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EVIDENCE OF COMMON ANCESTRY reflect Are birds related to dinosaurs? Years ago, this question spawned heated debates and controversy among many scientists. Today, the scientific community generally accepts the idea that birds and dinosaurs share a common ancestor. What changed? Did scientists find evidence to support this theory? If so, what sort of evidence did they find? ancestor: an organism from which other organisms evolved The Fossil Record Many pieces of evidence support the theory that birds and dinosaurs are related. One such piece of evidence, the fossil record, is the accumulation of all fossilized remains that scientists have collected around the world. While the fossil record provides evidence to support the theory of common ancestors among different organisms, it is not a complete record. From fossils, scientists can identify the habitats and ecosystems that extinct organisms once occupied. They can also group similar extinct organisms together and arrange them in the order in which they lived, from oldest to youngest. The relative age of a fossil can be determined by comparing its location to other fossils within the same rock layers. Older fossils appear in rock layers below younger fossils. Scientists also use radioactive dating to determine the age of fossils. Radioactive materials have a fixed rate at which they decay. By measuring how much radioactive material is left in a fossil, scientists know how much has decayed and can then determine how long it took to decay. Fossils can give clues about how organisms changed over time. Scientists study the similarities and differences in bones, shell shapes, or other features over time. For example, scientists discovered melanosomes in an ancient bird feather. Melanosomes are structures that contain melanin, which helps determine a creature’s coloring. Scientists also found melanosomes in the feathery areas of a dinosaur fossil, which suggests an ancestral link between birds and dinosaurs. © 2013-2014 Accelerate Learning - All Rights Reserved 1 EVIDENCE OF COMMON ANCESTRY Sometimes the fossil record reveals shifts from one entire species to another. After comparing the skeletons of birds to those of Coelurosaurian dinosaurs, paleontologists concluded that many birds are likely direct descendants of the group called coelurosaurs (especially the Velociraptors.) They paleontologist: a point to characteristics such as the S-shaped neck, hollow scientist who studies bones, large eye sockets, five or more vertebrae in the hip, and prehistoric life many other similarities. what do you think? Take a look at the photographs below. Do you think the elephant on the left is a direct descendant of the woolly mammoth on the right? Explain your reasoning. Studying Homologies of Different Organisms A 2005 study of mitochondrial DNA confirmed that woolly mammoths and African and Asian elephants share a common ancestor. Scientists compared the mitochondrial DNA sequences of both animals and found strong similarities. Mitochondrial DNA is particularly valuable to make these common ancestor: determinations because only the mother passes mitochondrial a predecessor that a DNA down to her offspring. Scientists looked at the DNA creature shares with sequences of each organism and compared their similarities another creature. or differences. Similarities in anatomical structure, gene sequences, developmental stages, or any other criteria that point to a common ancestor are called homologies. © 2013-2014 Accelerate Learning - All Rights Reserved 2 EVIDENCE OF COMMON ANCESTRY Paleontologists have compared bone structures in organisms for years to determine common ancestry among organisms. A likeness between the bone structure of one creature and that of another may indicate a common ancestor. Mammals, birds, and reptiles all show similar anatomical patterns. Although the function of each is different, this diagram shows similarities in the anatomical structure of forelimbs in mammals (bat and human), birds (penguin), and reptiles (alligator). Species with similar origins also show similar embryonic development patterns. Paleontologists can learn a lot about a species by studying the embryo and patterns of development. For example, certain snake embryos have small buds that look like limbs. The buds disappear in later developmental stages. This suggests that snakes evolved from an ancestor that had limbs. look out! Although evidence may indicate certain information, it may not be readily accepted. One hypothesis that has received mixed reactions is the idea that Tyrannosaurus rex is a predecessor of the chicken. Paleontologists found protein (collagen) in a 68 million-year-old T. rex bone. In 2007, they reported that five of the seven collagen fragments closely