Istenert
14-06-2005, 03:05
This is to be read with an open mind. You dont have to agree that this is how things are, but I think this is a very interested read even for a creationist. Im typing half the chapter (partially because im bored, partially to memorize it, and partially because I like to be productive in my period of procrastination) so I have coloured the very important areas for those of you who dont feel like reading pages of type.
Originating Events
Primordial Earth
Earth, when it formed some 4.6 billion years ago, was extreamly hot. Heat generated by asteroid impacts, internaiton compression, and radioactivity melted most of the rocky material. Dense materials, composed of such heavy elements as iron and nickel, formed Earth's inner core, while less dense materials formed a thick mantle. The least dense rock, composted mostly of lighter elements, floated on the surface and cooled to form a crst.
Mot gases formed Earth's primitive atmosphere. When, after some 500 million to 800 million years, the asteroid bombarrdment slowed the surface temperature cooled below 100*C, cast wuantities of water vapour condensed. Hundreds of years of torrential rains pooled in surface depressions to form ocean basins. The atmosphere of primordial Earth would have comtianed large amounts of nitorgen gas, carbon dioxide, carbon monoxide, and water vapor. Other hydrogen compounds - such as hydrogen sulfide, ammonia, and methane - would have been present. It is probable, though not certain, that this early atmosphere also contained hydrogen gas. Oxygen gas is highly reactive and, with the high temperatures present then, would have compbined with many other elements to form oxides; for this reason, the atmosphere would have contained little, if any, free oxygen. The surface of Earth would have been exposed to many intense sources of energy: radioactivity, intense ultraviolet light, visible light, and cosmic radiation from a young sun; heat from volcanic activity; and electrical energy from violent lightning storms.
Organic Molecules
In the mid 1930s, a Russian biochemist Alexander Oparin and British biologist J.B.S. Galdane independently proposed the theory of primary abiogenesis - that the first living thing on Earth arose from nonliving material. They reasoned that the first complex chemicals of life must have formed spontaneously on a primordial Earth and, at some point, arranged themselves into cell-like structures with a membrane separating them from the outside environment.
Although extreamly harsh, the early conditions on Earht were ideal for triggering chemical reactions and the formation of complex organic compounds. What molecules might have formed from the reactions of gases in the primordial atmosphere? In 1953, the Nobel Prize=winning astronomer Harold Urey and his student, Stanley Miller, investigated possible reactions. Their apparatus modelled the water cycle by using a condenser to produce precipitation and a heater to cause evaporation. Since Urey and Miller suspected that the early atmosphere would have contained water vaour, ammonia and methane and hydrogen gases, they combined the gases and exposed them to electrical sparks, thereby modelling early conditions on Earth [see picture (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure2.jpg)]. After one week, 15% of the original carbon in the methane had been converted to a variety of compounds, including aldehydes, carboxylic acids, urea, and - most interestingly - two amino acids: glycine and alanine.
More recent evidence suggests that the specific combination of gases chosen by Urey and Miller was not likely to have existed in the primordial atmosphere. In responce, many other scientists have continued this investigation with experiments that use the combination of gases now throught to have been present. These experiments have produced an even greater variety of simple organic compounds, including essential sugars, all 20 amino acids, many vitamis, and all four nitrogenous bases found in RNA and DNA. The most abundant nitrogen base, adenine, was the easiest to produce under laboratory conditions. These results suggest that many of the building blocks of life likely formed spontanously in Earths primordial environment.
Chemical Evolution
For the first molecules to have produced living cells, they had to have been able to form more complex chemical and physical arrangements. Polymerazation of early monomers may have occurred in numerous ways. Monomers may have become concentrated on hot surfaces as water evaporated, and the increased concentrations and heat energy may have triggered polymerization reactions. Under simialr conditions, in 1977, Sidney Fox at the University of Miami was able to trigger the spontanious production of thermal proteinoids consisting of chains of more than 200 amino acids. Other scientisits have discovered that such materials as clay particles and iron pyrite form electrostatically charged surfaces that are also capable of binding monomers and catalyzing polymerization rectoins. These findings suggest mechanisms for the formation of the first polymers. Could any polymers have then influenced their own formation?
The most fundemental characteristic of living things is organized self-replication. To self-replicate, molecules must demonstrate catalytic activity, that is, the ability to influence a chemical reactoin. But can a molecule act as a catalyst for its own formation?In the 1980's, Thomas Cech, working at the University of Colorado, discovered RNA molecules, called ribozymes, that act as catalysts in living cells. In other experiments, simple systems of RNA molecules have been created that are able to replicate themselves. In 1991, while working at the Massachusetts Institute of Technology, chemist Julius Rebek, Jr., created synthetic nucliotidelike molecules that could replicate themselves - and make mistakes, wich resulted in nonliving molecular systems that mutated and underwent a form of natural selection in a test tube. As demonstrated by such experiments, Earth's first self-replicating and evolving system may have been RNA molsecules. RNA is also likely to have been the first hereditary molecule. Its catalytic activity and the roles of tRNA, mRNA and rRNA suggest that it is likely to have played a direct role in the synthesis of proteins. Current scientific thinking about DNA is that it evolved later, perhaps by the reverse transcription of RNA.
