The question of how life evolved from matter is one of the ultimate questions of science, and is far from being solved. It is even difficult to clearly answer the question, what is life Gerald Joyce, a NASA exobiologist, defined life as a self-sustained chemical system capable of undergoing Darwinian evolution (Hazen, 2006) Two fundamental characteristics that distinguish life from non-life are metabolism and reproduction. Life grows and sustains itself metabolism is needed for this. Life replicates itself and evolves this involves reproduction. Life as we know it is based on carbon and our bodies consist of complex organic molecules. A class of organic molecules called amino acids are known as the building blocks of life. Amino acids that form proteins are essential for carrying out metabolism in living organisms. However, proteins are produced via the agency of nucleic acids or genes. Genes are needed for all kinds of duplication and reproduction in living cells. both proteins and genes are therefore essential for maintaining life.
The long process of evolution on earth traces itself back to single-celled organisms. Evidence suggests that life existed on earth as early as 3.5 billion years ago. More recently scientists have been able to push back the date up to 3.83 billion years ago (Eurekalert.org, 2009). Some kind of rudimentary life forms may have existed hundreds of millions even before that, though evidence may never be found for such primitive life forms bordering on inanimate matter. It would seem like somewhere around 4 to 4.4 billion years ago, complex organic molecules came together driven by pure chance, coalesced, and kicked into life. This is the traditional primordial soup theory of the origin of life. This was originally proposed by Darwin.
There are currently two major theories of origin of life, both involve the primordial soup with its complex molecules but in different degrees. The first theory suggests that the complexity of the randomly self-arranging organic molecules became ever greater until it reached a state where complex metabolic processes could be sustained within it. The second theory suggests that simpler forms of metabolism started at an intermediate stage of complexity of molecular organization and drove the level of molecular complexity steadily higher (Schirber, 2006).
1a) Genes-first RNA World theory
The assemblage of amino acids in the primordial soup could have taken place in some type of extreme conditions that prevailed on the early earth. A question arises here as to whether the first highly complex molecules of life were proteins or DNA (Deoxyribonucleic Acid). In living organisms, proteins and DNA are intricately tied up with each other DNA can replicate itself only by means of proteins, and proteins can be built only on the basis of the blue-print provided by DNA. DNA needs proteins, proteins need DNA but RNA (Ribonucleic Acid) can replicate itself. The possible role of RNA in the origins of life was first suggested by Francis Crick, the co-discoverer of DNA. RNA could conduct metabolism and carry out self-replication on its own. How could RNA that could catalyze its own replication have spontaneously formed from the amalgam of complex organic molecules And what was the likelihood of its happening The RNA World hypothesis concerns itself with these questions. It postulates that before life originated on the earth, the planet could have been rich in RNA material which worked as a substratum from which precellular and cellular life forms evolved. RNA-based catalysis and information storage were the first steps for the emergence of what we can recognize as life. From the RNA world evolved the more familiar protein-and-DNA world of lifes evolution on earth. DNA has greater stability than RNA, and has taken over the information storage function of RNA, while proteins which are much more efficient and flexible than RNA in the process of catalysis took over RNAs metabolic function.
1b) Metabolism-first theory
Objecting to the Genes-first RNA world theory, the metabolism-first theory of the origin of life states that a tremendously complex molecule such as RNA could not have emerged out of random conglomeration of organic molecules the theory instead proposes the initial emergence of metabolic cycles that could have been carried out even without very complex protein and genetic substances. A conglomeration of molecules much simpler than RNA could have first developed the ability to generate or absorb energy and maintain their organization via metabolism. Gradually the complexity of the metabolic processes could have developed and paved the way for the rise of the RNA world. Instead of molecules coming together and growing in complexity and generating metabolism once a level of complexity is reached, according to the metabolism-first approach metabolic processes kept growing in complexity and formed the basis of the eventual synthesis of nucleic acids.
2a) Evidence for Genes-first theory
It would be very difficult or impossible to gather any direct evidence for the origins of life. Fossil records underpin our knowledge of the course of evolution on earth, but the likelihood of gathering such clear indications regarding the mode of the origin of life on earth is very little. We can only favor a theory largely depending on the convincing power of its logic, and the ease of its demonstration and replication in a laboratory simulating conditions of the early earth. The primacy of RNA in the emergence of life on earth can be prima facie supported on the basis of RNAs ability to both store information and act as a catalytic agent an enzyme or a protein in chemical reactions. In theory, parts of the RNA molecule could have been spontaneously synthesized in the conditions that prevailed on the primitive earth with relative ease.
