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Not for Sale NOT FOR SALE © Roberts and Company Publishers, ISBN: 9781936221448, due August 23, 2013, For examination purposes only FINAL PAGES NOT FOR SALE The earliest whales, such as the 47-million-year-old Ambulocetus, still had legs. Their anatomy gives us clues to how whales made the transition from land to sea. © Roberts and Company Publishers, ISBN: 9781936221448, due August 23, 2013, For examination purposes only FINAL PAGES NOT FOR SALE Walking 1 Whales Introducing Evolution ne of the best feelings paleontologists can ever have is to realize that they’ve just found a fossil that will fll an empty space in our understanding Oof the history of life. Hans Thewissen got to enjoy that feeling one day in 1993, as he dug a 47-million-year-old fossil out of a hillside in Pakistan. As he picked away the rocks surrounding the FIGURE 1.1 bones of a strange mammal, he suddenly realized what he had Paleontologist Hans Thewissen has discovered many of the bones of found: a whale with legs. Ambulocetus in Pakistan. A hundred million years ago, not a single whale swam in all the world’s oceans. Whales did not yet exist, but their ancestors did. At the time, they were small, furry mammals that walked on land. Over millions of years, some of the descendants of those ancestors underwent a mind-boggling transformation. They lost their legs, traded their nostrils for a blowhole, and became crea- tures of the sea. This profound change was the result of evolution. 3 © Roberts and Company Publishers, ISBN: 9781936221448, due August 23, 2013, For examination purposes only FINAL PAGES NOTThis bookFOR is an introduction to that SALE process. Over the course of some 3.5 billion years, life has evolved from humble beginnings into the tremendous diversity found today on Earth—from whales to tulips to viruses. Indeed, to understand life on Earth—its diversity of forms, of biochemistry, of behav- iors—it’s crucial to understand evolution. The great twentieth-century biolo- gist Theodosius Dobzhansky summed up the importance of this branch of science in 1971 when he declared, “Nothing in life makes sense except in the light of evolution” (Dobzhansky 1973). The Tangled Bank is intended for anyone with a curiosity about how life works. To explain mathematical concepts, for example, it uses graphs and diagrams rather than detailed equations. This book presents the research of scientists like Thewissen not in technical detail, but through their own experi- ences of discovery. When Charles Darwin formulated the theory of evolution in the mid-1800s, the most sophisticated tool he could use was a crude light microscope. As we’ll see in this book, scientists today can study evolution in many ways—by analyzing our DNA, for example, by probing the atoms of an- cient rocks to determine the age of fossils, or by programming powerful com- puters to decipher how a thousand diferent species are related to each other. Understanding evolution is too important to be limited only to evolu- tionary biologists. Evolution underlies many important issues society faces, and a better understanding of evolution can help us to grapple with them. The world’s biodiversity faces many threats, from deforestation to global warming. By studying past mass extinctions, we can better understand our current crisis. Evolution has produced bacteria that can resist even the most powerful antibiotics. Scientists can observe this resistance as it evolves in lab- oratory experiments. The evolution of pests and pathogens poses a serious risk to our food supply. But evolution can also help us to address some of the biggest questions we ask about the universe and ourselves: How did we get here? What does it mean to be human? What Is Evolution? The short answer is descent with modifcation. The patterns of this modifcation, and the mechanisms by which it unfolds, are what evolutionary biologists study. They have much left to learn—but at this point in the history of science, there is no doubt that life has evolved and is still evolving. 4 CHAPTER 1 | WALKING WHALES © Roberts and Company Publishers, ISBN: 9781936221448, due August 23, 2013, For examination purposes only FINAL PAGES TheNOT fundamental principles that makeFOR evolution possible are prettySALE straight- forward. Organisms inherit traits from their ancestors because they receive a molecule called DNA from them (see page tk for a discussion of heredity). Cells use DNA as a guide to building biological molecules, and, when organisms reproduce, they make new copies of DNA for their offspring. Living things do not replicate their DNA perfectly; sometimes sequence errors occur. Such errors are referred to as mutations (Chapter 5). A mutation may be lethal; it may be harmless; or it may be benefcial in some way. A benefcial mutation may help an organism to fght off diseases, to thrive in its environment, or to