Formation of Protocells
The evolution of self-replicating molecular system and cell-like structures is a vital area of investigatoin among scientists who study the origin of life. All living things are composed of cells. For chemicals in cells ot remain concentrated enough for metabolic processes to occur, they must be separated from the surrounding dilute environment. How might the first membranes have formed and arranged themselves into cell-like packages with an interior separate from the surrounding environment?
Lipid membranes can and do form spontaneously. Because of their hydrophobic tails, fatty acids and phospholipids naturally arrange themselves into spherical double-layer liposomes, or clusters. These can increase in size by the addition of more lipid and, with gentle shaking, can form buds and devide. Their membranes also act as a semi-permeable boundaries, so that any large molecules initially trapped within them, or produced by internal chemical activity, are unable to escape, thereby increasing in concentration. Although they are not alive, they can respond to environmental changes or reproduce in a controlled way, which means protocellsdo share many traits of living cells. Additional experimental evidence has shown that semipermeable liquid-filled spheres can also form from proteinlike chains. Researchers have discovered that if amino acids are heated and placed in hot water, they form proteinoid spheres, which are capable of picking up lipid molecules from their surroundings, as show in Figure 3 [that I cant scan in so you dont get to see :P]. These protocells are also able to store energy in the form of an electrical potential across their membrane, a trait found in all living cells. Although these findings are the subject of debate and many unanswered questions remain, there is evidence that chemical evolution could have given rise to molecular systems and cellular structures that are characterestic of life.
Procayotic Oranisms: The First True Cells
The oldest known fossils of cells on Earth - accurately dated to 3.465 billion years ago - were found in western Australia in layered formations called stromatolites. These microscopic fossils resemble present-day anaerobic cyanobacteria (Figures 4 and 5, that, once again, I cant scan in). Even the world's oldest-known sedimentary rock formations located in greenland - dating to 3.8 billion years ago - show chemical traces of microbial life and activity.
Although the oldest fossil bacteria resemble photosynthetic cyanobacteria, which use oxygen, the very first prokaryotic cells would certainly have been anaerobic, as the atmosphere would then have contained little or no free oxygen. These first prokaryotic organisms would likely have relied on abiotic sources of organic compounds. They would have been chemoautotrophic, obtaining their energy and raw materials from the metabolism of such chemicals in their environment as hydrogen sulfide, released at high temperatures and in large quantities from ocean-floor vents. These organisms would have adapted to living under harsh conditions of extreme heat and pressure and may have resembled present-day thermopilic archaebacteria. As the first cells reporduced and became abundant, these chemicals would have gradually become depleted. Any cell that was able to use simple inorganic molicules and an alternative energy source would have had an advantage. Fossil evidence suggests that, by 3 billion years ago, photosynthetic autotrophs wree doing just that.
Although the first photosynthetic organsims may have also used hydrogen sulfide as a source of hydrogen, those that used water would have had virtually unlimited suplly. As they removed hydrogen from water, they would have released free oxygen gas into the atmosphere - a process that would have had a dramatic effect. The accumulatoin of oxygen gas, which is very reactive, would have been toxic to many of the anaerobic organisms on Earth. While these photosynthetic cells prospered, others would have had to adapt to the steadily increasing levels of atmospheric oxygen of perish. Some of the oxygen hags reaching the upper atmosphere would have reacted to form a layer of ozone gas, having the potential to dramtically reduce the amounts of damaging ultraviolet radiation raching Earth. At the same time, the very success of the photosynthetic cells would have favoured the evolution of many heterotrophic organisms.
These are early life forms and evolutionary stages produced the necessary conditions to support the dramtic success of life on Earth powered and supplied by energy from the sun and chemical products of photosynthesis.
Diversification and Extinctions
Endosymbiosis in Eukaryotic Cells
Not much fossil evidence of the early evolution of single-celled organisms exists. Compariosons of present-day prokaryotic and eukaryotic DNA, however, suggests that the earliest prokaryotic cells probably gave rise to eubacteria and archaeacteria. It is likely that photosynthesis and aerobic respiratoin first evolved among eubacteria. Present-day archaebacteria are adapted to survive in extreme environments not unlike those that may have been widespread on ancient Earth. Archaebactera may have then given rise to eukaryotic cells. Although present-day eukaryotic organisms still share many genetic traits with modern archaebacteria, the eukaryote lineage and archaebactera lineage are thought to have separated about 3.4 billion years ago. while eventually evolving into eukaryotes, this lineage still consists of prokaryotic organisms for another billion years. These proposed lineages are shown here (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure2-2.jpg) .
The appearance of eukaryotic cells marks the ekey event in the evolutionary history of life. In rocks older than 1.5 billion years, most fossils are of microorganisms that appear to be very similar and small in size [more pictures you cant see]. Recent fossil discoveries from the Empire Mine in Michigan appear to be those of early eukaryotic algae, dating between 1.85 and 2.1 billion years old. Although eukaryotic cells likely evolved more than 2 billion years ago, rock dated to about 1.4 billion years old offers the earlies clear evidence of much larger cells that appear to have membraine - bound internal structures and elaborate shapes.
The distinguishing feature of eukaryotic cells is the presence of membraine-bound organelles, such as the neucleus and vacuols. A nuclear membrane and endoplasmic reticulum may have evolved from infolding of theo uter cell membrane (pretty diagram (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure3-2.jpg)). Initially, such folding may have been an adaption that permitted more efficient exchange of materials between the cell and its surroundings by increasing surace area, and it may also have provided more intimate chemical communicatoin between the genetic material and the environment.
