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Observations, Notes, Essays, Images, and Ideas About Our Universe, Our Planet, and the Life it Nourishes
Earth, Life, & The Natural Sciences
Observations, Notes, Essays, Images, and Ideas About Our Universe,
Our Planet, and the Life it Nourishes
Reef -dwelling Sea Urchin (Echinodermata) Nassau Bahamas
Eocene fish fossil collected by the author in Warfields Quarry
Kemmerer, Wyoming in 2006
The Origin Of Life On Earth:
THE THIRD DAY
AN ANALYSIS OF THE GEOLOGICAL
AND BIOLOGICAL EVIDENCE FOR
THE ORIGIN OF LIFE ON EARTH
Steven J. Wamback
The mystery of the events and processes that led to the origin of life on Earth has long captivated the imaginations of people and constitutes, to a large degree, the dogma of a number of the world's religions. Science, too, has systematically explored the mystery of life from its generally, but not always, less dogmatic perspective. In this paper, the unique cosmic, astronomical, geological, and chemical circumstances on the early Earth, which are believed to be responsible for the synthesis of living matter, are examined. Additionally, several of the biological and biochemical factors that operate on living things are reviewed in order to explain the ways in which organisms and biomolecules can undergo change and thereby achieve increasing levels of complexity. The experiments and theories of several prominent workers in the fields of evolutionary science and protolife research are also examined. From these examinations a model for the biochemical synthesis of life is suggested. It is concluded that life arose from a series of complex endothermic reactions. When energy is taken into a system, its entropy (randomness) decreases. This increasing order led to the development of large biomolecules which, like DNA, were able to replicate themselves. The environment then selected those molecules that were best adapted to survive and replicate further. Certain associations between groups of these molecules (symbiosis) improved their chances for survival. The process of evolution was thus established early in, and was responsible for, the origin of life on Earth.
And God said, “Let the waters bring forth abundantly the moving creature that hath life; and fowl that may fly above the earth in the
open firmament of heaven." (Genesis 1:20)
Since prehistoric times, humans have attempted to explain much of that which they could not understand by relying upon traditional tales, myths, superstition, and religion. Among the most perplexing of mysteries have been the origins of the Earth and of the life which inhabits it. Various religious dogma and cultural myths have suggested these beginnings as ranging from the oral issuance of mighty celestial dragons to the verbal command of a nameless god who commanded, "Let it be...” Great debates and even bloody wars have been waged, and continue to be waged, between groups of people who espouse differing systems of beliefs, with most incorporating explanations for the mysteries of life. Thus, it appears that the mystery of the origin of life has held, and still holds, a position of great importance to many of the world's people. As noted above, it is the subject of the opening chapter of the book of Genesis in the Judeo-Christian Bible, and it is the subject of this paper.
Throughout the ages, humans have acquired much of the knowledge and skill that is necessary to explain the things and forces of the universe in an accurate, orderly and, therefore, rational fashion. This is what is called science. It is science that objectively attempts to draw us closer and closer to that perfect but unattainable truth. The myths, superstitions, and religious dogma of our ancestors have purported to have already attained that truth and have not generally been subject to change, even when contradictory evidence is in plain view. A growing number of modern progressive religions, however, have accepted the fact that science and religion are not necessarily mutually exclusive. In this paper we will approach the subject of the origin of life from the scientific point of view with the understanding that evidence and data, yet to be accumulated, can be used to refine, refute, and even replace any currently-accepted or proposed models.
THE ORIGIN AND AGE OF THE EARTH
An inquiry into the probable origin and age of the Earth itself is fundamental to our discussion on the origin of life for, as will be pointed out later, these are intimately related. The most widely held theory on the origin of the Earth and the solar system suggests that the Sun and the planets formed from the gravitational coalescence of cosmic debris following the explosion of a giant star... probably a supernova. This occurred about 11 billion years after the event that is popularly known as "The Big Bang" which was responsible for the formation of the universe. With the accretion of this debris within distinct orbits around a central mass and gravity source, the solar system was formed. Nuclear reactions due to the attainment of critical masses of radioactive isotopes were responsible for the heating, melting, and shaping of these rapidly rotating bodies. As radioactive decay proceeded the planets began to cool. This explains the hot molten core and tectonic activity of the still-cooling Earth (Press and Siever, 1978).
