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13.1 Originating Events Earth, and life on it, originated billions of years ago. Scientists have pieced together a scientific description of the initial conditions and events that may have resulted in the origin of life. Much ongoing research focuses on testing hypotheses about the origins and evolution of the earliest life forms. Primordial Earth Figure 1 Thermal mud pools, such as those in Yellowstone National Park, are suggestive of conditions on the surface of primordial Earth. Earth, when it formed some 4.6 billion years ago, was extremely hot. Heat generated by asteroid impacts, internal 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, composed mostly of lighter elements, floated on the surface and cooled to form a crust (Figure 1). Hot gases formed Earth’s primitive atmosphere. When, after some 500 million to 800 million years, the asteroid bombardment slowed and surface temperatures cooled below 100°C, vast quantities 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 contained large amounts of nitrogen gas, carbon dioxide, carbon monoxide, and water vapour. 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 combined with many other elements to form oxides; for this reason, the atmosphere would have contained little, if any, free oxygen gas. 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 primary abiogenesis theory that the first living things on Earth arose from nonliving material 586 Chapter 13 In the mid-1930s, the Russian biochemist Alexander Oparin and British biologist J.B.S. Haldane independently proposed the theory of primary abiogenesis—that the first living things 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 extremely harsh, the early conditions on Earth 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 vapour, ammonia, and methane and hydrogen gases, they combined these gases and exposed them to electrical NEL Section 13.1 sparks, thereby modelling early conditions on Earth (Figure 2). 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 response, many other scientists have continued this investigation with experiments that use the combination of gases now thought to have been present. These experiments have produced an even greater variety of simple organic compounds, including essential sugars, all 20 amino acids, many vitamins, and all four nitrogenous bases found in RNA and DNA. The most abundant nitrogenous base, adenine, was the easiest to produce under laboratory conditions. These results suggest that many of the building blocks of life likely formed spontaneously in Earth’s primordial environment. electrodes to vacuum pump DID YOU KNOW ? Building Blocks from Space Evidence from outer space offers new information regarding Earth’s history. The Murchison meteorites discovered in Australia are rich in amino acids, and many asteroids and comets contain significant quantities of organic compounds. These findings provide additional evidence of the formation of important organic compounds in the solar system. Some scientists hypothesize that heavy bombardment of the young Earth by such bodies may have been the source for much of the organic material that would eventually form the first living cells. spark discharge CH4 NH3 H2O H2 gases water out condenser water in water droplets boiling water water containing organic compounds liquid water in trap Figure 2 The Miller and Urey experimental apparatus 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. Polymerization 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 similar conditions, in 1977, Sidney Fox at the University of Miami was able to trigger the spontaneous production of thermal proteinoids consisting of chains of more than 200 amino acids. Other scientists 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 reactions. These findings suggest mechanisms for the formation of the first polymers. Could any polymers have then influenced their own formation? NEL thermal proteinoids a polypeptide that forms spontaneously when amino acids polymerize on hot surfaces; proteinoids are able to selfassemble into small cell-like structures in cool water The Evolutionary History of Life 587 ribozymes an RNA molecule able to catalyze a chemical reaction The most fundamental characteristic of living things is organized self-replication. To self-replicate, molecules must demonstrate catalytic activity, that is, the ability to influence a chemical reaction. But can a molecule act as a catalyst for its own formation? In the 1980s, 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 nucleotidelike molecules that could replicate themselves—and make mistakes, which 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 systems may have been RNA molecules. 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 liposomes spherical arrangements of lipid molecules that form spontaneously in water ACTIVITY 13.1.1 Observing Liposome Formation (p. 629) How do nonliving collections of phospholipids behave in water? In this activity, you can model the formation of cell-like structures as the process may have occurred on primordial Earth. 588 Chapter 13 The evolution of self-replicating molecular systems and cell-like structures is a vital area of investigation among scientists who study the origin of life. All living things are composed of cells. For chemicals in cells to 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 separated 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 doublelayered liposomes, or clusters. These can increase in size by the addition of more lipid and, with gentle shaking, can form buds and divide. Their membranes also act as a semipermeable 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 protocells do 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 shown in Figure 3. 