matched the collagen of chickens. © 2013-2014 Accelerate Learning - All Rights Reserved 3 EVIDENCE OF COMMON ANCESTRY Other researchers questioned this conclusion and the research process. No other scientist had ever found protein that had survived even one million years, let alone 68 million. Could the proteins have come from contamination? A computerized process determined the match. Some scientists wondered if different results might have occurred if the instructions were written differently. Acceptance of this hypothesis will take more research and evidence. Biogeography Biogeography is another type of evidence that supports the theory of evolution by natural selection. Biogeography is the study of past and present geographical distribution of species. Organisms have characteristics that allow them to survive in their environment. If the environment changes, some species may evolve over time to cope with the differences. Sometimes part of a population is geographically separated from the main group. The isolated population is forced to adapt to different conditions and evolve separately. The lemur is an example. Some scientists believe lemurs crossed from Africa to the island of Madagascar millions of years ago by floating on vegetation. By 20 million years ago, the continents had drifted so far apart that the lemurs became permanently isolated on Madagascar. The lemur is an example of an animal whose ancestors were geographically separated from their native population. Continental drift, the breaking apart and movement of continents from one massive landform named “Pangaea,” explains why many species evolved into new species. As the land masses of Pangaea drifted apart, newly isolated species developed characteristics that set them apart from their common ancestor. Crocodiles have not changed from their original forms in 200 million years. They are an example of stasis. © 2013-2014 Accelerate Learning - All Rights Reserved 4 EVIDENCE OF COMMON ANCESTRY Rates and Patterns of Evolution Charles Darwin proposed the theory of evolution by natural selection in 1859, which suggested that all creatures descended from common ancestors. His theory contended that organisms evolve through natural selection, a gradual process by which subsequent generations of organisms develop characteristics that help them survive in their surrounding conditions through mutations. Mutations that mutation: an benefit an organism are passed on to subsequent generations. abrupt change in However, it is important to note that natural selection is not a the genotype of an deliberate process. Organisms do not choose which traits to organism pass on to their offspring—it is a natural process that favors certain traits in a particular environment. Organisms that do not have these favorable characteristics will gradually decrease in number, and the gene pool of the population changes. As Darwin’s theory was accepted, so was the theory of gradualism. This is the idea that evolution occurs gradually. Changes or adaptations appear slowly and sequentially over time. In 1972, Niles Eldredge and Stephen Jay Gould contended that the fossil record shows very few gradual changes and intermediate forms of evolutionary change. They proposed a new theory of evolutionary change consisting of two general ideas: stasis and punctuated equilibrium. Eldredge and Gould stated that some species tend to stay in a state of stasis, or a period during which no evolutionary changes occur for a long time. Stasis can last for millions of years. When evolutionary change does occur, it happens quickly in bursts, called punctuated equilibrium. Punctuated equilibrium results in new species appearing suddenly in the fossil record and leaves little fossil evidence behind of the transition from the ancestral species to the new form. The Origin of Life and the Complexity of Cells Much of this lesson has been devoted to understanding how Earth’s organisms have changed over time. But how did Earth’s first, single-celled organisms arise? Which scientific theories explain the origin of life on Earth? In the 1920s, two scientists working in different countries proposed the same hypothesis on the origin of life. Russian biochemist Alexander Oparin and British geneticist J.B.S. Haldane proposed that life began spontaneously from combinations of nonliving matter exposed to energy from lightning, volcano eruptions, or the Sun. © 2013-2014 Accelerate Learning - All Rights Reserved 5 EVIDENCE OF COMMON ANCESTRY Oparin and Haldane proposed that life began in the oceans where organic molecules were everywhere, thus resulting in a “primordial soup.” The two scientists suggested that Earth’s early atmosphere lacked oxygen. Instead, it was composed of nitrogen, hydrogen, and other compounds, including water vapor, ammonia, and methane. Life