Experiments have corroborated this notion. Short strings of self-replicating RNA molecules have been produced artificially (Johnston et al, 2001). However, these were not generated spontaneously from simple inorganic molecules as were the complex organic molecules in the classic Miller-Urey experiment of the 1950s. It has been shown that smaller strings of catalytic RNA could concatenate by themselves under the right conditions and form into self-replicating RNA. Darwinian-type natural selection could have operated in the realm of RNA that existed on the primitive earth. The self-catalyzing structures which were more efficient at reproducing themselves could have proliferated leading to the eventual emergence of a full-fledged RNA molecule. Also, there are certain identified RNA enzymes which are self-sustaining and self-replicating.
2b) Evidence for Metabolism-first theory
The support for metabolism-first theory is based on the fact that spontaneous synthesis of RNA in the laboratory was not effectively demonstrated, and to the extent it was achieved it was accompanied by serious difficulties. That is to say, even when some kind of spontaneous synthesis was achieved it involved contrived conditions that were highly unlikely to have existed on the primitive earth. Also, in conditions of spontaneously growing complexity even if some set of molecules were heading in the right direction, they would be cut by other molecules long before they could achieve any semblance of RNA. Therefore some scientists felt the need to support the notion that metabolism could have first occurred in molecules much simpler than RNA. Envisaging smaller and simpler molecules interacting with other in a closed system of chemical reactions within some kind of membrane is easier than envisaging the miraculous appearance of RNA all by itself. Also, we can conceive these chemical interactions to be gradually growing in complexity without taxing credulity, say the proponents of metabolism-first theory. These gradually growing chemical reactions could have produced more complex molecules over time.
Incidentally, metabolism-first models have been first suggested in 1920s, many years before DNA was discovered. In the 1980s and 90s several different RNA-alternative models were proposed for the occurrence of complex chemical reactions in a metabolic cycle, each associated with its own degree of plausibility.
An undersea microbe which is also found in a variety of other environments, Methanosarcina acetivorans, can be used to support this theory. This Archaeon eats carbon monoxide and ejects methane and acetate. Many existing oldest forms of bacteria on earth can convert carbon monoxide into methane, but this microbe can additionally produce acetate. It does this with the help of two common enzymes. In undersea environments where there is the presence of the mineral ironsulfide, the expelled acetate reacts with the mineral forming a sulfide-containing compound known as acetate thioester. The microbe ingests this compound and again breaks it down to acetate creating ATP (adenosine triphosphate) in the process. Living organisms derive energy from ATP. One of the enzymes in the microbe can even synthesize ATP directly. This is perhaps the simplest self-sustaining metabolic process happening on the earth, requiring just two simple proteins or enzymes. This process was discovered by Biologist James Ferry and geochemist Christopher House in 2006. It very much bolsters the plausibility of metabolism-first theory (Mason 2006).
3) A theory of personal preference
The objections laid against the RNA world hypothesis by the proponents of metabolism-first approach are quite valid. The replication of RNA is far too complex process to have developed spontaneously within the first 500 million years of earths formation. The early earth was an exceedingly violent and tumultuous environment bearing little resemblance to the latter-day planet. It was not a warm, exotic haven maintaining the right conditions for complex organic molecules to bounce around and self-organize themselves into forms of ever increasing complexity.
Until recently it was believed that even if protocellular life could form by itself, it could not have withstood the intense meteoric bombardment that the planet was subjected to in those times, especially around 3.9 years ago. However, last year it was found out in NASA experiments that the bombardment would not have sterilized the earth completely, however intense it might have been. Therefore scientists now suppose that proto-life could have occurred in some deep and extreme environments of the planet from removed from the violent surface, as early as 4.4 billion years ago, soon after the moon formed during a colossal collision of the primitive earth with another same-sized planetary body. Simpler metabolic processes must have developed in complex organic compounds for 600 to 800 million years before the stage of RNA World was reached.