The assumed rate of cooling of the Earth led Lord Kelvin, in the nineteenth century, to erroneously calculate the age of the Earth at about 30 million years based on the second law of thermodynamics. This, however, did not account for the energy liberated during radioactive decay. Becquerel discovered radioactivity in 1896.
Rutherford, in 1902, found that radioactive isotopes degenerate to more stable elements. In 1905, Boltwood determined that the ratio of (SEE HARD COPY) lead to uranium was higher in older uranium minerals. Thus the means of radioactive age dating were established. Currently, potassium-argon dating has determined the oldest rocks on Earth to be from Greenland with an age of about 3.8 billion years. (In this paper one billion equals 1000 million.) This age most likely corresponds with the original cooling of the Earth's crust. Radiometric dating has determined the age of meteorites and the moon rocks to be about 4.5 billion years. These are thought to have formed at about the same time as the Earth. This suggests that the Earth is between 4.5 and 5.0 billion years old and is certainly no younger than about 4.0 billion years (Foster, 1983).
The point of the preceding is that the Earth is unique, at least in our solar system, in having the proper amounts of solar energy, internal energy, temperature, and liquid water (as consequences of its composition, age, and distance from the sun) necessary to support life as we know it at this time. Before beginning our discussion of the fossil record of life, it is important that we discuss the events that occurred between the time of the proposed origin of the Earth and the appearance of the first fossil-like objects.
THE EARLY EARTH
One of the attributes of the Earth that enables it to sustain life is its gaseous atmosphere. On the newly formed Earth, one of the most important sources of its atmosphere was the "degassing" of the crust. This was accomplished by meteoric impact and volcanic activity which provided vents for the escaping gases. The Principle of Uniformitarianism (the present is the key to the past), as applied to the exudates of modern volcanoes, indicates to us that these gases were predominantly carbon dioxide and water vapor with lesser amounts of carbon monoxide, methane, and nitrogen in the form of ammonia.
Owing to the Earth's proper distance from the Sun, photolysis of the water vapor was not permitted and the rising gas cooled in the upper atmosphere, condensed, and fell as torrential rains. Dissolved carbon dioxide in the precipitate was buffered by the calcium in clay minerals at the surface and was also precipitated as calcium carbonate. With the subsequent reduction of the early Earth's radioactivity, via the passing of many half-lives, cooling at the surface occurred, thus causing the formation of lakes, rivers, and oceans. This established the water cycle and erosive processes at an early stage in the history of the planet, leading to the formation of the earliest sedimentary rocks (Walker, 1977).
The primordial seas consisted of liquid water with dissolved minerals and possibly some simple organic molecules as evidenced by their presence in some meteorites from outer space. This early aqueous environment was bombarded by a variety of energy types that included the remaining radioactivity in the crust, lightning from the tremendous atmospheric disturbances, cosmic rays from space, and ultraviolet rays from the Sun. Owing to the lack of the ozone (O3) layer, since large quantities of molecular oxygen had not yet been formed, ultraviolet radiation was quite intense. Hence, despite the strongly reducing environment, the proverbial "hot thin soup" was cooking and the scene was set for the subsequent evolution of life as evidenced by controlled laboratory experiments (discussed later) and by the fossil record (Zubay, 1984).
Both halves... the Cast and the Mold of the Eocene fish fossil shown above.
THE GEOLOGICAL RECORD OF LIFE
Fossils are the traces and remains of prehistoric life. Their modes of preservation range between unaltered, replacement by various minerals, molds, and faint carbon films between layers of sedimentary rocks. Most fossils are, in fact, preserved in rocks and as noted above, rocks can be dated. Sedimentary rocks that contain fossils are not generally used for radiometric age dating as these are clastically derived by erosion and deposition from much older sources. However, absolute ages can be determined from igneous rocks which can be chronologically correlated with fossil bearing sedimentary strata. Most useful are volcanic ash layers that fall between sedimentary beds. An ash fall generally represents an isochronous event of widespread distribution. Employing the Law of Superposition (oldest layer on the bottom; youngest on top) the relative ages of rocks and fossils can also be determined.
