Why evolution is true pdf download

What we read in the news today is full of subjectivity, half-truths, and blatant falsehoods; and thus it is more necessary now than ever to safeguard the truth with facts. In his provocative new book, evolutionary biologist Jerry A.

Coyne aims to do exactly that in the arena of religion. In clear, dispassionate detail he explains why the toolkit of science, based on reason and empirical study, is reliable, while that of religion—including faith, dogma, and revelation—leads to incorrect, untestable, or conflicting conclusions.

The author examines the interaction between science and the Christian faith and explores the place of faith in an age of science. John Weaver, himself a scientist, explores the responses of the Christian faith to scientific advances, particularly as they impinge upon an understanding of God and human nature.

Contemporary issues such as cloning, stem cell research, GM crops, global climate change and ecological destruction, new research on the origins of life and the issue of suffering brought about by 'natural evil' such as the Boxing Day tsunami, are covered in this accessible and student-friendly textbook.

It is designed to communicate information clearly and accessibly, using chapter summaries, diagrams and questions for further reading as well as suggestions for further reading at the close of chapters. This relationship is fascinating, complex and often very controversial, involving myriad issues that are difficult to keep separate from each other.

Evolution, Chance, and God introduces the reader to the main themes of this debate and to the theory of evolution, while arguing for a particular viewpoint, namely that evolution and religion are compatible, and that, contrary to the views of some influential thinkers, there is no chance operating in the theory of evolution, a conclusion that has great significance for teleology.

One of the main aims of this book is not simply to critique one influential contemporary view that evolution and religion are incompatible, but to explore specific ways of how we might understand their compatibility, as well as the implications of evolution for religious belief.

This involves an exploration of how and why God might have created by means of evolution, and what the consequences in particular are for the status of human beings in creation, and for issues such as free will, the objectivity of morality, and the problem of evil. By probing how the theory of evolution and religion could be reconciled, Sweetman says that we can address more deeply key foundational questions concerning chance, design, suffering and morality, and God's way of acting in and through creation.

Score: 5. In fact, Sara Sybesma Tolsma, an award-winning scientist, and Jason Lief, a leading practical theologian, argue that youth ministry needs science to help young people explore their relationship to God and engage their world faithfully. Jesus Loves You and Evolution Is True invites the church and its leaders to open their minds and hearts to what science can tell us about our human lives and our connections to, and role in, our natural world.

Why would a SID create creatures like underground moles with eyes that cannot see? Why create humans with an appendix and a vestigial tail? Another line of evidence against ID is what Coyne calls dead genes. Out of about 30, genes, humans have more than 2, pseudo- or dead genes. Why would a SID include thousands of genes in humans and other organisms that seem to have no function? ID has no sensible answer but evolutionary theory explains dead genes quite easily.

When a trait is no longer useful, evolution simply inactivates genes that made the trait rather than snipping them out of the DNA. If organisms were made from scratch by a SID, no such silenced or dead genes should exist. Good Also in this chapter comparative embryology is used to further illustrate the over- whelming evidence for evolution and Coyne observes that Darwin considered embryology the strongest evidence when he wrote The Origin.

Some species retain their evolutionary history during embryonic development. In certain ways human fetuses resemble embryonic fish and reptiles and this makes sense only if organisms have an evolutionary history. Even without our current knowledge of continental drift and molecular taxonomy, Darwin understood that the Earth was dynamic and changes like glacial expansion could answer many questions about the distribution of species.

All they can do is invoke the inscrutable whims of the creator. Many examples are provided and then Coyne gets to what he calls his favorite lecture on Darwin and island biogeography. Oceanic islands, including the Galapagos, are missing certain types of species that are found on both continents and continental islands, and there is a pattern to the missing groups.

Why are oceanic islands missing land mammals, reptiles, amphibians, and freshwater fish? Darwin eventually determined that species native to oceanic islands could have, given enough time, traveled the hundreds of miles over or on the water and then gradually evolved into different species like the finches seen on the Galapagos, for example.