could more easily arise in this environment because electrons from energy sources such as lightning could react with molecules such as nitrogen to form organic compounds. An oxidized atmosphere would have simply absorbed these electrons. Their ideas became known as the OparinHaldane Hypothesis. In 1953, Stanley Miller and Harold Urey tested this hypothesis with the Miller/Urey experiment. They built an apparatus to simulate Earth’s early atmosphere, shown in the diagram at right. They applied a continuous electric current to mimic the lightning storms on Earth. Within a week, small amounts of carbon had formed into organic molecules, and some had even formed into small amino acid chains. Amino acids are the building blocks of proteins. Many scientists are skeptical of this theory, partly because lightning storms probably did not occur often enough to generate the quantities of organic molecules necessary for the origin of life. So, while amino acids and other organic compounds may have been formed, they would not have been formed in the amounts produced by the Miller/Urey experiment. Iron-Sulfur World Theory In 1985, German chemist Günter Wächtershäuser proposed the Iron-Sulfur World Theory. He claimed that life began on the surfaces of mineral deposits near volcanic vents on the ocean floor. There, carbon reacted with metals such as iron and nickel, creating organic compounds (carbon monoxide, carbon dioxide, hydrogen cyanide, and hydrogen sulfide) from gases spewed by the underwater volcanoes. This process took place under high pressure and high temperatures. According to Wächtershäuser’s theory, these were the perfect conditions for organic synthesis. An evolving cycle of chemical reactions eventually formed acetic acid and pyruvic acid, two key elements in the citric acid cycle that organisms use to make energy. This theory accounts for metabolism, or how chemical energy is transferred within a cell, but it does not describe how the organism could duplicate itself. For this, the RNA World hypothesis offers a possible explanation. © 2013-2014 Accelerate Learning - All Rights Reserved 6 EVIDENCE OF COMMON ANCESTRY RNA World Hypothesis The first two hypotheses explain how inorganic molecules become organic monomers, such as amino acids, sugars, phosphates, and bases. The more difficult problem was describing how these simple chemical systems became complex enough to form organisms—how did monomers become polymers, such as proteins, carbohydrates, and lipids. In the 1980s, American scientist Thomas Cech and his colleagues discovered that RNA (ribonucleic acid) can act as a catalyst, helping to drive the chemical reactions necessary for many processes in an organism. Ribosomal RNA and transfer RNAs are the primary molecules responsible for protein synthesis. RNA is also able to store and pass on information, allowing molecules to replicate, which is another essential aspect of living organisms. Thus, the RNA World Hypothesis was introduced, proposing that RNA existed before DNA. This theory explains how simple molecules could become complex selfreplicating systems. However, scientists do not understand how RNA was originally formed. Debate and research still continue on many aspects of this theory. The First Cells Once the basic chemicals came together under the right circumstances, how did they form into cells? Over millions of years, primitive organic systems associated with lipids that formed membranes that held them together and separated them from the external environment. The information-containing molecules (e.g. RNA) inside the primitive cells enabled the cells to replicate. Many scientists now believe the earliest forms of life were ancient bacteria and archaea that could live in extreme temperatures and thrive in an environment without oxygen. catalyst: a substance that helps start or increase the rate of a reaction Did life begin in extreme environments like this thermal pool? © 2013-2014 Accelerate Learning - All Rights Reserved 7 EVIDENCE OF COMMON ANCESTRY Scientists in the Spotlight: Lynn Margulis In the 1960s, biologist Lynn Margulis proposed the endosymbiotic theory. She suggested that mitochondria, which convert food into energy in cells, and chloroplasts, which convert sunlight into energy in plant cells, have more in common with prokaryotes than eukaryotes. In fact, she suggests that mitochondria and chloroplasts were once free-living bacteria that evolved a mutualistic relationship with early eukaryotic cells. Over time, the reproductive cycle of the endosymbiont became completely tied to that of the host, and the endosymbionts lost the ability to live outside of the host cell. Support for the endosymbiotic theory is found in the DNA and ribosomes of mitochondria – both of which are similar to that found in Rickettsia, a