friend, “Wow! Put another way, if we find life elsewhere in the universe—or even our own solarlook at that? What are the chances that my storage basis and this you system—will it have the same information ball would land on replicate in the sameprobability must be a billion to one— that’sare blade of grass? The way as ours? Not likely—many pathways almost available 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 some kind of life—given the right conditions such as exist on Earth—could indeed be 100%.
  8. 8. Spontaneous Emergence of Life—Yes! A current school of thought suggests that life is an inevitable consequence of ―cosmic evolution,‖ shaped by the fundamental laws of physics. All around us we see evidence of self-organization that has been taking place on a cosmic scale since the Universe began. Superclusters, clusters and galaxies have formed out of stars and giant clouds of gas and dust—pulled together by the action of gravity. Stars and their solar systems formed in the same way. This large scale organization of matter was driven by the energy expended during gravitational contraction. the Sun and other stars, gravitational In the case of contraction heated them enough for nuclear fusion to start in their cores—unleashing another source of energy that now bathes all the planets in any solar systems that accompany them. This energy source ultimately drove the emergence of complexity 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) suitable the 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 period different 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 waters spontaneously, built from a random conglomeration of whatever happened to be around. …The collective system does possess a stunning property not possessed by any of its parts. It is able to reproduce and evolve. The collective system is alive. Its parts are just chemicals. …If all this is true, life is vastly more probable than we have supposed. Not only are we at home in the Universe, but we are far more likely to share it with unknown companions.‖
  10. 10. Stepping Towards Complexity 1. Synthesis of simple organic molecular building blocks 2. Polymerization—assembly of simple building blocks into long chains. 3. Origin of the first replicator—the RNA world 4. The emergence of genetically encoded protein synthesis 5. The emergence of DNA as the vehicle of information storage 6. Last Universal Common Ancestor (LUCA)—the first cell 7. Origin of the eukaryotic cell 8. Origin of multicellular life— specialization 9. The Cambrian Explosion 10.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 of or the organics formed slowly in GMC‘s and then fell to H2O, CO2, CO, CH4, formaldeh Earth, like manna from heaven…and the time required for The icy mantles facilitate yde (H2CO) (which might play formation 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—if captured by Earth‘s gravity—would float in the atmosphere for many years―ultimately falling to the surface—and the molecules would remain intact! Thus the delivery of cometary material 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! Murchison meteorite, which Complex organic molecules formed in fell near the town space are able to survive passage of through Earth‘s atmosphere as well as Murchison, Austral ground impact. ia in 1969 Slight excess of L over D-amino acids → processing on dust grains by circularly polarized light → explains chirality of amino acids and other biotic compounds!
  14. 14. This 4.5 billion-year-old rock, labeled meteorite ALH84001, is believed to have once been a part of Mars and to contain fossil evidence that primitive life may have existed on Mars more than 3.6 billion years ago. The rock is a portion of a meteorite that was dislodged from Mars by a huge impact about 16 million years ago and that fell to Earth in Antarctica 13,000 years ago. The meteorite was found in Allan Hills ice field, Antarctica, by an annual expedition of the National Science Foundation's Antarctic Meteorite Program in 1984. It is preserved at the Johnson Space Center's 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 living organisms did not require proteins at all. Could some simple single- stranded RNA-like polymer have spontaneously formed that could catalyze its own replication without the aid of proteins? Eventually, such a structure would have evolved the ability to make proteins that would greatly accelerate the replication process. Later on DNA appeared as a more robust form of information storage, thanks to its superior chemical stability. This idea is known as the ―RNA world hypothesis.‖
  18. 18. Evidence for RNA First It received support with the discovery of ribozymes, which are types of RNA that act as catalysts. RNA enzymes are ‗living fossils‘, still found in today's DNA-based life. In 2001, the 3-d structure of the ribosome was deciphered—they consist of RNA and proteins—but the key catalytic sites of ribosomes were revealed to be composed of RNA. The proteins are of peripheral functional importance. The formation of the peptide bond that binds amino acids together into proteins, is now known to be catalyzed by an adenine residue in the rRNA of the ribosome: thus, the ribosome is a ribozyme. This finding suggests that RNA molecules were most likely capable of generating the first proteins, i.e., they came first. The existence of the ribosome supports the hypothesis that a simple RNA replicator appeared before DNA because the RNA in the ribosome contains in its structure ‗fossil‘ evidence of the existence of an earlier ‗RNA world.‘ The hypothesis received further support from experiments in which ribozymes were produced in the laboratory that catalyze their own synthesis—such as the RNA polymerase ribozyme.