It is indeed possible for duplication and transmission of characteristics to occur in molecules known as compound genomes or composomes, which are far less complex than RNA or DNA. These genomes could have acted as some kind of metabolic systems which gradually grew in complexity and created a pathway for the formation of the first protocell. However, just two months ago NASA scientists demonstrated that Darwinian type evolution could not occur in molecules that were less complex than RNA and DNA, after carrying out rigorous analyses (physorg.com, 2010). This finding controverts a fundamental assumption of metabolism-first proponents that metabolic cycles could gradually increase in complexity. NASAs finding pushes us back to RNA world theory, but the degree of implausibility associated with RNA strands emerging spontaneously out of purely random coalition of molecules in the extremely chaotic environment of primitive earth still remains very high.
Some scientists believe that certain forms of RNA could have occurred on the planet over 4 to 4.4 billion years ago however, an important point to consider about this scenario is that except for the existence of liquid water, earth was not very different from other terrestrial planets in the solar system and millions of other planets elsewhere in the universe. If RNA could promptly emerge on the earth all by itself very little time after the planet solidified into existence, it could have also formed on millions of other planets and equally well led to life on all these places. But this does not seem to be the case. Spontaneous formation of RNA is not easy and may need several billions of years trial and error in stable environments with sufficient energy sources and not just a few tens or hundreds of millions of years in an intensely turbulent environment.
This consideration leaves us with the possibility that RNA molecules or even primitive cellular organisms could have evolved on some other distant planets over billions of years and could have subsequently drifted in cosmic debris toward our solar system carrying seeds of life with them. If the great interstellar distances pose a problem, we can alternatively picture another scenario the entire solar system formed from the debris of a previous supernova, primitive life forms could have developed over billions of years of evolution on some planet of our progenitor star that exploded and may have survived the explosion. Therefore primitive life could have existed in frozen forms in the very material from which the solar system formed, and later on it could have taken root on the earth. On the other earth-like planets of the solar system, Venus and Mars, it could have been sterilized.
Last year, NASA scientists identified an amino acid, glycine, in the material sample obtained from Comet Wild2 during its 2004 encounter with Stardust probe. This has corroborated the widely held belief that comets can typically contain complex organic molecules. More of such discoveries in the future can fairly substantiate the theory of extraterrestrial origin of life on earth. The metabolism-first approach seems to have hit a dead-end with the pronouncement of NASAs latest verdict on the issue. Future research on the genes-first hypothesis may only confirm how unlikely it is for RNA molecule to spontaneously evolve under very rough conditions within a relatively very short period of time. But any discovery of primitive life forms anywhere other than the planet earth can revolutionize our conception of life and how it could have originated on earth.
The long process of evolution on earth traces itself back to single-celled organisms. Evidence suggests that life existed on earth as early as 3.5 billion years ago. More recently scientists have been able to push back the date up to 3.83 billion years ago (Eurekalert.org, 2009). Some kind of rudimentary life forms may have existed hundreds of millions even before that, though evidence may never be found for such primitive life forms bordering on inanimate matter. It would seem like somewhere around 4 to 4.4 billion years ago, complex organic molecules came together driven by pure chance, coalesced, and kicked into life. This is the traditional primordial soup theory of the origin of life. This was originally proposed by Darwin.
There are currently two major theories of origin of life, both involve the primordial soup with its complex molecules but in different degrees. The first theory suggests that the complexity of the randomly self-arranging organic molecules became ever greater until it reached a state where complex metabolic processes could be sustained within it. The second theory suggests that simpler forms of metabolism started at an intermediate stage of complexity of molecular organization and drove the level of molecular complexity steadily higher (Schirber, 2006).
1a) Genes-first RNA World theory
The assemblage of amino acids in the primordial soup could have taken place in some type of extreme conditions that prevailed on the early earth. A question arises here as to whether the first highly complex molecules of life were proteins or DNA (Deoxyribonucleic Acid). In living organisms, proteins and DNA are intricately tied up with each other DNA can replicate itself only by means of proteins, and proteins can be built only on the basis of the blue-print provided by DNA. DNA needs proteins, proteins need DNA but RNA (Ribonucleic Acid) can replicate itself. The possible role of RNA in the origins of life was first suggested by Francis Crick, the co-discoverer of DNA. RNA could conduct metabolism and carry out self-replication on its own. How could RNA that could catalyze its own replication have spontaneously formed from the amalgam of complex organic molecules And what was the likelihood of its happening The RNA World hypothesis concerns itself with these questions. It postulates that before life originated on the earth, the planet could have been rich in RNA material which worked as a substratum from which precellular and cellular life forms evolved. RNA-based catalysis and information storage were the first steps for the emergence of what we can recognize as life. From the RNA world evolved the more familiar protein-and-DNA world of lifes evolution on earth. DNA has greater stability than RNA, and has taken over the information storage function of RNA, while proteins which are much more efficient and flexible than RNA in the process of catalysis took over RNAs metabolic function.
