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Principles of Biology 73 contents Origin of Life Life on Earth likely began as macromolecules that developed into self-replicating protocells. Lightning during a storm. Scientists hypothesize that forces such as lightning could have provided the energy and circumstances needed for forming complex organic molecules. NOAA Research. Topics Covered in this Module From Organic Molecules to Self-Replicating Protocells How Did Macromolecules Assemble from Organic Molecules? Major Objectives of this Module Explain how organic molecules self-assemble into macromolecules. Describe how macromolecules could replicate using templates. Discuss hypotheses about the formation of self-replicating protocells. page 375 of 989 4 pages left in this module Principles of Biology 73 Origin of Life How did life begin? The short answer is we don't quite know. Evidence comes from a variety of sources: the anatomy and physiology of current life forms and fossils, geologic findings that show how different the Earth was earlier in its history, data from astronomy describing how stars and planets are created, and laboratory experiments that attempt to replicate early life on a molecular level. Current hypotheses fit the evidence we have, but because the evidence is limited, they are still hypotheses. There is no coherent, widely accepted scientific theory about the origins of life. From Organic Molecules to Self-Replicating Protocells How are plants and animals different from rocks and water? Living organisms reproduce themselves. Even the simplest bacteria produce copies of their single cells. How could this complicated process have evolved? Scientists generally agree on a basic trajectory. First, organic molecules (molecules containing carbon) developed, then macromolecules, then protocells. At what point in this process did the molecules become self-replicating, or living? How did organic molecules originate? Our best interpretation of the scientific evidence available is that a sequence of events like the following might have occurred: About 4.6 billion years ago, dust and rocks around our Sun condensed to form our solar system, including Earth. The planet would have been battered by rocks and ice hurtling through space, keeping it too hot for water to form until conditions calmed down between 4.2 and 3.9 billion years ago. The atmosphere was probably made up primarily of carbon dioxide and nitrogen, but volcanic eruptions might have contributed methane, ammonia, and hydrogen sulfide. As the planet cooled, water vapor in the atmosphere condensed, forming seas. Most of the hydrogen could have escaped into space. Geological evidence suggests that almost as soon as the oceans formed, life began. In the 1920s, two scientists — J. B. S. Haldane, in Britain, and A. I. Oparin, in Russia — independently hypothesized that early Earth might have had a reducing atmosphere that, along with energy from ultraviolet radiation or volcanic explosions, could create organic molecules. A reducing environment is one that adds electrons to molecules. Haldane called this early environment a "primitive soup." In 1953, Stanley Miller, a graduate student working in Harold Urey's lab at the University of Chicago, tried to replicate the atmospheric conditions of early Earth to determine how life could have begun (Figure 1). He simulated lightning and volcanic eruptions, both conditions that create reducing environments. Under the conditions he created, simple chemicals changed into complex organic molecules, including amino acids (the building blocks of protein). Other scientists have replicated these results. One of Miller's former graduate students, Jeffrey Bada, reanalyzed Miller's original samples in 2008 using modern equipment and found additional amino acids Miller had not detected. contents Figure 1: The Miller-Urey experiment. This illustration of the Miller-Urey experiment shows a series of glass tubes and compartments containing water, water vapor, methane, and electric charge. © 2011 Nature Education All rights reserved. Figure Detail Miller and Urey might have produced the wrong atmospheric conditions for replicating early Earth. More recent evidence suggests that the early atmosphere was mostly nitrogen and carbon dioxide, making it closer to neutral: neither electron-adding nor electron-removing. Researchers have repeated experiments like Miller's with a simulated atmosphere made primarily of nitrogen and carbon dioxide, and these researchers have also found organic molecules. Based on these findings, a reducing atmosphere does not appear to be necessary for synthesizing organic molecules. Another possibility is that life did evolve in a reducing atmosphere but not in the atmosphere that was typical on most of the planet. How would that have been possible? About 30 years