Abiogenesis Is a Likely Explanation for Life on Earth

Composition of the Early AtmosphereThe early Earth's atmosphere was different than today's. For one thing, there was much less free oxygen. The composition of the atmosphere is very important in determining how life could have been created naturally. "Taking a clue from Earth's sibling planets, we might expect high concentrations of carbon dioxide (CO2),with small amounts of methane (CH4), and ammonia (NH3). Hydrogen and nitrogen probably also were present, as H2 and N2, as was hydrogen sulfide (H2S). Hydrogen has a tendency to leave the planet. Thus, it would have quickly reached low levels. "[1]The Origin of Life From Simple Organic CompoundsThe first step was the creation of amino acids, the building blocks of proteins, from the contents of Earth's atmosphere. The Miller-Urey experiment showed how this could occur. "Essential to the spontaneous origin of life was the availability of organic molecules as building blocks. The famous prebiotic soup’ experiment by Stanley Miller had shown that amino acids, the building blocks of proteins, arose among other small organic molecules spontaneously by reacting a mixture of methane, hydrogen, ammonia and water in a spark discharge apparatus. These conditions were assumed to simulate those on the primitive Earth. Already in 1922 Oparin had proposed that the early Earth had such a reducing atmosphere. .. It was suggested that only in a reducing atmosphere like this, synthesis of organic molecules – also sugars and organic bases, building blocks of nucleotides – would have been possible in large amounts. "(Note that when the author uses the term "spontaneously", he refers to the reaction of molecules into compouds; it's not like the compounds suddenly came into existence. )[2][3][4]An illustration of the Miller-Urey experiment can be seen here: The second step was the creation of RNA. Again, this could have been achieved in the conditions of the early Earth. On the early Earth, RNA was dominant for storing genetic information. "Here we show that activated pyrimidine ribonucleotides can be formed in a short sequence that bypasses free ribose and the nucleobases, and instead proceeds through arabinose amino-oxazoline and anhydronucleoside intermediates. The starting materials for the synthesis - cyanamide, cyanoacetylene, glycolaldehyde, glyceraldehyde and inorganic phosphate—are plausible prebiotic feedstock molecules, and the conditions of the synthesis are consistent with potential early-Earth geochemical models. ""[5]This chart below shows how this process could occur: Caption: "Pyrimidine ribonucleotide assembly options. Previously assumed synthesis of b-ribocytidine-2’,3’-cyclic phosphate 1 (blue; note the failure of the step in which cytosine 3 and ribose 4 are proposed to condense together) and the successful new synthesis described here (green). p, pyranose; f, furanose. (5 = Cyanoacetaldehyde, 6 = urea 6, cyanoacetylene 7, 8= cyanamide, 9 = glyceraldehyde, 10 = glycolaldehyde, 11 = 2-amino-oxazole, 12= pentose amino-oxazoline, arabinose derivative, 13 = anhydroarabinonucleoside). "[6] The creation of long strands of these ribonucleotides, important for the large chains needed to store genetic information, could have been catalyzed in a number of ways, one of which is a particular type of clay present on the early Earth. "Research from James Ferris' group at Rensselaer Polytechnic Institute suggests that the formation of long strands of RNA may have been catalyzed by clays such as montmorillonite. The charged clay surface attracts the nucleotides and the increased local concentration of nucleotides causes bond formation between nucleotides, forming a polymer of RNA. "[7]Another important component of RNA is the ability to replicate. Otherwise, new organisms could not be created. Ribozymes are RNA molecules that catalyze certain biochemical reactions, including RNA replication (this specific type of ribozyme is called a replicase). These replicases, being RNA molecules themselves, could easily have been found in addition to other types of RNA molecules. "Recently, however, the Hollinger group (MRC, UK) discovered an ice-water stabilized RNA polymerase ribozyme that is capable of copying strands of RNA that were over 200 basepairs long - longer than the ribozyme itself - suggesting that a self-replicating RNA is indeed possible. "[7]The origin of DNA is a bit murkier. While theoretically possible, it is much more likely that DNA evolved from RNA than DNA evolving all by itself. How this could happen is very complex, but the research has been done. "We are reasonably sure now that DNA and DNA replication mechanisms appeared late in early life history, and that DNA originated from RNA in an RNA/protein world. The origin and evolution