Chemical Evolution Proposes life began with the formation of a self-replicating molecule Origin-of-life researchers are testing the steps of the theory by simulating conditions found in atmosphere and ocean of the early Earth

Miller’s experiments Start with simple molecules like CH4, NH3, CO2 and CO In the presence of ultraviolet radiation or lightning, these simple molecules can form more complex organic molecules like H2C0 and HCN The results suggest these nonspontaneous reactions could have occurred on ancient Earth

Figure 3.1 Electrode Large glass flask Glass tubing Spark discharge Stopcock for taking samples Gases Condenser Figure: 3.1 Caption: This schematic diagram shows the important elements in Stanley Miller’s apparatus for conducting spark-discharge experiments. The arrows indicate the flow of water vapor or liquid, starting with the 200 milliliters in the small, boiling flask. The large glass flask can contain any mixture of gases desired; when a voltage is applied across the electrodes in the flask, a spark jumps across the gap. The condenser consists of a jacket with cold water flowing through it.  Exercise Label the parts of the apparatus that mimic the ocean, the atmosphere, rain, and lightning. Water droplets Water Trap Heat

More complex organic molecules Chemical evolution continues as HCN and H2CO react to form amino acids, sugars and nucleotides These molecules are the building blocks or monomers needed to make the complex macromolecules found in living organisms Experiments have shown that amino acids, sugars and purines are readily produced under early Earth conditions

Condensation reaction: monomer in, water out Figure 3.8a Condensation reaction: monomer in, water out HO Monomer H HO H Figure: 3.8a Caption: (a) In a condensation reaction, a monomer is added to a polymer to make a longer polymer. The new bond that forms results in the formation of a water molecule.  HO Monomer H + H OH Water

Polymerization of monomers into macromolecules These condensation reactions occur readily when growing polymers stick to clay particles Researchers have found polypeptide formation and RNA formation from the polymerization of amino acids and ribonucleic acids, respectively

Figure 3.6a-c Nucleotide Pyrimidines Purines NH2 O O H3C O N NH NH N 5 O Nitrogenous base N O N O N O O– 4 1 Phosphate group H H H 3 2 Cytosine (C) Uracil (U) Thymine (T) 5-carbon sugar Purines NH2 O Ribose Deoxyribose N N N NH HO5CH2 Figure: 3.6a-c Caption: (a) This sketch shows the relationship between the phosphate group, the sugar, and the nitrogenous base found in a nucleotide. The numbers indicate the positions of the five carbons in the ring. Note that the nitrogenous base is bonded to carbon number 1 in the ring, while the phosphate is bonded to carbon number 5. The bond between the phosphate group and the sugar is called a 5’ linkage; the “prime” symbol indicates that the carbon being referred to is part of the sugar and not the attached nitrogenous base. Also notice that while hydrogen atoms are bonded to the carbon atoms in the ring (see part b), biologists routinely omit them to make the diagrams less cluttered. (b) Ribose and deoxyribose are similar sugars that are found in nucleotides. (c) Purines and pyrimidines are nitrogen-containing bases. A C–N bond links them to the sugar in a nucleotide. This bond forms at the nitrogen atom that is highlighted on each base. Note that purines are substantially larger molecules than pyrimidines. O OH HO5CH2 O OH 4C H H 1C 4C H H 1C N N N N NH2 C3 2C C3 2C H H H H H H OH OH OH H Adenine (A) Guanine (G)

Final step - a self-replicating molecule A self-replicating molecule must: Be able to catalyze polymerization reactions Furnish a mechanism for making a copy Most origin-of-life researchers propose the first self-replicating molecule was RNA RNA can catalyze a variety of chemical reactions Complementary base pairing provides a mechanism for making a copy

Other candidates for first self-replicating molecule Proteins are excellent catalysts but are not capable of self- replication DNA is an excellent template for its own replication but has no catalytic abilities

Figure 3.16, left RNA FORMS A TEMPLATE FOR ITS SYNYTHESIS 5´ C A G U 5´ 3´ 2. Copied strand polymerizes. Template strand Copied strand 5´ 3´ 3. Copy and template separate. G C A U Template strand Copied strand 3´ C G A U Template strand G C 3´ A U 5´ G C Figure: 3.16, left Caption: This diagram is a hypothesis for how RNA molecules can be copied. The copying process is based on complementary base pairing between ribonucleotides. 3´ 5´ 1. Complementary bases pair.

Figure 3.16, right Copied strand = new template New copy strand 5´ 3´ 5´ 3´ C A G U 5. New copy polymerizes. New template strand New copy strand 6. New copy is identical to original template. 5´ 3´ C A U G New copy strand New template strand C G 3´ A 5´ U Copied strand = new template G C 3´ U 5´ A G C Figure: 3.16, right Caption: This diagram is a hypothesis for how RNA molecules can be copied. The copying process is based on complementary base pairing between ribonucleotides. 3´ 5´ 4. Copy serves as new template.

What Constitutes Life? First living organism was a cell in which controlled reactions could occur in an enclosed environment, leading to replication of the cell. Those simple organisms would have been acted on by natural selection Biological evolution overtakes chemical evolution

The earth’s environment - 4. 6 billions years ago until 2 The earth’s environment - 4.6 billions years ago until 2.5 billion years ago The earth’s interior was much hotter and volcanism more frequent Materials like iron and other unoxidized minerals were brought to the earth’s surface These materials were were quickly oxidized Little free oxygen remained in the environment

The oldest fossils Taken from rocks in Greenland 3.8 billion years old The Greenland fossils appear to be Archaea

Filamentous cyanobacteria - 3.5 billion years old

Fossil bacteria - 3.5 billion years old

Stromatolites

Stromatolites Work of communities of photosynthetic bacteria The bacteria secrete a sticky gel that protects them from uv radiation Gel also causes sediment to stick Periodically, the bacteria have to creep outward to be exposed to sufficient sunlight The result is a cabbage-like formation

Stromatolites

The earth’s environment - beginning 2.5 billion years ago Heat production tapered off Crustal movements slowed and larger land masses began to form and persist Shallow seas spread over these continental expanses, providing habitat for photosynthetic cyanobacteria These bacteria relentlessly pumped out free oxgyen

The result of oxygen availability was that the earth rusted, visible in banded iron formations

Once the iron was all oxidized, oxygen began to accumulate first dissolved in water then escaping into the atmosphere These organisms fundamentally changed the earth: The earlier atmosphere of methane and hydrogen sulfide was replaced with an atmosphere of oxygen

The oxygen boom drove organisms without oxygen-handling enzymes into anaerobic habitats (stagnant waters, dead organic material, sediments) Other bacteria evolved the ability to use oxygen to break down food into carbon dioxide and water