Even for highly improbable events where the chance of occur- rence of a bird or a seed or an insect reaching an oceanic island is only one in a million each year, given a million years Coyne notes there is a 63 per cent chance that the island would be colonized at least once. In geological and evolutionary terms, a million years is not a long time. Camouflage works to protect prey from predators and to give predators an advantage in capturing prey.

One of the questions that faced Darwin is why animals like the male peacock develop what seem to be maladaptive traits like huge tail feathers. Scientists since Darwin have confirmed his hypothesis by studying many different species and by doing experimental studies where certain features are altered and mating behavior is observed. Coyne notes that about 90 per cent of bird species are socially monogamous but in about three-quarters of these species both males and females mate with other individuals.

In polyandrous species such as seahorses and pipefish, the females are more brightly colored and they compete for males. Darwin explained how a single species changes over time but he never explained how one species splits into two, and if speciation did not occur there would be no biodiversity—only a single, long-evolved descendent of that first species.

This requires the third idea of evolution: that of splitting, or, more accurately, speciation. An example showing common ancestors in reptiles. X and Y are species that were the common ancestors between later-evolved forms.

What exactly happened when node X, say, split into the lineage that leads to modern reptiles like lizards and snakes on the one hand and to modern birds and their dinosaurian relatives on the other?

Node X represents a single ancestral species, an ancient reptile, that split into two descendant species. One of the descendants went on its own merry path, eventually splitting many times and giving rise to all dinosaurs and modern birds. The other descendant did the same, but produced most modern reptiles. But although common ancestors are no longer with us, and their fossils nearly impossible to document after all, they represent but a single species out of thousands in the fossil record , we can sometimes discover fossils closely related to them, species having features that show common ancestry.

What happened when ancestor X split into two separate species? Nothing much, really. Millions of years later, and after more splitting events, one of the descendant dinosaur species, node Y, itself split into two more species, one eventually producing all the bipedal, carnivorous dinosaurs and the other producing all living birds.

It is only in retrospect that we can identify species Y as the common ancestor of T. These evolutionary events were slow, and seem momentous only when we arrange in sequence all the descendants of these diverging evolutionary streams. Others, like gingko trees, live millions of years without pro- ducing many new species.

But each time one species splits into two, it doubles the number of opportunities for future speciation, so the number of species can rise exponentially.

Speciation was so important to Darwin that he made it the title of his most famous book. And that book did give some evidence for the split- ting. And if life began with one species and split into millions of descendant species through a branching process, it follows that every pair of species shares a common ancestor sometime in the past. Closely related species, like closely related people, had a common ancestor that lived fairly recently, while the common ancestor of more distantly related species, like that of distant human relatives, lived farther back in the past.

So reptiles and mammals must have had a more recent common ancestor that itself possessed such an egg. But this group also contains two subgroups, one with species that all have hair, are warm-blooded, and produce milk that is, mammals , and another with species that are cold-blooded, scaly, and produce watertight eggs that is, reptiles. Like all species, these form a nested hierarchy: a hierarchy in which big groups of species whose members share a few traits are subdivided into smaller groups of species sharing more traits, and so on down to species, like black bears and grizzly bears, that share nearly all their traits.

A phylogeny evolutionary tree of vertebrates, showing how evolution produces a heirarchical grouping of features, and thus of species containing these features. The dots indicate where on the tree each trait arose.

Actually, the nested arrangement of life was recognized long before Darwin. This means that these groupings are not subjective artifacts of a human need to classify, but that they tell us something real and fundamental about nature. But nobody knew what that something was until Darwin came along, and showed that the nested arrangement of life is precisely what evolution predicts. Creatures with recent common ancestors share many traits, while those whose common ancestors lay in the distant past are more dissimilar.

Take cardboard books of matches, which I used to collect. You could, for example, sort matchbooks hier- archically beginning with size, and then by country within size, color within country, and so on. Or you could start with the type of product advertised, sorting thereafter by color and then by date. There is no sorting system that all collectors agree on. Matchbooks resemble the kinds of creatures expected under a cre- ationist explanation of life.

