The Rise And Fall Of The Third Chimpanzee

More than 98 percent of human genes are shared with two species of chimpanzee. The ‘third’ chimpanzee is man. Jared Diamond surveys out life-cycle, culture, sexuality and destructive urges both towards ourselves and the planet to explore the ways in which we are uniquely human yet still influenced by our animal origins.

Jared Diamond is among America’s most remarkable scholars. While, by appointment, a Professor of Physiology at the University of California Medical School, he is equally celebrated for his brilliant contributions to ecology and evolutionary biology, and for his explorations of remote parts of New Guinea. The Rise and Fall of the Third Chimpanzee won the Rhône-Poulenc Science Book Prize in 1992, as did Guns, Germs and Steel in 1998.

Guns, Germs and Steel: A Short History of
Everybody for the Last 13,000 Years
Collapse: How Societies Choose to Fail or Survive
Why is Sex Fun? The Evolution of
Human Sexuality

Dedicated to my sons
Max and Joshua,
to help them understand
where we came from
and where we may be heading

How the human species changed, within a short time,
from just another species of big mammal
to a world conqueror;
and how we acquired the capacity
to reverse all that progress overnight

It is obvious that humans are unlike all animals. It is also obvious that we are a species of big mammal, down to the minutest details of our anatomy and our molecules. That contradiction is the most fascinating feature of the human species. It is familiar, but we still have difficulty grasping how it came to be and what it means.

On the one hand, between ourselves and all other species lies a seemingly unbridgeable gulf that we acknowledge by defining a category called ‘animals’. It implies that we consider centipedes, chimpanzees, and clams to share decisive features with each other but not with us, and to lack features restricted to us. Among these characteristics unique to us are the abilities to talk, write, and build complex machines. We depend completely on tools, not just on our bare hands, to make a living. Most of us wear clothes and enjoy art, and many of us believe in a religion. We are distributed over the whole Earth, command much of its energy and production, and are beginning to expand into the ocean depths and into space. We are also unique in darker attributes, including genocide, delight in torture, addictions to toxic drugs, and extermination of other species by the thousands. While a few animal species have one or two of these attributes in rudimentary form (like tool use), we still far eclipse animals even in those respects.

Thus, for practical and legal purposes, humans are not animals. When Darwin intimated in 1859 that we had evolved from apes, it is no wonder that most people initially regarded his theory as absurd and continued to insist that we had been separately created by God. Many people, including a quarter of all American college graduates, still hold to that belief today.

On the other hand, we obviously are animals, with the usual animal body parts, molecules, and genes. It is even clear what particular type of animal we are. Externally, we are so similar to chimpanzees that eighteenth-century anatomists who believed in divine creation could already recognize our affinities. Just imagine taking some normal people, stripping off their clothes, taking away all their other possessions, depriving them of the power of speech, and reducing them to grunting, without changing their anatomy at all. Put them in a cage in the zoo next to the chimp cages, and let the rest of us clothed and talking people visit the zoo. Those speechless caged people would be seen for what we all really are: a chimp that has little hair and walks upright. A zoologist from outer space would immediately classify us as just a third species of chimpanzee, along with the pygmy chimp of Zaire and the common chimp of the rest of tropical Africa.

Molecular genetic studies over the last half-a-dozen years have shown that we continue to share over ninety-eight per cent of our genes with the other two chimps. The overall genetic distance between us and chimps is even smaller than the distance between such closely related bird species as red-eyed and white-eyed vireos, or willow warblers and chiffchaffs. So we still carry most of our old biological baggage with us. Since Darwin’s time, fossilized bones of hundreds of creatures variously intermediate between apes and modern humans have been discovered, making it impossible for a reasonable person to deny the overwhelming evidence. What once seemed absurd – our evolution from apes – actually happened.

Yet the discoveries of many missing links have only made the problem more fascinating, without fully solving it. The few bits of new baggage we acquired – the two per cent of our genes that differ from those of chimps – must have been responsible for all of our seemingly unique properties. We underwent some small changes with big consequences rather quickly and recently in our evolutionary history. In fact, as recently as a hundred thousand years ago that zoologist from outer space would have viewed us as just one more species of big mammal. Granted, we had a couple of curious behavioural habits, notably our control of fire and our dependence on tools, but those habits would have seemed no more curious to the extraterrestrial visitor than would the habits of beavers and bowerbirds. Somehow, within a few tens of thousands of years – a time that is almost infinitely long when measured against one person’s memory but is only a tiny fraction of our species’ separate history – we had begun to demonstrate the qualities that make us unique and fragile.

