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220 12Lecturedetails15 EAS 2200 Spring 2011 The Earth System Lecture 15 Evolution of the Atmosphere and Biosphere II The RNA World One macromolecule, 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 Macromolecules to Protocells Life as we know it requires a barrier between itself and its surroundings. Present cell walls are composed of a bilayer of phospholipids -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 micelles. 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 fatty acid vesicle would be permeable to nucleotides, so the material needed to synthesize additional RNA could accumulate in the cell. The protocell would grow as it accreted micells and accumulated nucleotides, eventually 1 EAS 2200 Spring 2011 The Earth System Lecture 15 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 “metabolism first” A contained (perhaps in a lipid vesicle) chemical system or cycle that exploits an energy 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 carbon monoxide to carbon dioxide. Transition out of the RNA World RNA has some disadvantages: Not as chemically stable as proteins 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 viruses, 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 bacteria - they are significantly evolved beyond “first life”. The Earth (and solar system) 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 Oxygen 2.0-2.3 Ga First Eukaryotes 2.7-1.2 Ga Evolution Evolution is the process by which novel traits arrive in populations and are passed on from generation to generation. The fundamental mechanisms driving this are mutation and natural selection. 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 organisms (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 2 EAS 2200 Spring 2011 The Earth System Lecture 15 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 Charles Darwin – whose joint paper was presented in 1858. Evidence for Evolution and Common Ancestry Darwin’s evidence Taxonomy Comparative anatomy & embryology Observed variation in domesticated and non-domesticated organisms Biogeography Fossil Record Subsequent Evidence Similarities of cellular biochemistry 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 tree of life 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, cat’s leg, whale’s fin, bat’s wing) Comparative Embryology Early development of all vertebrates is similar Small Scale Changes - Domestic Animals In a few thousand years, selective breeding has produced widely varying characteristics of domestic animals such as the dog. 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: 3 EAS 2200 Spring 2011 The Earth System Lecture 15 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 genes 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 Genetics Darwin was unaware of the mechanism of inheritance. The term genetics was not coined until the early 20th century. Gregor Mendel (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 prokaryotes. Compared to even single-celled eukaryotes, prokaryotes are: Morphologically simple Small Internally simple. Nevertheless, they profoundly changed the Earth. Evolution of photosynthesis 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 4 EAS 2200 Spring 2011 The Earth System Lecture 15 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, mitochondrion. Some of these internal structures have their own DNA. Eukaryota comprise all other organisms (plants, 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 Archaea, rather than Bacteria. Eukaryotes may have arisen through “Endosymbiosis”. Evidence: Both mitochondria and chloroplasts
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