Research postulated that a process of endosymbiosis may have given rise to mitochondria and chloroplasts, two unusual organelles. According to this new widely accepted theory, early eukaryotic cells engulred aerobic bacteria in a process similar to phagocytosis in amoeba (Clicky (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure4-2.jpg)). Having been surrounded by a plasma membrabe, the bacteria were not digested but, instead, entered into the symbiotic relationship with the host cell. The bacteria would have continued to preform aerobic respiratoin, providing excess ATP to the host eukaryotic cell, which would have continued to seek out acquire energy-rich molecules from its surroundings. Endosymbiotic bacteria, benifiting from this chemical-rich environment, would have begun to reporduce independently within this larger cell. Subsequently, photosynthetic bacteria - such as cyanobacteria - may have become endosymbiotic in a similar way within aerobic eukaryotic cells. Such a relatoinship would have benefited the bacteria by providing a richer supply of carbon dioxide for photosynthesis, and the eukaryotic cells by providing excess glucose or other energy-rich products of photosynthesis.
The theory of endosymbiosis is supported by examinations of the organelles themselves. Mitochondria and chloroplasts have features that are different from those of other organelles. They are typically surrounded by two membranes. Although the outer membrane is similar to all other eukaryotic cellular membranes, the chemistry of the internal membrane resembles that of eubacteria plasma membranes. These organelles also have their own DNA, which appears to be remnants of circular eubacteral chromosomes, and contains genetic coding sequences for various proteins and RNA which resemble bacteral genes more than eukaryotic genes. Mitochondria and chloroplasts replicate their own NDA and undergo division indiependently of their host cell's division. They have, however, lost many vital genes and are no longer able to live independently of the host cell.
The evolutoin of both aerobic heterotrophic and aerobic photosynthetic eukaryotic cells likely occured thorugh endosymbiosis. Heterophic eukaryotic cells could have evolved into various protists and, later, into fungi and animals, while photosynthetic eukaryotic cells could have been the ancestors of photosynthetic protists and, eventually, plants. It is probable that chloroplasts originated by endo symbiosis in more than one lineage of eukaryotic organisms. One way to represent these hypothetical evolutionary steps is shown here (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure5-2.jpg) (bad diagram, sorry).
Endosymbiosis has been discovered to occur in many moder organisms. Some ciliates and marine slugs are known to ingest algae and store their chloroplasts, which continues to preform photosynthesis for a few weeks. Coral organisms house living photosynthetic protests within their tissues, and many insects are known to host prokaryotic cells within their cells. One protozoan, [i]pelomyxa, replies on three different endosymbiotic bactera species for respiration. Researchers have even documented the engulfing of one eukaryotic cell by other; for example, the crypromonad Guillardia theta, a eukaryotic alga, contians chloroplasts that are surrounded by four membranes rather than the usual two. Between the outer and inner pairs are renants of the first host cell, including a small but functioning nucleus complete with eukaryotic DNA. In this case, photosynthetic eubactera became endosymbiotic within eukaryotic cells, which later also became endosymbiotic. [see here (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure6-2.jpg)]
Multicellular Organisms and the Cambrian Explosion
For the first 3 billion years of life on Earth, all organisms were unicellular. Eubacteria gave rise to aerobac and photosynthetic linages, which archaebacteria evolved into three main groups: methanogens, extreme halphiles, and extreme thermophiles. Once eukaryotic organisms evolved complex structures and processes, including mitosis and sexual reporduction, they would have had the benifit of much more extensive genetic recombination than would have been possible among prokaryotic cells. Photosynthesis continued to increase the oxygen concentration in the atmosphere to benifit of aerobic organisms. Multicellular organisms, including plants, fungi, and animals, are thought to have evolved less than 750 million years ago.
The oldest fossils of multicellular animals date form about 640 million years ago. However, during a 40-million-year period beginning about 565 million years ago, a massive increase in animal diversity occurred, referred to as the cambrian explosion. Fossil evidence dating from this period shows the appearance of early arthropods, such as trilobites, as well as echinoderms and molluscs; primitive chordates - which were precursors to the vertebrates - also appeared. Animals represeting all present-day major phyla, as well as many that are now extinct, first appeared during this period, a time span that represents less than 1% of Earths history.
Deversification and Mass Extinction
[Big fucking diagram showing 4 things that I am NOT going to draw out] provides a geological time scale and summarizes some of the most significant events in the evolutionary history of earth since the Cambrian explosion. Geologists have established a geological time scale devided into five eras, each of which is further subdivided into periods and, in some cases, epochs. These time intervals are based on their distincitve fossil records, and dramatic changes in the fossil records mak the boundaries between these intervals. The ears of Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life), are remarkable for rapid diversification of life forms, as well as awidespread extinctions. The Paleozoic era, for instance, beigns with the Cambrian explosion and ends with the Permian extinction believed to be the most massive extinction in Earths history.
