2 Key Events in Evolution of Life on Earth Origins of lifeOxygen revolutionEukaryotes (Endosymbiogenesis)MulticellularityCambrian ExplosionMajor Habitat TransitionsInvasion of land
3 Key Events in Evolution of Life on Earth 1.2 bya:First multicellular eukaryotes535–525 mya:Cambrian explosion(great increasein diversity ofanimal forms)500 mya:Colonizationof land byfungi, plantsand animals2.1 bya:First eukaryotes (single-celled)3.5 billion years ago (bya):First prokaryotes (single-celled)5004,0003,5003,0002,5002,0001,5001,000PresentMillions of years ago (mya)
4 Outline Origins of life Evolution of Eukaryotes Evolution of Animals Major Habitat Transitions
5 Outline Origins of life Evolution of Eukaryotes Evolution of Animals Major Habitat Transitions
6 EUKARYA BACTERIA ARCHAEA Fig. 26-21 Land plants Dinoflagellates Green algaeForamsCiliatesDiatomsRed algaeAmoebasCellular slime moldsEuglenaTrypanosomesAnimalsLeishmaniaFungiSulfolobusGreen nonsulfur bacteriaThermophiles(Mitochondrion)Figure The three domains of lifeSpirochetesHalophilesChlamydiaCOMMONANCESTOROF ALLLIFEGreensulfur bacteriaBACTERIAMethanobacteriumCyanobacteriaARCHAEA(Plastids, includingchloroplasts)
7 The 3 Domains of Life Eukarya Phylogenetic relationships based on 18S rRNA sequences, showing clade separation of bacteria, archaea, and eukaryotes.
8 Origins of Life (Herron & Freeman, Chapter 17) 4/14/2017The Earth was formed ~4.6 billion years agoEarth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide)
9 What were the earliest life forms? Prokaryotes are the earliest life forms we know of, but what might have preceded them?
10 Protobionts20 µmFig. 25-3Glucose-phosphateGlucose-phosphatePhosphataseStarchAmylasePhosphateMaltoseFigure 25.3 Laboratory versions of protobionts(a) Simple reproduction byliposomesMaltose(b) Simple metabolismProtobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structureProtobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment
11 Protobionts Replication and metabolism are key properties of life Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compoundsFor example, small membrane-bounded droplets called liposomes can form when lipids or other organic molecules are added to water
12 Self-Replicating RNA and the Dawn of Natural Selection The early genetic material might have formed an “RNA world”The first genetic material was probably RNA, not DNAEarly life forms might have consisted of protobionts with RNA as the genetic codeEarly protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection
13 RNA WorldRNA can both serve as the genetic code (hereditary material) and also perform enzymatic functions (ribozymes)RNA molecules called ribozymes have been found to catalyze many different reactionsFor example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNANatural selection in the laboratory has produced ribozymes capable of self-replication (Herron& Freeman written prior to this discovery)http://www.rsc.org/chemistryworld/News/2009/January/ asp
14 Self-Replicating RNA and the Dawn of Natural Selection Ribozymes (= RNA enzyme or catalytic RNA)RNA that can catalyze chemical reactions. Many natural ribozymes catalyze either the breaking or synthesis of phosphoester bonds in DNA or RNA. Such capacity would enable the capacity to self replicate and propagate life.The functional part of the ribosome, the molecular machine that translates RNA into proteins, is fundamentally a ribozyme
15 Figure 17.2a The ribozyme from Tetrahymena thermophila (a) The primary nucleotide sequence, which is the genotype of this catalytic RNA. This RNA is an intron (an intervening sequence between two genes) that separates two regions of the Tetrahymena genome that code for ribosomal RNA (rRNA) genes. Tom Cech and colleagues found that this sequence has the catalytic capability to splice itself out from between the two adjacent rRNAs after they have been transcribed (Kruger et al. 1982). The sequence shown here is a 413-nucleotide version, shortened from the naturally occurring form by the use of a restriction enzyme. A secondary structure of this molecule is formed when nucleotides base pair with each other as the molecule folds back upon itself during transcription from DNA. Note that in RNA, uridine (U) replaces thymidine (T), and that three types of base pairs are common: A-U, G-U, and G-C. The secondary structure shown here is drawn such that no RNA strands cross each other and, as such, does not accurately reflect how the molecule folds further into a tertiary (three-dimensional) structure.
