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EAS 2200 Spring 2011 The Earth System Lecture 15

Evolution of the Atmosphere and II The RNA World One , RNA (ribonucleic acid), is the prime suspect because it can Store, transmit and duplicate genetic information (like DNA) Catalyze chemical reactions (unlike DNA) Furthermore, certain riboenzymes have been shown to catalyze their own synthesis under specific conditions. This, however, is one idea of many. It remains controversial and there is not yet a consensus on this matter. From to as we know it requires a barrier between itself and its surroundings. Present walls are composed of a bilayer of -each of which has a hydrophobic and hydrophilic end. In water, phospholipids spontaneously arrange such that the tails are shielded from the water, resulting in the formation of structures such as bilayers, vesicles, and . Fatty acids (just hydrocarbon chains with a COOH on the end) have the same properties and could have played this role in the first protocells. Fatty acids formation could be catalyzed by clays in hydrothermal systems. A vesicle would be permeable to , so the material needed to synthesize additional RNA could accumulate in the cell.

The would grow as it accreted micells and accumulated nucleotides, eventually

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becoming distorted and splitting. Learn more and get videos at http://exploringorigins.org/. The ‘metabolism first’ hypothesis Some scientists, while agreeing it preceded DNA, think RNA is still far too complex for “first life”. One line of thinking is “ first” A contained (perhaps in a vesicle) chemical system or cycle that exploits an source (perhaps redox) to sustain the cycle, grow, and reproduce. Catalysis of reactions is key. Example: the combination and separation of amino acids in the presence of metal sulfide catalysts with energy supplied by the oxidation of to carbon dioxide.

Transition out of the RNA World RNA has some disadvantages: Not as chemically stable as or DNA Not as good a catalyst as proteins Not as good at storing information as DNA. Single-stranded RNA (if was indeed the basis of first life) was eventually replaced by double-stranded DNA. Some think DNA first evolved in , which then infected RNA-protocells. Some Observations about the Origin of Life Improbable or not, it did happen (in one way or the other). It happened relatively quickly. Possibly present 3.8 Ga ago - and arguably far more complex than mere macromolecules. The fossils and trace fossils of 3.5 Ga are remarkably similar to modern - they are significantly evolved beyond “first life”. The Earth (and ) were likely quite hostile places for the first few hundred million years. The time scale for the origin of life is no more than a couple of hundred million years - and quite possibly less than one hundred million years. Simple life might not be all that improbable after all. Life later became more complex, but apparently in a series of ‘giant steps’ rather than steadily. Key Events in the Proterozoic Huronian Glaciation 2.3-2.5 Ga Rise of Atmospheric 2.0-2.3 Ga First 2.7-1.2 Ga Evolution is the process by which novel traits arrive in and are passed on from generation to generation. The fundamental mechanisms driving this are and . There is certainly no question as to whether mutation, natural selection, and evolution occur (witness antibiotic-resistant bacteria, HIV, and H1N1 flu). And it has been observed in higher (fruit flies, fish, birds) in the laboratory and in the field. Whether evolution has led to the biological diversity we see today (and to us) has been

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controversial in broader society, but not within science. Development of Evolutionary Theory Modern evolutionary theory has its roots in the 18th century, contributors include James Hutton, Erasmus Darwin (Charles’ grandfather), and Pierre Maupertuis. Ideas of Jean-Baptiste Lamarck (1744-1829) particularly influential. He thought, however, than inherited traits could be passed on. Modern theory, which identifies natural selection as the mechanism, is due to Alfred Russel Wallace and – whose joint paper was presented in 1858. Evidence for Evolution and Common Ancestry Darwin’s evidence Comparative & Observed variation in domesticated and non-domesticated organisms Fossil Record Subsequent Evidence Similarities of cellular DNA sequences Chirality Taxonomy That organisms can be classified (Linnean system) based on their similarities in a hierarchical manner suggests evolutionary relationship (Why not a Jackalope?) More on taxonomy and the http://www.sciencemag.org/feature/data/tol/ http://tolweb.org/tree/ Comparative Anatomy Fundamental similarities in anatomy of widely different organisms (e.g., human arm, ’s leg, whale’s fin, bat’s wing) Comparative Embryology Early development of all is similar Small Scale Changes - Domestic In a few thousand years, has produced widely varying characteristics of domestic animals such as the . Variation & Selection Light and Dark Moths in Britain Biogeography Distribution implies a history: Species are different in widely separate, but similar, environments. Species are absent from environments they could inhabit. Closely related species are often found in close proximity. The Fossil Record Overall pattern on the long time scale is one of increasing complexity. Biological succession:

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individual species are (usually) restricted to limited periods of geologic time. It is possible to trace the ancestry of present species to ancient ones through a succession of forms. Species becoming increasingly diverse through time (with notable and important reversals). Only a tiny fraction of organisms are fossilized, so the fossil record provides only a glimpse of ancient life. Post-Darwin Evidence At a cellular level, all life is remarkably similar All rely on same fundamental set of chemicals & reaction pathways: DNA, RNA, similar proteins; all use ATP, all autotrophs rely on the Calvin cycle, etc. Complex chemicals (e.g., hemoglobin) are similar (but not identical) as well. DNA/RNA sequencing DNA/RNA sequences generally match the phylogenic tree based on morphology (but there have been surprises) You share >98% of your with chimpanzees and 94% with baboons. Amino Acids Of the many amino acids possible, life uses only 20. Universal Chirality All biologically generate amino acids are left-handed (abiotic amino acids can be either). All nucleotides are right-handed. Post-Darwin Darwin was unaware of the mechanism of inheritance. The term genetics was not coined until the early 20th century. (1822-1884), who was Darwin’s contemporary, worked out fundamentals of genetics, but Darwin was unaware of it. Molecular basis of inheritance, DNA, not discovered until 1950’s. The points, in this context, are: Once life emerged, it increased in diversity and complexity. All organisms today are related and are descendents of a common Archean (or possibly Hadean) ancestor. Archean Life Based on morphology and size, all Archean fossils appear to have been . Compared to even single-celled eukaryotes, prokaryotes are: Morphologically simple Small Internally simple. Nevertheless, they profoundly changed the Earth. Evolution of Speculation that Isua carbon was the product of photosynthesis. Early Archean microfossils look like photosynthetic cyanobacteria. Cyanobacteria-related hydrocarbons (methylhopanes) found in 2.7 Ga Fortescue Group of W. Australia. Rise of Atmospheric Oxygen A good deal of evidence suggests the oxygen became a significant component of the

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atmosphere around 2.3-2.0 Ga. Before that time: Banded Iron Formations Detrital pyrite and uranite in sediments & paleosols Mass-independent sulfur isotope fractionation (implying UV penetration of the atmosphere) Iron-poor paleosols After that: Red beds (basically, hematite-rich sandstones) Iron-rich paleosols Banded Iron Formations (BIFs) Banded iron formations are thought to form when deep water containing soluble Fe2+ upwelled and mixed with oxygen-bearing shallow water. The iron was oxidized to insoluble Fe3+ and precipitated. They are most abundant in the late Archean/early Proterozoic. Prokaryotes & Eukaryotes Prokaryotes are small, morphologically simple and internally simple, with little internal differentiation (most notably, no nucleus). The Eubacteria and Archea are prokaryotes Eukaryotes are larger, morphologically diverse and internally differentiated, i.e., nucleus, . Some of these internal structures have their own DNA. Eukaryota comprise all other organisms (, animals, monera). Eukaryotic Roots Key observation: “the archaeal DNA replication machinery has striking similarity to that in eukaryotes and is evolutionarily distinct from that in bacteria.” “Many archaeal DNA replication proteins are more similar to those found in eukarya than bacteria.” This would suggest we eukaryotes are most likely descended from the , rather than Bacteria. Eukaryotes may have arisen through “Endosymbiosis”. Evidence: Both mitochondria and can arise only from preexisting mitochondria and chloroplasts. They cannot be formed in a cell that lacks them. (Your mitochondrial DNA comes exclusively from your mother.) Mitochondria (and chloroplasts) have own set of genes (a single circular of DNA) that are more like bacterial genes than eukaryotic genes. DNA replication in mitochondria is independent of nuclear DNA replication and cellular . Both mitochondria and chloroplasts have their own -synthesizing machinery, and it more closely resembles that of bacteria than that found in the cytoplasm of eukaryotes. ³ntibiotics that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria. Conversely, inhibitors of protein synthesis by eukaryotic ribosomes that do not affect bacterial protein synthesis do not affect protein synthesis within mitochondria. When did Eukaryotes Appear? First undisputed fossils are Meso- (middle) Proterozoic red algae (bangiophytes) from the Canadian Arctic – 1.2 Ga. Abundant Acritarchs (resting stage cysts) are known from the Neo (Late) Proterozoic.

