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Benchwarmer
Abiogensis (Scientifc Creationism if you will pardon the pun)
In the natural sciences, abiogenesis, or origin of life, is the study of how life on Earth emerged from inanimate organic and inorganic molecules. Scientific research theorizes that abiogenesis occurred sometime between 4.4 billion years ago, when water vapor first liquefied, and 2.7 billion years ago, when the ratio of stable isotopes of carbon (12C and 13C), iron (56Fe, 57Fe, and 58Fe) and sulfur (32S, 33S, 34S, and 36S) points to a biogenic origin of minerals and sedimentsand molecular biomarkers indicate photosynthesis.
Until the early 19th century people frequently believed in spontaneous generation of life from non-living matter.
Classical notions of abiogenesis, now more precisely known as spontaneous generation, held that complex, living organisms are generated by decaying organic substances, e.g. that mice spontaneously appear in stored grain, maggots spontaneously appear in meat, or moderlieschens and eels are produced by mud in ephemeral ponds.
According to Aristotle it was a readily observable truth that aphids arise from the dew which falls on plants, fleas from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so forth.
In the 17th century such assumptions started to be questioned; such as that by Sir Thomas Browne in his Pseudodoxia Epidemica, subtitled Enquiries into Very many Received Tenets, and Commonly Presumed Truths, of 1646, an attack on false beliefs and "vulgar errors." His conclusions were not widely accepted, e.g. his contemporary, Alexander Ross wrote:
"To question this (i.e., spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants".
In 1546 the physician Girolamo Fracastoro theorized that epidemic diseases were caused by tiny, invisible particles or "spores", which might not be living creatures, but this was not widely accepted.
Next, Robert Hooke published the first drawings of a microorganism in 1665. He is also credited for naming the cell which he discovered while observing cork samples.
Then in 1676 Anthony van Leeuwenhoek discovered microorganisms that, based on his drawings and descriptions are thought to have been protozoa and bacteria. This sparked a renewal in interest in the microscopic world.
The first step was taken by the Italian Francesco Redi, who, in 1668, proved that no maggots appeared in meat when flies were prevented from laying eggs.
From the 17th century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be omne vivum ex ovo: that every living thing came from a pre-existing living thing (literally, everything from an egg).
In 1768 Lazzaro Spallanzani proved that microbes came from the air, and could be killed by boiling.
Yet it was not until 1861 that Louis Pasteur performed a series of careful experiments which proved that organisms such as bacteria and fungi do not appear in nutrient rich media of their own accord in non-living material, and which supported cell theory.
By the middle of the 19th century Pasteur and other scientists discovered the theory of Biogenesis by demonstrating that living organisms do not arise spontaneously from non-living matter.
No real progress was made until 1924 when Aleksandr Ivanovich Oparin reasoned that atmospheric oxygen prevented the synthesis of the organic molecules that are the necessary building blocks for the evolution of life. In his The Origin of Life,Oparin argued that a "primeval soup" of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight.
There is no truly "standard model" of the origin of life. But most currently accepted models build in one way or another upon a number of discoveries about the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence:
1. Plausible pre-biotic conditions result in the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller-Urey experiment by Stanley L. Miller and Harold C. Urey in 1953.
2. Phospholipids (of an appropriate length) can spontaneously form lipid bilayers, a basic component of the cell membrane.
3. The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis).
4. Selection pressures for catalytic efficiency and diversity result in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent.
5. Proteins outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use.
The origin of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic chemicals from which life is thought to have formed are:
1. Methane (CH4),
2. Ammonia (NH3),
3. Water (H2O),
4. Hydrogen sulfide (H2S),
5. Carbon dioxide (CO2) or carbon monoxide (CO), and
6. Phosphate (PO43-).
Origin of organic molecules
There are three sources of organic molecules on the early Earth:
1. organic synthesis by other energy sources (such as ultraviolet light or electrical discharges) (eg.Miller's experiments).
2. delivery by extraterrestrial objects (eg carbonaceous chondrites);
3. organic synthesis driven by impact shocks.
Recently estimates of these sources suggest that the heavy bombardment before 3.5 Gyr ago within the early atmosphere made available quantities of organics comparable to those produced by other energy source.
In 1953 a graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that demonstrated how organic molecules could have spontaneously formed on early Earth from inorganic precursors. The now-famous ?Miller-Urey experiment? used a highly reduced mixture of gases ? methane, ammonia and hydrogen ? to form basic organic monomers, such as amino acids.