This was based on the reasonable assumption that organisms with similar features also have similar genes, and thus are more closely related. But now we have a powerful new and independent way to estab- lish ancestry: we can look directly at the genes themselves. By sequencing the DNA of various species and measuring how similar these sequences are, we can reconstruct their evolutionary relationships. This is done by making the entirely reasonable assumption that species having more similar DNA are more closely related—that is, their common ancestors lived more recently.

These molecular methods have not produced much change in the pre-DNA-era trees of life: both the visible traits of organ- isms and their DNA sequences usually give the same information about evolutionary relationships. The idea of common ancestry leads naturally to powerful and testable predictions about evolution.

This idea was not in fact unique to Darwin—his contemporary, the naturalist Alfred Russel Wallace, came up with it at about the same time, leading to one of the most famous simultaneous discoveries in the history of science. The idea of natural selection is not hard to grasp.

Over time, the population will gradually become more and more suited to its environment as helpful mutations arise and spread through the population, while deleterious ones are weeded out. Ultimately, this process produces organisms that are well adapted to their habitats and way of life. The wooly mammoth inhabited the northern parts of Eurasia and North America, and was adapted to the cold by bear- ing a thick coat of hair entire frozen specimens have been found buried in the tundra.

Mutations in the ancestral species led to some individual mammoths—like some modern humans—to be hairier than others. This enriched the population in genes for hairiness. In the next generation, the average mammoth would be a bit hairier than before. Let this process continue over some thousands of generations, and your smooth mammoth gets replaced by a shaggy one. The process is remarkably simple.

It requires only that individuals of a species vary genetically in their ability to survive and reproduce in their environment. Given this, natural selection—and evolution—are inevitable. As we shall see, this requirement is met in every species that has ever been examined.

Natural selection is not a master engi- neer, but a tinkerer. Mutations for a perfect design may not arise because they are simply too rare. The African rhinoceros, with its two tandemly placed horns, may be better adapted at defending itself and sparring with its brethren than is the Indian rhino, graced with but a single horn actually, these are not true horns, but compacted hairs.

But a mutation producing two horns may simply not have arisen among Indian rhinos. Still, one horn is better than no horns. This, by the way, poses an enormous problem for theories of intelligent design. A conscientious designer might have given the turtles an extra pair of limbs, with retractable shovel-like appendages, but turtles, like all reptiles, are stuck with a developmental plan that limits their limbs to four.

Mutations are changes in traits that already exist; they almost never create brand-new features. This means that evolution must build a new species starting with the design of its ancestors.

Evolution is like an archi- tect who cannot design a building from scratch, but must build every new structure by adapting a preexisting building, keeping the structure habitable all the while. This leads to some compromises. When the fetus is six or seven months old, they migrate down into the scrotum through two channels called the inguinal canals, removing them from the damaging heat of the rest of the body. Those canals leave weak spots in the body wall that make men prone to inguinal hernias.

These hernias are bad: they can obstruct the intestine, and sometimes caused death in the years before surgery. No intelligent designer would have given us this tortuous testicular journey.

And although selection gives the appearance of design, that design may often be imper- fect. This leads to evolutionary change that, being random, has nothing to do with adaptation.

Natural selection remains the only process that can produce adaptation. These, then, are the six parts of evolutionary theory. If speciation is true, for instance, then common ancestry must also be true. But some parts are independent of oth- ers. Evolution might occur, for example, but it need not occur gradu- ally. Such claims can be tested. Mutationism predicts that new groups should arise instantly from old ones, without transitions in the fossil record.

But the fossils tell us that this is not the way evolution works. Alternatively, evolution might be true, but natural selection might not be its cause. This kind of drive was said to have propelled the evolution of the huge canine teeth of saber-toothed tigers, making the teeth get larger and larger, regardless of their usefulness, until the animal could not close its mouth and the species starved itself to extinction.

