Chapter 5: Origins of Life Lesson 27

Chapter 5 • Lesson 27 The Origins of Life on Earth Objective: 3,4,1 Key Terms atmosphere • fossil • cyanobacteria • photosynthesis • cellular respiration Getting the Idea Earth is the only planet known to support life. One reason why Earth can support life is because it has plenty of liquid water. Water is essential to life as we know it. Scientists searching for life elsewhere look for evidence of water on other worlds. As you have read, most organisms alive today also need oxygen, but some organisms can survive without it. Conditions Necessary for Life Earth has several conditions that enable it to sustain life as we know it. These conditions include the following: • presence of liquid water • a moderate temperature range • free oxygen in the atmosphere • adequate sunlight • absence of toxic substances in the atmosphere • absence of lethal radiation A Changing Earth Earth is made up mostly of rock. High temperatures on early Earth caused most of the rock to melt and separate. Dense materials, such as iron and nickel, tended to sink, forming Earth's core. Lighter materials settled above the core, forming Earth's mantle and crust. There were many active volcanoes on early Earth. The volcanoes released water vapor, which built up in the atmosphere. Much of the water vapor condensed and fell back to Earth as rain. Over hundreds of millions of years, the liquid water collected in depressions on Earth's rocky surface and eventually formed oceans. In addition to water vapor, the erupting volcanoes released other gases. These gases-methane, hydrogen, nitrogen, ammonia, carbon dioxide, and carbon monoxide—formed Earth's early atmosphere. The atmosphere is the mass of gases that surrounds Earth. Today's atmosphere contains free oxygen (oxygen gas that is not combined with other elements) and a layer of ozone that protects living things from harmful radiation. The atmosphere also keeps Earth at a suitable temperature for life. Earth's early atmosphere was very different. That atmosphere lacked free oxygen, so it could not support life as it is known today. The early atmosphere also contained enough carbon monoxide, carbon dioxide, methane, and ammonia to be toxic to many of today's organisms. The Beginning of Life on Earth The lack of free oxygen in Earth's early atmosphere would have prevented the survival of most modern organisms. So where did the first living things come from? No one knows for sure. However, in the 1950s, American chemists Stanley Miller and Harold Urey tried to model early Earth conditions. Miller and Urey designed and carried out an experiment to test the hypothesis that mixtures of organic compounds needed for life could have formed from simpler compounds that were present on early Earth. The Miller-Urey experiment produced organic compounds, but not cells. Scientists are still not sure how cells arose. However, most scientists agree that before organic molecules could join together to create life, they had to be enclosed in a membrane. Experiments have shown that polypeptides (chains of amino acids that join together to form proteins) can join together to form small fluid-filled spheres. When the spheres are exposed to solutions that contain some types of lipids, the lipids organize themselves into a two-layered membrane. This structure, which is selectively permeable, may have been the precursor of a plasma membrane. Miller and Urey's experiment showed that amino acids could form from matter present on early Earth. In the 1960s, Juan Oro also showed that amino acids could be made from chemicals present on early Earth. In addition, Oro was able to form adenine, a nitrogen base found in RNA and DNA. Later experiments by other scientists showed that the other bases in RNA and DNA could also be made from substances present on early Earth. Several years later, Francis Crick and Leslie Orgel experimented with the nucleic acid RNA. They showed that in addition to playing a role in the formation of proteins, RNA can act as an enzyme. Crick and Orgel conducted other experiments that led them to develop the Crick-Orgel hypothesis, which is that RNA evolved much earlier than DNA and was the primary nucleic acid in Earth's first organisms. Some other scientists think that many of the chemicals necessary for life formed in space. They suggest that the molecules may have been carried to Earth on meteorites or comets. This idea is supported by telescopes that have detected some of these molecules in space. Primitive Organisms and the Endosymbiont Theory Organisms obtain the materials needed for life from their environment. Environmental conditions on Earth likely played a key role in how the first cells evolved and the types of organisms that developed over time. There is much evidence that this sequence of development took the following path: A fossil is the remains or evidence of an organism that lived in past ages. Fossil evidence indicates that the first organisms appeared in Earth's oceans about 3.5 billion years ago. These organisms, which formed in the absence of oxygen, were prokaryotes. Some scientists have hypothesized that life on Earth began in deep-ocean hydrothermal vents. A hydrothermal vent is a crack in the ocean floor that emits extremely hot, mineral-rich water. Hydrothermal vents are located in regions where large pools of magma lie a short distance below the ocean floor. These deep-ocean environments contain both an energy source and the chemicals that would be required for life. Archaea and bacteria living near hydrothermal vents today use the chemicals ejected from the vents to produce food by chemosynthesis. Most of these organisms use sulfur compounds as the source of energy for chemosynthesis. Many scientists think Earth's earliest living cells probably resembled chemosynthetic bacteria. Fossils show that cyanobacteria became common in shallow areas of Earth's oceans about 3 billion years ago. Cyanobacteria are prokaryotes that make food by photosynthesis. Recall that photosynthesis is a process by which organisms use the energy in sunlight to form glucose from water and carbon dioxide. The process produces oxygen as a waste product. Photosynthesis by cyanobacteria gradually increased the amount of oxygen in the atmosphere. This change was important to the development of life on Earth. By about 1.8 billion years ago, the atmosphere contained abundant free oxygen. The oxygen made possible the development of more complex, oxygen-breathing life forms. Although most modern organisms depend on oxygen, many primitive organisms could not survive in the presence of this gas. When oxygen became common, some organisms developed the ability to use it for cellular respiration. Recall that cellular respiration is the process by which organisms use oxygen to get energy from carbohydrates and other food molecules. Most organisms that could not adapt to the oxygen in the atmosphere died out. A few survive in places where they are not exposed to oxygen gas. Prokaryotic cells do not contain internal structures bound by membranes. How then did eukaryotic cells evolve from prokaryotes? The endosymbiont theory explains how mitochondria, chloroplasts, and other organelles of eukaryotic cells evolved. According to this theory, free-living bacteria were engulfed, or taken in, by other prokaryotes. The engulfed bacteria and the host cell formed a symbiotic relationship in which each organism benefited from the other. (The prefix endo- means "inside.") Over time, the bacteria inside the host cell evolved into the organelles of modern eukaryotic cells. Several pieces of evidence support this theory. Mitochondria and chloroplasts are about the size of bacteria and have similar ribosomes. In addition, both mitochondria and chloroplasts have their own DNA and, like bacteria, are able to make copies of themselves by dividing. The diagram shows how this process may have taken place. Early Eukaryotes Scientists estimate that the first eukaryotes evolved around 2 billion years ago. These organisms included algae. Photosynthesis by algae added more oxygen to the atmosphere. Sexual reproduction evolved around 1.2 billion years ago and increased the rate of change. The oldest known fossils of multicellular organisms are about 580 million years old. Most of the fossils are hard to classify, but they may have included animals, plants, and protists. Life became much more varied and complex about 540 million years ago. This event is known as the Cambrian explosion. All the major animal groups first appeared during the Cambrian period. These groups have evolved and become more complex in the hundreds of millions of years since then. Plant life became more complex at around the same time. The fossil evidence suggests that plants started living on land about 480 million years ago. Land plants are a major source of atmospheric oxygen today. The oxygen released by photosynthesis is used by organisms for respiration. Photosynthesis and respiration are related because the products of each process are the reactants in the other. As organisms carry out these essential functions, they maintain the oxygen and carbon dioxide levels in Earth's atmosphere. In fact, as a result of these processes, the composition of Earth's atmosphere began to stabilize about 350 million years ago.