What were those few key ingredients that made us human? Since our unique properties appeared so recently and involved so few changes, those properties or at least their precursors must already be present in animals. What are those animal precursors of art and language, of genocide and drug abuse?

Our unique qualities have been responsible for our present biological success as a species. No other large animal is native to all the continents, or breeds in all habitats from deserts and the Arctic to tropical rainforests. No large wild animal rivals us in numbers. But among our unique qualities are two that now jeopardize our existence: our propensities to kill each other and to destroy our environment. Of course, both propensities occur in other species: lions and many other animals kill their own kind, while elephants and others damage their environment. However, these propensities are much more threatening in us than in other animals because of our technological power and exploding numbers.

There is nothing new about prophecies to the effect that the end of the world is near if we do not repent. What is new is that such a prophecy is now true, for two obvious reasons. First, nuclear weapons give us the means to wipe ourselves out quickly: no humans possessed this means before. Second, we already appropriate about forty per cent of the Earth’s net productivity (that is, the net energy captured from sunlight). With the world’s human population now doubling every forty-one years, we will soon have reached the biological limit to growth, at which point we will have to start fighting each other in deadly earnest for a slice of the world’s fixed pie of resources. In addition, given the present rate at which we are exterminating species, most of the world’s species will become extinct or endangered within the next century, but we depend on many species for our own life support.

Why rehearse these familiar depressing facts? Why try to trace the animal origins of our destructive qualities? If they really are part of our evolutionary heritage, that seems to imply that they are genetically fixed and hence unchangeable.

In fact, our situation is not hopeless. Perhaps the urge to murder strangers or sexual rivals is innate in us, but that still has not prevented human societies from attempting to thwart those instincts, and from succeeding in sparing most people the fate of being murdered. Even taking two world wars into account, proportionately far fewer people have suffered violent deaths in twentieth-century industrialized states than in stone-age tribal societies. Many modern populations enjoy longer lifespans than did humans of the past. Environmentalists do not always lose in battles with developers and destroyers. Even some genetic infirmities, such as phenylketonuria and juvenile-onset diabetes, can now be mitigated or cured. Therefore, my purpose in rehearsing our situation is to help us avoid repeating our mistakes – to use knowledge of our past and our propensities in order to change our behaviour. That is the hope behind the dedication of this book. My twin sons were born in 1987 and will reach my present age in the year 2040. What we are doing now is shaping their world.

It is not the goal of this book to propose specific solutions to our predicament, because the solutions we should adopt are already clear in broad outline. Some of those solutions include halting population growth, limiting or eliminating nuclear weapons, developing peaceful means for solving international disputes, reducing our impact on the environment, and preserving species and natural habitats. Many excellent books make detailed proposals on how to carry out these policies. Some of these policies are being implemented in some cases now; we ‘just’ need to implement them consistently. If we all became convinced today that they were essential, we would already know enough to start carrying them out tomorrow.

What is lacking is the necessary political will. Hence I seek to foster that will, by tracing in this book our history as a species. Our problems have deep roots tracing back to our animal ancestry. They have been growing for a long time with our increasing power and numbers, and are now steeply accelerating. We can convince ourselves of the inevitable outcome of our current short-sighted practices just by examining the many past societies that destroyed themselves by destroying their own resources, despite having less potent means of self-destruction than ours. Political historians justify the study of individual states and rulers by the opportunity to learn from the past. That justification applies even more so to the study of our history as a species, because the lessons of that study are simpler and clearer.

The story of our rise and fall divides into five natural parts. In the first part (Chapters One and Two) we shall follow our history from several million years ago until just before the appearance of agriculture ten thousand years ago. These two chapters deal with the evidence of bones, tools, and genes – the evidence that is preserved in the archaeological and biochemical record, and that gives us our most direct information about how we have changed. Fossilized bones and tools can often be dated, permitting us to deduce just when we changed. We shall examine the basis of the conclusion that we are still ninety-eight per cent chimps in our genes, and try to figure out what in the remaining two per cent was responsible for our great leap forward.

The second part (Chapters Three to Seven) deals with changes in the human life-cycle, which were as essential to the development of language and art as were the skeletal changes discussed in Part One. It is restating the obvious to mention that we feed our children after the age of weaning, instead of leaving them to find food on their own; that most adult men and women associate in couples; that most fathers as well as mothers care for their children; that many people live long enough to experience being grandparents; and that women undergo menopause. To us, these traits are the norm, but by the standards of our closest animal relatives they are bizarre. They constitute major changes from our ancestral condition, though they do not fossilize and so we do not know when they arose. For that reason they receive much briefer treatment in human paleontology texts than do our changes in brain size and pelvis, but they were crucial to our uniquely human cultural development, and merit equal attention.

