EPSC201 Lecture 22 Notes.doc

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McGill University
Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC 201 Lecture 22 Notes In the last class we looked at where the origin of life might have started. Could life come from other parts of our solar system. There was a Martial meteorites that may have contained fossils. There was also nucleic acids found in the meteorites. Carbonaceous chondrites have been found on earth with many different com- plex molecules. Life may have existed on Mars, so there are space exploration mis- sions dedicated to this. Water was present on Mars, and formed evaporites and canals. Miller envisioned an early atmosphere with no oxygen, but rich in ammonia and methane. Light- ning could have provided the energy to produce amino acids and other complex molecules that are building blocks of life. He showed that a closed system with the gases shown above, when subjected to electricity, can produce amino acids. This provided some energy to say that life may have started in the ocean interface. There are long strands of DNA in the nucle- us of the cell. There are four bases, which comprise genes when ordered the right way. Different sequences give different proteins. The backbone of DNA is made from sugars and phosphorus. The DNA is tran- scripted into an RNA molecule. The RNA exits the nucleus, and is translated into a protein by the ribo- some’s. This occurs in the cytoplasm of the cell. The RNA is like a book, being read by the ribosome. The early Earth was almost entirely ocean. Around 2.5 billion years ago, there started to be land on the surface. In the 1970s we discovered black smok- ers in mid ocean rideges. There was plenty of thriv- ing species in these areas, that should have been too deep to support life. Depths of several kilome- ters don’t allow for photosynthesis. So the metabolic pathway then, is much different. It is called chemosynthesis. The animals used the thermal energy at the spreading centers, hot spots, and subduction zones (all hot) to synthesis chemicals necessary for life. Black smokers have the ability to produce com- plex molecules. There is iron present, which acts as a catalyst to produce big organic mole- cules from simple gases, such as carbon monoxide, oxygen, sulphur, nitrogen, and water. These building blocks are catalyzed by metallic iron, or iron sulphide, to make complex mole- cules. At a spreading center, we have rift faults. Water moves downwards into the mantle, and is even- tually heated up. The water moves back up, and generates these black smokers. They are black due to all the precipitates coming out of the hot water. There is iron sulphide, pyriditite, and other precipitates. There are lots of iron complexes and sulphates which make the smoker look black. The water comes out of the smoker at about 300 degrees, but stays in the liquid form due to the underwater pressure of the ocean. The mineral olivine is converted into serpentine, magnetite, and hydro- gen. This hydrogen can be used as a precursor. This reaction takes place in the mantle. The hydrogen gas will work its way up towards the surface, where it can take place in the reactions in black smokers. The mantle also has lots of carbon. There will be nitrogen gas, carbon monoxide, sulphur gases present in the black smokers. All of the reac- tants necessary to make the building blocks of life were present. Olivine -> serpentine + magnetite + hydrogen An important feature of this structure is an ability to omit metal atoms with the total fraction up to 1/8, thereby creating iron vacancies. One of such structures is pyrrhotite-4C (Fe7S8). These irregularities could be causing the pyrrhotite to act as a template to form specific complex or- ganic molecules. We know this mineral is found in black smokers. There is a link between the geological processes and biological origin. Life is divided into two basic categories, eukaryotic (EU) and prokaryotic (P). P are mostly single cells and don’t have a defined nucleus. EU are mostly multicellular organisms that do have a nu- cleus, and evolved from singled celled organisms. The prokaryotic life form evolved first, and is much simpler. The EU have more complex cellular machinery, and much more DNA. There is a defined nucleus in the center of the cell, containing all the DNA. The prokaryotic cells have less DNA, which is circular in structure and called a plas- mid. There is a theory that the eukaryotic cell’s mitochondria was a separate prokaryotic cell, that got swallowed up by another prokaryotic cell, and formed a symbiotic relationship. How do cells replicate? The prokaryotic cells undergo fission, where the cell basically splits in half. This is very straightforward, as it only takes one parent to produce an offspring. There is less genetic variation between generations, compared to organisms that have two parents. All early life was prokaryotic. The first EUs occurred around 1.8 billion years ago. This is inter- preted from fossil representation, but we are essentially positive that the first life forms were prokaryotic. At 1.8 billion years ago, we have both EU and P. As soon as eukaryotics life came to be, evolution proceeded much more rapidly. Note how the oxygen is increasing in content as time progresses forward. Oxygen built up in the atmosphere. Banded iron formation represents a sedimentary layer of iron oxide. There is lots of banded iron about 2.4 billion years ago, which is thought to be a result of bacteria producing oxygen. There was a lot of BIF until about 2 billion years ago, when it stopped being pro- duced. We get a perfect coincidence of BIF and oxygenation of the atmosphere. In countries with younger rock, we don’t see and BIF. Only countries with old rocks will have BIF. BIF formation stopped around 2 billion years ago. It was a result of cyanobacteria undergoing photosynthesis, which produced oxygen. These bacteria ap- peared about 2.5 billion years ago. This oxygenation of the atmosphere allowed more complex organisms to form, and be dependent on oxygen for respiration to oc- cur. Chert is biochemical sediment, SiO2, which is microcrystalline and may contain small fossils. Jasper jewelry is from banded iron forma- tion and is probably about 2.5 billion years old. BIF – Fe2O3 – iron oxide The early oceans were loaded with iron. Largely Fe(2+) reduced form. It can also be in Fe(3+) can also form, but is very insoluble. The 2+ oxida- tion state is very soluble. The early oceans were loaded with the 2
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