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.
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?