With Parts One and Two having surveyed the biological underpinnings of our cultural flowering, Part Three (Chapters Eight to Twelve) considers the cultural traits that we believe distinguish us from animals. Those that come to mind first are the ones of which we are proudest: language, art, technology, and agriculture, the hallmarks of our rise. Yet our distinguishing cultural traits also include black marks on our record, such as abuse of toxic chemicals. While one can debate whether all these hallmarks rank as uniquely human, they at least constitute huge advances on animal precursors. But animal precursors there must have been, since these traits flowered only recently on an evolutionary time scale. What were those precursors? Was their flowering inevitable in the history of life on Earth, for example, so inevitable that we expect there to be many other planets out in space, inhabited by creatures as advanced as ourselves?

Besides chemical abuse, our self-destructive traits include two serious enough that they may lead to our fall. Part Four (Chapters Thirteen to Sixteen) considers the first of these: our propensity for xenophobic killing of other human groups. This trait has direct animal precursors – namely, the contests between competing individuals and groups that, in many species besides our own, may be resolved by murder. We have merely used our technological prowess to improve our killing power. In Part Four we shall consider the xenophobia and extreme isolation that marked the human condition before the rise of political states began to make us more homogenous culturally. We shall see how technology, culture, and geography affected the outcome of two of the most familiar historical sets of contests between human groups. We shall then survey the worldwide recorded history of xenophobic mass murder. This is painful material, but here above all is an example of how our refusal to face up to our history condemns us to repeat past mistakes on a more dangerous scale.

The other dark trait that now threatens our survival is our accelerating assault on our environment. This too has its direct animal precursors. Animal populations that for one reason or another escaped control by predators and parasites have in some cases also escaped their own internal controls on their numbers, multiplied until they damaged their resource base, and occasionally have eaten their way into extinction. Such a risk applies with special force to humans, because predation on us is now negligible, no habitat is beyond our influence, and our power to kill individual animals and destroy habitats is unprecedented.

Unfortunately, many people still cling to the Rousseau-esque fantasy that this tendency appeared in us only with the Industrial Revolution, before which we lived in harmony with Nature. If that were true, we would have nothing to learn from the past except how virtuous we once were, and how evil we have now become. Hence Part Five (Seventeen to Nineteen) seeks to dismantle this fantasy by facing up to our long history of environmental mismanagement. In Part Five as in Part Four, the emphasis is on recognizing that our present situation is not novel, except in degree. The experiment has already been run many times, and the outcome is there for us to learn from.

This book concludes with an epilogue that traces our rise from animal status. It also traces the acceleration in our means to bring about our fall. I would not have written this book if I thought that the risk was remote, but I also would not have written it if I considered our situation hopeless. Lest any readers get so discouraged by our track record and present predicament that they overlook this message, I point out the hopeful signs and the ways in which we can learn from the past. For those of you who would like suggestions for further reading, a section at the end will guide you to more books and articles on the material of each chapter.

A volume that ranges over such a broad canvas as this one has to be selective. Every reader is bound to find some absolutely crucial favourite subjects omitted and some other subjects pursued in inordinate detail. So that you will not feel you were misled, I shall lay out at the start my own particular interests, and where they come from.

My father is a physician, my mother a musician with a gift for languages. Whenever I was asked as a child about my career plans, my response was that I wanted to be a doctor like my father. By my last year in college, that goal had become gently transformed into the related goal of medical research, and so I trained in physiology, the area in which I now teach and do research at the University of California Medical School in Los Angeles.

However, I had also become interested at the age of seven in bird-watching, and I had been fortunate to go to a school that let me delve into languages and history. After I got my PhD., the prospect of devoting the rest of my life to the single professional interest of physiology began to look increasingly oppressive. At that point a happy constellation of events and people gave me the chance to spend a summer in the highlands of New Guinea. Ostensibly, the purpose of my trip was to measure nesting success of New Guinea birds, a project that collapsed dismally within a few weeks when I found myself unable to locate even a single bird’s nest in the jungle. Yet the real purpose of the trip succeeded completely: to indulge my thirst for adventure and bird-watching in one of the wildest remaining parts of the world. What I saw then of New Guinea’s fabulous birds, including its bowerbirds and birds of paradise, led me to develop a parallel second career in bird ecology, evolution, and biogeography. Since then, I have returned to New Guinea and the neighbouring Pacific islands a dozen times to pursue my bird research.