16 Figure 17.2b The ribozyme from Tetrahymena thermophila (b) A cartoon of the catalysis performed by the Tetrahymena ribozyme in vitro, which is its phenotype. A short oligonucleotide substrate (orange) binds to the 5' end of the ribozyme (red) through complementary base pairing (green ticks). In the presence of a divalent cation, such as Mg2+, the ribozyme catalyzes the breakage of a phosphoester bond in the substrate and the ligation of the 3' fragment to its own 3' end. This "pick-up-the-tail" event can be used to discriminate catalytically active mutant sequences from less active or inactive mutants during in vitro evolution experiments (see Figure 17.4).
17 RNA WorldRNA can both serve as the genetic code and also perform enzymatic functions (ribozymes)But, can RNA self replicate? If so, it would have all components to be considered “life”
18 Figure 17.6 Test-tube selection scheme for identifying ribozymes that can synthesize RNA See text for explanation. After Bartel and Szostak (1993).
19 Figure 17.8a A ribozyme that can catalyze template-based RNA synthesis. (a) Shows a schematic diagram of a ribozyme evolved in the lab by Wendy Johnston and colleagues (2001). The RNA of which the ribozyme is made is represented by green, black, magenta, and blue lines. The RNA sequence shown in red is the template that the researchers challenged their ribozyme to copy. The sequence in orange is a primer, complementary to the beginning of the template, which gives the ribozyme a place to start. The primer is also radioactively labeled (asterisk), which allowed the researchers to detect it, and any longer strands built from it, in an electrophoresis gel.(b) Shows the full sequence of the template. The nucleotides that extend beyond the primer are numbered.(c) Shows an electrophoresis gel revealing the products that the ribozyme had made after various lengths of time. By the end of 24 hours, the ribozyme had made complete copies of the template, 14 base pairs longer than the primer. From Johnston et al. (2001).
20 Synthetic LifeHumans can now create synthetic life, by constructing a genome on the computer, synthesizing the DNA and injecting the genetic code into a cell
21 The First Single-Celled Organisms ProkaryotesBillions ofyears ago4321The First Single-Celled OrganismsThe oldest known fossils are stromatolites, rock-like structures composed of many layers of bacteria and sedimentStromatolites date back 3.5 billion years agoProkaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago
22 StromatolitesThe oldest known fossils are of cyanobacteria from Archaean rocks of western Australia, dated 3.5 billion years old
23 Prokaryotes Prokaryote: No internal membranes, organelles, nucleus Bacteria + Archaea, not a EukaryoteNot a coherent categoryArchaea and Bacteria are not closer to each other than to Eukaryotes
24 EUKARYA BACTERIA ARCHAEA Land plants Dinoflagellates Green algae ForamsCiliatesDiatomsRed algaeAmoebasCellular slime moldsEuglenaTrypanosomesAnimalsLeishmaniaFungiSulfolobusGreen nonsulfur bacteriaThermophiles(Mitochondrion)Figure The three domains of lifeSpirochetesHalophilesChlamydiaCOMMONANCESTOROF ALLLIFEGreensulfur bacteriaBACTERIAMethanobacteriumCyanobacteriaARCHAEA(Plastids, includingchloroplasts)
25 Figure 17.18 An estimate of the phylogeny of all living organisms This tree is based on the analysis of nucleotide sequences of small-subunit rRNAs. The photos show E. coli representing the Bacteria, Methanococcus jannaschii representing the Archaea, and an ant and an aphid representing the Eucarya. From Woese (1996).