ago, geologists discovered hydrothermal vents on the ocean floor. Now many of these vents have been identified, in the Pacific, Atlantic, Indian, and Arctic Oceans. They arise near volcanoes or where tectonic plates are moving apart, in places where the magma beneath the Earth's crust is exposed, and heat the seawater to temperatures as high as 405°C (761°F). Seawater sinks into the ocean crust, leaches minerals and metals from the rocks, and then boils up and emerges into the cooler seawater as hot springs. The dissolved material precipitates out, often causing what's known as a "black smoker." Hydrogen sulfide and methane create a reducing environment. Nearby, temperatures drop dramatically and the chemical environment changes. A very hot, acidic environment may sound inhospitable, but many organisms live in or near hydrothermal vents. Archaebacteria in black smokers obtain their energy from hydrogen sulfide and replicate at temperatures as high as 121°C (250°F), hotter than any other known organism. These microorganisms are among the most evolutionarily stable organisms known (Figure 2). Their presence in hydrothermal vents constitutes one piece of evidence supporting the idea that life evolved there. Fossilized microorganisms have also been found in fossilized black smokers, including sulfide deposits dated to over 3 billion years ago. Hydrothermal vents would have existed on earth as soon as water did, around 4.2 billion years ago. Some of the organisms found on hydrothermal vents today, such as siboglinid tubeworms, might be "living fossils," some of the least-changing organisms to have evolved. Figure 2: Unique organisms living in a hydrothermal vent. Some organisms found in hydrothermal vents might be "living fossils" of some of the earliest organisms to evolve on Earth. Panel a): Siboglinid tubeworms (Sclerolinum contortum) spread across the side of a hydrothermal vent in front of white mats of microbes. Panel b): A close-up of the siboglinid tube worms with small gastropods (Pseudosetia griegi and Skenea spp.) on the tubes. © 2010 Nature Publishing Group Pedersen, R. B., et al. Discovery of a black smoker vent field and vent fauna at the Arctic Mid-Ocean Ridge. Nature Communications 1 (2010) doi:10.1038/ncomms1124. Used with permission. Hydrothermal vents also house bacteria that obtain their energy from methane. The process that these bacteria use to fix carbon dioxide and produce ATP, the molecule that provides energy to cells, is similar to the pathway the mitochondria in our own cells use. It is, however, more efficient, producing more energy-containing molecules per cycle. This bacterial process might resemble a metabolic pathway that could have sustained early-evolving microorganisms. An alternate theory of how organic molecules arose on Earth is that they did not form on Earth but simply landed here. A meteorite, called the Murchison meteorite, landed in Australia in 1969. It contained more than 70 amino acids, many in large quantity and some that were unusual. Some of these amino acids were present in two configurations, called L and D. D and L isomers are different forms of the same molecule, like mirror images or right and left hands. While nearly all of the amino acids on Earth are in the L configuration, many of those on the meteorite were in the D configuration. Later research found hints of terrestrial contamination of the meteorite but also uncovered evidence that at least some of the amino acids and some other molecules had an extraterrestrial origin. The Murchison meteorite also contained other organic compounds, including lipids, nitrogenous bases, and simple sugars, for a count of more than 14,000. Of course, the real answer may be a combination of both theories — it is entirely possible that macromolecules both arrived on asteroids and evolved in deep-sea vents. In 2010, astronomers found something else exciting on an asteroid: ice. Scientists don't know exactly when the current supply of water on Earth developed. One hypothesis is that an extraterrestrial object collided forcefully with the proto-Earth and began to orbit proto-Earth as our moon. That event would have vaporized any water on the planet at the time. Could all of our water have been delivered by asteroids? Astronomers once thought the asteroids traveled too close to the Sun to maintain ice (Figure 3). But then, Andrew Rivkin and Joshua Emery, looking through an infrared telescope at Mauna Kea, Hawaii, found a pattern in the radiation reflecting off an asteroid that indicated ice and organic materials. The astronomers monitored the asteroid, called 24 Themis, over six years. Another team of scientists, led by Humberto Campins, independently confirmed Rivkin and Emery's results. The asteroid 24 Themis is about three times further from the Sun than the earth is, but because asteroids have no atmosphere, that