I found it hard to work in New Guinea amid the accelerating destruction of the birds and forests that I loved, without getting involved in conservation biology. So I began to combine my academic research with practical work as a consultant for governments, by applying what I knew about animal distributions to designing national park systems and surveying their proposed national parks. It was also hard to work in New Guinea, where languages replace each other every twenty miles, and where learning bird names in each local language proved to be the key to tapping New Guineans’ encyclopedic knowledge of their birds, without returning to my earlier interest in languages. Most of all, it was hard to study the evolution and extinction of bird species without wanting to understand the evolution and possible extinction of Homo sapiens, by far the most interesting species of all. That interest, too, was especially difficult to ignore in New Guinea, with its enormous human diversity.

Those are the paths by which I came to be interested in the particular aspects of humans that are emphasized in this book. I do not feel as if I am thereby making excuses for inappropriately slanted coverage. Numerous excellent books by anthropologists and archaeologists already discuss human evolution in terms of tools and bones, which this book can therefore summarize more briefly. However, those other volumes devote much less space to my particular interests of the human life-cycle, human geography, human impact on the environment, and humans as animals. Those subjects are as central to human evolution as are the more traditional subjects involving tools and bones.

What may at first seem here to be a plethora of examples drawn from New Guinea is also, I believe, appropriate. Granted, New Guinea is just one island, located in a particular part of the world (the tropical Pacific), and hardly providing a random cross-section of modern humanity. But New Guinea harbours a much bigger slice of humanity than you would at first guess from its area. About a thousand of the world’s approximately 5,000 languages are spoken only in New Guinea. Much of the cultural diversity that survives in the modern world is contained within New Guinea. All highland peoples in New Guinea’s mountainous interior were stone-age farmers until very recently, while many lowland groups were nomadic hunter-gatherers and fishermen practising somewhat casual agriculture. Local xenophobia was extreme, cultural diversity correspondingly so, and travel outside one’s tribal territory would have been suicidal. Many of the New Guineans who have worked with me are deadly and expert hunters who lived out their childhood in the days of stone tools and xenophobia. Thus, New Guinea is as good a model as we have left today of what much of the rest of the human world was like until recently.

THE CLUES ABOUT when, why, and in what ways we ceased to be just another species of big mammal come from three types of evidence. Part One considers some of the traditional evidence from archaeology, which studies fossil bones and preserved tools, plus newer evidence from molecular biology. Other evidence from studies of living apes and people will be taken up in Parts Two and Three.

One basic question concerns just how extensive the genetic differences between ourselves and chimps are. That is, do we differ in ten, fifty, or ninety-nine per cent of our genes? Merely looking at humans and chimps or counting up visible traits would not be any help, because many genetic changes have no visible effects at all, while other changes have sweeping effects. For example, the visible differences between breeds of dogs such as great danes and pekinese are far greater than those between chimps and ourselves. Yet all dog breeds are interfertile, breed with each other (insofar as it is mechanically feasible) when given the opportunity, and belong to the same species. To a naive observer, the appearance of great danes and pekinese would suggest that they are genetically much further apart than chimps are from humans. Those visible differences among dog breeds in size, proportions, and hair colour depend on relatively few genes which have negligible consequences for reproductive biology.

How, then, can we estimate our genetic distance from chimps? Chapter One describes how this problem has been solved only within the past half a dozen years by molecular biologists. The answer is not just intellectually surprising but may also have some practical ethical implications for how we treat chimps. We shall see that gene differences between us and chimps, although large compared to those among living human populations or among breeds of dogs, are still small compared to differences among many other familiar pairs of related species. Evidently, changes in only a small percentage of chimpanzee genes had enormous consequences for our behaviour. It has also proved possible to work out a calibration between genetic distance and elapsed time, and thereby to get an approximate answer to the question of when we and chimps split apart from our common ancestor. That turns out to be somewhere around seven million years ago, give or take a few million years.

While the molecular biological story of the first chapter yields overall measures of genetic distance and elapsed time, it tells us nothing about how specifically we differ from chimps, and when those specific differences appeared. Hence Chapter Two will consider what more can be learned from bones and tools left by creatures variously intermediate between our ape-like ancestor and modern humans. The changes in bones constitute the traditional subject matter of physical anthropology. Especially important were our increase in brain size, skeletal changes associated with walking upright, and decreases in skull thickness, tooth size, and jaw muscles.