26 Incredible Metabolic Diversity Some can use inorganic chemicals as the electron donor in the redox reaction of making ATP (respiration)(animals use organic carbon, food, as the electron donor)Eukaryotes can use only oxygen as the terminal electron acceptor when making ATP in the electron transport chain (aerobic respiration)Prokaryotes can use NO3-, NO2-2, SO4-2, etc. (anaerobic respiration)Some are photosyntheticSome can fix nitrogen, converting atmospheric N2 NH3
27 Outline Origins of life Evolution of Eukaryotes Evolution of Animals Major Habitat Transitions
28 Oxygen Revolution “Oxygen revolution” from 2.7 to 2.2 billion yrs ago AtmosphericoxygenBillions ofyears ago4321Oxygen Revolution“Oxygen revolution” from 2.7 to 2.2 billion yrs agoO2 in the atmosphere was generated by photosynthetic activity of cyanobacteriaCyanobacteria were the first organisms to use H2O (instead of H2S or other compounds) as a source of electrons and hydrogen for fixing CO2Photosynthesis: 6 CO2 + 6 H2O + photons → C6H12O6 + 6 O2
29 Oxygen RevolutionO2 allows the production of more energy and more efficient metabolismC6H12O O2 → 6 CO H2O, ΔG = kJ per mole of C6H12O6Aerobic metabolism: 36 ATP molecules per glucoseAnaerobic metabolism: 2 ATP molecules per glucoseAerobic and anaerobic metabolism share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation.However, Oxygen is also ToxicOxygen is highly toxic to many speciesElectrons that escape from the electron transport chain can bind to oxygen and produce reactive oxygen speciesReactive oxygen species (H2O2, O2−) can damage DNA and other molecules
30 Evolutionary History dictates type of Metabolism The fact that much of prokaryotic life evolved BEFORE the oxygen revolution is reflected in the diverse forms of anaerobic metabolismMost bacteria do not require oxygen, and for many it is toxicMost bacteria use other molecules as the terminal electron acceptor when making ATPThe fact eukaryotic life evolved AFTER the oxygen revolution is reflected in the fact that all eukaryotes use aerobic metabolismThey use oxygen as the terminal electron acceptor in energy production
31 Lynn MargulisEndosymbiotic TheoryWrote the seminal paper“The Origin of Mitosing Eukaryotic Cells”1966Certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts.Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteriaBoth mitochondria and plastids contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (in being circular in shape, its size, and DNA composition)
32 Covered in Chapter 15, Freeman and Herron Figure 25.9 A model of the origin of eukaryotes through serial endosymbiosisProbably ArchaeaThe organelles (mitochondria and chloroplasts) greatly aid in energy production in eukaryotes.
33 Given the evolutionary history of endosymbiosis (or endosymbiogenesis) leading to eukaryotes, eukaryotes will inevitably share traits with both archaea and bacteriaTable 27.2
34 Outline Origins of life Evolution of Eukaryotes Evolution of Animals Major Habitat Transitions
36 (3) Origins of Animal Phyla Cambrian Explosion (570 MYA) 4/14/2017(3) Origins of Animal PhylaCambrian Explosion (570 MYA)Burgess Shale
37 Radiations: Extinctions???? Cambrian Explosion 4/14/2017(1) Precambrian-Paleozoic (570 MYA) BoundaryCambrian ExplosionRadiations:Evolution of hard body partsDiversification of body formsRadiation of InvertebratesExtinctions????