Fossil evidence of diversification of marine invertebrates early in the Paleozoic era is very extensive. The first vertebrates are thought to have evolved later, followed by bony fish and amphibians. By the mid-Paleozoicera, plants have invaded land surfaces and the first reptiles and isnects have evolved. Around 245 millino years ago, a series of cataclysmic events eradicated more than 90% of known marine species, as indicated by their disappearance form the fossil record after this period. Although uncertainty remains about causes of the Permian extinction, many scientists suspect that tectonic movemtns were a primary cause. The formaiton of the supercontintent Pangea, which occured during the Permian period, would have produced major changes in terrestrial and costal environments as well as in global climate. Ongoing research by Kunio Kaiho of Tohoku University, Japan, and his colleagues has uncovered evidence in sothern China of 69-kn-wide asteroid that may have collided with Earth in this period. These researchers believe that the impact may have vaporised enough sulfur to consume a third of the atmospheric oxygen and generate enough acide rain to make the ocean surface water as acidic as lemon juice. If such a catastrophic impact did occur, it would have been the primary cause of the biggest extinction event in history.
Despite the harsh conditions responsible for mass extinctions, life on earth continued. The Mesozoic era is well known for dinasaurs, a divers group of often veyr large animals that dominated earth for about the mid Trassic to the late Cretacious period. Oceans were home to many bony fish, hard-shelled molluscs, and crabs. On land, at first dominated by gymnosperms, early mammals evolved alongside dinasaurs and insects. Placental mammals, birds, and flowering plants also evolved within the Mesozoic era. After this time, the remaining dinasurs and many other species suddenly disappeared from the fossil reocrd. Considerable evidence supports the hypothesis that an asteroid ijmpact caused the best-known mass extinction. The chiczulub Crater, almost 10km deep and 200km in diameter at the edge of the Yucatan peninsula is thought to be the impact zone for such an asteroid. Some theories that the asteroid would have been moving at about 160 000 km.h and would have blasted 200 000 km3 of vaporized debris and duest into Earths atmosphere. The debris and energy released in the resulting fireball - equivalent to 100 million nuclear bombs - would have killed most of the plants and animals in the contental Americas within minutes. Tidal waves 120m high would ahve devistated costlines around the world and atmospheric debris would have blocked out the sunlight for months. Among the strong evidence for the impact hypothesis is the presence of unusually high concerntrations of iridium in sedimentary rock dated at 65 millino years old, the boundary beteen the Mesozoic and Cenzoic eras. Rock samples from 95 loctaions world wide show these same elevated levels. Iridium, a rare metal in Earths crus, is abundant in meterite samples. These findings suggest that a large asteroid may have been the source of a great quantity of iridium - bearing dust, deposited on a global scale.
Although the mass extinctions that ended the Permian and the Mesozoic eras are dramatic in scope, it is important to kepe in mind that most species extinctions results from ongoing evolutionary forces of competition and environment change. Amazingly, even the five major mass exticnitons events since the Cambrian explosion account for about only 4% of all extictions that took place during this time. Scientsits have also noted that periods of widespread extinciton are folllowed by periods of vary rapid diversification. In the present Cenozoic era, life forms have attained the greatest diversity in Earths history. Flowering plants have out-competed gymnosperms in many habitats and now number more than 250 000 species. Millions of species of insects now dominate the animal kingdom. Are natural extincitons as much a prt of evolutoin as diversificaton? It is probable that, had the dimasours not become extinct, the ancesotrs of humans may not have met with later successes - which means that humans might havenever existed.
Rate of Evolution
Biologists are keenly interested in the pace at which evolution may be occurring. Until recently, most supported the idea that changes to species were slow and steaily paces over time. The theory of gradualism contends that when new species first evolve, they appear very simialr to the orginiator species and only gradually become more distinctive, as natural seleciotn and genetic drift act independently on both species. One would expect to find, according to this theory, as a result of slow incremental changes, numerous fossil species representing transitional forms (also called intermediate forms). Many veyr distinct species, however, seem to appear suddenly in the fossil record with little evidence of grdual transition from ne species to another. There sudden appearence is often followed by little change over very long periods of time. The most accepted explanation for these deviations from the gradualism model was that the fossil record is incomplete, and intermediate forms may not have been preserved.
Niles Eldredge of the American Museum of Natural History and Stephen Jay Gould of Harvard University rejected this explanation and, in 1972, proposed an alternative theory called the theory of punctuated equilibrium. It consists of three main assertions:
~~Species evolve very rapidly in evolutionary time
~~Speciation usually occurs in small isolated populations and thus intermediate fossils are very rare
~~After the initial burst of evolutoin, species do not change significantly over long periods of time
These contrasting theories about the rate of evolutoin are represented in Figure 10 (that I cant draw out). To some extent, the differences between them are a matter of perspective. To many population biologists, the word rapid in relation to species evolution suggest changes that can be measured in a few generations or, perhpas, decades. To paleontologists, rapid might represent the appearence of a new species in a fossil record within a thousand gernatoins or 100 000 years. In fact, both theories are needed to understand the fossil record while remaning compatible wiht many other forms of evidence. Consider, for instance, how both thoeries apply to the evolutoin of species before and after a major extiction event.
~~Before the event, an environment might be host to many well-adapted species that have evolved to occupy specific ecological niches. They are largely exposed to the pressures of stabalizing selection and evolutoinary changes would be ver slow
~~An environmental crisis results in the extinction of most species, leaving many miches empty.
~~Surviving species have many new opportunities and experiences strong disruptive selectoin. These survivors can evolve rapidly into many new species, filling these empty niches.
~~Once the new species become well adapted to their new niches in a relatively stable environment, they again experience stabalizing selsection pressures. Thereafter, they show little, or more gradual, change until another crisis opens opportunities for diversification.