Our large brain was surely prerequisite for the development of human language and innovativeness. One might therefore expect the fossil record to show a close parallel between increased brain size and sophistication of tools. In fact, the parallel is not at all close. This proves to be the greatest surprise and puzzle of human evolution. Stone tools remained very crude for hundreds of thousands of years after we had undergone most of our expansion of brain size. As recently as 40,000 years ago, Neanderthals had brains even larger than those of modern humans, yet their tools show no signs of innovativeness and art. Neanderthals were still just another species of big mammal. Even for tens of thousands of years after some other human populations had achieved virtually modern skeletal anatomy, their tools too remained as boring as those of Neanderthals.

These paradoxes sharpen the conclusion drawn from Chapter One. Within the modest percentage of genes that differs between us and chimps, there must have been an even smaller percentage of genes which were not involved in the shapes of our bones, but which were responsible for the distinctively human traits of innovation, art, and complex tools. At least in Europe, those traits appear unexpectedly suddenly, at the time of the replacement of Neanderthals by Cro-Magnons. That is the time when we finally ceased to be just another species of big mammal. In Chapter Two I shall speculate about what those few changes were that triggered our steep rise to human status.

By what percentage of our genes do we differ from (the other two) chimpanzees? And what implications does that number have? Darwin himself would have been surprised by the answers.

THE NEXT TIME that you visit a zoo, make a point of walking past the ape cages. Imagine that the apes had lost most of their hair, and imagine a cage nearby holding some unfortunate people who had no clothes and couldn’t speak but were otherwise normal. Now try guessing how similar those apes are to ourselves genetically. For instance, would you guess that a chimpanzee shares ten, fifty, or ninety-nine per cent of its genes with humans?

Then ask yourself why those apes are on exhibit in cages, and why other apes are being used for medical experiments, while it is not permissible to do either of those things to humans. Suppose it turned out that chimps shared 99.9% of their genes with us, and that the important differences between humans and chimps were due to just a few genes. Would you still think it is okay to put chimps in cages and to experiment on them? Consider those unfortunate mentally-defective people who have much less capacity to solve problems, to care for themselves, to communicate, to engage in social relationships, and to feel pain, than do apes. What is the logic that forbids medical experiments on those people, but not on apes?

You might answer that apes are ‘animals’, while humans are humans, and that is enough. An ethical code for treating humans should not be extended to an ‘animal’, no matter what percentage of its genes it shares with us, and no matter what its capacity for social relationships or for feeling pain. That is an arbitrary but at least self-consistent answer that cannot be lightly dismissed. In that case, learning more about our ancestral relationships will not have any ethical consequences, but it will still satisfy our intellectual curiosity to understand where we come from. Every human society has felt a deep need to make sense of its origins, and has answered that need with its own story of the Creation. The Tale of Three Chimps is the creation story of our time.

For centuries it has been clear approximately where we fit into the animal kingdom. We are obviously mammals, the group of animals characterized by having hair, nursing their young, and other features. Among mammals we are obviously primates, the group of mammals including monkeys and apes. We share with other primates numerous traits lacking in most other mammals, such as flat fingernails and toenails rather than claws, hands for gripping, a thumb that can be opposed to the other four fingers, and a penis that hangs free rather than being attached to the abdomen. Already by the Second Century AD, the Greek physician Galen deduced our approximate place in Nature correctly when he dissected various animals and found that a monkey was ‘most similar to man in viscera, muscles, arteries, veins, nerves and in the form of bones’.

It is also easy to place us within the primates, among which we are obviously more similar to apes than to monkeys. To name only one of the most visible signs, monkeys sport tails, which we lack along with apes. It is also clear that gibbons, with their small size and very long arms, are the most distinctive apes, and that orangutans, chimpanzees, gorillas, and humans are all more closely related to each other than any of them is to gibbons. But to go further with our relationships proves unexpectedly difficult. It has provoked an intense scientific debate, which revolves around three questions including the one that I posed in the first paragraph of this chapter:

What is the detailed family tree of relationships among humans, the living apes, and extinct ancestral apes? For example, which of the living apes is our closest relative?

When did we and that closest living relative, whichever ape it is, last share a common ancestor?

At first, it would seem natural to assume that comparative anatomy had already solved the first of those three questions. We look especially like chimpanzees and gorillas, but differ from them in obvious features such as our larger brains, upright posture, and much sparser body hair, as well as in many more subtle points. However, on closer examination these anatomical facts are not decisive. Depending on what anatomical characters one considers most important and how one interprets them, biologists differ on whether we are most closely related to the orangutan (the minority view), with chimps and gorillas having branched off our family tree before we split off from orangutans, or whether we are instead closest to chimps and gorillas (the majority view), with the ancestors of orangutans having gone their separate way earlier.