So much for the claims of evolutionary theory. The implication is that there is something not quite right about a theory—that it is a mere speculation, and very likely wrong. There are two points I want to emphasize here. First, in science, a theory is much more than just a speculation about how things are: it is a well-thought-out group of propositions meant to explain facts about the real world.

This brings us to the second point. That is, we must be able to make observations about the real world that either support it or disprove it. Atomic theory was initially speculative, but gained more and more credibility as data from chemistry piled up, sup- porting the existence of atoms. To be technical, the gravity of such a body distorts space-time, which distorts the path of nearby photons. After all, there are usually several explanations for a given phenomenon.

Scientists try to make key observations, or conduct decisive experiments, that will test one rival explanation against another. Continental drift then became more certain as fossils accumulated and paleontologists found that the distribution of ancient species suggested that the continents were once joined. And although plate tectonics was also greeted with skepticism by geologists, it was subject to rigorous testing on many fronts, yielding convincing evidence that it is true.

When Darwin wrote The Origin, most Western scientists, and nearly everyone else, were creationists. Much of that book presents evidence that not only supports evolution but at the same time refutes creationism. So how do we test evolutionary theory against the still popular alterna- tive view that life was created and remained unchanged thereafter? There are actually two kinds of evidence. The deepest and oldest layers of rock would contain the fossils of more primitive species, and some fossils should become more complex as the layers of rock become younger, with organisms resembling present-day species found in the most recent layers.

Evolutionary theory, then, makes predictions that are bold and clear. Darwin spent some twenty years amassing evidence for his theory before publishing The Origin. So much knowledge has accumulated since then! And whole new branches of science, undreamt of by Darwin, have arisen, including molecular biology and systematics the study of how organisms are related. Paleontologists have worked tirelessly to piece together the tangible his- torical evidence for evolution: the fossil record.

Time-consuming, expensive, and risky expeditions to remote and inhospitable corners of the world are often involved. My Chicago colleague Paul Sereno, for instance, studies African dinosaurs, and many of the most interesting fossils lie smack in the middle of the Sahara Desert.

Such discoveries involve true dedication to science, many years of painstaking work, persistence, and courage—as well as a healthy dose of luck. To biologists, fossils are as valuable as gold dust. All we could do would be to study living species and try to infer evolutionary relationships through similarities in form, development, and DNA sequence. We would know, for example, that mammals are more closely related to reptiles than to amphibians.

Fortunately, advances in physics, geology, and bio- chemistry, along with the daring and persistence of scientists throughout the world, have provided these precious insights into the past. How can we decipher that history? First, of course, you need the fossils—lots of them. Then you have to put them in the proper order, from oldest to youngest. Each of these requirements comes with its own set of challenges.

What remains is a cast of a living creature that becomes compressed into rock by the pressure of sediments piling up on top. Bones and teeth are abundant, as are shells and the hard outer skeletons of insects and crustaceans. Then it must be discovered. How incomplete?

The total number of species that ever lived on Earth has been estimated to range between seventeen million probably a drastic underestimate given that at least ten million species are alive today and four billion. Ironically, the fossil record was originally put in order not by evolu- tionists but by geologists who were also creationists, and who accepted the account of life given in the book of Genesis.

Younger rocks lie atop older ones. But not all layers are present at any one place—sometimes they are not formed or are eroded away. So far, so good. But this tells you only the relative ages of rocks, not their actual ages.

Several radioisotopes usually occur together, so the dates can be cross-checked, and the ages invariably agree. But we can obtain the ages of fossils by bracketing the sedimentary layers with the dates of adjacent igneous layers that contain radioisotopes. Opponents of evolution often attack the reliability of these dates by saying that rates of radioactive decay might have changed over time or with the physical stresses experienced by rocks.

But it is specious. There are yet other ways to check the accuracy of radiometric dating. One of them uses biology, and involved an ingenious study of fossil corals by John Wells of Cornell University.

Each day—one revolution of the Earth— is a tiny bit longer than the last one. But corals can do this, for as they grow they record in their bodies how many days they experience each year. Living corals produce both daily and annual growth rings. In fossil specimens, we can see how many daily rings separate each annual one: that is, how many days were included in each year when that coral was alive. There are several types.

