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Origin of Life

Origin of Life

How did life begin on Earth? The fact is that no one knows the answer yet, and it remains one of the primary unsolved questions of biology. We may never know with certainty because life began on Earth nearly four billion years ago. The events that initiated life no longer occur, and even the conditions of that the early Earth are not known with any certainty.

We do know one thing with reasonable certainty: Even bacteria, the simplest forms of life today, are so complex that they could not have appeared spontaneously on the early earth. More likely there were even simpler forms of life that required several hundred million years to evolve into bacterial life, complete with deoxyribonucleic acid (DNA) genes , metabolic pathways, ribonucleic acid (RNA) machinery, and protein catalysts .

The first life probably appeared several hundred million years after Earth was formed as a planet in the early solar system 4.5 billion years ago. There are many lines of evidence that support this statement, but the simplest to understand is the fossil record. Even bacteria leave fossils, and such micro-fossils were discovered in Australian rocks that are about 3.5 billion years old.

Something else scientists know with certainty is that Earth was very different when life first began. For example, the large number of craters on the moon's surface were produced by giant impacts of comets and asteroidsized objects that were part of the accretion process by which the moon formed. The collisions continued until about 3.9 billion years ago. During that time Earth was also being hit by objects many kilometers in diameter, and the first life could not have begun until the violent bombardment ceased. Therefore, scientists estimate that the simplest form of life probably was present about 3.8 billion years ago, and over a few hundred million years evolved into the bacteria that left the Australian microfossils.

What sorts of chemical and physical processes might have produced the first forms of life? A brief summary of the properties of life today can tell scientists a lot about how life began. All life is cellular, from single-celled bacteria to multicellular human beings. Cells have anywhere from a thousand or so genes (bacteria) to thirty thousand genes (human beings) and each gene carries the information to synthesize a specific protein. The synthesis of proteins requires energy and occurs on ribosomes . RNA carries genetic information transcribed from DNA to the ribosomes where it is used to direct the synthesis of proteins.

Properties of Life

The most basic activity of life is a process called polymerization. During this process organized systems of molecules use energy and nutrients to grow by linking smaller molecules into larger molecules. The chemical reactions involving energy and nutrients are collectively called metabolism , and the individual reactions of metabolism are catalyzed, meaning their rates are increased in a controlled way by specific molecules (proteins, in the case of all living organisms). Second, a living organism has the potential to reproduce itself at some point in its life cycle. Third, because mutations can lead to variations among individuals, populations of living organisms can evolve over time from generation to generation, responding to changes in their environment through natural selection. When one talks about the origin of life, one therefore must think about how organized systems of organic molecules could have appeared on the early Earth, and how they could take on the basic properties of the living state defined above.

Early Proposals

Louis Pasteur was the first scientist to think about how life can begin. In 1865, Pasteur showed that bacteria do not occur spontaneously in sterile culture media, and concluded that life can only appear from preexisting microorganisms. This view was accepted by the scientific community for more than fifty years, until a young Russian scientist named Alexander Oparin realized that preexisting organisms could not have been present on the early earth, which must have been sterilized by the heat of its formation. And yet, somehow life began. Oparin suggested that organic molecules could spontaneously aggregate into larger structures he called coacervates, one of which could have happened to have the basic properties of life. In general, Oparin's proposal about aggregation remains a viable hypothesis for the origin of life, but his coacervates are no longer considered to be plausible models of the first forms of life.

The next advance came with a better understanding of chemistry and biochemistry. Life as it is known in the twenty-first century requires organic compounds containing carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, and these are present in four kinds of biochemical compounds and their polymers: amino acids and proteins, nucleotides and nucleic acids, simple sugars and polysaccharides like starch and cellulose , and lipids , which self-assemble into cell membranes. Were such compounds available for the origin of life? The answer seems to be yes. Even today, certain meteorites fall to Earth that contain thousands of different organic compounds, including amino acids, synthesized by nonbiological processes. Scientists also know from the early experiments of Stanley Miller and Harold Urey that organic compounds can be synthesized under simulated prebiotic conditions, so it seems reasonable to assume that simple organic compounds were present on the early Earth.

