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Supported by a wide variety of evidence, including comparative anatomy, DNA sequences, and the fossil record.

Figure 8.20 Four hypotheses about the origin of modern organisms. The result of our evaluation of these hypotheses is to reject all of them except the theory of common descent.

unambiguous evidence that the species that have inhabited this planet have changed over time.

Of the remaining three hypotheses, transformation is the poorest explanation of the observations. If organisms arose separately, and each changed on its own path, there is no reason to expect that different species would share structures—especially if these structures are vestigial in some of the organisms. There is also no reason to expect similarities among species in DNA sequence. The hypothesis of transformation predicts that we will find little evidence of biological relationship among living organisms. As our observations have indicated, evidence of relationship abounds.

Both the hypothesis of common descent and the hypothesis of separate types contain a process by which we can explain observations of relationship. That is, both hypothesize that modern species are descendants of common ancestors. The difference between the two theories is that common descent hypothesizes a single common ancestor for all living things, while separate types hypothesizes that ancestors of different groups arose separately and then gave rise to different types of organisms. Separate types seems more reasonable than common descent to many people. It seems impossible that organisms as different as pine trees, mildew, ladybugs, and humans share a common ancestor. However, several observations indicate that these disparate organisms are all related.

The most compelling evidence for the single origin of all life is the universality of both DNA and of the relationship between DNA and proteins. As you learned in Chapter 7, genes from bacteria can be transferred to plants, and the plants will make a functional bacterial protein. This is only possible because both bacteria and plants translate genetic material into functional proteins in an identical manner. If bacteria and plants arose separately, we could not expect them to translate genetic information identically.

The fact that organisms as different as pine trees, mildew, ladybugs, and humans contain cells with nearly all the same components, processes of cell division, and biochemistry is also evidence of shared ancestry. A mitochondrion could have many different possible structural forms and still perform the same function; the fact that the mitochondria in a plant cell and an animal cell are essentially identical implies that both groups of organisms received these mitochondria from a common ancestor.

Pine trees, mildew, ladybugs, and humans are very different. Proponents of the hypothesis of separate types argue that the differences among these organisms could not have evolved in the time since they shared a common ancestor. (See Essay 8.2 to explore hypotheses about the origin of the common ancestor of all life.) But the length of time pine trees, mildew, ladybugs, and humans have been diverging is immense—at least 1,000,000,000 years. The remaining basic similarities among all living organisms serve as evidence of their ancient relationship (Figure 8.21).

Scientists favor the theory of common descent because it is the best explanation for how modern organisms came about. The theory of evolution— including the theory of common descent—is robust, meaning that it is a good explanation for a variety of observations and well supported by a wide variety of evidence from anatomy, geology, molecular biology, and genetics. The theory of common descent is no more tentative than atomic theory; few scientists disagree with the models that describe the basic structures of atoms, and few disagree that the evidence for the theory of common descent is overwhelming. Most scientists would say that both of these theories are so well supported that we can call them fact.

Evolutionary theory helps us understand the functions of human genes, comprehend the interactions among species, and predict the consequences of

Figure 8.21 The unity and diversity of life. The theory of evolution, including the theory of common descent, provides the best explanation for how organisms as distinct as pine trees, humans, mildew, and ladybugs can look very different while sharing a genetic code and many aspects of cell structure and cell division.

Figure 8.21 The unity and diversity of life. The theory of evolution, including the theory of common descent, provides the best explanation for how organisms as distinct as pine trees, humans, mildew, and ladybugs can look very different while sharing a genetic code and many aspects of cell structure and cell division.

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