Topic D.1 Evolution Origin of Life on Earth

D.1.1 Describe the 4 processes needed for the spontaneous origin of life on Earth 1. Chemical reactions to produce simple organic molecules from inorganic molecules: Amino acids, Water, CO2, Ammonia. 2. Assembling of these organic molecules into polymers: Polypeptides from amino acids. Formation of polymers that can self replicate- this allows inheritance of characteristics. 4. Development of membranes, to form spherical droplets, with an internal chemistry different from the surroundings, including polymers that held genetic information. Conditions of the Pre-biotic Earth: Continents of solid rock forming Oceans of water forming High Temperatures High UV levels Reducing atmosphere (no O2) Frequent storms with lightning

D.1.2 Outline the experiments of Miller and Urey into the origin of organic compounds Reproduction of environment that existed before on Earth : 1. Methane, Hydrogen, Ammonia inserted 2. Water cycle simulation; heat –evaporate-cooled-condensed 3. UV radiation (no ozone layer before) 4. Electrical sparks = lightning They simulated conditions on the early Earth by constructing an apparatus that contained a warmed flask of water simulating the primeval sea and an atmosphere of water, hydrogen gas, CH4 (methane), and NH3 (ammonia). Sparks were discharged in the synthetic atmosphere to mimic lightning. A water was boiled while a condenser cooled the atmosphere, raining water and any dissolved compounds back to the miniature sea. The simulated environment produced many types of amino acids and other organic molecules leading them to conclude the pre-biotic synthesis of organic molecules was possible. However, the question of the concentration of methane and other chemicals is in doubt so the applicability of the results is uncertain.

D.1.3 State that comets may have delivered organic compounds Panspermia- the theory concerned with the arrival of material from outer space. Hundreds of meteorites and comets hitting the early Earth brought with them organic molecules formed by abiotic reactions in outer space. Extraterrestrial organic compounds, including amino acids, have been found in modern meteorites, and it seems likely that these bodies could have seeded the early Earth with organic compounds.

Deep sea thermal vents (heat, minerals in water) D.1.4 Discuss possible locations where conditions would have allowed the synthesis of organic compounds Tidal pools (clay-catalyst polymerisation, minerals) Deep sea thermal vents (heat, minerals in water) Volcanoes (water vapours, minerals, heat) In space (amino acids in dust) There are hydrothermal vents deep in the oceans, with chemicals welling up from the rocks below. Around these vents, there are unusual chemical conditions, which might have allowed the spontaneous synthesis of the first organic compounds. There had been a belief that the bottom of the ocean was lifeless due to the fact that sunlight could not reach here but nothing could be farther from the truth.  There are communities living and thriving around these smokers.  An example is the red and white tube worms.  They trap the minerals in the water and transfer them to symbiotic bacteria who use the minerals to make food which feeds the tube worms. The discoveries of these deep water organisms gives credibility to the idea that the earliest forms of life may have began deep in the ocean around these thermal vents. Another possible location where the synthesis of organic compounds may have occurred is around volcanoes.  When a volcano erupts it releases water vapors and, other gases and a variety of minerals; all of which could be used to form organic matter.  The temperature surrounding the volcano may have also assisted in the formation of the compounds. Polymers of amino acids can be created by dripping solutions of amino acids onto hot clay.  Clay minerals are very variable and can catalyze the formation of polypeptides from amino acids.  Although the polymers are unlike proteins in that they are composed of linked and cross-linked amino acids, they may still have played a key role in reactions early in the Earth’s history.  These conditions could be found on the tidal pools, seashores or flood plains of a river where there is an alternation of wet and dry conditions. Astronomers claimed to have found glycine, the simplest amino acid in comic dust particles.  Laboratory experiments have been carried out  in an environment that recreates the low pressure, low temperature environment of space and  the synthesis of amino acids has been possible.

