Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Mysteries of Life`s Origins
Document related concepts
Transcript
Chapter 19 Sect 3 Biogenesis Production of new or living organisms. Living things come from living things. Ex: Spider eggs become spiders Scientists Louis Pasteur came up with this idea. Endosymbiosis Explains how eukaryotes came from prokaryotes. Primordial Soup Theory a solution rich in organic compounds in the primitive oceans of the earth, from which life is hypothesized to have originated. Miller Urey Experiment Used Earth’s early atmosphere composition to show that life can spontaneously happen. The Mysteries of Life’s Origins What do scientists hypothesize about early Earth and the origin of life? Geological and astronomical evidence suggests Earth forms as pieces of cosmic debris collided with one another. For millions of years, violent volcanic activity shook Earth’s crust along with comets and asteroids bombarding the surface. About 4.2 billion years ago Earth cooled allowing solid rocks to form and water to condense and fall as rain. Earth’s early atmosphere contained little or no oxygen. It was principally composed of carbon dioxide, water vapor, and nitrogen, with smaller amounts of carbon monoxide, hydrogen sulfide, and hydrogen cyanide. A few deep breaths would have killed you! The First Organic Molecules 1/2 Could organic molecules assemble under conditions on early Earth? Stanley Miller and Harold Urey tried to answer that question. They filled a sterile flask with water and boiled it – illustrating the oceans. To the water vapor they added methane, ammonia, and hydrogen – illustrating the atmosphere. They passed the gases listed above through electrodes to simulate lightning. They then passed the gases through a condensation chamber where cold water cooled them, causing drops to form. T The liquid circulated for a week. The end result was 21 amino acids! The First Organic Molecules 2/2 Miller and Urey’s experiment suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds on a primitive Earth. Were they wrong??? Yes they were wrong. More research showed that the composition of Earth’s early atmosphere was different than what they used in their experiment. But recent experiments using the correct ingredients in the atmosphere still produced organic compounds. In fact, in 1995 one of Miller’s more accurate mixtures produced cytosine and uracil which are two bases found in RNA. Formation of Microspheres A stew of organic molecules is a long way from a living cell. Geological evidence suggests that during Earth’s Archean Eon (200-300 million years ago) after Earth cooled enough to carry liquid water, cells similar to bacteria were common. Where did these come from though? Large organic molecules form tiny bubbles called proteinoid microspheres under certain conditions. Microspheres are not cells, but have characteristics of living systems. They have selectively permeable membranes and have simple means of storing and releasing energy. Several hypotheses suggest that structures similar to proteinoid microspheres acquired the characteristics of living cells as early as 3.8 billion years ago. Evolution of RNA and DNA Where did RNA and DNA come from? Scientists haven’t solved this yet, but that doesn’t mean they won’t. A number of experiments however simulating conditions on early Earth show small sequences of RNA forming from simple molecules. Is this good news? The answer is yes! Scientists know that under the right conditions some RNA sequences help DNA replicate, some RNA process mRNA, some catalyze reactions, and some even grow and replicate on their own! The “RNA World” hypothesis proposes that RNA existed by itself before DNA. From this simple RNA-based system, several steps could have led to DNA-directed protein synthesis. It is still being tested today. Production of Free Oxygen Microscopic fossils of prokaryotes that resemble bacteria have been found in Archaeon rocks more than 3.5 billion years old. These evolved in the absence of oxygen because early Earth hardly had any oxygen. During the Proterozoic Eon, photosynthetic bacteria became common. 2.2 billion years ago, these organisms were churning out oxygen. The oxygen combined with the iron in the oceans creating iron oxide (rust) and sank to the bottom of the ocean forming great bands or iron that are the source of most iron mined today. Without iron, the oceans changed color from brown to blue-green. Oxygen began to accumulate in the atmosphere, the ozone layer formed, and the skies turned to blue. Over several hundred million years, oxygen concentration levels rose to those of today. Early life forms died because they couldn’t breathe oxygen. Other organisms evolved to breathe oxygen. Origin of Eukaryotic Cells What theory explains the origin of eukaryotic cells? One of the most important events in the history of life was the evolution of eukaryotic cells from prokaryotic cells. How did these complex organisms evolve? Researchers hypothesize that about 2 billion years ago some ancient prokaryotes began evolving internal cell membranes. Endosymbiotic Theory – prokaryotic cells entered ancestral prokaryotes. These intruders didn’t infect their host and the host didn’t digest them. Small prokaryotes began living inside larger cells. The Endosymbiotic Theory proposes that a symbiotic relationship evolved over time, between primitive eukaryotic cells and the prokaryotic cells within them. This was proposed about a century ago. Looking under the microscope the membranes of mitochondria and chloroplasts look like the cell membranes of free living prokaryotes. Endosymbiotic Theory This was proposed about a century ago. Looking under the microscope the membranes of mitochondria and chloroplasts look like the cell membranes of free living prokaryotes. This led to two hypotheses. The first says that mitochondria evolved from endosymbiotic prokaryotes that were able to use oxygen to generate ATP. These energy generating prokaryotes evolved into mitochondria. The other hypothesis says that chloroplasts evolved from endosymbiotic prokaryotes that had the ability to do photosynthesis. Over time these prokaryotes turned into chloroplasts of plants and algae. Modern Evidence During the 1960s, Lynn Margulis gathered evidence supporting the endosymbiotic theory. Margulis found that mitochondria and chloroplasts contain DNA similar to bacterial DNA. She also found that mitochondria and chloroplasts have ribosomes whose size and structure look similar to that bacteria mentioned above. Mitochondria and chloroplasts reproduce through binary fission when cells containing them divide by mitosis. These similarities show strong evidence. Sexual Reproduction and Multicellularity What is the evolutionary significance of sexual reproduction? Sometime after eukaryotic cells arose, they began to reproduce sexually. The development of sexual reproduction sped up evolutionary change because sexual reproduction increases genetic variation. Asexual reproduction produces identical genetic cells. Sexual reproduction leads to a “shuffling” of genes which is why offspring look different from their parents. Genetic variation increases the likelihood of a population’s adapting to new or changing environmental conditions. Multicellular organisms evolved a few hundred million years after the evolution of sexual reproduction. Early multicellular organisms underwent a series of adaptive radiations, resulting in greater diversity.
