Chemical evolution Abiogenesis Focus 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 selfreplicating 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 complex organic 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 selfreplicating polymers housed in a lipid bilayer protocell bacterium Your 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 Reference page 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 to polymerize 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 without enzymes 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 selfreplicate 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 Reference pages 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 Born The next step of the abiogenesis theory is that self replicating molecules found themselves inside bubbles made of lipid bilayers These were protobionts or protocells the precursors of cells Life has officially arrived on Earth Reference pages 93 94 Membranes and Chemical Evolution Eventually organisms using self replicating RNA were outcompeted by