Abiogenesis
An overview of Abiogenesis
Abiogenesis is a theory that explains the original evolution of life or living organisms from inorganic or inanimate substances. Abiogenesis proposes that the first life-forms generated were very simple and through a gradual process became increasingly complex. Biogenesis, in which life is derived from the reproduction of other life, was presumably preceded by abiogenesis, which became impossible once Earth’s atmosphere assumed its present composition.
Much details of this process are still unknown,
A prevailing hypothesis is that the transition from non-living to living entities was not a single event, but a process of continuously increasing complexity that involved molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes.
The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions different from those on Earth today
Abiogenesis due to the recent technological developments has branched into an interdisciplinary field, taking aid from biology, chemistry, and geophysics, astrobiology, biochemistry, biophysics, geochemistry, molecular biology, oceanography, and paleontology.
Now let's talk about the steps involving abiogenesis:
Scientists have proposed a four-stage process of formation for the first life:
1). formation of small organic molecules (amino acids, nucleic acid bases,…),
2). and these combine to make larger biomolecules (proteins, RNA, lipids,…),
3). which self-organized, by a variety of interactions, into a semi-alive system
4). that gradually transformed into a more sophisticated form, a living organism.
Let’s dwell into each stage in detail:
Stage 1 — PreBiotic Chemistry (Miller-Urey and more):
Modern studies of prebiotic (pre-biological) biochemistry — to form organic molecules and biomolecules in stages 1 and 2 — began in 1953 with the Miller-Urey Experiment. Early MU experiments used a reducing atmosphere with reactive chemicals (CH4, H2, and NH3) plus H2O. Within two decades, most geologists thought the early earth had a non-reducing neutral atmosphere (mainly CO2 and N2 plus H2O) that was much less reactive; when these chemicals were used in later variations of MU the yields of organic molecules were much lower. But geological questions about earth's early atmosphere continued through the 1990s, and in 2005 calculations about gas from chordates indicated that the atmosphere might have been reducing, similar to the early MUs. Currently, the chemistry of the early atmosphere is in doubt.
There have been questions about other aspects of Miller-Urey experiments, such as the choice of energy sources and why newly formed products were isolated (before they could be broken down by further reactions), to ask whether the MUs were realistic simulations of conditions on the early earth.
In response to these questions and their own, researchers studied a wide variety of Miller-Urey variations, using different reactant mixtures, energy sources, and conditions, and in the reaction products, they observed a variety of organic compounds, in amounts that spanned a wide range but usually were fairly low.
In addition, scientists discovered that objects from outer space (meteors, comets,…) contain interesting organic compounds, plus H2O, and these compounds would have become "part of the reaction mixture" when the space-objects landed on earth.
Stage 2 — Polymer Chemistry (to make proteins, RNA,…):
The Miller-Urey experiments are about stage 1, forming small organic molecules.
In stage 2, problems occur due to energetics — because in water the reactions to form larger biomolecules (proteins, RNA, and DNA) are energetically unfavorable — and also due to competition.
For example, during protein synthesis, a prebiotic reaction mixture would contain many different chemicals (L-amino acids and R-amino acids, plus many other molecules) and the majority of newly formed bonds would not be the special peptide bonds (linking only L-amino acids) found in natural proteins. The scarcity of L-peptide bonds is partly due to the fact that in a watery "soup" the formation of these bonds is energetically unfavorable. Therefore, abiogenesis researchers have searched for and studied non-aqueous reaction sites, such as evaporated ponds or on the surface of minerals.
Similar difficulties would arise in the prebiotic formation of other important biomolecules. Problems occur in both stages of forming RNA, in forming ribose sugars and some nucleotide bases (in stage 1) and connecting these together (in stage 2). The prebiotic synthesis of RNA has been especially unsuccessful, but perhaps special environments (such as the surface of minerals) could help with the reactions.
Stages 3 and 4 — Chemical Evolution into the First Life:
Chemical evolution is the sequence of chemical changes in originally nonliving matter that gives rise to life. The phrase "chemical evolution" is also used, in astronomy and cosmology, to describe the changing makeup of the Universe's stock of chemical elements through deep time since the Big Bang, from hydrogen and helium immediately after the Big Bang to the full array of elements observed today.
According to prevalent theory, chemical evolution occurred in four stages. In the first stage of chemical evolution, molecules in the primitive environment formed simple organic substances, such as amino acids. This concept was first proposed in 1936. It was considered that hydrogen, ammonia, water vapor, and methane to be components in the early atmosphere. Oxygen was lacking in this chemically-reducing environment. ultraviolet radiation from the Sun provided the energy for the transformation of these substances into organic molecules.
Proof for Abiogenesis:
The Miller-Urey Experiment: The Miller-Urey experiment was the first attempt to explore the origin of life. Stanley Miller simulated conditions proposed to be common on the ancient Earth. The purpose was to test the idea that the complex molecules surrounding life ( amino acids in this case) could have arisen on our young planet through simple, natural chemical reactions.
The experiment was a success in which amino acids, the building blocks of life, were produced during the simulation. The finding was so significant that it kick-started an entirely new discipline of study: Prebiotic Chemistry.
Scientists now have reason to believe that the gases used in the Miller-Urey simulation were not actually the same as those of the ancient atmosphere. Because of this, many experiments have since been done, testing a wide variety of atmospheres and different environmental conditions. The results are fantastic: the molecules of life can form under a wide variety of ancient Earth-like conditions. The Miller-Urey Experiment stands as the biggest conformation for Abiogenesis.
Fischer-Tropsch Process: The Fischer-Tropsch process is a gas to liquid (GTL) polymerization technique that turns a carbon source into hydrocarbons chains through the hydrogenation of carbon monoxide by means of a metal catalyst. The feedstock is typically coal or natural gas, though more exotic (and carbon neutral) possibilities such as removing CO2 from the ocean or the atmosphere have been considered. This process is speculated to have been involved in the formation of hydrocarbons in the early Earth’s atmosphere.
Let's have an outline of problems surrounding the field of Abiogenesis:
There are chemistry problems in Stages 1 and 2, due to some energetically unfavorable reactions and unproductive competitive reactions.
But the toughest problems are biological, in Stages 3 and 4, because the simplest possible "living system" seems to require hundreds of components interacting in an organized way to achieve self-replication and energy production, and this organized complexity would have to occur before natural selection (which depends on self-replication) was available. For these stages we can ask, "Are scientists learning that what is required for life is greater than what is possible by the natural process?" or is our current knowledge insufficient to answer this question because we don't yet know enough about what is required and what is possible?
Apart from the problems in these stages, we can also generalize some problems, these are:
There is no detailed theoretical path to go from complex organic molecules to a life form.
There are no successful experiments supporting the formation of molecules more complex than amino acids.
There is no mechanism for RNA building blocks to develop into the purine/pyrimidine bases of full RNA.
There is no consensus on how the replicating/metabolizing molecules become life forms.
Citations/More to Read:-
The Vital Question
https://archive.org/details/genesisscientifi0000haze
https://web.archive.org/web/20151010074651/http://wwwdca.iag.usp.br/www/material/fornaro/ACA410/Kasting%201993_EarthEarlyAtmos.pdf,
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3131571/
Pic credit- @google
©Tanmay
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