The measurement of time and the dating of geological events belong to the science of geochronology. The major time-rock unit is the eon. Time is divided into five of these with the most recent being the Phanerozoic. This eon began about 600 million years ago at the dawn of the Cambrian Period with the widespread diversification and an explosive increase in the abundance of fossil life. Precambrian time is divided into four eons which cover the time span between the Phanerozoic and the origin of the solar system. Table 1, taken from Cloud (1978), Hargraves (1976), Margulis (1981), and Schopf (1978), summarizes the geochronology, events, and interpretation of the history of the Earth and the evolution of life.
The oldest known sedimentary rocks are from Isua, Greenland. They have been dated at 3.8 billion years and in many ways resemble more recent sedimentary rocks. The ratio of carbon-12 to carbon-13 in these rocks indicates the possibility that it may have been organically derived. Carbon in this ratio is known only from biological sources. Metamorphic alteration, however, has made it unlikely that biological origins can ever be proven for the carbon traces in these oldest rocks (Zubay, 1984). The relatively new science of organic geochemistry does show some promise in enabling scientists to determine biological processes from residues left in younger, less metamorphosed rocks.
Another interesting type of sedimentary deposit is the banded iron formations (BIFs). They are Archean to Proterozoic in age and are the world's chief source of iron ore. The banding is caused by alternation of iron-rich and iron-poor strata. A reducing environment (lacking oxygen) favors the non-deposition of iron while an oxidizing environment (oxygen present) favors its deposition. In the absence of oxygen, iron remains dissolved in sea water. Upon the addition of oxygen, iron is converted from the ferrous to the ferric state and precipitates from sea water as iron oxide (rust). Schopf (1978) notes that the majority of BIFs were deposited in a space of only a few hundred million years. This may indicate that the geologically-sudden oxygenation of the water due to the evolution of photosynthesis was responsible for this "rusting out" of the world's oceans. It should also be noted that BIFs are not found to be younger than about 2.0 billion years.
Only when the iron was depleted from the sea, was the oxygenation of the atmosphere able to occur. According to Fischer (1965), "The only source that could account for the accumulation of oxygen in the Proterozoic atmosphere about two billion years ago is biological photosynthesis," (cited in Margulis, 1981, p. 129). The presence of terrestrial redbeds, that are always younger than 2.0 billion years, also suggests increased levels of atmospheric oxygen. Once present, oxygen was modified by intense lightning to form the ozone layer which shields out lethal ultraviolet rays, permitting other forms of life to evolve, diversify, and thus, occupy a greater variety of ecological niches.
The oldest actual fossils known are from the Warrawoona Group of Western Australia and have been dated at 3.5 billion years. These are well preserved microfossils contained in chert and are morphologically similar to modern blue-green algae. Slightly younger are the 3.4 billion year old Swaziland microfossils of South Africa. Foster (1983) has proposed that these oldest forms are not truly biological and represent the abiotic precursors of life which will be discussed below. The Gunflint Chert from the north shore of Lake Superior in Canada contains 2.0 billion year old microfossils that were certainly photosynthetic.
The oldest known eukaryotic fossils are the green algae of the 1.3 billion year old Pahrump Group in California and of the 1.0 billion year old rocks at Bitter Springs in Northern Australia. These fossils also show signs of cell division. Thus, with the evolution of a true nucleus, and its neatly-packaged genetic material, the stage was set for the rapid evolution of more diverse and complex organisms leading to the Ediacarin type biota of the Late Precambrian and the subsequent "Cambrian Explosion".
Throughout the above transition over time one can visualize the conversion from simple to increasingly more complex life forms. Based on these observations in the rock record, one can infer the improved ability of organisms to adapt in response to a variety of changes in the environment. This leads us to an analysis of the process of evolution and to the key question: Just what exactly is it that constitutes life anyway?
THE MEANING OF LIFE
In the course of any scientific inquiry it is appropriate to provide working definitions for any terms that are generally ambiguous or have alternate meanings. As noted in the previous section, some workers question the bioticity of some of the earliest apparent microfossils. Similar arguments can be made regarding modern viruses which, due to their simplicity, may be similar to the protolife forms. For our purposes here, life can be defined as a system of chemical and physical processes that are capable of self-duplication and mutation. Self-duplication provides for the formation of new generations, while mutations allow organisms to evolve, under the pressure of natural selection, to forms that can compete more readily for energy, nutrients, and subsequent reproductive success. Incidentally, by this definition, viruses can be considered living entities even though their self-replication is accomplished through the use of the host's cellular machinery and mutations occur by alteration and incorporation of, or insertion into, the host's genetic material.
