Chapter 5 Evidence for Evolution Lesson 29

Chapter 5 • Lesson 29 Evidence for Evolution Objective: 3.4.1 Key Terms fossil • homologous structures • analogous structures • vestigial structure • embryology embryo Getting the Idea The history of life on Earth shows that as species evolve, they develop new traits and lose some older traits. Scientists use many kinds of data to piece together the evolutionary histories of organisms. Data come from fossils, comparative anatomy, embryology, and the biochemistry of different life forms. This information helps scientists learn how and why adaptations arise, how new species develop, and why many past species are now extinct. Fossil Evidence of Evolution Paleontology is the study of the fossil record. Paleontologists examine fossils, the remains or traces of organisms that lived in past ages, to understand past life forms. Fossils also provide clues about when different species lived. Scientists can estimate the age of a fossil or rock by comparing it with another fossil or rock. Most fossils are found in layers of sedimentary rock. Fossils and rocks in the same rock layer are about the same age. Because of the way sedimentary rock forms, older layers are usually found under younger layers. The diagram below shows fossils in undisturbed layers of sedimentary rock. Scientists looking at these layers would conclude that the fossil shown in layer B of the illustration is older than the rocks and fossils in the shallower rock layers (C and D) and younger than the rocks and fossils below it (layer A). Relative dating with these comparisons gives scientists an approximate idea of when a fossil formed. To determine the age of a fossil more precisely, scientists use radioisotope dating. This technique calculates the age of a material by comparing the different forms of certain radioactive elements it contains. Because these elements decay at a constant rate, the ratios of different forms can be used to estimate when a material, such as bone, formed. Organizing fossils by age helps scientists study how organisms and their environments changed over time. However, the fossil record is incomplete. Scientists have found fossils of only a small number of the many kinds of organisms that have lived on Earth. Despite these gaps in the record, scientists have found some transitional fossils, fossils that show a transition from one species to another. One group of organisms whose development is well documented by transitional fossils is whales. Modern whales are descended from hoofed mammals that lived on land. Fossil evidence indicates that those animals walked on land and could also swim. Later fossils suggest that over time, the hind limbs of the whales' ancestors shrank. Their forelimbs were modified to become flippers, and their hind limbs evolved into a powerful tail-like structure called a fluke. The illustration below shows one possible sequence of events in which modern whales evolved. Scientists have discovered relatively few transitional fossils. One reason why the discovery of transitional fossils is rare is that the discovery of fossils in general is rare. Most organisms, especially those without hard body parts such as teeth and bones, do not become fossils. In addition, the intermediate forms of organisms, which could become transitional fossils, do not appear to exist for very long. However, over decades of excavations and research, newly unearthed fossils have slowly filled in many of the gaps in the fossil record. Physical Features: Comparative Anatomy Many species of organisms have similar structures. For example, both turkeys and cardinals have feathers. The feathers suggest that both species are descended from an animal with feathers. The turkey and the cardinal are more closely related to each other than either is to animals without feathers. Scientists often study the physical features and anatomical structures of organisms to try to discover how organisms are related. Homologous structures are body parts of different organisms that have a similar structure but may have different functions. The human arm, the wing of a bird, and the flipper of a whale are examples of homologous structures. Although each of these appendages is used differently, they all are composed of similar bones. Homologous structures indicate that organisms share an ancestor that had a similar structure. The number of homologous structures that two species share is evidence of how closely related they are. Analogous structures are body parts that have a similar function but not a similar structure. Analogous structures do not indicate shared ancestry. Examples of homologous and analogous structures are shown in the diagram. Scientists also examine vestigial structures when they compare anatomic features to look for shared ancestry. A vestigial structure is a body part that does not seem to play a major role in an organism's life functions but was more important in the organism's ancestors. A rat has an appendix that is an important part of its digestive system. Humans also have an appendix. However, its function is unclear, and the human appendix is not essential for survival. It is a vestigial structure. That both rats and humans have an appendix suggests that their common ancestor had an appendix. Embryology is the study of embryos. An embryo is an early stage in the development of an organism. Scientists have discovered that the embryos of related organisms develop in similar ways. The diagram shows that the embryos of fish, chickens, and humans look very similar. They all have folds called gill pouches in the neck region. In fish, these folds develop into gills. Although adult chickens and humans do not have gills, the similarities in their embryos suggest that these organisms are at least distantly related. Comparisons of embryos are further evidence of evolution and are used to figure out relationships between organisms. Biochemical Evidence of Evolution The biochemicals found in organisms also provide clues about common ancestry. Recall that all organisms contain the nucleic acid DNA. DNA directs the production of proteins, which are made up of amino acids. Scientists compare both the sequences of bases in DNA and the sequences of amino acids in the resulting proteins to infer relationships between organisms. The similarities between nucleotide sequences in DNA and amino acid sequences in proteins indicate how closely related two organisms are. Biochemistry is especially useful for figuring out relationships between organisms that are so closely related that their structures look identical. For example, there are species of birds that look alike but cannot interbreed. The method is also useful for comparing organisms that are so distantly related that they have few structures in common. Consider hemoglobin, an important protein found in red blood cells. The table compares a certain amino acid chain in the hemoglobin of humans and the hemoglobin of other organisms. The proteins in humans and gorillas are almost identical: the sequences differ by only one amino acid. In contrast, the human sequence differs by 67 amino acids from the sequence in frogs. The more similar the amino acid sequences, the more closely related the organisms are and the more recently they shared an ancestor. The data show that humans and gorillas are closely related and shared an ancestor much more recently than humans and frogs did. Cytochrome c is another protein that scientists use to determine how closely related organisms are. This protein is found in all organisms. The amino acid sequence of cytochrome c is identical in humans and chimpanzees, and the sequence in rhesus monkeys differs by only one amino acid. This indicates that chimpanzees, which are a type of ape, are more closely related to humans than they are to rhesus monkeys. Similar studies have shown that chickens and turkeys have exactly the same amino acid sequence for cytochrome c, which indicates these birds are very closely related. The amino acid sequence in ducks, however, differs by one amino acid from that in chickens and turkeys, indicating that ducks are slightly less closely related to either chickens or turkeys. Discussion Question Why might examinations of similar structures in different species lead to incorrect conclusions about the evolution of those organisms? How can scientists avoid this kind of error?