  19. 19. Polymerization of RNA A dilemma—the sugar of one nucleotide must form a bond spontaneously with the phosphate of the next…and so on. However, this process of polymerization in water is not thermodynamically favored since it involves the release of water into surrounding water and thus the process will not happen spontaneously—in fact, the reaction goes the other way—polymers in water will undergo hydrolysis and break down eventually into individual monomers! However, if there is some external energy source to drive the binding of monomers—along with the presence of some catalyst to facilitate the process—then polymerization 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 layer containing aluminum sandwiched between two silicate layers. They concentrate nucleotides and provide metal ions to catalyze their polymerization.
  21. 21. Thermodynamic Argument for the Emergence of RNA and DNA Life is ‗an irreversible thermodynamic process‘ which arises and persists to produce entropy. The production of entropy is not merely incidental to the process of life, but in fact it is the very reason for its existence. The absorption and transformation of sunlight into heat is the most important irreversible process generating entropy in the biosphere. Thus, life most probably began as a catalyst for the absorption and dissipation of sunlight on the surface of Archaean seas. The resulting heat could then be 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 for absorbing the intense UV that penetrated the early atmosphere and are remarkably rapid in transforming this light into heat in the presence of liquid water. Thus, the origin and evolution of life were inseparable from water and the water cycle and would have resulted from the natural thermodynamic imperative of increasing the entropy production of the Earth in its interaction with its solar environment.
  22. 22. Entropy—an Aside The Second Law of Thermodynamics tells us which configuration came first—Nature tends from order to disorder in isolated systems—a statement that paraphrases the Second Law of Thermodynamics—or more technically— the entropy of isolated systems always increases.
  23. 23. Entropy and Life An infusion of energy is needed to generate and support any system that is more complex than its surroundings—order has to be maintained. Earth a system morelong wavelength photons back into space Life is emits more complex than its surroundings. Its order needs to than short wavelengthenergy—fromreceives from the Sun.sink‘. be maintained by a flow of photons it a ‗hot source‘ to a ‗cooler The Sun, the Earth and surrounding of Thermodynamics,an isolated According to the Second Law space—or universe— is this process increasesthe entropy of the Universe. system—more entropy must be returned into space from the Earth than it receives from the Sun—in other words, the organization of complexity in Earth‘s biosphere requires the removal of entropy.
  24. 24. The First Replicator Simple nucleotides could have ‗polymerized‘ forming short strands of RNA in a variety of possible ways. Many combinations that formed would break apart because the strength of their bonds wasn‘t very great and the energy required to break the nucleotide chain was correspondingly small. However, certain base pair sequences have catalytic properties that strengthen the bonds of a forming chain, allowing them to stay together for longer periods of time. Such chains The idea of emergence of RNA ribozymes has recently been given would grow longer Lincoln andmore matching nucleotides faster until credence by Tracy and attract Gerald Joyce of the Scripps Research they eventually formed two RNA ribozymes in the lab from individual RNA Institute who ‗evolved‘ at a faster rate than they broke down. Thus, they would begin catalyzed the on early Earth. each other. The evolution of strands that to proliferate reproduction of these two RNA ribozymes capable of self-replication took about an hour. Theiremergence of an example of natural selection in action—it the The emergence was such chains would mark the origin of occurred as a result of of life—the beginning of ancandidate enzyme first primitive form molecular competition between ‗RNA world‘ in mixtures which different forms of RNA compete with each other for free nucleotides and are subject to natural selection. The most ‗fit‘ RNA molecules—ribozymes—the ones able to efficiently catalyze their own replication—would survive and evolve, ultimately forming modern RNA.