41 What is an Animal? Multicellular (metazoan) 4/14/2017What is an Animal?Multicellular (metazoan)Heterotrophic (eat, not photo or chemosynthetic)EukaryoteNo Cell Walls, have collagenNervous tissue, muscle tissueParticular Life History-developmental patterns(this lecture)
42 Origins of Multicellularity 4/14/2017Origins of MulticellularityIt is thought that metazoans arosefrom Colonial flagellate protistsOxygen Revolution:allow higher metabolic ratelarger body sizepowered motion
43 Cells of sponges are similar to flagellate protists 4/14/2017Cells of sponges are similar to flagellate protists
44 Doushantuo formation Southern China 4/14/2017Doushantuo formation Southern China570 million years agoClusters of cells (embryos?)SpongesFossilized metazoan embryo at the 256 cell stage
51 Was the Cambrian Explosion an Explosion? 4/14/2017Was the Cambrian Explosion an Explosion?Geology: YESSudden Appearanceof Animal FossilsGenetics: NODNA DivergencesPredate Cambrian
52 Precambrian Annelida Arthropoda Mollusca Agnatha Gnathostomata 4/14/2017AnnelidaMolluscaAgnathaMillion Years AgoArthropodaEchinodermataGnathostomata200Cambrian600Based on phylogeny of animals based on DNA sequence data, the radiation of animals predates the geological record of the Cambrian Explosion8001000Precambrian1200Wray et al. 19961400
53 Fossil Record vs Molecular Clock 4/14/2017Fossil Record vs Molecular ClockMolecular clock and fossil record are not always congruentFossil record is incomplete, and soft bodied species are usually not preservedMutation rates can vary among species (depending on generation time, replication error, mismatch repair)But they provide complementary informationFossil record contains extinct species, while molecular data is based on extant taxaMajor events in fossil record could be used to calibrate the molecular clock
54 Fossil Record vs Molecular Clock 4/14/2017Fossil Record vs Molecular ClockSo it’s highly likely that the divergence of animal lineages (phyla) was more gradual than the fossil record would suggest and that the time of divergence happened earlier
55 Evolution of Animal Body Plans 4/14/2017Evolution of Animal Body PlansTrue TissuesTissue Layers (Diplo vs Triploblasts)Body SymmetryEvolution of body cavity (Coelom)Evolution of Development
57 Of course, evolution continued after the Cambrian Explosion 4/14/2017Of course, evolution continued after the Cambrian ExplosionBut major body plans evolved within a short period of time
58 How could this happen? (genetic mechanism?) 4/14/2017How could this happen? (genetic mechanism?)
59 4/14/2017Genetic Mechanisms?We talked about how selection and drift could result in speciationThe formation of novel body plans calls for changes (mutation, selection, drift) at loci that cause fundamental morphological changes
60 The Evolution of Development (Freeman& Herron, Chapter 19) The tremendous increase in diversity during the Cambrian explosion appears to have been caused by evolution of developmental genesChanges in developmental genes can result in radically new morphological formsDevelopmental genes control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult
61 Changes in a few regulatory genes could have big impacts 4/14/2017Changes in a few regulatory genes could have big impactsMost new features of multicellular organisms arise when preexisting cell types appear at new locations or new times in the embryo.Changes in the specification of cell fates is a major mechanism for the evolution of different organismal forms.
62 Example of Evolution Developmental Genes that could result in radically new body forms Hox genes are a class of homeotic genes that provide positional information during developmentIf Hox genes are expressed in the wrong location, body parts can be produced in the wrong locationFor example, in crustaceans, a swimming appendage can be produced in a segment instead of a feeding appendage
63 Mutations in a Hox gene causing legs to grow out of the head In this case, the identity of one head segment has been changed to that of a thoracic segment.Figure 19.2b Homeotic mutants in Drosophila(b) Mutations in the antp gene can produce adult flies with legs growing from the head. In this case, the identity of one head segment has been changed to that of a thoracic segment.
64 Hox Clusters Gene family formed by gene duplication events 4/14/2017Hox ClustersGene family formed by gene duplication eventsHox genes, usually found clustered next to each other along animal chromosomesHox gene products are regulatory proteins that bind to DNA and control the transcription of other genes
65 Figure 19.1 Hox genes in Drosophila In these diagrams,T1-T3 and A1-A8 indicate the three thoracic and eight abdominal segments, respectively. The int, mx, and 1a regions of the embryo form head structures. The bottom part of the figure shows the relative locations of genes in the HOM cluster. Each gene is color coded to indicate where it is expressed. Expression of pb, for example, influences the identity of cells that make the proboscis or mouthparts of the fly; both are shaded green. From Gerhart and Kirschner (1997).
67 Christiane Nüsslein-Volhard and Sean Carroll 4/14/2017Christiane Nüsslein-Volhard and Sean Carroll
68 4/14/2017Hox ClustersHox genes specify the developmental fate of individual regions within the body (modules), and usually the genes are activated and expressed in the body in the same order as their position in the clusterThese genes regulate timing and location of gene expression during development
70 Figure 19.3 Hox genes from various animal phyla The boxes on the right represent homeotic loci that have been found in each of the taxa on the tree. Genes that are thought to be homologous are lined up vertically; the vertical white lines group genes that are clearly homologous. Modified from Valentine et al. (1996), de Rosa et al. (1999), and Hoegg and Meyer (2005).
