For much of modern science's existence, the question of how life originated on Earth has vexed researchers and scientists alike. One of the most followed theories has been extraterrestrial in nature and has been placed at point in time some 3.9 billion years ago. That was when the orbital patterns of the gas giant planets of the solar system (Saturn and Jupiter) changed and allowed a swarm of comets, asteroids and other small celestial bodies to plunge into the gravitational attraction of the sun.
As these chunks of solar system debris fall toward the sun, they collided with the inner planets and moons that were in their path causing violent collisions that left huge craters in their wake, and in regards to the Earth; the impacts had a devastating effect. The Earth lost its primordial oceans as the surface of our planet was turned into a moltant cauldron of liquefied rock and debris.
With the cessation of the celestial impacts, the Earth cooled and rocks formed a global crust of silicic rocks embedded in a basaltic crustal layer. Oceans quickly formed and weathering produced the first sedimentary rocks worldwide. Protocontenents took early shape from a process of metamorphosis and the melting of accumulated debris. Plate tectonics began to shift the early continental structures and an atmosphere made up mostly of nitrogen, with some carbon dioxide, ammonia, methane, and water took shape.
The combination of all of these materials and ingredients could have possibly resulted in the formation of organic molecues. And amazingly life began to stir some 3.85 billion years ago when primitive single-celled bacteria appeared. This startling development has lead intrigued observers of the Earth's past in deep time to question how and why life was able to develop so quickly.
Although scientists feel comfortable with the time frame for the development of life outlined above, they have been unable or unwilling to agree on the source of the organic molecules that came together to form life. Leading the proponents that suggest that life had extraterrestrial origins has been such noteworthy scientists such as Francis Crick who is credited with discovering the structure of DNA. These theories have developed because there has yet to exist a plausible explanation that links the formation of life on Earth to terrestrial sources.
Among those who have worked to discover a terrestrial origin to life; recent developments have surfaced that give plausible reasons why Earth is where the origin of life occurred and break down into four component parts.
The first part of this terrestrial theory concerns the existence of fatty chemicals available on our primordial world and their relationship to the co-development of a 'genetic system' and a 'cell membrane.' Researcher Jack W. Szostak, according to the New York Times, discovered that "(s)imple fatty acids, of the sort likely to have been around on the primitive Earth, will spontaneously form double-layered spheres, much like the double-layered membrane of today’s living cells. These protocells will incorporate new fatty acids fed into the water, and eventually divide. Living cells are generally impermeable and have elaborate mechanisms for admitting only the nutrients they need. But Dr. Szostak and his colleagues have shown that small molecules can easily enter the protocells. If they combine into larger molecules, however, they cannot get out, just the arrangement a primitive cell would need. If a protocell is made to encapsulate a short piece of DNA and is then fed with nucleotides, the building blocks of DNA, the nucleotides will spontaneously enter the cell and link into another DNA molecule."
The New York Times explains that: "Dr. Szostak’s experiments have come close to creating a spontaneously dividing cell from chemicals assumed to have existed on the primitive Earth. But some of his ingredients, like the nucleotide building blocks of nucleic acids, are quite complex. Prebiotic chemists, who study the prelife chemistry of the primitive Earth, have long been close to despair over how nucleotides could ever have arisen spontaneously.
"Nucleotides consist of a sugar molecule, like ribose or deoxyribose, joined to a base at one end and a phosphate group at the other. Prebiotic chemists discovered with delight that bases like adenine will easily form from simple chemicals like hydrogen cyanide. But years of disappointment followed when the adenine proved incapable of linking naturally to the ribose.
"Last month, John Sutherland, a chemist at the University of Manchester in England, reported in Nature his discovery of a quite unexpected route for synthesizing nucleotides from prebiotic chemicals. Instead of making the base and sugar separately from chemicals likely to have existed on the primitive Earth, Dr. Sutherland showed how under the right conditions the base and sugar could be built up as a single unit, and so did not need to be linked.
"Once a self-replicating system develops from chemicals, this is the beginning of genetic history, since each molecule carries the imprint of its ancestor. Dr. Crick, who was interested in the chemistry that preceded replication, once observed, “After this point, the rest is just history.”"
Another step in the process of understanding of how terrestrial life developed from earthly origins has been made by Gerald F. Joyce of the Scripps Research Institute in La Jolla, California who stands as a preeminent expert life's origin on Earth. Among a number of accomplishments, Dr. Joyce has most recently worked with RNA; investigating it's capacity for reproduction. In a major article in Science, Joyce announced his development of two RNA molecules that assisted each other's synthesis from the four neucleotides that constitute RNA. Joyce reacted to his acomplishment by exclaming that: “We finally have a molecule that’s immortal,” since the information has an indefinite timeframe for indefinitely passing on instructions. And even though his synthetic system is not alive it can mimic life functions such as self replication and adapting to the environment.
The New York Times explains: "Another striking advance has come from new studies of the handedness of molecules. Some chemicals, like the amino acids of which proteins are made, exist in two mirror-image forms, much like the left and right hand. In most naturally occurring conditions they are found in roughly equal mixtures of the two forms. But in a living cell all amino acids are left-handed, and all sugars and nucleotides are right-handed.
"Prebiotic chemists have long been at a loss to explain how the first living systems could have extracted just one kind of the handed chemicals from the mixtures on the early Earth. Left-handed nucleotides are a poison because they prevent right-handed nucleotides linking up in a chain to form nucleic acids like RNA or DNA. Researchers such as: "Donna Blackmond of Imperial College London have discovered that a mixture of left-handed and right-handed molecules can be converted to just one form by cycles of freezing and melting.
The New York Times explains: "With these four recent advances — Dr. Szostak’s protocells, self-replicating RNA, the natural synthesis of nucleotides, and an explanation for handedness — those who study the origin of life have much to be pleased about, despite the distance yet to go. “At some point some of these threads will start joining together,” Dr. Sutherland said. “I think all of us are far more optimistic now than we were five or 10 years ago.”
"One measure of the difficulties ahead, however, is that so far there is little agreement on the kind of environment in which life originated. Some chemists, like Günther Wächtershäuser, argue that life began in volcanic conditions, like those of the deep sea vents. These have the gases and metallic catalysts in which, he argues, the first metabolic processes were likely to have arisen.
"But many biologists believe that in the oceans, the necessary constituents of life would always be too diluted. They favor a warm freshwater pond for the origin of life, as did Darwin, where cycles of wetting and evaporation around the edges could produce useful concentrations and chemical processes."
The origin of life on Earth may forever remain an inexact proposition spread across millions of years. The most ancient evidence of fossilized bacteria has been found in the Gunflint Formation of Ontario and is estimated to be some 1.9billion years old. There also exist rock formations in Greenland that contain evidence of carbon isotopes related to 3.830 billion years old biological processes. Researchers are convinced that the 'late heavy bombardment' of celestial bodies could have left some life untouched and that is what accounts for the wide span of time across which evidence of life has been found.
The new York Times reports: "Recent evidence from very ancient rocks known as zircons suggests that stable oceans and continental crust had emerged as long as 4.404 billion years ago, a mere 150 million years after the Earth’s formation. So life might have had half a billion years to get started before the cataclysmic bombardment."
The conclusion drawn by the New York Times is that: "Chemists and biologists are thus pretty much on their own in figuring out how life started. For lack of fossil evidence, they have no guide as to when, where or how the first forms of life emerged. So they will figure life out only by reinventing it in the laboratory."
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