It is now widly accepted that both gradual and rapid evolutoinary process are at work, Although many species have evolved rapdily at times, the fossil records of some organisms show very gradual changes over extended periods of time.
Originating Events
Primordial Earth
Earth, when it formed some 4.6 billion years ago, was extreamly hot. Heat generated by asteroid impacts, internaiton compression, and radioactivity melted most of the rocky material. Dense materials, composed of such heavy elements as iron and nickel, formed Earth's inner core, while less dense materials formed a thick mantle. The least dense rock, composted mostly of lighter elements, floated on the surface and cooled to form a crst.
Mot gases formed Earth's primitive atmosphere. When, after some 500 million to 800 million years, the asteroid bombarrdment slowed the surface temperature cooled below 100*C, cast wuantities of water vapour condensed. Hundreds of years of torrential rains pooled in surface depressions to form ocean basins. The atmosphere of primordial Earth would have comtianed large amounts of nitorgen gas, carbon dioxide, carbon monoxide, and water vapor. Other hydrogen compounds - such as hydrogen sulfide, ammonia, and methane - would have been present. It is probable, though not certain, that this early atmosphere also contained hydrogen gas. Oxygen gas is highly reactive and, with the high temperatures present then, would have compbined with many other elements to form oxides; for this reason, the atmosphere would have contained little, if any, free oxygen. The surface of Earth would have been exposed to many intense sources of energy: radioactivity, intense ultraviolet light, visible light, and cosmic radiation from a young sun; heat from volcanic activity; and electrical energy from violent lightning storms.
Organic Molecules
In the mid 1930s, a Russian biochemist Alexander Oparin and British biologist J.B.S. Galdane independently proposed the theory of primary abiogenesis - that the first living thing on Earth arose from nonliving material. They reasoned that the first complex chemicals of life must have formed spontaneously on a primordial Earth and, at some point, arranged themselves into cell-like structures with a membrane separating them from the outside environment.
Although extreamly harsh, the early conditions on Earht were ideal for triggering chemical reactions and the formation of complex organic compounds. What molecules might have formed from the reactions of gases in the primordial atmosphere? In 1953, the Nobel Prize=winning astronomer Harold Urey and his student, Stanley Miller, investigated possible reactions. Their apparatus modelled the water cycle by using a condenser to produce precipitation and a heater to cause evaporation. Since Urey and Miller suspected that the early atmosphere would have contained water vaour, ammonia and methane and hydrogen gases, they combined the gases and exposed them to electrical sparks, thereby modelling early conditions on Earth [see picture (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure2.jpg)]. After one week, 15% of the original carbon in the methane had been converted to a variety of compounds, including aldehydes, carboxylic acids, urea, and - most interestingly - two amino acids: glycine and alanine.
More recent evidence suggests that the specific combination of gases chosen by Urey and Miller was not likely to have existed in the primordial atmosphere. In responce, many other scientists have continued this investigation with experiments that use the combination of gases now throught to have been present. These experiments have produced an even greater variety of simple organic compounds, including essential sugars, all 20 amino acids, many vitamis, and all four nitrogenous bases found in RNA and DNA. The most abundant nitrogen base, adenine, was the easiest to produce under laboratory conditions. These results suggest that many of the building blocks of life likely formed spontanously in Earths primordial environment.
Chemical Evolution
For the first molecules to have produced living cells, they had to have been able to form more complex chemical and physical arrangements. Polymerazation of early monomers may have occurred in numerous ways. Monomers may have become concentrated on hot surfaces as water evaporated, and the increased concentrations and heat energy may have triggered polymerization reactions. Under simialr conditions, in 1977, Sidney Fox at the University of Miami was able to trigger the spontanious production of thermal proteinoids consisting of chains of more than 200 amino acids. Other scientisits have discovered that such materials as clay particles and iron pyrite form electrostatically charged surfaces that are also capable of binding monomers and catalyzing polymerization rectoins. These findings suggest mechanisms for the formation of the first polymers. Could any polymers have then influenced their own formation?
The most fundemental characteristic of living things is organized self-replication. To self-replicate, molecules must demonstrate catalytic activity, that is, the ability to influence a chemical reactoin. But can a molecule act as a catalyst for its own formation?In the 1980's, Thomas Cech, working at the University of Colorado, discovered RNA molecules, called ribozymes, that act as catalysts in living cells. In other experiments, simple systems of RNA molecules have been created that are able to replicate themselves. In 1991, while working at the Massachusetts Institute of Technology, chemist Julius Rebek, Jr., created synthetic nucliotidelike molecules that could replicate themselves - and make mistakes, wich resulted in nonliving molecular systems that mutated and underwent a form of natural selection in a test tube. As demonstrated by such experiments, Earth's first self-replicating and evolving system may have been RNA molsecules. RNA is also likely to have been the first hereditary molecule. Its catalytic activity and the roles of tRNA, mRNA and rRNA suggest that it is likely to have played a direct role in the synthesis of proteins. Current scientific thinking about DNA is that it evolved later, perhaps by the reverse transcription of RNA.
Formation of Protocells
The evolution of self-replicating molecular system and cell-like structures is a vital area of investigatoin among scientists who study the origin of life. All living things are composed of cells. For chemicals in cells ot remain concentrated enough for metabolic processes to occur, they must be separated from the surrounding dilute environment. How might the first membranes have formed and arranged themselves into cell-like packages with an interior separate from the surrounding environment?
