Origins of Life on Earth

AP/IB 11 Biology

Origins of Life on Earth
There are multiple theories as to how life began on earth, beginning with the findings of various revolutionary scientists. This paper will discuss various scientists' theories and experiments as they strove to discover how life began, and the processes that might have occurred in order to develop and create the world we are now familiar with today.
Recent discovered functions of RNA have suggested that RNA provides a crucial framework in the evolution of the first cells, which may have been assembled from RNA comprised of nucleotides. Sidney W. Fox, a scientist at the University of Miami produced membrane-bound structures known as proteinoid microspheres. These microspheres can carry out some chemical reactions that resemble real living cells. They grow slowly and eventually bud off smaller microspheres. They're not living cells, but they are key in determining the kinds of processes that could have given rise to “self-sustaining protein entities”. It has even been hypothesized that comets and meteorites containing carbonaceous material hit the Earth with great force, and may have been a source of organic compounds such as lipid-like materials. David W. Deamer at the University of California and his co-workers extracted these organic materials from the carbon containing meteorites. Inevitably, some of the molecules assembled into membrane-bound spheres, possibly signifying that this might be how first primitive cells conglomerated. This then brings us to the theories of influential scientists, Russian A.I. Oparin and British scientist J. B. S. Haldane, who hypothesized that earth's organic molecules were formulated from simple molecules.
Both Oparin and Haldane postulated that earth was a reducing agent, in that electrons could be added; the energy from this could have come from lightning and intense UV radiation. In 1953, Stanley Miller and Harold Urey conducted an experiment to test what was deemed as the Oparin-Haldane Hypothesis. They created their very own mimic of a closed system, utilizing the gases hydrogen (H2), methane (CH4), ammonia (NH3), and water vapor. They used sparks between the gases to mimic lightning, and used a condenser in order to cool the gases that yielded water with any dissolved compounds. The dissolved compounds were therefore hypothesized to have “rained” into the “sea” on early earth. In result, Miller and Urey had yielded amino acids and other organic molecules, supporting their hypothesis that earth most likely formed these organic molecules in which to help create our planet. Further experiments have observed that organic molecules may have formed near submerged volcanoes, rich with organic sulfur and iron. Carbonaceous chondrites, a type of meteorite, contain 1-2% carbon compounds; a 4.5 million year old meteorite found in Australia in 1969 even contained over 80 amino acids that were similar to those produced by Miller and Urey.
In addition to formulating organic molecules, there are numerous theories as to how primitive cells were created. There were early earth's protobionts, or aggregates of abiotically produced molecules surrounded by a membrane/membrane-like structure. Early ones incorporated polymers of amino acids into their membranes, making them more permeable to organic molecules in their surrounding environment. Experiments have demonstrated that protobionts could have formed spontaneously from organic compounds, such as one that used lipids and organic molecules in water that thus resulted in liposomes. This was yielded because of the lipid bilayer being selectively permeable and shrinking/swelling when located in different environments (i.e. hypotonic, hypertonic, or isotonic solutions). These are perhaps crucial because protobionts represent similar properties to life, including reproduction and metabolism, as well as homeostasis. It is why liposomes are believed to have selectively taken up organic molecules from their environment.
Aside from organic compounds and lipids possibly having formulated more complex organisms on earth, there are also the theories as to how genetic material and eukaryotic cells were created. The hypothesis for how genetic material was created stems from the RNA hypothesis, which states that short virus-like RNA sequences were the first genetic material on earth, not DNA. The characteristics that led to this hypothesis was that RNA: 1) plays a central role in protein synthesis, 2) acts as a catalyst (otherwise known as a ribozyme in this case), 3) comes in a variety of specific 3D shapes (and therefore has its own phenotype and genotype (nitrogenous base sequences), and 4) can replicate themselves in transcription. These facts alone are rather justified reasons for why RNA could have been the first genetic material. Just how RNA formed genetic material starts with the incorporation of RNA into protobionts. The RNA then acts as a template in which DNA nucleotides assembled. DNA eventually became more stable in replicating than RNA could have ever done before. This is why the “RNA world” gives way to the “DNA world”, as scientists state. Last comes the theory pertaining to how eukaryotic cells developed.
Eukaryotic cells are believed to have developed about 2.1 billion years ago, and developed from prokaryotes according to the process of endosymbiosis. In this process, mitochondria and plastids may have advanced whereby mitochondria and plastids were originally former small prokaryotes living within larger cells, otherwise known as endosymbionts. The ancestors of mitochondria may have been aerobic heterotrophic (where these organisms depend on organic molecules for energy) prokaryotes that became endosymbionts. The proposed ancestors of plastids may have been photosynthetic prokaryotes that eventually became endosymbionts. In forming eukaryotic cells, first comes the endomembrane system formulation, then the formation of mitochondria, and lastly, the formation of chloroplasts. All eukaryotes contain mitochondria or genetic remnants of such organelles, but not all contain plastids. Thus, the hypothesis of serial endosymbiosis supposes that mitochondria may have evolved before plastids. Systematists have focused on the nucleotide sequence of RNA in one of the ribosomal subunits in order to determine just which prokaryotic lineage gave rise to mitochondria and plastids. Inevitably, alpha proteobacteria were the closest found relatives to mitochondria, and that cyanobacteria were the closest relatives of plastids.
Now, we've discussed the possible theories as to how organic materials, primitive cells, genetic material, and eukaryotic cells developed. However, the question remains: why is earth important in containing living organisms and life in general? A major factor is that earth is neither too close nor too far from the sun. Life depends heavily on liquid and water, and many other planets are just too cold to support any form of life. Amino acids must be at a reasonable temperature in order to prosper. Earth's size and mass are also important in sustaining life; planets other than earth do not have the right gravitational pull to hold a protective atmosphere. Thus denser planets do not have the ability to receive light from the sun, which provide crucial energy rays for photosynthetic organisms. On the other hand, earth is able to block out the most energetic and harmful radiations from the sun, that could be otherwise capable of breaking covalent bonds between carbon atoms. The admission of important sun rays are crucial in the process of photosynthesis, a significant process making up the steps of evolution of complex living systems and organisms.