71 Evolution of Hox clusters 4/14/2017Evolution of Hox clustersHOX-clusters undergo essential rearrangements in evolution of main taxaDuplication, deletion, divergence of the genes lead to differentiation in body plansOther regulatory genes/gene families are also important
73 Evolutionary changes in Hox Genes New morphological forms likely come from gene duplication events that produce new developmental genesA possible mechanism for the evolution of six- legged insects from a many-legged crustacean ancestor has been demonstrated in lab experimentsSpecific changes in the Ubx gene have been identified that can “turn off” leg development
74 Hox gene 6 Hox gene 7 Hox gene 8 Ubx About 400 mya Drosophila Artemia Figure Origin of the insect body planDrosophilaArtemia
75 Figure 19.4 The arthropod radiation Note the diversity of form and function in the segments and limbs of these representative taxa. Onychophorans are the closest living relative of arthropods.
76 Differences in Hox gene expression distinguish the various arthropod segmentation patterns Figure 19.5 Differences in Hox gene expression distinguish the various arthropod segmentation patternsThe Hox cluster for all arthropods is similar, represented by the color-coded boxes on the left. The evolutionary tree shows the relationships between some of the major arthropod groups and the Onychophora. The diagrams on the right show where the various Hox genes are expressed in representatives of these taxa. Appendages such as antennae, mouthparts, wings, and legs are shown for some segments. Each unique body plan is associated with a unique pattern of Hox gene expression. From Knoll and Carroll (1999).
77 Evolution of Vertebrates (Phylum Chordata) Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genesTwo duplications of Hox genes are thought to have occurred in the vertebrate lineageThese duplications may have been important in the evolution of new vertebrate characteristics
78 Polyploidization is probably the single most important mechanism for the evolution of major lineages and for speciation in plantsMultiple rounds of polyploidization might have occurred during the early evolution of vertebrates
79 Hypothetical vertebrate ancestor (invertebrate) with a single Hox clusterFirst HoxduplicationHypothetical earlyvertebrates (jawless)with two Hox clustersSecond HoxduplicationFigure Hox mutations and the origin of vertebratesVertebrates (with jaws)with four Hox clusters
80 Outline Origins of life Evolution of Eukaryotes Evolution of Animals Major Habitat Transitions
81 Habitat Transitions Freshwater invasions Invasion of Land Key innovations with the invasion of land
82 Geological record of timing of terrestrial invasions
83 Example of extraordinary environmental change: Transitions betweenSaltwater Freshwater andWater LandConstitute Major Evolutionary Transitions in the History of LifeSo a striking example of an extraordinary environmental change:are Transitions between major habitat boundariessuch as the invasion from saltwater freshwater habitats,and the invasion from aquatic habitats onto landThese major habitat transitions Constitute Major Evolutionary Transitions in the History of Life
84 Sea Fresh LandProtista X XPorifera X XCnideria X XCtenophora XPlatyhelminthes X X XNemertea X X XRotifera X XGastrotricha X XKinorhyncha XNematoda X XNematomorpha X XEntoprocta X XAnnelida X X XMollusca X X XPhoronida XBryozoa X XBrachiopoda XSipunculida XEchiuroida XPriapulida XTardigrada X XOnychophora XArthropoda X X XEchinodermata XChaetognatha XPogonophora XHemichordata XChordata X X XLife evolved in the sea, and freshwater and terrestrial habitats impose profound physiological challenges for most taxa (Hutchinson 1957; Lee & Bell 1999)Of ~40 Animal Phyla, only 16 invaded fresh water, and only 7 invaded landAs you all know, Life evolved in the sea, and freshwater and terrestrial habitats impose profound physiological challenges for most taxaThe bulk of animal diversity is in the sea, and of the ~40 Animal Phyla, only 16 have been able to invade fresh water, and only 7 have been able to invade land
85 Habitat Invasions Sea Fresh water Soil Land Protista X X X 4/14/2017Sea Fresh water Soil LandProtista X X XPorifera X XCnideria X XCtenophora XPlatyhelminthes X X X XNemertea X X XRotifera X X XGastrotricha X XKinorhyncha XNematoda X X XNematomorpha X XEntoprocta X XAnnelida X X X XMollusca X X X XPhoronida XHabitat Invasions
86 Habitat Invasions Sea Fresh water Soil Land Bryozoa X X Brachiopoda X 4/14/2017Sea Fresh water Soil LandBryozoa X XBrachiopoda XSipunculida XEchiuroida XPriapulida XTardigrada X X XOnychophora X XArthropoda X X X XEchinodermata XChaetognatha XPogonophora XHemichordata XChordata X X X XHabitat Invasions
87 4/14/2017OsmoregulationThe regulation of water and ions poses among the greatest challenges for surviving in different habitats.Marine habitats pose the least challenge, while terrestrial habitats pose the most. In terrestrial habitats must seek both water and ions (food).In Freshwater habitats, ions are limiting while water is not.