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Possible molecular fossils (steranes) found in the Fortescue Group of W. Australia - 2.7 Ga Molecular clocks suggest eukaryotes originated between 2.7 and 1.8 Ga Grypania Oldest macrofossil is Grypania in 2.1 Ga rocks from Michigan (and China). What it is? Colonial ? (blue-green algae) Colonial eukaryote? (colonial algae) Multi-cellular ? How do we know the Earth was Glaciated? Three evidences of glaciation: Glacial Till (tillite or diamictite): poorly sorted, containing angular blocks Striations Glacial marine sediments: large clasts in otherwise fine-grained, well-bedded marine sediment. Huronian Glaciation First clear evidence of a glacial event occurs in 2.3 Ga Huronian rocks of Canada (near Lake Huron). Curiously, these are sandwiched between pyrite- and urananite-bearing sediments of the Archean and Proterozoic red beds. So what caused the glaciations? What was the atmosphere like in the Archean? Rich in CO2, possibly rich in CH4. What kind of gases are these? gases. So why was the Earth not a “hot house”? “Faint Young Sun Paradox” Stars grow brighter as they age. The Sun would have been about 30% less luminous during the Hadean than it is now. So the paradox is that the sedimentary record shows liquid water, not ice, has existed throughout almost all of Earth’s history. How do we resolve it? CO2 and CH4 Enter Photosynthetic Life What does photosynthesis do? Consumes CO2 Produces O2 What does O2 do to methane? Oxidizes it to water and CO2 What happens to the greenhouse effect and climate when the atmosphere becomes oxidizing? It crashes. Late Proterozoic Glaciations: Snowball Earth? Peculiar Rock Sequences in the Late Proterozoic Massive beds of limestone immediately overly tillites (i.e., very poorly sorted sediments deposited by glaciers). Carbonates are unusual in that they appear to be abiologic.

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Snowball Earth Hypothesis Continents cluster near the equator where rainfall and rock weathering scrubs carbon dioxide out of the air. Global temperatures fall, and large ice packs form in the polar oceans. Albedo increases, driving temperatures even lower, etc. Average global temperatures plummet to -50˚ C. The oceans freeze to depth of more

than a km, marine organisms die. With no rainfall, CO2 is not removed from the atmosphere. As CO2 from volcanic eruptions accumulates, the planet warms and sea ice slowly thins.

Concentrations of atmospheric CO2 increase 1,000-fold as a result of volcanic activity. In a matter of centuries, surface temperatures soar to more than 50˚ C. An intense cycle of evaporation and carbonic acid-rich rain erodes the rock and washes bicarbonate into the oceans, where they form carbonate sediment. New life-forms-- engendered by prolonged genetic isolation and selective pressure--populate the world as global climate returns to normal.

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