  25. 25. RNA Replication Without Enzymes Thermodynamic arguments hypothesize that RNA became self- replicating when the temperature of the primitive seas had cooled to somewhat below the denaturing temperature of RNA (around 70±15 °C). During the night, the surface water temperature would be below the denaturing temperature and single-stranded RNA could act as a template for the formation of double-stranded RNA. During the daylight, double-stranded RNA and DNA would absorb UV light and convert this directly to heating of the ocean surface, raising the local temperature enough to allow for denaturing of RNA (breakup into 2 single strands). The copying process would be repeated during the cool period overnight. Such a temperature assisted mechanism of replication is similar to polymerase chain reaction (PCR), a routine laboratory procedure to multiply DNA segments. Thus, RNA/DNA at the beginning of life did not require enzymes for self- replication—reproduction was instead promoted by the day/night fluctuation of the sea-surface skin temperature about the denaturing temperature of RNA/DNA!
  26. 26. Proteins Enter Eventually, however the first replicator appeared in Earth‘s primordial soup, RNA chains developed catalytic properties that help amino acids bind together. These amino acid chains were primitive proteins that assisted the synthesis of RNA, giving those RNA chains that had this catalytic property a highly selective evolutionary advantage. The ability to catalyze one step in protein synthesis (aminoacylation) of RNA has been demonstrated in the lab in a short (five-nucleotide) segment of RNA. Furthermore, competition between various RNA chains may have favored the emergence of chains that acted cooperatively—which could have opened a pathway to the formation of the first proto-cell!
  27. 27. The Emergence of the Cell No other ‗mechanism of life‘ has proved to be ‗more fit‘ than the cell. It is virtually impossible to imagine any other mechanism that offers more advantages than those derived from the cell. For example— Molecules synthesized within a cell‘s membrane do not escape into the surrounding environment and their concentrations are maintained at desired levels by regulating their formation and break-up. Transport of molecules thru the cell is regulated by other cellular molecules that act together in a coordinated way. Since the molecules that are produced by the cellular machinery are based upon genetic information stored in the cell‘s DNA and since these molecules interact with the cellular membrane, those cells with the most favorable interaction are the ones that tend to proliferate—in other words genes evolve based upon their products. This physical interaction with cell membranes allows for joint maintenance of different genes within the cell‘s DNA and makes possible co-evolution towards enhancing synergistic function. In essence, the cell membrane makes it possible for all cellular components to function and evolve 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, which spontaneously formed vesicles in water. Recent experiments have demonstrated that membranes formed out of amphiphilic-lipid chains spontaneously assemble into bi-layers and then spontaneously form vesicles. Such vesicles could have enclosed double-stranded RNA segments which then served as compartments for their replication.
  29. 29. Recent Experiments Certain membranes made of simple fatty acids, such as oleic acid, were shown to be semipermeable—small molecules like nucleotides and amino acids pass through them, but large polymers do not. Thus, vesicles that form from such fatty acids would spontaneously take in these simple monomers but would keep the larger polymers that formed inside from passing back out. The formation of vesicles that grow—and divide—has been demonstrated in the lab. Experimenters have incorporated short segments of single-stranded DNA into such vesicles immersed in an aqueous solution containing DNA nucleotides. The nucleotides passed through the vesicle membrane spontaneously and once inside the ‗model protocell‘ lined up its matching nucleotides on the DNA template and then reacted with each other to form a complementary DNA strand. The experiment demonstrated that the first protocells that spontaneously formed contained some RNA-like polymer carrying genetic information and replicated, grew and divided without the aid of any enzymes. Proteins were 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 of chemical reactions that enable protocells to tap cell membrane. ribozymes accelerate replication and strengtheninto nutrients from the environment 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 across membrane that made proteinsentry could carry outand nutrients needed DNA to store genetic information that thenucleotides RNA would be to act as ribozymes allowing selective and of main task of a variety of tasks, such tobridge between DNA and proteins. The RNA world would be taken over by a a support chemical reactions within the cell. In the cell membrane even as assisting with replication and strengthening addition some proteins would act as enzymes taking over the task of ribozymes. DNA world more.