In the natural sciences, abiogenesis, or origin of life, is the study of how life on Earth emerged from inanimate organic and inorganic molecules. Scientific research theorizes that abiogenesis occurred sometime between 4.4 billion years ago, when water vapor first liquefied, and 2.7 billion years ago, when the ratio of stable isotopes of carbon (12C and 13C), iron (56Fe, 57Fe, and 58Fe) and sulfur (32S, 33S, 34S, and 36S) points to a biogenic origin of minerals and sedimentsand molecular biomarkers indicate photosynthesis.
Until the early 19th century people frequently believed in spontaneous generation of life from non-living matter.
Classical notions of abiogenesis, now more precisely known as spontaneous generation, held that complex, living organisms are generated by decaying organic substances, e.g. that mice spontaneously appear in stored grain, maggots spontaneously appear in meat, or moderlieschens and eels are produced by mud in ephemeral ponds.
According to Aristotle it was a readily observable truth that aphids arise from the dew which falls on plants, fleas from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so forth.
In the 17th century such assumptions started to be questioned; such as that by Sir Thomas Browne in his Pseudodoxia Epidemica, subtitled Enquiries into Very many Received Tenets, and Commonly Presumed Truths, of 1646, an attack on false beliefs and "vulgar errors." His conclusions were not widely accepted, e.g. his contemporary, Alexander Ross wrote:
"To question this (i.e., spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants".
In 1546 the physician Girolamo Fracastoro theorized that epidemic diseases were caused by tiny, invisible particles or "spores", which might not be living creatures, but this was not widely accepted.
Next, Robert Hooke published the first drawings of a microorganism in 1665. He is also credited for naming the cell which he discovered while observing cork samples.
Then in 1676 Anthony van Leeuwenhoek discovered microorganisms that, based on his drawings and descriptions are thought to have been protozoa and bacteria. This sparked a renewal in interest in the microscopic world.
The first step was taken by the Italian Francesco Redi, who, in 1668, proved that no maggots appeared in meat when flies were prevented from laying eggs.
From the 17th century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be omne vivum ex ovo: that every living thing came from a pre-existing living thing (literally, everything from an egg).
In 1768 Lazzaro Spallanzani proved that microbes came from the air, and could be killed by boiling.
Yet it was not until 1861 that Louis Pasteur performed a series of careful experiments which proved that organisms such as bacteria and fungi do not appear in nutrient rich media of their own accord in non-living material, and which supported cell theory.
By the middle of the 19th century Pasteur and other scientists discovered the theory of Biogenesis by demonstrating that living organisms do not arise spontaneously from non-living matter.
No real progress was made until 1924 when Aleksandr Ivanovich Oparin reasoned that atmospheric oxygen prevented the synthesis of the organic molecules that are the necessary building blocks for the evolution of life. In his The Origin of Life,Oparin argued that a "primeval soup" of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight.
There is no truly "standard model" of the origin of life. But most currently accepted models build in one way or another upon a number of discoveries about the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence:
1. Plausible pre-biotic conditions result in the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller-Urey experiment by Stanley L. Miller and Harold C. Urey in 1953.
2. Phospholipids (of an appropriate length) can spontaneously form lipid bilayers, a basic component of the cell membrane.
3. The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis).
4. Selection pressures for catalytic efficiency and diversity result in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent.
5. Proteins outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use.
The origin of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic chemicals from which life is thought to have formed are:
1. Methane (CH4),
2. Ammonia (NH3),
3. Water (H2O),
4. Hydrogen sulfide (H2S),
5. Carbon dioxide (CO2) or carbon monoxide (CO), and
6. Phosphate (PO43-).
Origin of organic molecules
There are three sources of organic molecules on the early Earth:
1. organic synthesis by other energy sources (such as ultraviolet light or electrical discharges) (eg.Miller's experiments).
2. delivery by extraterrestrial objects (eg carbonaceous chondrites);
3. organic synthesis driven by impact shocks.
Recently estimates of these sources suggest that the heavy bombardment before 3.5 Gyr ago within the early atmosphere made available quantities of organics comparable to those produced by other energy source.
In 1953 a graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that demonstrated how organic molecules could have spontaneously formed on early Earth from inorganic precursors. The now-famous ?Miller-Urey experiment? used a highly reduced mixture of gases ? methane, ammonia and hydrogen ? to form basic organic monomers, such as amino acids.