distance is usually too close for ice to stay on the surface. Researchers think the ice may be emerging gradually from a reservoir under the asteroid's surface. Perhaps asteroids brought water to Earth, too, and not just organic molecules. Of interest, even a planet as close to Sun as Mercury might, according to recent findings, have pockets of ice in craters at its poles. Figure 3: The asteroid Gaspra, photographed by the Galileo spacecraft. One hypothesis about the origin of life holds that ice and organic molecules came to Earth on asteroids. NASA. Figure Detail The possibility of asteroids bringing the building blocks of life to Earth raises an intriguing question: if life could happen on Earth, could it also be happening on other planets? After all, we have known since the 17th century that our Sun is just one of many Suns within our galaxy. Cosmologists Carl Sagan and Stephen Hawking, among other prominent scientists, have argued that life on Earth is unlikely to be the only life in the universe. After all, if it could happen here, why not on another planet with similar conditions? Test Yourself What is one explanation for why most isomers of amino acids in living things are those in the L configuration? Submit Future perspectives and open questions. 24 Themis might not be representative of asteroids near Earth. It could have formed much farther from the Sun and later been knocked in closer. If asteroids near Earth rarely contain water or organic molecules, it would be less probable that those asteroids played a role in the origin of life. Another factor is the isotope ratio of the ice. If deuterium, or heavy hydrogen (which contains an extra neutron and is therefore "heavy"), were found in the same ratio on the asteroid as on Earth, this result would support an asteroid origin hypothesis. Julie Castillo-Rogez, an astrophysicist at NASA, has suggested that NASA send robotic and manned missions to search for water and ice on asteroids near Earth. Now that researchers have also pretty clearly established the presence of ice and water on Mars, the question of extreplanetary delivery becomes even more interesting. Scientists speculate not just about the origin of organic molecules in general, but also about the origins of particular molecules that would have been necessary at particular stages in the evolution of life. For example, RNA formation requires both ribose and phosphate. Where did these ingredients come from? Ribose will form from two simple sugars, but it breaks down rapidly in an alkaline, or basic, solution. Alonso Ricardo, a researcher at Harvard University, has studied ways to stabilize ribose. Phosphorus, a critical component of the phosphate backbone connecting ribose molecules in RNA, is abundant in the Earth's crust but usually isn't released into water in large quantities, even around modern volcanoes. In 2005, Matthew Pasek and Dante Lauretta at the University of Arizona discovered that soluble phosphate is released from certain meteors. The origin of these molecules is a field of active research. IN THIS MODULE From Organic Molecules to Self-Replicating Protocells How Did Macromolecules Assemble from Organic Molecules? Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? SCIENCE ON THE WEB Stanley Miller Explains Do your own Miller-Urey experiment Up Close With Hydrothermal Vents Browse through underwater images captured by exploring scientists page 376 of 989 3 pages left in this module Principles of Biology contents 73 Origin of Life How Did Macromolecules Assemble from Organic Molecules? How does a simple, single-celled organism survive and reproduce? It needs macromolecules like DNA and RNA to make proteins. It needs proteins, such as enzymes, to make DNA and RNA. How were the first molecules made if there were no enzymes? The answer might have been that jack-of-all-trades molecule, RNA, or another type of flexible nucleic acid. BIOSKILL Was tPNA the First Genetic Material? Scientists have tried to create self-replicating chains that resemble RNA or DNA, but once bound together, the chains usually do not rearrange their base pairs to copy a template. In 2009, the chemist Reza Ghadiri at the Scripps Research Institute succeeded in creating a synthetic DNA-like molecule with nucleic acids reversibly bound to a backbone. They created a backbone from two repeating amino acids, and then attached the bases to it using adenine thioesters. The molecule is called thioester peptide nucleic acid (tPNA). When put in solution with a DNA template, tPNA rearranges its nucleic acids to match up with that particular template. Although tPNA doesn't look like current nucleic acids, the way it works does seem like a step toward replication. The next step is to find a way to keep the tPNA together so it will act as a template for another DNA or PNA strand. PNA is more chemically