PSC182 Life in the Universe Lecture Notes (Ch. 6) PDF

2024-10-22 PSC182 (Life in the Universe, Ch.6) Lectures 15+ Development of Life on Earth A brief history of life on Earth 1 1 Lecture plan: 2 2...

2024-10-22 PSC182 (Life in the Universe, Ch.6) Lectures 15+ Development of Life on Earth A brief history of life on Earth 1 1 Lecture plan: 2 2 1  2024-10-22 How did Life start? The essential organic materials (CO2, CH4, NH3, water, H2, N2,…) already here. Comets & meteorites may have brought more complex organic molecules ready-made from outer space and additional water. BUT... Big steps still needed to explain how life started: 1. Formation of (specific?) organic molecules: amino acids & nucleotides. 2. Joining of large molecules into (functional) proteins and nucleic acids. 3. Aggregation into droplets (cells?) with chemical properties different from what is in the environment. 4. Replication & heredity: via RNA & DNA (or their percursor). 3 3 Earth's Early Atmosphere Did not contain oxygen; dominated by CO2, with hydrogen compounds. Volcanic gases: carbon dioxide (CO2, ~35% ?, ~1,000× more than now), nitrogen (N2), hydrogen sulfide (H2S), steam (H2O), methane (CH4). Red or orange in colour, thick and opaque. Heat deposited through frequent hits by comets and meteors. Some hits vaporized the oceans, but some brought additional water (ice) from the outer solar system. Rapid changes in level of solar illumination: - Early Sun was more variable and 30% fainter than today. Rapid rotation of the globe (8–12 hour days?). Violent lightning storms. 4 4 2  2024-10-22 Earth, a water planet ◆ Ocean crust eventually sinks (subducts) back into mantle ◆ Continental crust floats on top and grows over time. ◆ The total amount of water on Earth is 0.1% of Earth's total mass. compare this with bodies in the outer solar system: (e.g.- Europa may be 20% water) ◆ Height-variation of rocky surface: 8 km above to 5 km below sea level (today) average ocean depth: 4km. If Earth had twice as much water as it does today (i.e.- 0.2% of mass), oceans would cover all the land. 5 5 The Miller – Urey experiment In the 1920s, realization (by Oparin, Haldane) that Earth's early atmosphere was without any oxygen. In the 1950s (Stanley Miller & Harold C. Urey), conduct experiments with electric discharges in an atmosphere of water vapour, methane, ammonia and hydrogen gases......after 1 week, it produced many amino acids (among the 18 produced, 6 occur in living organisms). 6 6 3  2024-10-22 Origins of Life: Early ideas Some early ideas for where life originated no longer fit current scientific knowledge… ◆“Pungent ponds of tidal water full of stuff”: Vulnerable to UV radiation (no oxygen, hence no ozone-layer) Vulnerable to impacts ◆“Soup of organic molecules” formed by sunlight or lightning Miller-Urey experiment used (based on what we now know) an unrealistic atmosphere composition products created would be too diluted in the ocean Lack of chemical disequilibrium Chemical reactions always go both ways. So need a dis-equilibrium to force reactions in a preferential direction. Doesn’t work in a uniform "primordial soup". 7 Origin of Life: in Mid-Ocean Ridges? ◆One good possibility: ◆ deep-ocean hydrothermal fields For example: Lost City 9 8 4  2024-10-22 Fig. 1. (A) The Atlantis massif is located ∼15 km to the west of the MAR axial valley. D S Kelley et al. Science 2005;307:1428-1434 Published by AAAS 9 9 Fig. 2. Three-dimensional view, looking toward the northeast, of the LCHF. This image is based on 17 ABE missions using the SM2000 sonar system in a down- looking and side-looking mode. D S Kelley et al. Science 2005;307:1428-1434 Published by AAAS 10 10 5  2024-10-22 Fig. 4. Hydrothermal deposits at Lost City. 10’s of meters Hydrogen, H2S, and other chemicals and minerals percolate out of the ocean floor and form structures. Steady stream of chemicals creates dis-equilbrium D S Kelley et al. Science 2005;307:1428-1434 Published by AAAS 11 11 Fig. 5. Active vent deposits from Lost City. D S Kelley et al. Science 2005;307:1428-1434 Formations have many small crevices to concentrate chemicals, with walls that may hold minerals that can act as catalysts. Published by AAAS 12 12 6  2024-10-22 BUT a long way from molecules to cells First single cell organisms, even if simple by current standards, were much more complex than just ensembles of organic molecules. A prokaryotic cell typically contains one long DNA molecule in the central parts and several other components. First cells were probably much simpler. The inner content (cytoplasm) is confined within a membrane. 13 13 Complex organic molecules Lipids (fats & oils) have ends that “sense” the electric asymmetry of the water molecules. If put on water, they sit with hydrophilic end in the water. They can create a monolayer (one molecule thick) of oriented molecules. 14 14 7  2024-10-22 Complex pre-biotic molecules When the mixture is shaken, small spherical drops form, the “micelles”. This is still long way to life, yet they look and even behave like very simple cells… 15 15 Complex pre-biotic molecules Bilayer arrangement of lipid molecules, and the spherical vesicles that this can form. These structures may change form and behaviour according to external conditions like salinity (salt concentration), etc. 16 16 8  2024-10-22 The initial structures? Amino acid droplets Microscopic membranes made by cooling a warm made from lipids mixed water solution of amino acids with water 17 17 Some necessary steps 1. Amino acids and other organic materials. … 2. Strands of RNA form, perhaps capable of self-replication. (Viruses utilize RNA as genetic material.) 3. Membranes enclosing pre-cells, trapping organic molecules. 4. Natural selection among the (replicating) RNA molecules => increased complexity. … 5. Natural selection and formation of DNA. 18 18 9  2024-10-22 The tree of life possibly grew this way Last Universal Common Ancestor LUCA 19 19 Oxygen and eukaryotes (2) 1 2 5 3 4 20 20 10  2024-10-22 Earth's atmosphere Earth's early atmosphere contained essentially no oxygen. Now 21%, the 2nd most abundant gas. Living organisms collectively produced oxygen in large amounts. Without constant replenishment, oxygen would disappear in ~2 Myr through oxidization of minerals; it would react with the more abundant atmospheric nitrogen to form nitric acid, HNO3 (present in "acid rain"). Current rate of production determines oxygen content of the atmosphere(averaged only over Earth's current atmo- ~2Myr). spheric composition 21 21 Photo-synthesis: Building organic material The first photo-synthesis (light-utilizing) bacteria utilized the common greenhouse gases, hydrogen sulfide and carbon dioxide (H2S, CO2), to produce carbohydrates : nCO2 + 2nH2S + solar energy → (CH2O)n + nH2O + 2nS Chemically, relatively easy. Today, the most common photosynthesis reaction is of the type: nCO2 + nH2O + solar energy → (CH2O)n + nO2 More efficient and uses widely available water, but chemically more difficult. 22 22 11  2024-10-22 Advantages of Oxygen ◆Aerobic metabolism more efficient than anaerobic metabolism Aerobic (oxygen-based): about 40% of possible energy is utilized Anaerobic (non-oxygen, sulfur-, hydrogen-, iron- based): only organic matter + O2 ◆But respiration (burning of organic matter to obtain energy) uses O2 organic matter + O2 => Energy + CO2 + H2O ◆Ifall organic matter is consumed by bacteria & animals, then NO net increase in Oxygen => Burial of organic matter is required to raise Oxygen levels 99.99% of organic matter is consumed 0.01% is buried