A pair of horseshoe crabs mating in the shallows of Pine Island Beach,
Hernando County. Florida.
For centuries humans have known that the traits of parent organisms can be passed on to their offspring. Agriculturalists have used this knowledge to their advantage in the selective breeding of their crops and livestock. In the 1860s, the Austrian monk, Gregor Mendel, developed the basic laws of heredity through his controlled scientific research on the cross-breeding of the garden pea (Pissum sativum). He chose a variety of characteristics and by careful experiment determined the ways in which parental traits appear in the offspring. For example, he found that self-pollinated tall parent plants produced tall and short offspring in the ratio of 3:1 respectively. His interpretation of this states that, "Each organism contains two factors for each characteristic and that the factors segregate during the formation of gametes so that each gamete contains only one of each pair of factors." Subsequent experiments involving the cross-pollination of plants with differing traits showed that the traits appear in the offspring separately and in combination with predictable regularity (Johnson, 1983). From these experiments he established the laws of dominant and recessive factors that are passed on to the offspring. What Mendel did not know is that these so-called factors were controlled by genes on the chromosomes in the nuclei of cells. The study of genes and chromosomes is the subject of Molecular Biology.
In 1902, Sutton and Boveri proposed independently that Mendel's factors, or genes, were located on the chromosomes in the nuclei of eukaryotic cells. Chromosomes are rod-shaped bundles of deoxyribonucleic acid (DNA) bound by proteins called histones. Prokaryotic cells also contain circular strands of DNA but they are not bound in a nucleus. During the first half of the 20th Century, biologists were aware of the gross chemical composition of DNA and its apparent relationship to the passing-on of genetic traits. They had not, however, been able to elucidate its structure.
In 1953, Watson and Crick published a proposed structure for DNA in a single page article in the journal Nature (Watson and Crick, 1953). Their conclusions, based on x-ray diffraction studies, showed DNA to be a double helix with crossmembers resembling a twisted ladder. Each of the helices consists of a chain of sugar (deoxyribose) molecules with phosphate groups facing outward. The crossbars consist of nucleotide pairs. Nucleotides are of two types... the two purines: adenine (A) and guanine (G) and the pyrimidines: thymine (T) and cytosine (C). Adenine always pairs with thymine (AT) and guanine always pairs with cytosine (GC). It is this relationship that allows for the double strands of DNA to separate during cell division; each single strand then gains a new set of complementary nucleotides yielding two double strands identical to the original.
It is the sequence of nucleotides that contains the genetic information in cells. To state it quite simply, DNA is the blueprint for proteins. Variation in protein type, structure, function, and arrangement is what accounts for the differences between living organisms. The more DNA, and therefore protein, that two organisms have in common, the more genetically related they are. Each three-nucleotide sequence along a strand of DNA codes for one amino acid. Proteins are chains of amino acids. The 64 possible three-base combinations using A, G, T, and C are called codons. Each codon codes for one amino acid or "stop" signal. Since there are only twenty amino acids some are coded for by more than one codon. This serves as a safety net so that in the case of the mutation of one nucleotide, there is a probability that the proper amino acid will still be coded for. Ribonucleic acid (RNA) serves as the ¬messenger between the DNA and the ribosomes where proteins are synthesized. The production of RNA strands complementary to the DNA strands is called transcription. The synthesis of proteins at the ribosomes is called translation.
Variation and changes in cells and in organisms are accomplished in two ways. In eukaryotic cells, arms of the chromosomes can be exchanged during gamete formation. This is called crossing over and provides for a nearly infinite number of possible combinations of dominant and recessive genes in the gametes and explains why the multiple offspring of two parents can be so different, each with a unique combination of parental traits. An analogous process in prokaryotes involves the transfer of segments of DNA, called plasmids, between cells. The other source of variation is mutation. Mutation can be defined as spontaneous changes in the DNA sequence. Mutations can be harmful if the protein produced is toxic to, or ineffective in, the organism or if an essential protein is not coded for. Most mutations have no effect because a variety of safety mechanisms are built into the genetic code. These include the multiplicity of codons, mentioned above and out-of-phase stop codons that prevent the misreading of DNA when bases are added or deleted. Occasionally mutations occur that give rise to enzymatic or structural changes that can benefit the organism. These changes can accumulate in and alter a population's gene pool over time. Ultimately, these can enhance an organism's or species' ability to compete effectively for energy, nutrients, and subsequent reproductive success. This process of evolution is the subject of the next section.
