Chemical evolution/AbiogenesisFocus questions: 1. Why do you think it’s important for a molecule to be self-replicating for evolution to occur?2. Why do we think RNA was the first self-replicating molecule? What are the two things a self-replicating molecule must be able to do?3. Why is protein not as good a candidate for a self-replicating molecule as RNA?4. At which point in this series of steps do we actually have “life?”5. What are some unfinished spots in this theory? Where are the holes?6. If we can’t yet fill all the holes, does this prove that abiogenesis is wrong? Evolution is a powerful theory, but it does have one major shortcoming- how do we explain the origins of the first cells? Any theory that claims that life was not spontaneously generated or created must explain how life could have self-assembled from nonlife under the conditions of primordial Earth. Enter chemical evolution, a branch of science that attempts to answer that question.Experiments that attempt to replicate early Earth conditions are hindered by debate over what those conditions actually were. It’s an exciting area of research with many different theories about how complexorganic molecules could have come about, but all of them have one thing in common: we aren’t talking about the spontaneous generation of DNA and RNA and proteins just by chance. No scientific theory claims that a cell, with membrane and genetic material and complex protein-making machinery, assembled itself from scratch without any intervening steps. The progression is, instead, thought to follow a much more complicated series of small, logical steps: Simple chemicals  organic molecules  monomers  polymers  self-replicating polymers  self-replicating polymers housed in a lipid bilayer  protocell  bacteriumYour book talks about this topic in multiple chapters; I’ve given you the bare facts and relevant page numbers below. The start of Chemical Evolution: Simple molecules floating in earth’s oceans are exposed to energy in the form of radiation, heat, or sunlight. By chemical processes we can easily replicate in laboratory conditions, the first organic molecules are formed. (Reference pages: 34-35 and figure 2.23, “Recent Origin of Life Experiments”)Early building blocks are formed:The classic Urey-Miller experiment demonstrated that under early-earth conditions, simple compounds like formaldehyde and hydrogen cyanide, again exposed to inputs of energy, begin to form more complex organic molecules: amino acids, and nucleic acids. (Reference pages: 32-34, “Investigating Chemical Evolution, Early Origin-of-Life Experiments”) Basic amino acids were reproduced in the Miller experiment. Basic nucleotides? Here we hit a snag—nobody has yet figured out how nucleotides could have formed under early earth conditions. (Reference page: 59 “Could Chemical Evolution Result in the Production of Nucleotides?”) Monosaccharides (simple sugars) are also readily synthesized under these conditions. (Referencepage 74, Monosaccharides and Chemical Evolution”) Lipids could have been formed under some conditions, or they could have arrived from space! (Reference page 88, “Were Lipids Present During Chemical Evolution?”)Monomers begin to form polymers (long chains):The next challenge is to demonstrate that it is possible, under early earth conditions, for these basic organic building blocks to start forming polymers without the help of the biological catalysts we have today.  Protein polymerization could have occurred in several different ways. (Reference page: 45, “Could Polymerization Occur in the Energy-Rich Environment of Early Earth?”) RNA/DNA polymerization. As mentioned, we have difficulty recreating the step that gave us the nucleotide monomers. But assuming they existed in abundance, it’s relatively easy to get them topolymerize. (Reference page 60 “Could Nucleic Acids Have Formed in the Absence of Cellular Enzymes?”) It’s unlikely that polysaccharides played much of a role in early life. The particular chemical bonds that join modern polysaccharides are unlikely to form under early-earth conditions withoutenzymes. (Reference page 76, “Polysaccharides and Chemical Evolution”.)Self-replicating polymers:The biggest hurdle is to find a molecule that is capable of replicating itself. Once a molecule can self-replicate, it is subject to the laws of natural selection. The quicker and more accurately replicating molecules win out over the less efficient over time. The best candidates for this task are proteins and RNA, because both are capable of catalytic activity (encouraging particular chemical reactions to take place, including self-polymerization). Which one was it? RNA is best suited because in addition to its enzyme-like activity, its structure provides a template for replication.  Protein discussion. (Reference page 55, “Was the First Living Entity a Protein Catalyst?) DNA is very stable and has no observed catalytic activity. (Reference pages 64-65, “Is DNA a Catalytic Molecule?”) RNA, however, still acts as an enzyme in several important ways. It would be more than capable of replicating itself. Scientists are working on creating a self-replicating RNA molecule. (Referencepages 67-69, “RNA Can Function as a Catalytic Molecule,” “In Search of the First Life-form”) Polysaccharides and lipids are right out. They don’t have catalytic activity, or carry information. (Reference page 76, “Polysaccharides and Chemical Evolution”.)Early membranes:Lipids may not carry out chemical reactions, but they are an essential part of abiogenesis theory. A particular kind of lipid that has long hydrophobic regions and a hydrophilic head is of special interest. Such a molecule is referred to as an amphipathic lipid. Such a molecule would have one end that formed hydrogen bonds with water, and one end that did not. (Reference page 87, “Phospholipids” and “The Structures of Membrane Lipids”) The unique property of amphipathic lipids is that when you put a drop of them into water, they spontaneously assemble into either lipid micelles (bubbles with all the hydrophobic tails facing in, and the hydrophilic heads facing the water), or lipid bilayers. All modern cell membranes are constructed of bilayers of amphipathic lipids called phospholipids. (Reference page 88, “Phospholipid Bilayers”) Life is