Lipid membranes can and do form spontaneously. Because of their hydrophobic tails, fatty acids and phospholipids naturally arrange themselves into spherical double-layer liposomes, or clusters. These can increase in size by the addition of more lipid and, with gentle shaking, can form buds and devide. Their membranes also act as a semi-permeable boundaries, so that any large molecules initially trapped within them, or produced by internal chemical activity, are unable to escape, thereby increasing in concentration. Although they are not alive, they can respond to environmental changes or reproduce in a controlled way, which means protocellsdo share many traits of living cells. Additional experimental evidence has shown that semipermeable liquid-filled spheres can also form from proteinlike chains. Researchers have discovered that if amino acids are heated and placed in hot water, they form proteinoid spheres, which are capable of picking up lipid molecules from their surroundings, as show in Figure 3 [that I cant scan in so you dont get to see :P]. These protocells are also able to store energy in the form of an electrical potential across their membrane, a trait found in all living cells. Although these findings are the subject of debate and many unanswered questions remain, there is evidence that chemical evolution could have given rise to molecular systems and cellular structures that are characterestic of life.
Procayotic Oranisms: The First True Cells
The oldest known fossils of cells on Earth - accurately dated to 3.465 billion years ago - were found in western Australia in layered formations called stromatolites. These microscopic fossils resemble present-day anaerobic cyanobacteria (Figures 4 and 5, that, once again, I cant scan in). Even the world's oldest-known sedimentary rock formations located in greenland - dating to 3.8 billion years ago - show chemical traces of microbial life and activity.
Although the oldest fossil bacteria resemble photosynthetic cyanobacteria, which use oxygen, the very first prokaryotic cells would certainly have been anaerobic, as the atmosphere would then have contained little or no free oxygen. These first prokaryotic organisms would likely have relied on abiotic sources of organic compounds. They would have been chemoautotrophic, obtaining their energy and raw materials from the metabolism of such chemicals in their environment as hydrogen sulfide, released at high temperatures and in large quantities from ocean-floor vents. These organisms would have adapted to living under harsh conditions of extreme heat and pressure and may have resembled present-day thermopilic archaebacteria. As the first cells reporduced and became abundant, these chemicals would have gradually become depleted. Any cell that was able to use simple inorganic molicules and an alternative energy source would have had an advantage. Fossil evidence suggests that, by 3 billion years ago, photosynthetic autotrophs wree doing just that.
Although the first photosynthetic organsims may have also used hydrogen sulfide as a source of hydrogen, those that used water would have had virtually unlimited suplly. As they removed hydrogen from water, they would have released free oxygen gas into the atmosphere - a process that would have had a dramatic effect. The accumulatoin of oxygen gas, which is very reactive, would have been toxic to many of the anaerobic organisms on Earth. While these photosynthetic cells prospered, others would have had to adapt to the steadily increasing levels of atmospheric oxygen of perish. Some of the oxygen hags reaching the upper atmosphere would have reacted to form a layer of ozone gas, having the potential to dramtically reduce the amounts of damaging ultraviolet radiation raching Earth. At the same time, the very success of the photosynthetic cells would have favoured the evolution of many heterotrophic organisms.
These are early life forms and evolutionary stages produced the necessary conditions to support the dramtic success of life on Earth powered and supplied by energy from the sun and chemical products of photosynthesis.
Diversification and Extinctions
Endosymbiosis in Eukaryotic Cells
Not much fossil evidence of the early evolution of single-celled organisms exists. Compariosons of present-day prokaryotic and eukaryotic DNA, however, suggests that the earliest prokaryotic cells probably gave rise to eubacteria and archaeacteria. It is likely that photosynthesis and aerobic respiratoin first evolved among eubacteria. Present-day archaebacteria are adapted to survive in extreme environments not unlike those that may have been widespread on ancient Earth. Archaebactera may have then given rise to eukaryotic cells. Although present-day eukaryotic organisms still share many genetic traits with modern archaebacteria, the eukaryote lineage and archaebactera lineage are thought to have separated about 3.4 billion years ago. while eventually evolving into eukaryotes, this lineage still consists of prokaryotic organisms for another billion years. These proposed lineages are shown here (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure2-2.jpg) .
The appearance of eukaryotic cells marks the ekey event in the evolutionary history of life. In rocks older than 1.5 billion years, most fossils are of microorganisms that appear to be very similar and small in size [more pictures you cant see]. Recent fossil discoveries from the Empire Mine in Michigan appear to be those of early eukaryotic algae, dating between 1.85 and 2.1 billion years old. Although eukaryotic cells likely evolved more than 2 billion years ago, rock dated to about 1.4 billion years old offers the earlies clear evidence of much larger cells that appear to have membraine - bound internal structures and elaborate shapes.
The distinguishing feature of eukaryotic cells is the presence of membraine-bound organelles, such as the neucleus and vacuols. A nuclear membrane and endoplasmic reticulum may have evolved from infolding of theo uter cell membrane (pretty diagram (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure3-2.jpg)). Initially, such folding may have been an adaption that permitted more efficient exchange of materials between the cell and its surroundings by increasing surace area, and it may also have provided more intimate chemical communicatoin between the genetic material and the environment.