THE PROCESS OF EVOLUTION
"There is a grandeur in this view of life, with its several powers, having been originally breathed by the creator into a few forms or into one; and that whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved." (Darwin, 1859)
Natural selection as proposed by Darwin accounts for the differences between, and the evolution of various species of living things. That is to say that nature selects those individuals that are best adapted to accommodate changes in their environments. As these selective pressures are applied to many successive generations of individuals, and especially if subpopulations become geographically (and therefore reproductively) isolated, the evolution of new species can occur. This is also known as survival of the fittest, for it is the most fit that survive to reproduce. Despite ongoing arguments over whether speciation occurs gradually over time (phyletic gradualism) or periodically in short bursts (punctuated equilibrium), understanding the overall process takes little more than common sense. There is no point in complicating the simple and the obvious. As noted previously, farmers have known this for centuries and used it to their advantage, selecting desirable traits themselves rather than waiting for chance or for nature.
The biochemical processes involved in developing the favorable traits to be selected were discussed in the previous section. It is important to make clear at this time that evolutionary change does not occur in response to changes in the environment. Change occurs because of mutations and genetic recombination. Nature (like the farmer) then selects, for survival and subsequent reproduction, the organisms that are best adapted and the most fit. Speciation occurs when two groups or subpopulations of an organism change sufficiently such that they can no longer interbreed. This separation of genetic identities constitutes reproductive isolation: a valid working definition for species. From this and the previous discussions it follows, then, that the more segments of DNA that two species have in common, the greater the evolutionary/genetic relationship between them.
A discourse on evolution would not be complete without the mention of extinction. Most would tend to agree that extinction is nature's way of giving an inadequate species the proverbial "deep six". Stephen Jay Gould, however, points out that, "Dinosaurs dominated the land for 100 million years, yet a species that measures its own life in but tens of thousands of years has branded dinosaurs as a symbol of failure... Like death and taxes, the final disappearance of every species is inevitable." (Gould, 1982). Statistically, 2.0 to 4.6 families of organisms become extinct each million years, while periodic mass extinction events cause 19.3 families to become extinct per million years (Raup and Sepkoski, 1986; Raup, 1986). Thus, it appears that it is the fate of every species to ultimately become extinct. Extinction occurs when the rate of need for adaptation exceeds the rate of genetic variation (via mutation and recombination) that occurs. Extinctions are indicative of a lack of sufficient variations from which nature can then select those best suited to adapt. Those that cannot adapt... die.
The past several sections have dealt with some of the basic biological principles that are fundamental to the understanding of what constitutes life and the means by which organisms evolve. With this knowledge at our disposal we can now proceed with our discussion of the process by which life originated.
The oldest known fossils are the microfossils in the 3.5 billion year old Warrawoona Cherts of Western Australia. These are thought to resemble modern blue-green algae. Also known as cyanobacteria, blue-green algae are colonial prokaryotic cells of very simple construction. They grow in sheet-like masses in shallow marine environments. Over time, they form club-shaped mounds. These mounds are called stromatolites because of their layered rocky structure. Only the outer layer contains living cells. Fine sediments become trapped and bound by the sticky living surface and subsequent growth of living cells occurs over the sediments. Repeated layering in this fashion accounts for their internal structure.
These organosedimentary structures were common in the Precambrian and appear to be the most abundant life form during that time. They began to decline in the Paleozoic, due to consumption by the other evolving organisms, and have continued to decline since that time. One of their last remaining strongholds is Hamlin Pool at Shark Bay in Western Australia. They survive there because the hypersaline conditions in that area preclude the existence of the other organisms that feed on them. Cyanobacteria are able to survive under a wide variety of conditions but grazing, by gastropods in particular, has limited their existence to the most inhospitable of environments. Playford (1980) suggests that the progressive suppression of this 3.5 billion year old group has forced it to the extreme limits of its ecological range by the evolution and adaptive expansion of animals that use it as a source of food.
Much work has been done in the study of ancient stromatolitic microfossils and fossil-like structures that resemble simple prokaryotic cells. Before we discuss one of the greatest undertakings in this field of study, let us describe the projects and theories of some of the workers that have led the way to our proposed model for the origin of life.