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 characteristic of life. Figure 3 Liquid-filled spheres can form spontaneously by various protein mixtures in water. NEL Section 13.1 Prokayotic Organisms: 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). 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. (a) (b) Figure 4 (a) Microfossils dating to 3.5 billion years ago were discovered by J. William Schopf of UCLA. They closely resemble present-day cyanobacteria (b). 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 thermophilic archaebacteria. As the first cells reproduced and became abundant, these chemicals would have gradually become depleted. Any cell that was able to use simple inorganic molecules and an alternative energy source would have had an advantage. Fossil evidence suggests that, by 3 billion years ago, photosynthetic autotrophs were doing just that. Although the first photosynthetic organisms may have also used hydrogen sulfide as a source of hydrogen, those that used water would have had a virtually unlimited supply. 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 accumulation 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 or perish. Some of the oxygen gas reaching the upper atmosphere would have reacted to form a layer of ozone gas, having the potential to dramatically reduce the amount of damaging ultraviolet radiation reaching Earth. At the same time, the very success of the photosynthetic cells would have favoured the evolution of many heterotrophic organisms. These early life forms and evolutionary stages produced the necessary conditions to support the dramatic success of life on Earth powered and supplied by energy from the sun and the chemical products of photosynthesis. NEL stromatolites shaped rock formations that result from the fossilization of mats of ancient prokaryotic cells and sediment Figure 5 The actions of cyanobacteria began forming these stromatolites in western Australia 2000 years ago. They are almost identical to those containing the fossils discovered by Schopf (Figure 4). chemoautotrophic describes an organism capable of synthesizing its own organic molecules with carbon dioxide as a carbon source and oxidizing an inorganic substance as an energy source heterotrophic describes an organism unable to make its own primary supply of organic compounds (i.e., feeds on autotrophs, other heterotrophs, or organic material) The Evolutionary History of Life 589 DID YOU KNOW ? The Panspermia Theory Proponents of the panspermia theory suggest that life may have arisen elsewhere in this solar system and travelled to Earth on a meteorite or comet. How much evidence is there to support this theory? A meteor from Mars made headlines in 1996 when an examination with a scanning electron microscope of samples revealed objects resembling bacteria fossils (Figure 6). Many scientists, however, suspect these structures are inorganic in origin. What conditions on Mars might have permitted or fostered abiogenesis? GO www.science.nelson.com Figure 6 SUMMARY Earliest Evolutionary Processes • Earth formed about 4.6 billion years ago. By about 4 billion years ago, less dense compounds had cooled to form a solid crust, water vapour had condensed, and ocean basins had filled. • Early anaerobic conditions on Earth likely resulted in the formation and polymerization of many small organic molecules. • Some RNA molecules act as catalysts for various reactions, including their own replication. As such, they are likely candidates for the first hereditary molecular systems. • Both lipid and protein compounds likely formed liquid-filled semipermeable vessels spontaneously. These vessels have some of the same properties as cells. • The first cells, which evolved at least 3.8 billion years ago, resembled modern prokaryotic cells. After photosynthetic prokaryotic cells evolved, at least 3 billion years ago, oxygen gas began to accumulate in Earth’s atmosphere. Section 13.1 Questions Understanding Concepts 1. Review the theory of natural selection as described by Darwin (Chapter 11, section 11.6). Given that chemical evolution occurred before life existed on primordial Earth, explain when you consider the process of natural selection to have begun. 2. Many scientists study chemical evolution. Suggest ways in which selective forces might have acted on chemicals before living cells existed on Earth. 3. Compare and contrast thermal proteinoids and liposomes. 4. In what way might the lack of oxygen in the early atmos- phere have influenced the formation of both complex organic molecules and the first living cells? Applying Inquiry Skills attempted to model chemical reactions in the atmosphere of ancient Earth. Even though other scientists have cast doubts on this particular combination of gases, why do we consider the findings of the Urey–Miller experiment to be relevant? 6. (a) In Activity 13.1.1, why was lecithin used to model the formation of protocells? (b) Did your observations permit you to determine whether the vesicles have double-layered membranes? Making Connections 7. Some scientists have different perspectives on the earliest evolutionary history on Earth. In print and electronic sources, find out more about their research, evidence, reasoning, and differing interpretations of experimental results. GO www.science.nelson.com 5. Suggest reasons that Urey and Miller selected the combina- tion of gases they did for their experiment in which they 590 Chapter 13 NEL