First, the big evolutionary picture: a scan through the entire sequence of rock strata should show early life to be quite simple, with more complex species appearing only after some time. Later species should have traits that make them look like the descendants of earlier ones. For example, nineteenth-century anatomists predicted that, from their bodily similarities, mammals evolved from ancient reptiles. When writing The Origin, Darwin bemoaned the sketchy fossil record. Some groups, like whales, appeared suddenly in the record, without known ancestors.

But Darwin still had some fossil evidence for evolution. This included the observation that ancient animals and plants were very dif- ferent from living species, resembling modern species more and more as one moved up to more recently formed rocks. He also noted that fossils in adjacent layers were more similar to each other than to those found in layers more widely separated, implying a gradual and continuous process of divergence.

This suggested that modern species descended from the fossil ones. We can now show continuous changes within lineages of animals; we have lots of evidence for common ancestors and transitional forms those missing ancestors of whales, for instance, have turned up ; and we have dug deep enough to see the very beginnings of complex life.

Instead, for most groups we see gradual evolution from earlier forms birds and mammals, for example, evolved over millions of years from reptilian ancestors. Earlier groups, of course often persisted: photosynthetic bacteria, sponges, and worms appear in the early fossil record, and are still with us.

Groups appear on the scene in an orderly evolutionary fashion, with many arising after known fossil transitions from ancestors. The sequence shown, along with the transitional forms, disproves creationist claims that all forms of life arose not only suddenly, but also at the same time. After the earliest mammals appear, they, along with insects and land plants, become ever more diverse, and as we approach the shallowest rocks, the fossils increas- ingly come to resemble living species.

Vari- ous imaginative analogies have been used to make this point, and it is worth making again. Although the fossil record of plants is sparser—they lack easily fos- silized hard parts—they show a similar evolutionary pattern.

So the appearance of species through time, as seen in fossils, is far from random. Simple organisms evolved before complex ones, predicted ancestors before descendants. The most recent fossils are those most similar to living species. And we have transitional fossils connecting many major groups. No theory of special creation, or any theory other than evolution, can explain these patterns.

Tracing a single fossil species through the core, you can often see it evolve. Evolutionary change of thorax size in the radiolarian Pseudocubus vema over a period of two million years.

Values are population averages from each section of the core. Evolution, though gradual, need not always proceed smoothly, or at an even pace. Environments themselves change sporadically and unevenly, so the strength of natural selection will wax and wane.

Trilo- bites were arthropods, in the same group as insects and spiders. Since they were protected by a hard shell, they are extremely common in ancient rocks you can probably buy one in your nearest museum shop.

Peter Sheldon, then at Trinity College Dublin, collected trilo- bite fossils from a layer of Welsh shale spanning about three mil- lion years. Unfortunately, we have no idea what selective pressures drove the evolutionary changes in these plankton and trilobites. It is always easier to document evolution in the fossil record than to understand what caused it, for although fossils are preserved, their environments are not. What we can say is that there was evolution, it was gradual, and it varied in both pace and direction.

Marine plankton give evidence for the splitting of lineages as well as evolution within a lineage. The number gives the population average at each section of the three-million-year sample of shale. And there are also examples of species that barely change over time. Remember that evolutionary theory does not state that all species must evolve! Instead, forms of life appear in the record in evolutionary sequence, and themselves evolve and split.

The points represent the width of the fourth segment, shown as the average of each species at each section of the core. In areas to the north of where this core was taken, an ancestral population of E. This divergence may have been the result of natural selection acting to reduce competition for food between the two species. If so, where is the fossil evidence? These include many species whose existence was predicted many years ago, but that have been unearthed in only the last few decades.

But what counts as fossil evidence for a major evolutionary tran- sition? The fossil record is simply too spotty to expect that. They speculated that there must have been a common ancestor that, through a speciation event, produced two lineages, one eventually yielding all modern birds and the other all modern reptiles.