88 The invasion of land occurred relatively recently (late in the history of life on earth)
89 The Colonization of Land Precambrian: colonization of land by bacteriaLate Ordovician–Late Devonian: Terrestrial ecosystems of plants, fungi, arthropods and eventually vertebratesLate Silurian: vascular plantsDevonian: evidence of interactions among groups (fungal-plant symbiosis, arthropod herbivory)Late Devonian: Two major megaclades that were major participants in the Cambrian explosion appear on landThe lophotrochozoans: gastropod and bivalve mollusks, oligochaete annelids, rotifers, etc.The ecdysozoans: arthropods, nematodes, etc.Invasion of land by crustaceans origin of insects (Hexapods)Early Carboniferous: radiation of tetrapods (vertebrates with 4 legs)
91 The invasion of land by crustaceans (terrestrial crustaceans = insects)
92 QuestionsWhat are some of the key biological revolutions that occurred during the history of life on earth, and how did they come about?Why is it hypothesized that early life on earth was in the form of RNA?How did the Oxygen Revolution come about?What are the implications of a high oxygen atmosphere for organismal physiology?How were Eukaryotes created?What are animals?About when did animals evolve? Is there congruence between fossil estimates versus molecular clock estimates of the timing?How did animals diversify (Cambrian Explosion)?Where did life evolve (land, sea)?About when did organisms begin to live on land?What are some challenges of living on land (relative to the sea)?
93 Which of the following is MOST likely to be true regarding evolutionary relationships among the major domains of life?(A) Bacteria and Archaea are both grouped into the category of “Prokaryotes” because they are more closely related to each other than to Eukaryotes(B) Archaea are ancestral to all other forms of Life(C) The Oxygen Revolution defines the emergence of life on Earth(D) Eukaryotes arose from a symbiotic relationship among prokaryotes(E) Eukaryotes evolved from Prokaryotes through Horizontal Gene Transfer
94 2. Which of the following is Least likely to be true regarding Prokaryotic Metabolism and the Oxygen Revolution?The oxygen revolution led to the proliferation of diverse types of prokaryotic metabolism(B) The oxygen revolution enabled the preponderance of more efficient metabolism that could produce more ATP per unit glucose(C) The oxygen revolution altered the atmosphere of the Earth(D) The oxygen revolution created an inhospitable habitat for many classes of prokaryotes that use molecules other than oxygen as their terminal electronic acceptor for energy production(E) The oxygen revolution was caused as a byproduct of photosynthetic activity of cyanobacteria
95 3. Which of the following does NOT make RNA a good candidate for early life on earth? (a) RNA could act as an enzyme (b) RNA could serve as the genetic code (c) RNA is much more stable than DNA in our current atmosphere (d) RNA can catalyze chemical reactions that break or synthesize phosphodiester bonds in RNA or DNA (e) Recent studies suggest that RNA is capable of self replication
96 4. Which is the following is NOT a consequence of the Oxygen Revolution? (a) The ability of organisms to metabolically produce more ATP by using oxygen as the terminal electron acceptor during respiration (b) Eukaryotes, which evolved after the Oxygen Revolution, all use aerobic metabolism (c) The transformation of the earth's atmosphere ~2.5 billion years ago from one that was chemically reducing to one that is oxidizing (d) The heightened risk of DNA and cellular damage in organisms through oxygen free radicals (e) The proliferation of bacteria that use oxygen as the terminal electron acceptor during respiration, such that nearly all bacterial species are aerobic