The big question, for this segment, is what happens when living systems break down? [MUSIC] When thinking about the complexity of life, and the evolution of complex life, we can see that there are quite a few points in the fossil record of the J Chemical record, where huge changes in the composition of the atmosphere and sediments occurred. We can also see chemical remnants in ancient sediments, and gain insights into changes that had occurred in the evolutionary history of life on Earth and the evolution of biochemical events or systems. In this lecture we will look at one evolutionary event where some very complex-looking molecules were utilized by primitive organisms to drastically change the face of the planet. Life evolved on Earth over 3 billion years ago. When we consider early life in the history of the Earth, 2 to 4 billion years ago, we think of simple bacteria with no complex internal structures. These bacteria evolved complex biochemical pathways, utilizing metal ions and complex cofactors to help them divide and reproduce, and utilize chemicals in their environment. Environment includes the atmosphere, the hydrosphere, and the lithosphere. They probably had limited resources. Simple amino acids, organic acids, and mineral salts dissolved in the hydrosphere, methane in the atmosphere, and solid low solubility salts in the lithosphere. There is no oxygen, and the chemical environment was highly reduced. They made use of complex molecular cofactors to drive and catalyze chemical reactions in order to make more complex molecules. So where did these cofactors come from, and what do they look like? Vitamin B12, bacteriochlorophyll, chlorophyll, heme and factor 430, are all examples of compounds known as tetrapyrroles. Looking at these structures, we can say they look very complex, but they do have some similar features that are ideal for coordinating methylines, which are used in catalyzing chemical reactions. These complex molecules also stabilize, and often improve the solubility of the metals. Vitamin B12 and factor 430 would've been used very early on in evolutionary times. They're used extensively in methane and single carbon metabolism which would have been some of the earliest biochemical processes. Bacteria chlorophylls and chlorophylls capture light for photosynthesis. Bacteria chlorophylls are more reduced in chlorophyl, and are only able to be used in anoxygenic photosynthesis. And it is possible that slightly different types of anoxygenic photosynthesis may have evolved a number of times in the evolutionary history. In contrast, it appears that oxygenic photosynthesis only evolved once, and it's dependent on using chlorophyll lights to work. So oxygenic photosynthesis is dependent on chlorophyll lights being synthesized. Surprisingly, tetrapyrroles can form spontaneously by acid catalysis from simple precursors like aminolevulinic acid. So despite appearing to be quite complex, the basic structure can form from simple starting precursors. When we look at these chemicals, we see in their chemical structure, how and why they were first utilized. Pyrroles were formed from condensation of some type of amino acids. And tetrapyrroles were formed from reaction of these pyrroles with the III Isomer being the most abundant. Organisms then involved to use and further modify these complex tetrapyrroles, and they obviously use the ones at the highest concentration. Thus all biological tetrapyrroles now used are derived from the III Isomer. Because of this, they got locked in to using these isomers. So when a biochemical pathway evolved to make them more efficiently, they had to evolve a complex mechanism to make the most common isomer found from spontaneous chemical synthesis. We now see aerobic and anaerobic pathways for these compounds, with the anaerobic pathways being found in more ancient bacteria while the aerobic pathways are found in some bacteria and higher organisms. It took a long time for photosynthesis and then oxygenic photosynthesis to evolve, talking 1.5 to 2 billion years after life first evolved. But when it did, it caused massive changes in the environment. There was a massive oxidation event, as oxygen appeared in the atmosphere about 2 to 2.5 billion years ago and this was due to oxygenic photosynthesis. Oxygen was a by-product of photosynthesis, and is an extremely toxic gas for organisms growing anaerobically. The gas was initially absorbed chemically. For example, soluble metal salts became oxidized and insoluble. But this buffering was short-lived on a geological time scale and lasted only about 100,000 years before significant quantities of oxygen and gas appeared in the atmosphere. Chemical processes in cells were inhibited by oxygen. So new systems had to evolve to cope while organisms retreated to anaerobic natures, thick, dark and deep away from the oxygen and the oxygen produces. So both of these things disrupted not only the atmosphere but also has a huge impact on the materials available for primitive bacteria to use. The composition of not only the atmosphere, but also the hydrosphere and part of the lithosphere changed dramatically due to this oxydation. This oxydation event was also the likely evolutionary driver for more complex eukaryotic organisms, which based on the fossil records, seem to appear around the same time. These eukaryote arose due to symbiotic relationships between a primitive anaerobic organism, and an aerobic organism, as the aerobic organism had developed mechanisms to detoxify, and also utilize the toxic oxygen gas. So, while one complex biological system almost vanished, as it had to retreat to natures where the oxygen concentration was low a new complex system evolved to take its place and harnessed the toxic material, which is oxygen gas, to thrive. [MUSIC]