1b) Metabolism-first theory
Objecting to the Genes-first RNA world theory, the metabolism-first theory of the origin of life states that a tremendously complex molecule such as RNA could not have emerged out of random conglomeration of organic molecules the theory instead proposes the initial emergence of metabolic cycles that could have been carried out even without very complex protein and genetic substances. A conglomeration of molecules much simpler than RNA could have first developed the ability to generate or absorb energy and maintain their organization via metabolism. Gradually the complexity of the metabolic processes could have developed and paved the way for the rise of the RNA world. Instead of molecules coming together and growing in complexity and generating metabolism once a level of complexity is reached, according to the metabolism-first approach metabolic processes kept growing in complexity and formed the basis of the eventual synthesis of nucleic acids.
2a) Evidence for Genes-first theory
It would be very difficult or impossible to gather any direct evidence for the origins of life. Fossil records underpin our knowledge of the course of evolution on earth, but the likelihood of gathering such clear indications regarding the mode of the origin of life on earth is very little. We can only favor a theory largely depending on the convincing power of its logic, and the ease of its demonstration and replication in a laboratory simulating conditions of the early earth. The primacy of RNA in the emergence of life on earth can be prima facie supported on the basis of RNAs ability to both store information and act as a catalytic agent an enzyme or a protein in chemical reactions. In theory, parts of the RNA molecule could have been spontaneously synthesized in the conditions that prevailed on the primitive earth with relative ease.
Experiments have corroborated this notion. Short strings of self-replicating RNA molecules have been produced artificially (Johnston et al, 2001). However, these were not generated spontaneously from simple inorganic molecules as were the complex organic molecules in the classic Miller-Urey experiment of the 1950s. It has been shown that smaller strings of catalytic RNA could concatenate by themselves under the right conditions and form into self-replicating RNA. Darwinian-type natural selection could have operated in the realm of RNA that existed on the primitive earth. The self-catalyzing structures which were more efficient at reproducing themselves could have proliferated leading to the eventual emergence of a full-fledged RNA molecule. Also, there are certain identified RNA enzymes which are self-sustaining and self-replicating.
2b) Evidence for Metabolism-first theory
The support for metabolism-first theory is based on the fact that spontaneous synthesis of RNA in the laboratory was not effectively demonstrated, and to the extent it was achieved it was accompanied by serious difficulties. That is to say, even when some kind of spontaneous synthesis was achieved it involved contrived conditions that were highly unlikely to have existed on the primitive earth. Also, in conditions of spontaneously growing complexity even if some set of molecules were heading in the right direction, they would be cut by other molecules long before they could achieve any semblance of RNA. Therefore some scientists felt the need to support the notion that metabolism could have first occurred in molecules much simpler than RNA. Envisaging smaller and simpler molecules interacting with other in a closed system of chemical reactions within some kind of membrane is easier than envisaging the miraculous appearance of RNA all by itself. Also, we can conceive these chemical interactions to be gradually growing in complexity without taxing credulity, say the proponents of metabolism-first theory. These gradually growing chemical reactions could have produced more complex molecules over time.
Incidentally, metabolism-first models have been first suggested in 1920s, many years before DNA was discovered. In the 1980s and 90s several different RNA-alternative models were proposed for the occurrence of complex chemical reactions in a metabolic cycle, each associated with its own degree of plausibility.