Within the majority, most biologists have thought that gorillas and chimps are more like each other than either is like us, implying that we branched off before the gorillas and chimps diverged from each other. This conclusion reflects the common-sense view that chimps and gorillas can be lumped in a category termed ‘apes’, while we are something different. However, it is also conceivable that we look distinct only because chimps and gorillas have not changed much since we shared a common ancestor with them, while we were changing greatly in a few important and highly visible features like upright posture and brain size. In that case, humans might be most similar to gorillas, or humans might be most similar to chimps, or humans and gorillas and chimps might be roughly equidistant from each other, in overall genetic make-up.

Hence, anatomists have continued to argue about the first question, the details of our family tree. Whichever tree one prefers, anatomical studies by themselves tell us nothing about the second and third questions, our time of divergence and genetic distance from apes. Perhaps fossil evidence might in principle solve the questions of the correct ancestral tree and of dating, though not the question of genetic distance. If we had abundant fossils, we might hope to find a series of dated proto-human fossils and another series of dated proto-chimp fossils converging on a common ancestor around ten million years ago, converging in turn on a series of proto-gorilla fossils twelve million years ago. Unfortunately, that hope for insight from the fossil record has also been frustrated, because almost no ape fossils of any sort have been found for the crucially relevant period between five and fourteen million years ago in Africa.

The solution to these questions about our origins came from an unexpected direction: molecular biology as applied to bird taxonomy. About thirty years ago, molecular biologists began to realize that the chemicals of which plants and animals are composed might provide ‘clocks’ by which to measure genetic distances and to date times of evolutionary divergence. The idea is as follows. Suppose there is some class of molecules that occurs in all species, and whose particular structure in each species is genetically determined. Suppose further that that structure changes slowly over the course of millions of years because of genetic mutations, and that the rate of change is the same in all species. Two species derived from a common ancestor would start off with identical forms of the molecule, which they inherited from that ancestor, but mutations would then occur independently and produce structural changes between the molecules of the two species. The two species’ versions of the molecule would gradually diverge in structure. If we knew how many structural changes occur on the average every million years, we could then use the difference today in the molecule’s structure between any two related animal species as a clock, to calculate how much time had passed since the species shared a common ancestor.

For instance, suppose one knew from fossil evidence that lions and tigers diverged five million years ago. Suppose the molecule in lions were ninety-nine per cent identical in structure to the corresponding molecule in tigers and differed only by one per cent. If one then took a pair of species of unknown fossil history and found that the molecule differed by three per cent between those two species, the molecular clock would say that they had diverged three times five million, or fifteen million, years ago.

Neat as this scheme sounds on paper, testing whether it succeeds in practice has cost biologists much effort. Four things had to be done before molecular clocks could be applied: find the best molecule; find a quick way of measuring changes in its structure; prove that the clock runs steady (that is, that the molecule’s structure really does evolve at the same rate among all species that one is studying); and measure what that rate is.

Molecular biologists worked out the first two of these problems by around 1970. The best molecule proved to be deoxyribonucleic acid (abbreviated to DNA), the famous substance whose structure James Watson and Francis Crick showed to consist of a double helix, thereby revolutionizing the study of genetics. DNA is made up of two complementary and extremely long chains, each made up of four types of small molecules whose sequence within the chain carries all the genetic information transmitted from parents to offspring. A quick method of measuring changes in DNA structure is to mix the DNA from two species, then to measure by how many degrees of temperature the melting point of the mixed (hybrid) DNA is reduced below the melting point of pure DNA from a single species. Hence the method is generally referred to as DNA hybridization. As it turns out, a melting point lowered by one degree centigrade (abbreviated: delta T = 1°C) means that the DNA’s of the two species differ by roughly one per cent.

In the 1970s most molecular biologists and most taxonomists had little interest in each other’s work. Among the few taxonomists who appreciated the potential power of the new DNA hybridization technique was Charles Sibley, an ornithologist then serving as Professor of Ornithology and Director at Yale’s Peabody Museum of Natural History. Bird taxonomy is a difficult field because of the severe anatomical constraints imposed by flight. There are only so many ways to design a bird capable, say, of catching insects in mid-air, with the result that birds of similar habits tend to have very similar anatomies, whatever their ancestry. For example, American vultures look and behave much like Old World vultures, but biologists have come to realize that the former are related to storks, the latter to hawks, and that their resemblances result from their common lifestyle. Frustrated by the shortcomings of traditional methods for deciphering bird relationships, Sibley and Jon Ahlquist turned in 1973 to the DNA clock, in the most massive application to date of the methods of molecular biology to taxonomy. Not until 1980 were Sibley and Ahlquist ready to begin publishing their results, which eventually came to encompass applying the DNA clock to about 1,700 bird species – nearly one-fifth of all living birds.