improve its ability to fnd mates. Evolution takes place because mutated genes become more or less com- mon within populations over the course of generations. Many mutations even- tually disappear, while others spread widely. Some mutations spread simply by chance. Others spread because they allow organisms to produce more off- spring. This nonrandom spread of benefcial genes is known as natural selec- tion. The effect of a mutation depends on more than just the mutation itself. It is also infuenced by all the other genes an organism carries. The environment in which an organism lives can also have a huge effect. As a result, the same mutation to the same gene may be harmful in one individual and harmless in another. Natural selection may favor a mutation in one set of circumstances and may drive it to oblivion in another. These processes are taking place all around us every day, and they have been accumulating genetic changes ever since life began at least 3.5 billion years ago (Chapter 3). Darwin argued that over such vast stretches of time, natural selec- tion could have produced complex organs, from the wings of birds to human eyes. Today, the weight of evidence overwhelmingly supports that conclusion. Changes in organs are not the only adaptations that have emerged through evo- lution; behavior and even language have evolved as well (Chapters 13 and 14). To trace the history of these traits, evolutionary biologists reconstruct how species diverged from a common ancestor (Chapter 4). Natural selection and other processes can make populations genetically distinct from one another. Over time, the populations become so different from one another that they can be considered separate species (Chapter 9). One way to picture this process is to think of the populations as branches on a tree. When two populations diverge, a branch splits in two. As the branches split again and again over billions of years (and sometimes came back together), the tree of life emerged. To reconstruct the tree, evolutionary biologists identify which species are closely related to each other. They do so by comparing anatomical traits and DNA among many different species. Close relatives share more traits inherited from their common ancestor. We humans have a bony skull, for example, as do all other mammals, as well as birds, reptiles, amphibians, and fshes (Chapter 4). We are more closely related to these animals than we are to animals without a skull, such as earthworms and ladybugs. Evolutionary biologists investigate not only adaptations, but the shortcom- ings of those adaptations. Life does not evolve by steadily progressing toward some particular goal. Our apelike ancestors did not evolve big brains because they “needed” them, for example; the conditions in which they lived—search- ing for food on the African savanna in big social groups—were such that some traits were favored over others (Chapter 14). Our ancestors benefted from a bigger, more complex brain, but in some ways it’s a spectacularly maladapted organ. The human brain is so big that it makes childbirth much more danger- ous for human mothers than for other female primates, for example. It also WHAT IS EVOLUTION? 5 © Roberts and Company Publishers, ISBN: 9781936221448, due August 23, 2013, For examination purposes only FINAL PAGES NOTrequires a hugeFOR amount of energy to supply—oneSALE out of every fve calories you eat is used to fuel your brain. Evolution is imperfect, because it does not invent things from scratch: it only modifes what already exists. Any organism can acquire only a limited number of benefcial mutations, and so evolution can produce new forms only under tight constraints. Because mutations can have several different effects at once, evolution also faces trade-offs. Mutations that increase the number of offspring an organism can have may also cut down its life span. These tradeoffs underlie some of the diseases that are common in the elderly, such as cancer. As a result, studying evolution can help researchers better understand—and perhaps even treat—these diseases (Chapter 15). Evolution has produced many helpful partnerships in nature, such as the one between fowering plants and the insects that pollinate them—a partner- ship we depend on for many of our crops (Chapter 12). But evolution does not produce a peaceful balance in the natural world. The same process species use in adapting to one another can also give rise to what looks to us like cruelty. Pred- ators are exquisitely adapted to fnding and killing prey. Parasites can devour their hosts from the inside out. Their adaptations are fnely honed, allowing them to manipulate individual molecules within their hosts. Yet parasites and predators are not evil. They are just part of a dynamic balance that is constantly shifting, driving the diversifcation of life but also leading to extinctions.
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