Research postulated that a process of endosymbiosis may have given rise to mitochondria and chloroplasts, two unusual organelles. According to this new widely accepted theory, early eukaryotic cells engulred aerobic bacteria in a process similar to phagocytosis in amoeba (Clicky (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure4-2.jpg)). Having been surrounded by a plasma membrabe, the bacteria were not digested but, instead, entered into the symbiotic relationship with the host cell. The bacteria would have continued to preform aerobic respiratoin, providing excess ATP to the host eukaryotic cell, which would have continued to seek out acquire energy-rich molecules from its surroundings. Endosymbiotic bacteria, benifiting from this chemical-rich environment, would have begun to reporduce independently within this larger cell. Subsequently, photosynthetic bacteria - such as cyanobacteria - may have become endosymbiotic in a similar way within aerobic eukaryotic cells. Such a relatoinship would have benefited the bacteria by providing a richer supply of carbon dioxide for photosynthesis, and the eukaryotic cells by providing excess glucose or other energy-rich products of photosynthesis.
The theory of endosymbiosis is supported by examinations of the organelles themselves. Mitochondria and chloroplasts have features that are different from those of other organelles. They are typically surrounded by two membranes. Although the outer membrane is similar to all other eukaryotic cellular membranes, the chemistry of the internal membrane resembles that of eubacteria plasma membranes. These organelles also have their own DNA, which appears to be remnants of circular eubacteral chromosomes, and contains genetic coding sequences for various proteins and RNA which resemble bacteral genes more than eukaryotic genes. Mitochondria and chloroplasts replicate their own NDA and undergo division indiependently of their host cell's division. They have, however, lost many vital genes and are no longer able to live independently of the host cell.
The evolutoin of both aerobic heterotrophic and aerobic photosynthetic eukaryotic cells likely occured thorugh endosymbiosis. Heterophic eukaryotic cells could have evolved into various protists and, later, into fungi and animals, while photosynthetic eukaryotic cells could have been the ancestors of photosynthetic protists and, eventually, plants. It is probable that chloroplasts originated by endo symbiosis in more than one lineage of eukaryotic organisms. One way to represent these hypothetical evolutionary steps is shown here (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure5-2.jpg) (bad diagram, sorry).
Endosymbiosis has been discovered to occur in many moder organisms. Some ciliates and marine slugs are known to ingest algae and store their chloroplasts, which continues to preform photosynthesis for a few weeks. Coral organisms house living photosynthetic protests within their tissues, and many insects are known to host prokaryotic cells within their cells. One protozoan, [i]pelomyxa, replies on three different endosymbiotic bactera species for respiration. Researchers have even documented the engulfing of one eukaryotic cell by other; for example, the crypromonad Guillardia theta, a eukaryotic alga, contians chloroplasts that are surrounded by four membranes rather than the usual two. Between the outer and inner pairs are renants of the first host cell, including a small but functioning nucleus complete with eukaryotic DNA. In this case, photosynthetic eubactera became endosymbiotic within eukaryotic cells, which later also became endosymbiotic. [see here (http://img.photobucket.com/albums/v199/Wee_Little_Me/Figure6-2.jpg)]
Multicellular Organisms and the Cambrian Explosion
For the first 3 billion years of life on Earth, all organisms were unicellular. Eubacteria gave rise to aerobac and photosynthetic linages, which archaebacteria evolved into three main groups: methanogens, extreme halphiles, and extreme thermophiles. Once eukaryotic organisms evolved complex structures and processes, including mitosis and sexual reporduction, they would have had the benifit of much more extensive genetic recombination than would have been possible among prokaryotic cells. Photosynthesis continued to increase the oxygen concentration in the atmosphere to benifit of aerobic organisms. Multicellular organisms, including plants, fungi, and animals, are thought to have evolved less than 750 million years ago.
The oldest fossils of multicellular animals date form about 640 million years ago. However, during a 40-million-year period beginning about 565 million years ago, a massive increase in animal diversity occurred, referred to as the cambrian explosion. Fossil evidence dating from this period shows the appearance of early arthropods, such as trilobites, as well as echinoderms and molluscs; primitive chordates - which were precursors to the vertebrates - also appeared. Animals represeting all present-day major phyla, as well as many that are now extinct, first appeared during this period, a time span that represents less than 1% of Earths history.
Deversification and Mass Extinction
[Big fucking diagram showing 4 things that I am NOT going to draw out] provides a geological time scale and summarizes some of the most significant events in the evolutionary history of earth since the Cambrian explosion. Geologists have established a geological time scale devided into five eras, each of which is further subdivided into periods and, in some cases, epochs. These time intervals are based on their distincitve fossil records, and dramatic changes in the fossil records mak the boundaries between these intervals. The ears of Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life), are remarkable for rapid diversification of life forms, as well as awidespread extinctions. The Paleozoic era, for instance, beigns with the Cambrian explosion and ends with the Permian extinction believed to be the most massive extinction in Earths history.
Fossil evidence of diversification of marine invertebrates early in the Paleozoic era is very extensive. The first vertebrates are thought to have evolved later, followed by bony fish and amphibians. By the mid-Paleozoicera, plants have invaded land surfaces and the first reptiles and isnects have evolved. Around 245 millino years ago, a series of cataclysmic events eradicated more than 90% of known marine species, as indicated by their disappearance form the fossil record after this period. Although uncertainty remains about causes of the Permian extinction, many scientists suspect that tectonic movemtns were a primary cause. The formaiton of the supercontintent Pangea, which occured during the Permian period, would have produced major changes in terrestrial and costal environments as well as in global climate. Ongoing research by Kunio Kaiho of Tohoku University, Japan, and his colleagues has uncovered evidence in sothern China of 69-kn-wide asteroid that may have collided with Earth in this period. These researchers believe that the impact may have vaporised enough sulfur to consume a third of the atmospheric oxygen and generate enough acide rain to make the ocean surface water as acidic as lemon juice. If such a catastrophic impact did occur, it would have been the primary cause of the biggest extinction event in history.