stable than RNA, and some researchers think that it might have been the first genetic material. Another popular hypothesis is that RNA was the first genetic material. Scientists Thomas Cech and Sidney Altman found that some types of RNA catalyze reactions, like enzymes. Cech called these RNA catalysts "ribozymes". Ribozymes also produce nucleotide sequences that directly complement a piece of RNA, and some ribozymes replicate themselves. Of those ribozymes, some replicate themselves faster and with fewer errors than others. David Bartels at the Massachusetts Institute of Technology and Alonso Ricardo and Jack Szostak at Harvard University directed ribozyme evolution by selecting the most efficient catalysts. After many rounds of selection they produced ribozymes that could copy short strands of RNA. Tracey Lincoln and Gerald Joyce at Scripps Research Institute evolved a pair of ribozymes that could copy each other, but these reactions were catalyzed by complex macromolecules. Could RNA catalyze its own polymerization? The research group at Harvard tried to make nucleotide polymers form a double strand without any catalysts. The reaction took weeks. Then they found that a slight change to the chemical structure of the sugar component sped up the reaction so that it occurred in only hours. Although this finding could be a clue to the origin of modern-day DNA and RNA, the precise structure of the earliest genetic material remains an open question. BIOSKILL How did self-replicating protocells develop? All cells replicate using DNA in a complicated procedure. The DNA double helix unwinds and separates into two strands, and proteins stabilize each strand while several enzymes work to create first an RNA primer, then new DNA strands that preserve the information contained in the genome. How, therefore, could a cell replicate without proteins or enzymes? Test Yourself If some ribozymes do not replicate, some replicate with errors, and some replicate quickly and accurately, what will happen to the population of ribozymes over time? Submit Both prokaryotic and eukaryotic cells are enclosed in cell membranes: lipid bilayers with embedded proteins that control what goes in and out of the cell. How could cell membranes have developed? Adding lipids or other organic molecules to water results in the creation of vesicles, or fluid-filled sacs. Vesicle self-assembly is much more likely when montmorillonite, a type of clay created by weathering volcanic ash, is added along with organic molecules to the water (Figure 4). The organic molecules cluster on the surface of the clay. When positioned close together, they are more likely to form vesicles. The molecules enclosing a vesicle will arrange themselves into a bilayer, shielding their hydrophobic ends like the lipid bilayer of a cell membrane. Figure 4: Nucleic acid polymerization is catalyzed by clay particles. Lab experiments have shown that nucleotides are attracted to clay particles, such as those that may have existed in primordial ponds. By concentrating nucleotides in a small area, the clay particles increase the rate of nucleotide polymerization, spontaneously forming nucleic acids. Instead of enzymes making a reaction happen more quickly, in this case, clay does the job. © 2012 Nature Education All rights reserved. Although they are not alive, vesicles have some properties in common with cells (Figure 5). Vesicles can divide to create more vesicles, just as cell membranes divide to create two cells. They can take in montmorillite particles, and if the particles are carrying RNA or other molecules, the vesicles will enclose those as well. Vesicles can have an internal environment different from the environment outside the lipid bilayer, and they can grow larger without diluting their contents. Some vesicles can take in molecules or chemicals selectively and use them in metabolic reactions. Figure 5: A laboratory-produced protocell that fights cancer. This electronmicrograph shows a protocell developed in the laboratory for encapsulating and delivering drugs to malignant tumors. Arrows point to the membrane enclosing the protocell (scale bar = 25 nm). © 2011 Nature Publishing Group Ashley, C. E. et al. The targeted delivery of multicomponent cargos to cancer cells by nanoporous particle-supported lipid bilayers. Nature Materials 10, 389–397 (2011) doi:10.1038/nmat2992. Used with permission. Imagine a vesicle that could grow, split into two vesicles, and carry RNA that could replicate accurately and catalyze reactions. Such a vesicle would be almost like a cell. In fact, vesicles like these could have been protocells that later evolved into true cells (Figure 6). Figure 6: RNA and protocell replication. The first protocell could have formed from the internalization and replication of nucleic acid molecules inside a phospholipid bilayer. © 2014 Nature