Our intuition is to say that it would have resembled something halfway between a modern reptile and a modern bird, showing a mixture of features from both types of animal. Because reptiles appear in the fossil record before birds, we can guess that the common ancestor of birds and reptiles was an ancient reptile, and would have looked like one. We now know that this common ances- tor was a dinosaur.

Some of those species went extinct, while others continued evolving into what are now modern birds. It is to these groups of ancient species, the relatives of species near the branch point, that we must look for evidence of common ancestry. Showing common ancestry of two groups, then, does not require that we produce fossils of the precise single species that was their common ancestor, or even species on the direct line of descent from an ancestor to descendant.

Rather, we need only produce fossils having the types of traits that link two groups together, and, importantly, we must also give the dating evidence showing that those fossils occur at the right time in the geological record.

In the reptile-to-bird transition, for instance, the transitional forms should look like early reptiles, but with some bird- like traits. Those later feathered dinosaurs still provide evidence for evolution, because they tell us something about where birds came from. The dating and—to some extent—the physical appearance of transi- tional creatures, then, can be predicted from evolutionary theory. This is the fossil species Tiktaalik roseae, which tells us a lot about how vertebrates came to live on the land.

Its discovery is a stunning vindica- tion of the theory of evolution. Invasion of the land. Shaded bones are those that will evolve into the arm bones of modern mammals: the bone with darkest shading will become our humerus, and the medium- and light-shaded bones will become the radius and ulna, respectively. This was the question that interested—or rather obsessed—my Chicago colleague Neil Shubin. This is where the prediction comes in.

Somewhere in between. Searching his college geology textbook for a map of exposed freshwa- ter sediments of the right age, Shubin and his colleagues zeroed in on a paleontologically unexplored region of the Canadian Arctic: Ellesmere Island, which sits in the Arctic Ocean north of Canada. But it also has amphibian-like features. This suggests that it lived in shallow water and could peer, and probably breathe, above the surface.

And, like the early amphibians, Tiktaalik has a neck. Most importantly, Tiktaalik has two novel traits that were to prove useful in helping its descendants invade the land. Clearly Tiktaalik was well adapted to live and crawl about in shal- low waters, peek above the surface, and breathe air. Given its struc- ture, we can envision the next, critical evolutionary step, which prob- ably involved a novel behavior. Tiktaalik itself was not ready for life ashore. For one thing, it had not yet evolved a limb that would allow it to walk.

And it still had inter- nal gills for breathing underwater. So we can make another prediction. And equally marvelous is that its discov- ery was not only anticipated, but predicted to occur in rocks of a certain age and in a certain place.

The best way to experience the drama of evolution is to see the fossils for yourself, or better yet, to handle them. This was, to them, the most tangible evidence that evolution was true. How often do you get to put your hands on a piece of evo- lutionary history, much less one that might have been your distant ancestor?

Ever since Darwin, that question has been raised to cast doubt on evolution and natural selection. Natural selection, creationists argue, could not explain this transition, because it would require intermediate stages in which animals have only the rudiments of a wing. This would seem more likely to encumber a creature than to give it a selective advantage. And gliding has evolved independently many times: in placental mammals, marsupials, and even lizards. Since the nineteenth century, the similarity between the skeletons of birds and some dinosaurs led paleontologists to theorize that they had a common ancestor—in particular, the theropods: agile, carnivorous dinosaurs that walked on two legs.

By seventy million years ago, we see fossils of birds that look fairly modern. Archaeopteryx is really more reptile than bird. Its skeleton is almost identical to that of some theropod dinosaurs. The bird-like traits number just two: large feathers and an opposable big toe, probably used for perching.

But its asymmetrical feathers—one side of each feather is larger than the other—suggest that it could. After the discovery of Archaeopteryx, no other reptile-bird intermedi- ates were found for many years, leaving a gaping hole between modern birds and their ancestors. Archeopteryx has a few features like those of modern birds feathers and an opposable big toe , but its skeleton is very similar to that of the dinosaur, including teeth, a reptilian pelvis, and a long bony tail.