Despite the harsh conditions responsible for mass extinctions, life on earth continued. The Mesozoic era is well known for dinasaurs, a divers group of often veyr large animals that dominated earth for about the mid Trassic to the late Cretacious period. Oceans were home to many bony fish, hard-shelled molluscs, and crabs. On land, at first dominated by gymnosperms, early mammals evolved alongside dinasaurs and insects. Placental mammals, birds, and flowering plants also evolved within the Mesozoic era. After this time, the remaining dinasurs and many other species suddenly disappeared from the fossil reocrd. Considerable evidence supports the hypothesis that an asteroid ijmpact caused the best-known mass extinction. The chiczulub Crater, almost 10km deep and 200km in diameter at the edge of the Yucatan peninsula is thought to be the impact zone for such an asteroid. Some theories that the asteroid would have been moving at about 160 000 km.h and would have blasted 200 000 km3 of vaporized debris and duest into Earths atmosphere. The debris and energy released in the resulting fireball - equivalent to 100 million nuclear bombs - would have killed most of the plants and animals in the contental Americas within minutes. Tidal waves 120m high would ahve devistated costlines around the world and atmospheric debris would have blocked out the sunlight for months. Among the strong evidence for the impact hypothesis is the presence of unusually high concerntrations of iridium in sedimentary rock dated at 65 millino years old, the boundary beteen the Mesozoic and Cenzoic eras. Rock samples from 95 loctaions world wide show these same elevated levels. Iridium, a rare metal in Earths crus, is abundant in meterite samples. These findings suggest that a large asteroid may have been the source of a great quantity of iridium - bearing dust, deposited on a global scale.
Although the mass extinctions that ended the Permian and the Mesozoic eras are dramatic in scope, it is important to kepe in mind that most species extinctions results from ongoing evolutionary forces of competition and environment change. Amazingly, even the five major mass exticnitons events since the Cambrian explosion account for about only 4% of all extictions that took place during this time. Scientsits have also noted that periods of widespread extinciton are folllowed by periods of vary rapid diversification. In the present Cenozoic era, life forms have attained the greatest diversity in Earths history. Flowering plants have out-competed gymnosperms in many habitats and now number more than 250 000 species. Millions of species of insects now dominate the animal kingdom. Are natural extincitons as much a prt of evolutoin as diversificaton? It is probable that, had the dimasours not become extinct, the ancesotrs of humans may not have met with later successes - which means that humans might havenever existed.
Rate of Evolution
Biologists are keenly interested in the pace at which evolution may be occurring. Until recently, most supported the idea that changes to species were slow and steaily paces over time. The theory of gradualism contends that when new species first evolve, they appear very simialr to the orginiator species and only gradually become more distinctive, as natural seleciotn and genetic drift act independently on both species. One would expect to find, according to this theory, as a result of slow incremental changes, numerous fossil species representing transitional forms (also called intermediate forms). Many veyr distinct species, however, seem to appear suddenly in the fossil record with little evidence of grdual transition from ne species to another. There sudden appearence is often followed by little change over very long periods of time. The most accepted explanation for these deviations from the gradualism model was that the fossil record is incomplete, and intermediate forms may not have been preserved.
Niles Eldredge of the American Museum of Natural History and Stephen Jay Gould of Harvard University rejected this explanation and, in 1972, proposed an alternative theory called the theory of punctuated equilibrium. It consists of three main assertions:
~~Species evolve very rapidly in evolutionary time
~~Speciation usually occurs in small isolated populations and thus intermediate fossils are very rare
~~After the initial burst of evolutoin, species do not change significantly over long periods of time
These contrasting theories about the rate of evolutoin are represented in Figure 10 (that I cant draw out). To some extent, the differences between them are a matter of perspective. To many population biologists, the word rapid in relation to species evolution suggest changes that can be measured in a few generations or, perhpas, decades. To paleontologists, rapid might represent the appearence of a new species in a fossil record within a thousand gernatoins or 100 000 years. In fact, both theories are needed to understand the fossil record while remaning compatible wiht many other forms of evidence. Consider, for instance, how both thoeries apply to the evolutoin of species before and after a major extiction event.
~~Before the event, an environment might be host to many well-adapted species that have evolved to occupy specific ecological niches. They are largely exposed to the pressures of stabalizing selection and evolutoinary changes would be ver slow
~~An environmental crisis results in the extinction of most species, leaving many miches empty.
~~Surviving species have many new opportunities and experiences strong disruptive selectoin. These survivors can evolve rapidly into many new species, filling these empty niches.
~~Once the new species become well adapted to their new niches in a relatively stable environment, they again experience stabalizing selsection pressures. Thereafter, they show little, or more gradual, change until another crisis opens opportunities for diversification.
It is now widly accepted that both gradual and rapid evolutoinary process are at work, Although many species have evolved rapdily at times, the fossil records of some organisms show very gradual changes over extended periods of time.