Education All rights reserved. Transcript In the same way that RNA now acts as a template to create proteins, singlestranded RNA might have acted as a template for DNA. Double-stranded DNA is more stable than RNA and is copied more accurately, so cells with double-stranded DNA would likely have had an evolutionary advantage. RNA in these cells could have adapted to serve a different function, such as translating genes into proteins. Protocells might have engulfed other cells that later became organelles. Mitochondria, which produce ATP using glucose and oxygen, and chloroplasts, which produce glucose and oxygen from light and carbon dioxide, are widely believed to have begun as different types of bacteria engulfed by host cells, perhaps bacteria much like those still living in hydrothermal vents. Speaking of hydrothermal vents, one way to speed up a reaction — in addition to enzymes or clay — is to apply heat. How much difference does heat make? Some reactions that might have been important for developing the molecules necessary for life on the early Earth are slow at 25°C (77°F). But heating to 100°C (212°F) speeds them up by a factor of 10 million. Of all possible solvents for the molecules that assembled to create life, hot water might be the very best. Boiling water is hot enough to speed biochemical reactions by an enormous rate, but not so hot that it causes most complex molecules to fall apart. This effect of heat is yet another reason to think life might have evolved in volcano vents on the ocean floor, with a reducing environment, inorganic nutrients from lava and ash, water, and a large amount of heat. Processes that influence the origin of species are not the same as those that influence the origin of life. Some people think that evolutionary processes such as natural selection, which is backed by overwhelming evidence and universally accepted as a cornerstone of biology, equally explains the origin of life on Earth. It doesn't. Evolution is a characteristic of life, once it exists, but does not explain how life formed in the first place. As soon as protocells became self-replicating, evolutionary processes such as natural selection and genetic drift would have acted. For example, natural selection would have favored cells that replicated more effectively than other cells, cells that could use the available energy sources more efficiently, and cells that could repair themselves. Evolutionary processes such as natural selection underlie how protocells evolved into cells, how single-celled organisms evolved into multicellular organisms, and how organisms diversified into the multitude of species that we see across the Earth's ecosystems today. It does not, however, explain how the ability to self-replicate developed in the first place. Evolutionary theory deals with the origin of species but not with the origin of life. Test Yourself Why doesn't the theory of natural selection apply to the origin of life? Submit How did life begin? Is there life on other planets? Despite long-standing human curiosity about these questions, research on the origin of life has yielded only a set of hypotheses, not a single accepted theory. Most scientists agree that organic molecules must have appeared first, then macromolecules, and then self-replicating protocells. Maybe life began with meteorites, or maybe it began in hydrothermal vents, or maybe both. Maybe RNA self-assembly was the first step toward replication, perhaps PNA was the first genetic material, or maybe metabolic processes developed first, followed by guided assembly of macromolecules. Chemists, geologists, astronomers, and biologists continue to explore many different ways to test these hypotheses. IN THIS MODULE From Organic Molecules to Self-Replicating Protocells How Did Macromolecules Assemble from Organic Molecules? Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? SCIENCE ON THE WEB Stanley Miller Explains Do your own Miller-Urey experiment Up Close With Hydrothermal Vents Browse through underwater images captured by exploring scientists page 377 of 989 2 pages left in this module Principles of Biology contents 73 Origin of Life Summary Explain how organic molecules self-assemble into macromolecules. The origin of the first organic molecules is currently being investigated. These molecules self-assemble in conditions that are likely similar to the environment of primitive Earth. Hydrothermal vent conditions are conducive to the formation of organic molecules. Organic molecules similar to those found on Earth are also found on asteroids. Once smaller organic molecules formed, they might be able to aggregate into larger macromolecules. RNA might self-assemble and will form relatively long chains in the presence of clay. This process occurs faster at high temperatures. OBJECTIVE Describe how macromolecules could replicate