Evolution after darwin original ancestor would have required a great deal more More evidence time. Using evidence obtained by studying the rates of ra dioactive decay, we now know that the physicists of Dar More than a century has elapsed since Darwins death in wins time were wrong, very wrong: the earth was formed 1882. During this period, the evidence supporting his the about 4.5 billion years ago ory has grown progressively stronger. There have also been many significant advances in our understanding of The Mechanism of Heredity how evolution works. Although these advances have not altered the basic structure of Darwins theory, they have Darwin received some of his sharpest criticism in the area of taught us a great deal more about the mechanisms by heredity. At that time, no one had any concept of genes or which evolution occurs. We will briefly explore some of of how heredity works, so it was not possible for Darwin to this evidence here; in chapter 21 we will return to the the- explain completely how evolution occurs. Theories of he- ory of evolution and examine the evidence in more detail redity in Darwins day seemed to rule out the possibility of genetic variation in nature, a critical requirement of Dar- wins theory. Genetics was established as a science only at The Fossil record the start of the twentieth century, 40 years after the publica- tion of Darwins On the Origin of Species. When scientists Darwin predicted that the fossil record would yield inter- began to understand the laws of inheritance(discussed in mediate links between the great groups of organisms, for chapter 13), the heredity problem with Darwin's theory anished. Genetics accounts in a neat and orderly way for have arisen from them, and between reptiles and birds. We the production of new variations in organisms able in the nineteenth century. Recent discoveries of mi- %copic fossils have extended the known history of life on Comparative Anatomy earth back to about 3. 5 billion years ago. The discovery of Comparative studies of animals have provided strong evi shed light on how organisms have, over this enormous time brates, for example, the same bones are presen, n verte- dence for Darwins theory. In many different types of span, evolved from the simple to the complex. For verte- heir evolutionary past. Thus, the forelimbs shown in brate animals especially, the fossil record is rich and exhib- 1.12 are all constructed from the same basic array of b its a graded series of changes in form, with the evolutionary modified in one way in the wing of a bat, in another way in parade visible for all to see(see Box: Why Study Fossils the fin of a porpoise, and in yet another way in the leg of a horse. The bones are said to be homologous in the differ The Age of the Earth ent vertebrates; that is, they have the same evolutionary ori- gin, but they now differ in structure and function. This con- In Darwins day, some physicists argued that the earth was trasts with analogous structures, such as the wings of birds only a few thousand years old. This bothered Darwin, be- and butterflies, which have similar structure and function cause the evolution of all living things from some single but different evolutionary origi FIGURE 1.12 Homology among vertebrate limbs. The forelimbs of these five vertebrates show the ways in which the relative roportions of the forelimb bones have changed in relation to the particular way of life of Porpoise each organism Chapter 1 The Science of Biology 15Evolution After Darwin: More Evidence More than a century has elapsed since Darwin’s death in 1882. During this period, the evidence supporting his the￾ory has grown progressively stronger. There have also been many significant advances in our understanding of how evolution works. Although these advances have not altered the basic structure of Darwin’s theory, they have taught us a great deal more about the mechanisms by which evolution occurs. We will briefly explore some of this evidence here; in chapter 21 we will return to the the￾ory of evolution and examine the evidence in more detail. The Fossil Record Darwin predicted that the fossil record would yield inter￾mediate links between the great groups of organisms, for example, between fishes and the amphibians thought to have arisen from them, and between reptiles and birds. We now know the fossil record to a degree that was unthink￾able in the nineteenth century. Recent discoveries of mi￾croscopic fossils have extended the known history of life on earth back to about 3.5 billion years ago. The discovery of other fossils has supported Darwin’s predictions and has shed light on how organisms have, over this enormous time span, evolved from the simple to the complex. For verte￾brate animals especially, the fossil record is rich and exhib￾its a graded series of changes in form, with the evolutionary parade visible for all to see (see Box: Why Study Fossils?). The Age of the Earth In Darwin’s day, some physicists argued that the earth was only a few thousand years old. This bothered Darwin, be￾cause the evolution of all living things from some single original ancestor would have required a great deal more time. Using evidence obtained by studying the rates of ra￾dioactive decay, we now know that the physicists of Dar￾win’s time were wrong, very wrong: the earth was formed about 4.5 billion years ago. The Mechanism of Heredity Darwin received some of his sharpest criticism in the area of heredity. At that time, no one had any concept of genes or of how heredity works, so it was not possible for Darwin to explain completely how evolution occurs. Theories of he￾redity in Darwin’s day seemed to rule out the possibility of genetic variation in nature, a critical requirement of Dar￾win’s theory. Genetics was established as a science only at the start of the twentieth century, 40 years after the publica￾tion of Darwin’s On the Origin of Species. When scientists began to understand the laws of inheritance (discussed in chapter 13), the heredity problem with Darwin’s theory vanished. Genetics accounts in a neat and orderly way for the production of new variations in organisms. Comparative Anatomy Comparative studies of animals have provided strong evi￾dence for Darwin’s theory. In many different types of verte￾brates, for example, the same bones are present, indicating their evolutionary past. Thus, the forelimbs shown in figure 1.12 are all constructed from the same basic array of bones, modified in one way in the wing of a bat, in another way in the fin of a porpoise, and in yet another way in the leg of a horse. The bones are said to be homologous in the differ￾ent vertebrates; that is, they have the same evolutionary ori￾gin, but they now differ in structure and function. This con￾trasts with analogous structures, such as the wings of birds and butterflies, which have similar structure and function but different evolutionary origins. Chapter 1 The Science of Biology 15 Human Cat Bat Porpoise Horse FIGURE 1.12 Homology among vertebrate limbs. The forelimbs of these five vertebrates show the ways in which the relative proportions of the forelimb bones have changed in relation to the particular way of life of each organism