parasitic endosymbiont. Like mitochondrial DNA, chloroplast DNA is also similar to prokaryotic photosynthetic cyanobacteria. prokaryote: singlecelled organism without a nucleus or organelles eukaryote: organism (usually multicellular) with a defined nucleus and membrane-bound organelles © 2013-2014 Accelerate Learning - All Rights Reserved 8 EVIDENCE OF COMMON ANCESTRY What Do You Know? Read each example in the left column of the chart. Then choose a term from the list below the chart that BEST matches each example. Use each term only once. Example Humans and chimpanzees share some DNA and protein sequences. Whales, bats, and birds have the same bone that connects their limbs to the middle of their bodies. Fossils of a shelled organism show slow changes in form over time within the species. A combination of lightning and an atmosphere composed of nitrogen, hydrogen, and other compounds, including water vapor, ammonia, and methane results in organic compounds. A scientist is studying the distribution of extinct and modern ferns in North America. Chicken, pig, and fish embryos all have pharyngeal folds. A series of reactions in underwater volcanoes results in the formation of pyruvic acid and acetic acid. Fossils of a bird species show no changes in form over millions of years, and then a new, similar bird species appears in the fossil record. RNA acts as a catalyst, helping long molecules to form and cells to replicate. Matching Term List of Terms: • Biogeography • Developmental Homology • Structural Homology • Gradualism • RNA World Hypothesis • Iron-Sulfur World Theory • Molecular Homology • Punctuated Equilibrium • Oparin-Haldane Hypothesis what do you think? Analyze the endosymbiotic theory and propose a depiction of the early earth if early eukaryotes had not developed an endosymbiotic relationship with chloroplast and mitochondria. © 2013-2014 Accelerate Learning - All Rights Reserved 9 EVIDENCE OF COMMON ANCESTRY connecting with your child Evolutionary Trees To help your child learn more about common ancestry, work together to create an evolutionary tree. Also called a phylogenetic tree or a tree of life, this diagram shows how species may be related to each other through physical characteristics, genetics, or character traits. According to Darwin’s theory of evolution by natural selection, all species share a common ancestor. First have your child choose 12 organisms to include in his or her evolutionary tree, including three aquatic animals, three land mammals, three insects, and three reptiles. Search for photographs of these creatures on the Internet or in old magazines. Get a piece of large poster board, a ruler, glue, and a pencil. This evolutionary tree will focus on the physical characteristics of organisms. Brainstorm with your child different characteristics to use on the tree. Some ideas include: • presence or absence of a backbone • presence of hair, fur, or scales • warm or cold bloodedness • lungs or gills • external structures such as fins, claws, or hooves • food preference (omnivore, carnivore, or herbivore) Turn the poster board to a landscape format. Write “Common Ancestor” at the bottom of the poster board and draw a large “Y” above the term. Leave plenty of room above the “Y” to draw your tree “branches.” Now choose a characteristic for each side of the “Y” (e.g., presence or absence of a backbone). Separate the 12 organisms into two piles according to this characteristic. Now look at the organisms in each group and choose a characteristic to separate each of the two groups into smaller groups. You might find that one group, such as the organisms without backbones, cannot be further separated. If this is the case, glue these organisms onto the correct side of the “Y” at the top. For any group(s) that can be further separated, draw a “V” on the correct side(s) of the “Y” at the top. Place the organisms on either side of the “V.” For characteristics such as hair, fur, or scales, you will need to draw an extra line or two on the “V” – one line for each form of the characteristic. Continue with this process until only one organism is left and glue the picture of the organism at the end of the branch. For ideas about how evolutionary trees are drawn and organized, you may want to do an Internet search prior to this activity using the term, “evolutionary tree.” © 2013-2014 Accelerate Learning - All Rights Reserved 10 EVIDENCE OF COMMON ANCESTRY Here are some questions to discuss with your child: • What surprised you the most about the relationships of the organisms in the evolutionary tree? • What surprised you the least? • Do you think an evolutionary tree based on genetic information might look different than the tree based on physical characteristics? Explain your reasoning. © 2013-2014 Accelerate Learning - All Rights Reserved 11