Self-Assembly

Now one can think about the actual process by which a living organism could appear on the early Earth. An important point to understand is that some organic molecules have properties that allow them to spontaneously organize into larger structures. A common example is the self-assembly of soap molecules into soap bubbles. A living cell resembles a microscopic bubble, and the same forces that produce a soap bubble also stabilize the membrane that surrounds all living cells like a skin and separates the cytoplasm from the outside world. It is easy to imagine that such microscopic bubbles were present on the early Earth, and it has been shown that some of the organic compounds in meteorites can in fact produce bubblelike structures.

Although the assembly of microscopic membranes from soaplike molecules is interesting, two other self-assembly processes are equally important. The first is that the long strings of polymerized amino acids called proteins can fold up into tightly packed balls that represent the functional proteins, such as enzymes . This folding process occurs in all cells as proteins are synthesized from amino acids on ribosomes. If proteinlike molecules were somehow produced on the early Earth, they would also have the capacity to fold into a variety of structures, some of which might act as catalysts.

The second self-assembly process is that long strings of polymerized nucleotides called nucleic acids can wind together into double stranded structures. The famous DNA double helix is an example, and this is the only way that scientists know that a molecule can reproduce itself. That is, one strand of DNA acts as a template , and a second strand is produced on the template when nucleotides bind to it and are then linked together. All life today depends on this process, which is called replication, and the earliest forms of life must have had a primitive version incorporated into their system of molecules.

Defining How Life Began

Given all this, scientists can hypothesize how life began on Earth. There is little doubt that mixtures of organic compounds became organized into complex systems by self-assembly processes, because the same thing happens in the organic compounds of meteorites, which are as old as the solar system. These self-assembled systems can be thought of as countless natural experiments that occurred all over Earth for hundreds of millions of years.

The next step occurred when a few of the microscopic systems had the particular set of molecules and properties that allowed them to capture energy and nutrients from the environment, and use them to produce larger polymeric molecules. In the next step toward life, one of the growing systems contained molecules that could be used as templates to direct further growth, so that a second polymeric molecule was in a sense a replica of the first molecule. DNA synthesis in cells is a primary example of molecular growth by polymerization, and also demonstrates how the information in one molecule can be reproduced in a second molecule. Because these processes can be reproduced under laboratory conditions, one can be reasonably certain that they are plausible reactions on the early Earth, even though scientists don't know yet how the first long polymers were produced.

The last step in the origin of life is that one or more of the growing, replicating systems happened to find a way to use the sequence of monomers in one molecule, such as a nucleic acid, to direct the sequence of monomers in another kind of molecule such as a protein. This was the origin of the genetic code and the beginning of life. It also marked the beginning of evolution, because molecular systems composed of two different interacting molecules like nucleic acids and proteins have the potential to undergo mutational change followed by selection.

It is amazing to think that this complex set of events occurred spontaneously on the early Earth, and that life was up and running only a few hundred million years after Earth had cooled sufficiently for liquid water to exist. And yet, this seems to be what happened, and if it happened on Earth it could also happen elsewhere, since the laws of chemistry and physics are believed to be universal. This larger understanding of life has led to a new scientific discipline called astrobiology, which is defined as the study of life in the universe.

Could Life Have Begun Elsewhere?

Could life have begun elsewhere? The simplest place to look is in the solar system and compare other planets with Earth. Scientists now have a better understanding of where life exists on Earth, and it is much more widely distributed than we might have guessed. Bacterial life exists over a remarkable temperature range, from near 0°C (32°F) on melting snow to over 115°C (239°F) in submarine hydrothermal vents. It exists in acidic environments as strong as battery acid or as alkaline as household ammonia. Bacterial life exists in the dark, in the absence of oxygen, and has even been found growing in the radioactive water of nuclear reactors. In fact, the only constant is that microbial life requires liquid water, and if liquid water exists elsewhere we might expect that life could have started as it did on Earth, and may even still be flourishing.