  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 growing Some simple protist was most likely the first eukaryote to seasons, infested all of Ireland‘s potato crops and led to the death of 1/3 emerge on Earth, approximately 2 Gya, which eventually gave of Ireland‘s population! rise to the entire line of eukaryotes
  35. 35. Endosymbiosis
  36. 36. Early Evolution and Rise of O2 First organisms had simple metabolism Atmosphere was O 2 free, must have been anaerobic Probably 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 Evolution Natural selection probably resulted in rapid diversification Modern DNA has enzymes that reduce the rate of mutations RNA is not so lucky, more likely to have copying errors Higher mutation rate in early evolution than now
  38. 38. Photosynthesis Most important new metabolic process evolved gradually Organisms that lived close to ocean surface probably developed means of absorbing sunlight (UV in particular) Once absorbed, developed method of turning it into 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. Photosynthesis Using water for photosynthesis developed later, perhaps 3.5 billion years ago First appearing in cyanobacteria (blue-green algae) By product of O2, released into atmosphere Changed the world!
  40. 40. Rise of O2 O2 is highly reactive All initial O2 would react with rock and minerals in water O2 could not accumulate in atmosphere until surface rock was saturated Rocks 2-3 bill. Yr old called banded iron formations, show atmosphere had <1% of current amount of O2 Rock evidence suggests that O2 amounts in atmosphere began to rise about 2.0 bill. Yr ago Clear evidence of O2 near current levels appears only 200 million yr ago • Find charcoal (fossil fuel)
  41. 41. Rise of O2 Rise of O 2 would have created a crisis for life O2 reacts with bonds of organic materials Surviving species avoided effects of O 2 because they lived or migrated to underground locations • Many anaerobic microbes found in such locales today
  42. 42. Early Eukaryotes Fossil evidence dates to 2.1 bill. Yr ago Dates to when O2 rising in atmosphere DNA evidence suggests that prokaryotes and eukaryotes separated from common ancestor much earlier O2 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 Explosion Animal branch of the tree of life Different classifications based on body plan All known body plans made appearance in fossil record in a time span of 40 million years • <1% of Earth’s age • Animal diversity began 545 mill. Yr ago
  44. 44. Colonization of Land Life flourished where liquid water exist Life on land was more complicated • Had to develop means of collecting solar energy above ground and nutrients below Life in shallow ponds or edges of lakes • Water evaporates • Natural selection favored that which could withstand periods of drought
  45. 45. Colonization of Land DNA evidence suggests that plants evolved from an algae It took only 75 mill. Yrs for animals to follow plants out of water
  46. 46. Mass Extinctions
  47. 47. Mass Extinctions Possible 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 Evolution Monkeys, apes, lemurs and humans have common ancestor that lived in trees Tree 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 Humans Did NOT evolve from gorillas or monkeys Share a common ancestor that lived just a few million years ago 98% of human genome is identical to genome of the chimpanzee 2% 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 Humans After hominids diverged from chimps and gorillas, evolution has followed a complex path Numerous hominids species existed, some during the same time period • All humans are the same species First skull fossils that are identical to modern human skulls dates to 100,000 yr old Our ancestors shared the Earth with Neanderthals • Went extinct 35,000 years ago
  52. 52. Emergence of Humans
  53. 53. Cultural and Technological Evolution Have not undergone biological evolution in 40,000 years • Mutation rates are slow Dramatic cultural changes • Transmission of knowledge between generations • Spoken to written word, thousands of years • agriculture Technological 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