D.1.5 Outline two properties of RNA that would have allowed it to play a role in the origin of life RNA almost certainly preceded DNA as the genetic material = "RNA world" RNA, like DNA, is a sequence of nucleotides that can carry a genetic code: RNA molecules have both a genotype (nucleotide sequence) and a phenotype (three dimensional shape) that interacts with surrounding molecules. RNA is structurally simpler than DNA RNA can self-assemble from nucleotides available from the environment RNA can self-replicate using an existing RNA molecule as a template, adding free nucleotides available from the environment copying mistakes = mutations RNA also has been shown to act as an enzyme, called ribozyme. RNA can enzymatically catalyze metabolic reactions RNA can catalyze the formation of more RNA (rRNA, tRNA, and mRNA) RNA can bind amino acids and form peptide linkages

1. RNA can self-replicate D.1.5 Outline two properties of RNA that would have allowed it to play a role in the origin of life 1. RNA can self-replicate Short polymers of ribonucleotides can be synthesized abiotically in the laboratory. If these polymers are added to a solution of ribonucleotide monomers, sequences up to 10 based long are copied from the template according to the base-pairing rules. If zinc is added, the copied sequences may reach 40 nucleotides with less than 1% error.  

RNA can act as an enzyme – Ribozymes. D.1.5 Outline two properties of RNA that would have allowed it to play a role in the origin of life RNA can act as an enzyme – Ribozymes. RNA can enzymatically catalyze metabolic reactions: RNA can catalyze the formation of more RNA RNA can bind amino acids and form peptide linkages RNA-directed protein synthesis may have begun as weak binding of specific amino acids to bases along RNA molecules, which functioned as simple templates holding a few amino acids together long enough for them to be linked.

D.1.6 State that living cells may have been preceded by protobionts, with an internal chemical environment different from their surroundings A Protobiont is an aggregate of abiotically produced organic molecules surrounded by a membrane or a membrane-like structure. Protobionts exhibit some of the properties associated with life, including simple reproduction, metabolism and excitability, as well as the maintenance of an internal chemical environment different from that of their surroundings. Coacervates: spherical aggregations of lipid molecules form spontaneously from certain dilute organic solutions Possess osmotic properties Microspheres: small spherical aggregations of proteins form spontaneously from heated and cooled amino acids maintains an osmotic potential Liposomes: spherical vesicles composed of a bilayer membrane. form spontaneously from phospholipid molecules in turbulent water capable of growth and division

D.1.7 Outline the contribution of prokaryotes to the creation of an oxygen-rich atmosphere Anaerobic bacteria are believed to be the first forms of life on Earth. They consumed organic materials and reproduced to such numbers that competition was high and food was scarce. It is unclear how it happened but some bacteria evolved to become photosynthetic.  Photosynthetic prokaryotes began photosynthesis: Free oxygen is accumulated in the Earth’s atmosphere. The formation of an ozone layer in the upper atmosphere started. Incidence of UV light reaching the Earth’s surface reduced thanks top ozone layer. Terrestrial existence (rather than life restricted to below the water surface) became a possibility. Stromatolites: Earth’s oldest fossil made of rock formed by growth of blue-green bacteria Modern Cyanobacteria

D.1.8 Discuss the endosymbiontic theory for the origin of eukaryotes Evidence for endosymbiotic theory: Mitochondria and Chloroplasts have their own DNA; a naked loop similar to that of prokaryotes.  They have their own double membrane.  They are able to replicate themselves They have the ability to perform protein synthesis using ribosomes. The theory of endosymbiosis is a widely accepted theory that attempts to explain how prokaryotes evolved into eukaryotes; cells with a nucleus and membrane bound organelles. According to the Endosymbiotic Theory proposed by Lynn Margulis, both the Mitochondria and Chloroplasts have evolved from independent prokaryotic cells, which were taken into a larger heterotrophic cell by endocytosis. Instead of being digested, the cells were kept alive and continued to carry out aerobic respiration and photosynthesis. This theory explains how mitochondria and chloroplast became part of eukaryotic cells and it is supported by characteristics of these two organelles which make them different from the others.  First of all they have their own DNA; a naked loop similar to that of prokaryotes.  Secondly, they have their own double membrane.  Thirdly, they are able to replicate themselves when more are needed and last they have the ability to perform protein synthesis using ribosomes.