Europa was discovered by Galileo Galilei in 1610. This moon of Jupiter is the sixth largest moon in our solar system.


Where in the solar system might one find liquid water? There are only two places that scientists know of: Mars and Europa. Mars certainly has water, but in the form of ice. Liquid water cannot exist for long on the surface of Mars, due to the cold temperature and low atmospheric pressure, but it could be locked up in ice beneath the surface, just as water is present in the permafrost of Arctic tundras. Recent images from the Mars Global Surveyor clearly show that liquid water occasionally breaks through the ice and pours down steep slopes on the edges of craters. Europa, a moon of Jupiter about the size of Earth's moon, also has water in the form of a thick sheet of ice, and beneath the ice is a global ocean of liquid water. On both Mars and Europa there is a distinct possibility that life similar to bacteria could be present, and future space missions may finally answer the age-old question: Does life exist elsewhere?

see also Cell Evolution; Evolution; Evolution, Evidence for; Life, What Is

David W. Deamer

Bibliography

Malin, M. C., and K. S. Edgett. "Evidence for Recent Groundwater Seepage and Surface Runoff on Mars." Science 288 (2000): 23302335.

Miller, S. L. "Production of Amino Acids Under Possible Primitive Earth Conditions." Science 117 (1953): 528529.

Miller, S. L., and Urey, H. C. "Organic Compound Synthesis on the Primitive Earth." Science 130 (1959): 245251.

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Origin of Life

Origin of Life

The origin of life has been a subject of speculation in all known cultures and indeed, all have some sort of creation idea that rationalizes how life arose. In the modern era, this question has been considered in terms of a scientific framework, meaning that it is approached in a manner subject to experimental verification as far as that is possible. Geological formations contain a wealth of information concerning the origin of life on Earth and provide abundant evidence of the relationships between physical and biological evolutionary processes.

Radioactive dating provides evidence that that Earth formed at least 4.6 billion years ago. Yet, the earliest known fossils of microorganisms, similar to modern bacteria, are only about 3.53.8 billion years old. The earlier prebiotic era (i.e., before life began) left no direct record, and so it cannot be determined from the geologic record exactly how life arose. It is possible, however, to at least demonstrate the kinds of abiotic reactions that may have led to the formation of living systems through laboratory experimentation. It is generally accepted that the development of life occupied three stages: First, chemical evolution, in which simple geologically occurring molecules reacted to form complex organic polymers. Second, collections of these polymers self organized to form replicating entities. At some point in this process, the transition from a lifeless collection of reacting molecules to a living system probably occurred. The third process following organization into simple living systems was biological evolution, which ultimately produced the complex web of modern life.

The underlying biochemical and genetic unity of organisms suggests that life arose only once, or if it arose more than once, the other life forms must have become rapidly extinct. All organisms are made of chemicals rich in the same kinds of carbon-containing, organic compounds. The predominance of carbon in living matter is a result of its tremendous chemical versatility compared with all the other elements. Carbon has the unique ability to form a very large number of compounds as a result of its capacity to make as many as four highly stable covalent bonds (including single, double, triple bonds) combined with its ability to form covalently linked carbon-carbon (CC) chains of unlimited length. The same 20 carbon and nitrogen containing compounds called amino acids combine to make up the enormous diversity of proteins occurring in living things. Moreover, all organisms have their genetic blueprint encoded in nucleic acids, either DNA or RNA. Nucleic acids contain the information needed to synthesize specific proteins from their amino acid components. Enzymes, catalytic proteins, which increase the speed of specific chemical reactions, regulate the activity of nucleic acids and other biochemical functions essential to life, while other proteins provide the structural framework of cells. These two types of molecules, nucleic acids and proteins, are essential enough to all organisms that they, or closely related compounds, must also have been present in the first life forms.