of DNA replication mechanisms thus occurred at a critical period of life evolution that encompasses the late RNA world and the emergence of the Last Universal Cellular Ancestor (LUCA) to the present three domains of life (Eukarya, Bacteria and Archaea. ). "[8]The third and final step came from the creation of fatty acids. These fatty acids would envelope an RNA strand. Fatty acids are a type of semi-permeable membrane, allowing the RNA strand access to the environment without it escaping or encountering harmful substances. In addition, without fatty acids, replicases would make copies of every RNA molecule around it, inhibiting the spread of RNA. Still further, fatty acids helped the early RNA molecules to store energy. "Some scientists have proposed that hydrothermal vents may have been sites where prebiotically important molecules, including fatty acids, were formed. .. Research has shown that some minerals can catalyze the stepwise formation of hydrocarbon tails of fatty acids from hydrogen and carbon monoxide gases -- gases that may have been released from hydrothermal vents. Fatty acids of various lengths are eventually released into the surrounding water. [9]Now that we have amino acids, RNA (and later DNA), and fatty acids, we can get to so-called "protolife" in the form of the protocell from the collection of these elements. The next process would be to start to coalesce these life materials into a living organism. "The group of Jack Szostak has performed extensive and plausible studies that these fatty acid vesicles as containers for RNA would have allowed growth and replication merely by physico-chemical mechanisms, until a more sophisticated membrane machinery, steered by the cell itself and more resembling what is found in current organisms, would have taken their place. .. The group of Szostak also has demonstrated that nucleotides can pass through prebiotically plausible fatty-acid based vesicles and that non-enzymatic template copying of a model oligo dC DNA template can take place within them, which, in connection with the studies of vesicle growth and division, reveals in principle how a heterotrophic protocell may have functioned. "[6][10]Further, "Extrapolating from all the above data, inside fatty-acid vesicles the first self-replicating RNA molecule could have started copying itself. During copying, various things would have been possible. High-fidelity copies would have yielded the same self-replicating molecule. Copies with errors would mostly have resulted in RNA that was non-functional, but in a minority of cases, they could have yielded RNA that copied itself faster. It has been shown that RNA/vesicle systems that contain more genetic material (which would have resulted from faster RNA replication) develop more internal tension than neighboring vesicles that do not contain as much RNA, and draw membrane material from them. Importantly, this would have allowed for natural selection of vesicles by competition even in the absence of the ability to synthesize their own membrane components and therefore to directly control their own growth. Thus, for the first time, a system would have had the ability to undergo Darwinian evolution by natural selection acting on variation. This would have been a new and crucial emergent property arising at the transition from non-life to life. . "[6][11]This process can be illustrated here: Caption: "Conceptual model of a heterotrophic protocell. Growth of the protocell membrane results from the incorporation of environmentally supplied amphiphiles, whereas division may be driven by intrinsic or extrinsic physical forces. Externally supplied activated nucleotides permeate across the protocell membrane and act as substrates for the copying of internal templates. Complete template replication followed by random segregation of the replicated genetic material leads to the formation of daughter protocells. "[6]From here, the evolution from protocell to modern cell was as simple as making the application of DNA and changing the structure a bit of the protocell to be more compatible for use in a multi-cellular organism. "In summary, based on available data a spontaneous origin of life as simple ‘cells’ containing a single genetic polymer, upon which natural selection could act, is feasible. A gradual evolutionary transition from these to common cellular complexity would have been possible. "[6]Sources[1]: . http://earthguide.ucsd.edu...[2]: . http://www.talkorigins.org...[3]: . http://www.sciencemag.org...[4]: . http://www.nature.com...[5]: . http://www.nature.com...[6]: . http://www.talkorigins.org...[7]: . http://exploringorigins.org...[8]: . http://www.ncbi.nlm.nih.gov...[9]: . http://exploringorigins.org...[10]: . http://www.pnas.org...[11]: . http://www.sciencedaily.com...