An undersea microbe which is also found in a variety of other environments, Methanosarcina acetivorans, can be used to support this theory. This Archaeon eats carbon monoxide and ejects methane and acetate. Many existing oldest forms of bacteria on earth can convert carbon monoxide into methane, but this microbe can additionally produce acetate. It does this with the help of two common enzymes. In undersea environments where there is the presence of the mineral ironsulfide, the expelled acetate reacts with the mineral forming a sulfide-containing compound known as acetate thioester. The microbe ingests this compound and again breaks it down to acetate creating ATP (adenosine triphosphate) in the process. Living organisms derive energy from ATP. One of the enzymes in the microbe can even synthesize ATP directly. This is perhaps the simplest self-sustaining metabolic process happening on the earth, requiring just two simple proteins or enzymes. This process was discovered by Biologist James Ferry and geochemist Christopher House in 2006. It very much bolsters the plausibility of metabolism-first theory (Mason 2006).
3) A theory of personal preference
The objections laid against the RNA world hypothesis by the proponents of metabolism-first approach are quite valid. The replication of RNA is far too complex process to have developed spontaneously within the first 500 million years of earths formation. The early earth was an exceedingly violent and tumultuous environment bearing little resemblance to the latter-day planet. It was not a warm, exotic haven maintaining the right conditions for complex organic molecules to bounce around and self-organize themselves into forms of ever increasing complexity.
Until recently it was believed that even if protocellular life could form by itself, it could not have withstood the intense meteoric bombardment that the planet was subjected to in those times, especially around 3.9 years ago. However, last year it was found out in NASA experiments that the bombardment would not have sterilized the earth completely, however intense it might have been. Therefore scientists now suppose that proto-life could have occurred in some deep and extreme environments of the planet from removed from the violent surface, as early as 4.4 billion years ago, soon after the moon formed during a colossal collision of the primitive earth with another same-sized planetary body. Simpler metabolic processes must have developed in complex organic compounds for 600 to 800 million years before the stage of RNA World was reached.
It is indeed possible for duplication and transmission of characteristics to occur in molecules known as compound genomes or composomes, which are far less complex than RNA or DNA. These genomes could have acted as some kind of metabolic systems which gradually grew in complexity and created a pathway for the formation of the first protocell. However, just two months ago NASA scientists demonstrated that Darwinian type evolution could not occur in molecules that were less complex than RNA and DNA, after carrying out rigorous analyses (physorg.com, 2010). This finding controverts a fundamental assumption of metabolism-first proponents that metabolic cycles could gradually increase in complexity. NASAs finding pushes us back to RNA world theory, but the degree of implausibility associated with RNA strands emerging spontaneously out of purely random coalition of molecules in the extremely chaotic environment of primitive earth still remains very high.
Some scientists believe that certain forms of RNA could have occurred on the planet over 4 to 4.4 billion years ago however, an important point to consider about this scenario is that except for the existence of liquid water, earth was not very different from other terrestrial planets in the solar system and millions of other planets elsewhere in the universe. If RNA could promptly emerge on the earth all by itself very little time after the planet solidified into existence, it could have also formed on millions of other planets and equally well led to life on all these places. But this does not seem to be the case. Spontaneous formation of RNA is not easy and may need several billions of years trial and error in stable environments with sufficient energy sources and not just a few tens or hundreds of millions of years in an intensely turbulent environment.
This consideration leaves us with the possibility that RNA molecules or even primitive cellular organisms could have evolved on some other distant planets over billions of years and could have subsequently drifted in cosmic debris toward our solar system carrying seeds of life with them. If the great interstellar distances pose a problem, we can alternatively picture another scenario the entire solar system formed from the debris of a previous supernova, primitive life forms could have developed over billions of years of evolution on some planet of our progenitor star that exploded and may have survived the explosion. Therefore primitive life could have existed in frozen forms in the very material from which the solar system formed, and later on it could have taken root on the earth. On the other earth-like planets of the solar system, Venus and Mars, it could have been sterilized.
Last year, NASA scientists identified an amino acid, glycine, in the material sample obtained from Comet Wild2 during its 2004 encounter with Stardust probe. This has corroborated the widely held belief that comets can typically contain complex organic molecules. More of such discoveries in the future can fairly substantiate the theory of extraterrestrial origin of life on earth. The metabolism-first approach seems to have hit a dead-end with the pronouncement of NASAs latest verdict on the issue. Future research on the genes-first hypothesis may only confirm how unlikely it is for RNA molecule to spontaneously evolve under very rough conditions within a relatively very short period of time. But any discovery of primitive life forms anywhere other than the planet earth can revolutionize our conception of life and how it could have originated on earth.
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