Scientists suspect that the primordial Earth's atmosphere was very different from what it is today. The modern atmosphere with its 79% nitrogen, 20% oxygen , and trace quantities of other gases is an oxidizing atmosphere. The primordial atmosphere is generally believed not to have contained significant quantities of oxygen, having instead rather small amounts of gases such as carbon monoxide, methane, ammonia and sulphate in addition to the water , nitrogen and carbon dioxide that it still contains today. With these combinations of gases, the atmosphere at that time would have been a reducing atmosphere providing the hydrogen atoms for the synthesis of compounds needed to create life. In the 1920s, the Soviet scientist Aleksander Oparin (18941980) and the British scientist J.B.S. Haldane (18921964) independently suggested that ultraviolet (UV) light, which today is largely absorbed by the ozone layer in the higher atmosphere, or violent lightning discharges, caused molecules of the primordial reducing atmosphere to react and form simple organic compounds (e.g., amino acids, nucleic acids and sugars). The possibility of such a process was demonstrated in 1953 by Stanley Millar and Harold Urey , who simulated the effects of lightning storms in a primordial atmosphere by subjecting a refluxing mixture of water, methane, ammonia and hydrogen to an electric discharge for about a week. The resulting solution contained significant amounts of water-soluble organic compounds including amino acids.

The American scientist, Norman H. Horowitz proposed several criteria for living systems, saying that they all must exhibit replication, catalysis and mutability. One of the chief features of living organisms is their ability to replicate. The primordial self-replicating systems are widely believed to have been nucleic acids, like DNA and RNA, because they could direct the synthesis of molecules complementary to themselves. One hypothesis for the evolution of self-replicating systems is that they initially consisted entirely of RNA. This idea is based on the observation that certain species of ribosomal RNA exhibit enzyme-like catalytic properties and also all nucleic acids are prone to mutation. Thus RNA can demonstrate the three Horowitz criteria and the primordial world may well have been an "RNA world." A cooperative relationship between RNA and protein could have arisen when these self-replicating protoribosomes evolved the ability to influence the synthesis of proteins that increased the efficiency and accuracy of RNA synthesis. All these ideas suggest that RNA was the primary substance of life and the later participation of DNA and proteins were later refinements that increased the survival potential of an already self-replicating living system. Such a primordial pond where all these reactions were evolving eventually generated compartmentalization amongst its components. How such cell boundaries formed is not known, though one plausible theory holds that membranes first arose as empty vesicles whose exteriors served as attachment sites for entities such as enzymes and chromosomes in ways that facilitated their function.

See also Atmospheric chemistry; Cambrian Period; Carbon dating; Earth (planet); Evolution, evidence of; Evolutionary mechanisms; Evolution; Geologic time; Miller-Urey experiment; Precambrian; Uniformitarianism

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Life, Origin of

Life, origin of

The origin of life has been a subject of speculation in all known cultures and indeed, all have some sort of creation idea that rationalizes how life arose. In the modern era, this question has been considered in terms of a scientific framework, meaning that it is approached in a manner subject to experimental verification as far as that is possible. Radioactive dating suggests that Earth formed at least 4.6 billion years ago. Yet, the earliest known fossils of microorganisms , similar to modern bacteria , are present in rocks that are 3.53.8 billion years old. The earlier prebiotic era (i.e., before life began) left no direct record, and so it cannot be determined exactly how life arose. It is possible, however, to at least demonstrate the kinds of abiotic reactions that may have led to the formation of living systems through laboratory experimentation. It is generally accepted that the development of life occupied three stages: First, chemical evolution , in which simple geologically occurring molecules reacted to form complex organic polymers. Second, collections of these polymers self organized to form replicating entities. At some point in this process, the transition from a lifeless collection of reacting molecules to a living system probably occurred. The third process following organization into simple living systems was biological evolution, which ultimately produced the complex web of modern life.