While Sibley’s and Ahlquist’s achievement was a monumental one, it initially caused much controversy because so few other scientists possessed the blend of expertise required to understand it. Here are typical reactions I heard from my scientist friends:

‘I’m sick of hearing about that stuff. I no longer pay attention to anything those guys write,’ (an anatomist).

‘Their methods are okay, but why would anyone want to do something so boring as all that bird taxonomy?’ (a molecular biologist).

‘Interesting, but their conclusions need a lot of testing by other methods before we can believe them,’ (an evolutionary biologist).

‘Their results are The Revealed Truth, and you better believe it,’ (a geneticist).

My own assessment is that the last view will prove to be the most nearly correct one. The principles on which the DNA clock rests are unassailable; the methods used by Sibley and Ahlquist are state-of-the-art; and the internal consistency of their genetic-distance measurements from over 18,000 hybrid pairs of bird DNA testifies to the validity of their results.

Just as Darwin had the good sense to marshal his evidence for variation in barnacles before discussing the explosive subject of human variation, Sibley and Ahlquist similarly stuck to birds for most of the first decade of their work with the DNA clock. Not until 1984 did they publish their first conclusions from applying the same DNA methods to human origins, and they refined their conclusions in later papers. Their study was based on DNA from humans and from all of our closest relatives: the common chimpanzee, pygmy chimpanzee, gorilla, orangutan, two species of gibbons, and seven species of Old World monkeys. The figure on this page summarizes the results.

As any anatomist would have predicted, the biggest genetic difference, expressed in a big DNA melting point lowering, is between monkey DNA and the DNA of humans or of any ape. This simply puts a number on what everybody has agreed ever since apes first became known to science: that humans and apes are more closely related to each other than either are to monkeys. The actual statistic is that monkeys share ninety-three per cent of their DNA structure with humans and apes, and differ in seven per cent.

Equally unsurprising is the next biggest difference, one of five per cent between gibbon DNA and the DNA of other apes or humans. This too confirms the accepted view that gibbons are the most distinct apes, and that our affinities are instead with gorillas, chimpanzees, and orangutans. Among those latter three groups of apes, most recent anatomists have considered the orangutan as somewhat separate, and that conclusion too fits the DNA evidence: a difference of 3.6% between orangutan DNA and that of humans, gorillas, or chimpanzees. Geography confirms that the latter three species parted from gibbons and orangutans quite some time ago: living and fossil gibbons and orangutans are confined to Southeast Asia, while living gorillas and chimpanzees plus early fossil humans are confined to Africa.

At the opposite extreme but equally unsurprising, the most similar DNAs are those of common chimpanzees and pygmy chimpanzees, which are 99.3% identical and differ by only 0.7%. So similar are these two chimp species in appearance that it was not until 1929 that anatomists even bothered to give them separate names. Chimps living on the equator in central Zaire rate the name ‘pygmy chimps’ because they are on average slightly smaller (and have more slender builds and longer legs) than the widespread ‘common chimps’ ranging across Africa just north of the equator. However, with the increased knowledge of chimp behaviour acquired in recent years, it has become clear that the modest anatomical differences between pygmy and common chimps mask considerable differences in reproductive biology. Unlike common chimps but like ourselves, pygmy chimps assume a wide variety of positions for copulation, including face-to-face; copulation can be initiated by either sex, not just by the male; females are sexually receptive for much of the month, not just for a briefer period in mid-month; and there are strong bonds among females or between males and females, not just among males. Evidently, those few genes (0.7%) that differ between pygmy and common chimps have big consequences for sexual physiology and roles. That same theme – a small percentage of gene differences having great consequences – will recur later in this and the next chapter in regard to the gene differences between humans and chimps.

In all the cases that I have discussed so far, anatomical evidence of relationships was already convincing, and the DNA-based conclusions confirmed what the anatomists had already concluded. But DNA was also able to resolve the problem at which anatomy had failed – the relationships between humans, gorillas, and chimpanzees. As the figure on here shows, humans differ from both common chimps and pygmy chimps in about 1.6% of their (our) DNA, and share 98.4%. Gorillas differ somewhat more, by about 2.3%, from us and from both of the chimps.