The cooling of the early Earth permitted the condensation of water vapor. Torrential rains filled the seas and erosive processes contributed to their mineral content. Also present were a variety of dissolved gases including carbon dioxide, carbon monoxide, methane, ammonia, and hydrogen. There also may have been some simple organic bio-like molecules as evidenced by their presence in meteorites from space (Oro et al., 1980). In 1953, Stanley Miller attempted to duplicate the conditions of the early Earth. He placed pure sterile methane, ammonia, hydrogen, and water in a sealed chamber. By application of an intense electrical discharge (simulated lightning), he was able to produce four amino acids: the building blocks of protein (Miller, 1953). Since that time, a number of workers have performed similar experiments using a variety of probable starting materials and energy sources. Catalysis via electrical discharge, ultraviolet light, heat, and radioactivity has produced a host of biomolecules including amino acids, sugars, nucleotides, fatty acids, aldehydes, ketones, alcohols, and urea (Margulis, 1981). Thus, it seems apparent that all of the necessary ingredients were contained in the primordial soup for the synthesis of more complex biomolecules and the subsequent genesis of living organisms.
Enzymes are protein catalysts that accelerate the rates of the chemical reactions which take place within living cells. They do this by directing the reactive sites on molecules toward the orientation that favors the reaction. These reactions would generally take place anyway, but in a fashion that is too random, and too slow, or which requires too high of an activation energy to sustain life. One can visualize, however, that chance collisions between organic molecules, in the presence of the primitive Earth's unique energy sources, could lead to the formation of more complex molecules such as chains of amino acids, sugars, and nucleotides. Experiments similar to those above have borne this out (Fox, 1965 and Fox et al., 1960).
We may conclude, then, that the list of ingredients in the primitive sea may have included small proteins, lipids, polysaccharides, and DNA-like chains of nucleic acids. These, along with water, constitute the bulk of the structural and functional components of living cells. What remains, is to assemble these components into an integrated, self-contained, and self replicating entity which is the subject of the next several sections.
Organic molecules or portions of molecules can be classified into two groups based on the way they interact with water. Those that have an affinity for water by the formation of hydrogen bonds or dipole-dipole interactions are called hydrophilic (water-loving). Hydrophobic (water-hating) interactions occur between water and non-polar molecules or parts of molecules. This explains the ability of substances, such as salts, to be dissolved in water (hydrophilic). Oils float on water because they are non-polar and get "squeezed out" (hydrophobic) by the polar water molecules. Detergents work because they have both hydrophilic and hydrophobic portions. The polar end associates with water and the non-polar end associates with grease. A rule of thumb is: Like dissolves like.
Thermodynamics tells us that, in water, it is favorable for two or more hydrophobic things to be displaced together than to be displaced separately due to a smaller combined surface area. Molecules that contain polar heads and non-polar tails form spheres in water with the non-polar tails directed inward. These spheres are called micelles. The outer polar portion has a surface tension and resembles a biological membrane. Biological membranes are largely made up of molecules that, like detergents, have both polar and non-polar ends.
In the primordial sea, macromolecules could have coalesced to form small droplets and micelles containing other polymers in an aqueous matrix. Oparin (1969) proposed that these coacervates, as he called them, could have trapped proteins, nucleic acids, and other molecules to yield a primitive sort of metabolism. If these cell-like micro spheres contained chains of nucleotides they may have been capable of self-replication and mutation.
According to our working definition established earlier, this constitutes life. Statistically, the odds against this sequence of events occurring must have been great but the conditions of having millions of years, a large variety and number of molecules and a unique energy situation may have aided in reducing these odds. And after all, it needs to have happened only once. Some workers have suggested that these odds may have been reduced even further by the concentration and ordering of biomolecules on crystal defects in clay minerals (Zubay, 1984).
Once formed, these protocells were free to undergo the phenomenon of mutation and the process of evolution thereby leading to an increased variety and complexity of forms. The remaining biomolecules in the sea served as nutrients for the newly formed biota. As the nutrient source became depleted by these fermentors, they evolved ways to use carbon dioxide by converting it to food energy. Thus heterotrophs begat autotrophs. The beginning stages in the development of photosynthesis and the subsequent oxygenation of the atmosphere were established.