Archaeopteryx was about the size of a raven, Compsognathus slightly larger. And its claws, teeth, and long, bony tail clearly show that this creature was far from being a modern bird. Still others have large feathers on the forelimbs and tail, much like mod- ern birds. The other fossil is a female theropod who met her end while sitting on her nest of twenty-two eggs, showing brooding behavior similar to that of birds. Feathered dinosaurs probably contin- ued to exist after one of their kin gave rise to birds.

And these conditions are very rare. Fossil behavior: the feathered theropod dinosaur Mei long top fossilized in a birdlike roosting position, sleeping with its head tucked under its forelimb. Middle: a reconstruction of Mei long from the fossil.

Bottom, a modern bird juvenile house sparrow sleeping in the same position. There were many feathered dinosaurs, and their feathers are clearly related to those of modern birds.

They could have been used for ornamentation or display—perhaps to attract mates. It seems more likely, though, that they were used for insulation. And what feathers evolved from is even more mysterious. The best guess is that they derive from the same cells that give rise to reptilian scales, but not everyone agrees. Despite the unknowns, we can make some guesses about how nat- ural selection fashioned modern birds.

Early carnivorous dinosaurs evolved longer forelimbs and hands, which probably helped them grab and handle their prey. Then followed the feathery covering, probably for insu- lation. There is evidence that some theropods lived at least partly in trees.

Longer wings could also have evolved as running aids. The chukar partridge, a game bird studied by Kenneth Dial at the University of Montana, represents a living example of this step. The obvious advantage is that uphill scrambling helps these birds escape predators. Then would come the other innovations shared by modern birds, including hollow bones for lightness and that large breastbone. While we may speculate about the details, the existence of transi- tional fossils—and the evolution of birds from reptiles—is fact.

Fossils like Archaeopteryx and its later relatives show a mixture of bird-like and early reptilian traits, and they occur at the right time in the fossil record. This resembles the half-a-wing argument against the evolu- tion of birds.

How stupid, they thought, could evolutionists be? A good candidate is the hippopotamus, which, although closely related to terrestrial mammals, is about as aquatic as a land mammal can get. Hip- pos spend most of their time submerged in tropical rivers and swamps, surveying their domain with eyes, noses, and ears that sit atop their head, all of which can be tightly closed underwater. Hippos mate in the water, and their babies, who can swim before they can walk, are born and suckle underwater.

This has given rise to the myth that hippos sweat blood. Whales happen to have an excellent fossil record, courtesy of their aquatic habits and robust, easily fossilized bones. And how they evolved has emerged within only the last twenty years.

This is one of our best examples of an evolutionary transition, since we have a chronologically ordered series of fossils, perhaps a lineage of ancestors and descendants, showing their movement from land to water. They are warm-blooded, produce live young whom they feed with milk, and have hair around their blowholes. And evidence from whale DNA, as well as vestigial traits like their rudimentary pelvis and hind legs, show that their ancestors lived on land.

Whales almost certainly evolved from a species of the artiodactyls: the group of mammals that have an even number of toes, such as camels and pigs. But whales have their own unique features that set them apart from their terrestrial relatives.

Sixty million years ago there were plenty of fossil mammals, but no fossil whales. There is no need to describe this transition in detail, as the drawings clearly speak—if not shout—of how a land-living animal took to the water. The sequence begins with a recently discovered fossil of a close relative of whales, a raccoon-sized animal called Indohyus. Living forty- eight million years ago, Indohyus was, as predicted, an artiodactyl.

It is clearly closely related to whales because it has special features of the ears and teeth seen only in modern whales and their aquatic ancestors. Although Indohyus appears slightly later than largely aquatic ancestors of whales, it is probably very close to what the whale ancestor looked like. And it was at least partially aquatic.

We know this because its bones were denser than those of fully terrestrial mammals, which kept the creature from bobbing about in the water, and because the isotopes extracted from its teeth show that it absorbed a lot of oxygen from water. It probably waded in shallow streams or lakes to graze on vegetation or escape from its enemies, much like a similar animal, the African water chevrotain, does today.

