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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.
PART ONE
JUST ANOTHER SPECIES OF BIG MAMMAL
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.
ONE
A TALE OF THREE CHIMPS
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.
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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?
What fraction of our genes do we share with that closest living relative?
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 ins
tance, 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.