These first cells were quite simple and resembled modern bacteria. Their simplicity itself was an adaptive advantage that allowed them to undergo rapid evolution just as adaptation in modern bacteria permits the evolution of more antibiotic and immuno-resistant strains involved in infectious diseases. How these simple forms gave rise to the more complex eukaryotic cell is the subject of the next section.
Organisms interact with each other in a number of ways. These interactions can be harmful to one of the organisms when the other deprives it of food and energy as in the case of parasitism. Some symbiotic associations have no effect on either organism and are merely matters of convenience or chance. This is commensalism. Often symbiotic relationships are formed whereby both organisms benefit. These mutualistic relationships can become so important that neither organism can survive without intimate association with the other. A classic example of this is the lichen which consists of symbiotic mutualism between an algae and a fungus. Together, each derives the benefits of heterotrophic and autotrophic lifestyles. Separately they die.
An October storm washes the shores of Lake Erie on Grandview Bay
Angola, Evans, New York.
Lynn Margulis (1981) has proposed that symbiotic relationships between some simple prokaryotic organisms gave rise to the more complex eukaryotic cells. In particular, she has observed that eukaryotic mitochondria and chloroplasts resemble prokaryotic cells in several ways. Mitochondria are the powerhouses where food is converted to energy in eukaryotic cells. Chloroplasts are the organelles in plants that convert carbon dioxide and water in the presence of sunlight to sugar. Both of these organelles contain their own kind of ribosomes (sites of protein synthesis) which are nearly identical to those found in bacterial cells. They also contain single-stranded circular DNA that, just as in bacteria, is free within their sub cellular membranes. Chloroplasts and mitochondria also divide independently of the "host" cell. Both of these organelles resemble free living bacterial forms in size, shape, internal structure, and function.
Margulis has suggested further that intimate symbiotic relationships between prokaryotic cells in the primitive seas became so vital that some organisms evolved together such that they could not survive separately. Subsequent evolutionary refinements and the development of a true nucleus (perhaps by symbiosis also) gave rise to eukaryotic cells and all "higher" forms of life.
In this paper and in the last several sections in particular we have described a model for the origin of life. Much of what we have conveyed has been speculative and based on circumstantial evidence but supported by scientific fact. As mentioned in the introduction to this paper, science is the search for the truth and is always subject to refinement as better and more detailed data become available. Many scientists and one group in particular, are dedicated to the search for the truth about the origin of life.
In 1977, J.W. Schopf of U.C.L.A. and several others of the most prominent scientists in the world formed the Precambrian Paleobiology Research Group (P.P.R.G.). This included specialists in the fields of biology, geology, chemistry, astronomy, physics, and paleontology to name just a few. Their mission was to establish an interdisciplinary approach to solving the mystery of the origin of life. The project lasted fourteen months. Data were collected from every available source. Precambrian rock samples from all around the world were studied and catalogued. Previous work in the field was evaluated. All of this research was compiled into a single volume with the goals of reporting the results of the project, placing these data in context with a summary of all of the previously available data, assessing the current data, and establishing the ground-work for future study (Schopf, 1983). The analysis and interpretation of that work is ongoing to the present time. These accumulated data provide a departure point for students, future scientists, and established researchers who are interested in pursuing protolife investigations.
An interdisciplinary approach such as the one discussed above is the key to solving the mystery of the origin of life, for as mentioned earlier, the synthesis of life is the net result of a unique set of cosmic, physical, chemical, and geological circumstances that constitute the planet called Earth. The mystery may never be completely solved. But in our search for that perfect but unattainable truth we can learn much about ourselves, our planet, and our universe. With that knowledge we can then accept and fulfill our self-assigned, and perhaps if we are so disposed, god-given, responsibility as guardians and caretakers of this paradise.
Some Echinoderm tests (shells) and a small sponge from Nassau Bahamas
About The Author:
Having graduated from The State University of New York College at Fredonia, NewYork with degrees in both the Geological and Biological Sciences, Steven J.Wamback has worked as a biologist, geologist, and environmental scientist on various projects within the realms of hazardous waste site remediation; wetlands identification, delineation, and mapping; groundwater exploration and protection; natural resource conservation; technical project writing and editing; and public education. He is looking forward to future projects and opportunities in Public Service and in conserving and protecting natural environmental resources, land, water, wetlands, fish, and wildlife. Steve finds himself at home with his family in Angola, New York on the shores of Lake Erie and enjoys fossil hunting and amateur radio when time permits.
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