Indohyus was not the ancestor of whales, but was almost certainly its cousin. It is a fossil skull from a wolf-sized creature called Pakicetus, which is a bit more whale-like than Indohyus, having simpler teeth and more whale-like ears. Then follows, in rapid order, a series of fossils that become more and more aquatic with time. Transitional forms in the evolution of modern whales from the ancient artiodactyl Indohyus Balaena is the modern baleen whale, with a vestigial pelvis and hindlimb, while the other forms are transitional fossils.

Relative sizes of the animals are shown in gray shading. Rodhocetus forty-seven million years ago is even more aquatic. Its nostrils have moved somewhat backward, and it has a more elongated skull. With stout extensions on the backbone to anchor its tail muscles, Rodhocetus must have been a good swimmer, but was handicapped on land by its small pelvis and hindlimbs.

The creature certainly spent most if not all of its time at sea. The evolution of whales from land animals was remarkably fast: most of the action took place within only ten million years. But why did some animals go back to the water at all? After all, millions of years earlier their ancestors had invaded the land. These creatures would not only have competed with aquatic mammals for food, but probably made a meal of them.

With their reptil- ian competitors extinct, the ancestors of whales may have found an open niche, free from predators and loaded with food. The sea was ripe for invasion. From anatomical similarities, ento- mologists had long supposed that ants evolved from nonsocial wasps.

Similarly, snakes have long been supposed to have evolved from lizard-like reptiles that lost their legs, since reptiles with legs appear in the fossil record well before snakes. Just as predicted, it had a small pelvic girdle and reduced hind legs. But it also had a head, brain, heart, and cartilaginous bar along the back—the notochord. This marks it as perhaps the earliest chordate, the group that gave rise to all vertebrates, including ourselves.

The fossil record teaches us three things. First, it speaks loudly and eloquently of evolution. There is no getting around this evidence, no waving it away. Evolution happened, and in many cases we see how. The earliest birds appear after dinosaurs but before modern birds. We see ancestral whales spanning the gap between their own landlubber ancestors and fully modern whales. If evolution were not true, fossils would not occur in an order that makes evolutionary sense.

Asked what observation could conceiv- ably disprove evolution, the curmudgeonly biologist J. Needless to say, no Precambrian rabbits, or any other anachronistic fossils, have ever been found. Finally, evolutionary change, even of a major sort, nearly always involves remodeling the old into the new.

The tiny middle ear bones of mammals are remodeled jawbones of their reptilian ancestors. The wings of birds were fashioned from the legs of dinosaurs. And whales are stretched-out land animals whose forelimbs have become paddles and whose nostrils have moved atop their head.

There is no reason why a celestial designer, fashioning organisms from scratch like an architect designs buildings, should make new species by remodeling the features of existing ones.

Each species could be con- structed from the ground up. But natural selection can act only by changing what already exists. This has proved critical in our understanding of the ancient world. Many ancient texts are in fact known to us only by peering beneath the stra- tum of medieval overwriting to recover the original words. This painstaking work yielded three mathematical treatises of Archimedes written in ancient Greek, two of them previously unknown and enormously important in the history of science.

In such arcane ways we recover the past. Like these ancient texts, organisms are palimpsests of history— evolutionary history. Within the bodies of animals and plants lie clues to their ancestry, clues that are testimony to evolution. And they are many. And when an ostrich becomes agitated—as it tends to do when you chase it around a corral—it runs straight at you, extending its wings in a threat display.

The lesson, though, goes deeper. The wings of the ostrich are a vestigial trait: a feature of a species that was an adaptation in its ancestors, but that has either lost its usefulness completely or, as in the ostrich, has been co- opted for new uses.

They help the bird maintain balance, mate, and threaten its enemies. Opponents of evolution always raise the same argument when ves- tigial traits are cited as evidence for evolution. But this rejoinder misses the point. A trait can be vestigial and functional at the same time.