using templates. Some macromolecules, such as RNA, can self-replicate as well as catalyze other reactions. Both RNA and PNA can produce nucleotide sequences that are complementary to a strand of nucleic acid without the addition of enzymes. OBJECTIVE Discuss hypotheses about the formation of self-replicating protocells. Phospholipids will self-assemble into bilayers when placed in an aqueous solution. These vesicles can divide, take in macromolecules, and maintain an internal environment that differs from their external environment. If these vesicles contain RNA, these protocells might replicate and catalyze reactions within their membrane. OBJECTIVE Key Terms protocell A theoretical vesicle that could grow, split into two identical vesicles, and carry RNA that could replicate and catalyze reactions. IN THIS MODULE From Organic Molecules to Self-Replicating Protocells How Did Macromolecules Assemble from Organic Molecules? Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? SCIENCE ON THE WEB Stanley Miller Explains Do your own Miller-Urey experiment Up Close With Hydrothermal Vents Browse through underwater images captured by exploring scientists page 378 of 989 1 pages left in this module Principles of Biology 73 Origin of Life Test Your Knowledge 1. When the Murchison meteorite that landed in Australia was found to contain amino acids, how could scientists tell they had arrived with the meteorite rather than having contaminated it after it landed? The ratio of hydrogen to deuterium didn't match the ratio found on Earth. The meteor could not have accumulated amino acids on Earth because it landed in the ocean. The meteor landed in a barren field where it couldn't have picked up any organic molecules. They were trapped under a crust and only seeped out when the ice under the crust melted. About half of the amino acids were D isomers and half were L isomers, whereas nearly all organic molecules on Earth are in the L configuration. 2. Approximately when do scientists believe life originated on Earth? 150 million years ago 4.2 billion years ago 6,000 years ago 90,000 years ago 2.2 billion years ago 3. What did the Urey-Miller experiment show that is still applicable today? Primitive bacteria can live and reproduce using methane and hydrogen sulfide as energy sources. RNA molecules will spontaneously self-assemble into polymers on a hot clay or rock surface. Amino acids and other complex organic molecules will spontaneously form in a reducing environment. Complex organic molecules will spontaneously form in an atmosphere like that of early Earth. Heating water to its boiling point speeds up reactions tremendously, sometimes by 10 million-fold. 4. Which piece of evidence supports the idea that life originated in or near submarine hydrothermal vents? Hydrothermal vents would have been present nearly as soon as Earth was created, about 2 billion years ago. On land, volcanic ash creates an environment inhospitable to living organisms. Archaebacteria fix carbon dioxide and produce ATP using the same pathway mitochondria use. Organisms that live near hydrothermal vents have specific adaptations to withstand these harsh environments. Some of the microorganisms that appear to have evolved earliest still live in hydrothermal vents. 5. What is the prevailing theory for how macromolecules assembled in the absence of enzymes? DNA catalyzed the formation of macromolecules. Simple reactions can occur quickly under any circumstances. Heat-catalyzed reactions led to the assembly of macromolecules. contents The reactions occurred slowly. Macromolecules catalyzed the formation of more macromolecules. 6. How have researchers encouraged longer RNA polymers to self-assemble? by cooling the solution to near freezing by adding the ribose sugars to the reaction last by dissolving them in water by adding microscopic particles of clay by changing the backbone structure 7. Which of the following statements best represents the most likely evolution of RNA? Its function changed over time. Its function has remained the same since the origins of life. It evolved after DNA. It evolved after protocells. Its biochemical structure has become completely different since it first originated. 8. Natural selection ___. did not play a role in the evolution of early life explains the formation of macromolecules determined the elements available for the evolution of life did not play a role in the origins of life eliminated meteorite-carrying organisms from Earth Submit IN THIS MODULE From Organic Molecules to Self-Replicating Protocells How Did Macromolecules Assemble from Organic Molecules? Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? SCIENCE ON THE WEB Stanley Miller Explains Do your own Miller-Urey experiment Up Close With Hydrothermal Vents Browse through underwater images captured by exploring scientists page 379 of 989