The underlying biochemical and genetic unity of organisms suggests that life arose only once, or if it arose more than once, the other life forms must have become rapidly extinct. All organisms are made of chemicals rich in the same kinds of carbon-containing, organic compounds. The predominance of carbon in living matter is a result of its tremendous chemical versatility compared with all the other elements. Carbon has the unique ability to form a very large number of compounds as a result of its capacity to make as many as four highly stable covalent bonds (including single, double, triple bonds) combined with its ability to form covalently linked CC chains of unlimited length. The same 20 carbon and nitrogen containing compounds called amino acids combine to make up the enormous diversity of proteins occurring in living things. Moreover, all organisms have their genetic blueprint encoded in nucleic acids, either DNA or RNA . Nucleic acids contain the information needed to synthesize specific proteins from their amino acid components. Enzymes , catalytic proteins, which increase the speed of specific chemical reactions, regulate the activity of nucleic acids and other biochemical functions essential to life, while other proteins provide the structural framework of cells. These two types of molecules, nucleic acids and proteins, are essential enough to all organisms that they, or closely related compounds, must also have been present in the first life forms.

Scientists suspect that the primordial Earth's atmosphere was very different from what it is today. The modern atmosphere with its 79% nitrogen, 20% oxygen, and trace quantities of other gases is an oxidizing atmosphere. The primordial atmosphere is generally believed not to have contained significant quantities of oxygen, having instead rather small amounts of gases such as carbon monoxide, methane, ammonia and sulphate in addition to the water, nitrogen and carbon dioxide, which it still contains today. With these combinations of gases, the atmosphere at that time would have been a reducing atmosphere providing the hydrogen atoms for the synthesis of compounds needed to create life. In the 1920s, the Soviet scientist Aleksander Oparin (18941980) and the British scientist J.B.S. Haldane (18921964) independently suggested that ultraviolet (UV) light, which today is largely absorbed by the ozone layer in the higher atmosphere, or violent lightning discharges, caused molecules of the primordial reducing atmosphere to react and form simple organic compounds (e.g., amino acids, nucleic acids and sugars). The possibility of such a process was demonstrated in 1953 by Stanley Miller and Harold Urey , who simulated the effects of lightning storms in a primordial atmosphere by subjecting a refluxing mixture of water, methane, ammonia and hydrogen to an electric discharge for about a week. The resulting solution contained significant amounts of water-soluble organic compounds including amino acids.

The American scientist, Norman H. Horowitz proposed several criteria for living systems, saying that they all must exhibit replication, catalysis and mutability. One of the chief features of living organisms is their ability to replicate. The primordial self-replicating systems are widely believed to have been nucleic acids, like DNA and RNA, because they could direct the synthesis of molecules complementary to themselves. One hypothesis for the evolution of self-replicating systems is that they initially consisted entirely of RNA. This idea is based on the observation that certain species of ribosomal RNA exhibit enzyme-like catalytic properties, and that all nucleic acids are prone to mutation. Thus, RNA can demonstrate the three Horowitz criteria and the primordial world may well have been an "RNA world". A cooperative relationship between RNA and protein could have arisen when these self-replicating protoribosomes evolved the ability to influence the synthesis of proteins that increased the efficiency and accuracy of RNA synthesis. All these ideas suggest that RNA was the primary substance of life and the later participation of DNA and proteins were later refinements that increased the survival potential of an already self-replicating living system. Such a primordial pond where all these reactions were evolving eventually generated compartmentalization amongst its components. How such cell boundaries formed is not known, though one plausible theory holds that membranes first arose as empty vesicles whose exteriors served as attachment sites for entities such as enzymes and chromosomes in ways that facilitated their function.

See also DNA (Deoxyribonucleic acid); Evolution and evolutionary mechanisms; Evolutionary origin of bacteria and viruses; Miller-Urey experiment; Ribonucleic acid (RNA)

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origin of life

origin of life The process by which living organisms developed from inanimate matter, which is generally thought to have occurred on earth between 3500 and 4000 million years ago. It is supposed that the primordial atmosphere was like a chemical soup containing all the basic constituents of organic matter: ammonia, methane, hydrogen, and water vapour. These underwent a process of chemical evolution using energy from the sun and electric storms to combine into ever more complex molecules, such as amino acids, proteins, and vitamins. Eventually self-replicating nucleic acids, the basis of all life, could have developed. The very first organisms may have consisted of such molecules bounded by a simple membrane. See proteinoid.

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Origin of Life

Origin of Life

See Life, Origins of

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