Let us pause to let some of the implications of these momentous numbers sink in.

The gorilla must have branched off from our family tree slightly before we separated from the common and pygmy chimpanzees. The chimpanzees, not the gorilla, are our closest relatives. Put another way, the chimpanzees’ closest relative is not the gorilla but the human. Traditional taxonomy has reinforced our anthropocentric tendencies by claiming to see a fundamental dichotomy between mighty man, standing alone on high, and the lowly apes all together in the abyss of bestiality. Now future taxonomists may see things from the chimpanzees’ perspective: a weak dichotomy between slightly higher apes (the three chimpanzees, including the ‘human chimpanzee’) and slightly lower apes (gorilla, orangutan, gibbons). The traditional distinction between ‘apes’ (defined as chimps, gorillas, etc.) and humans misrepresents the facts.

The genetic distance (1.6%) separating us from pygmy or common chimps is barely double that separating pygmy from common chimps (0.7%). It is less than that between two species of gibbons (2.2%), or between such closely related North American bird species as red-eyed vireos and white-eyed vireos (2.9%), or between such closely related and hard-to-distinguish European bird species as willow warblers and chiffchaffs (2.6%). The remaining 98.4% of our genes are just normal chimp genes. For example, our principal haemoglobin, the oxygen-carrying protein that gives blood its red colour, is identical in all 287 units with chimp haemoglobin. In this respect as in most others, we are just a third species of chimpanzee, and what is good enough for common and pygmy chimps is good enough for us. Our important visible distinctions from the other chimps – our upright posture, large brains, ability to speak, sparse body hair, and peculiar sexual lives (of which I will say more in Chapter Three) – must be concentrated in a mere 1.6% of our genes.

If genetic distances between species accumulated at a uniform rate with time, they would function as a smoothly ticking clock. All that would be required to convert genetic distance into absolute time since the last common ancestor would be a calibration, furnished by a pair of species for which we know both the genetic distance and the time of divergence as dated independently by fossils. In fact, two independent calibrations are available for higher primates. On the one hand, monkeys diverged from apes between twenty-five and thirty million years ago according to fossil evidence, and now differ in about 7.3% of their DNA. On the other hand, orangutans diverged from chimps and gorillas between twelve and sixteen million years ago and now differ in about 3.6% of their DNA. Comparing these two examples, a doubling of evolutionary time, as one goes from twelve or sixteen to twenty-five or thirty million years, leads to a doubling of genetic distance (3.6 to 7.3% of DNA). Thus, the DNA clock has ticked relatively steadily among higher primates.

With those calibrations, Sibley and Ahlquist estimated the following time scale for our evolution. Since our own genetic distance from chimps (1.6%) is about half the distance of orangutans from chimps (3.6%), we must have been going our separate way for about half of the twelve to sixteen million years that orangutans had to accumulate their genetic distinction from chimps. That is, the human and ‘other chimp’ evolutionary lines diverged around six to eight million years ago. By the same reasoning, gorillas parted from the common ancestor of us three chimpanzees around nine million years ago, and the pygmy and common chimps diverged around three million years ago. In contrast, when I took physical anthropology as a college freshman in 1954, the assigned textbooks said that humans diverged from apes fifteen to thirty million years ago. Thus, the DNA clock strongly supports a controversial conclusion also drawn from several other molecular clocks based on amino acid sequences of proteins, mitochondrial DNA, and globin pseudogene DNA. Each clock indicates that humans have had only a short history as a species distinct from other apes, much shorter than paleontologists used to assume.

What do these results imply about our position in the animal kingdom? Biologists classify living things in hierarchical categories, each less distinct than the next: subspecies, species, genus, family, superfamily, order, class, and phylum. The Encyclopaedia Britannica and all the biology texts on my shelf say that humans and apes belong to the same order, called Primates, and the same superfamily, called Hominoidea, but to separate families, called Hominidae and Pongidae. Whether Sibley’s and Ahlquist’s work changes this classification depends on one’s philosophy of taxonomy. Traditional taxonomists group species into higher categories by making somewhat subjective evaluations of how important the differences between species are. Such taxonomists place humans in a separate family because of distinctive functional traits like large brain and bipedal posture, and this classification would remain unaffected by measures of genetic distance.

VireoPhylloscopustheHylobatesHomoPanHomoHomo troglodytesHomo paniscusHomo sapiensHomo