The Rise and Fall of the Third Chimpanzee

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?

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 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 by¹ 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 page 17 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.

However, — another school of taxonomy, called cladistics, argues that classification should be objective and uniform, based on genetic distance or times of divergence. All taxonomists agree now that red-eyed and white-eyed vireos belong together in the genus Vireo, willow warblers and chiffchaffs in the genus Phylloscopus, the various species of gibbons in the genus Hylobates. Yet the members of each of these pairs of species are genetically more distant from each other than are humans from the other two chimpanzees, and diverged longer ago. On this basis, then, humans do not constitute a distinct family, nor even a distinct genus, but belong in the same genus as common and pygmy chimps. Since our genus name Homo was proposed first, it takes priority, by the rules of zoological nomenclature, over the genus name Pan coined for the 'other' chimps. Thus, there are not one but three species of genus Homo on Earth today: the common chimpanzee, Homo troglodytes; the pygmy chimpanzee, Homo paniscus; and the third chimpanzee or human chimpanzee, Homo sapiens. Since the gorilla is only slightly more distinct, it has almost equal right to be considered a fourth species of Homo.

Even taxonomists espousing cladistics are anthropocentric, and the lumping of humans and chimps into the same genus will undoubtedly be a bitter pill for them to swallow. There is no doubt, however, that whenever chimpanzees learn cladistics, or whenever taxonomists from outer space visit Earth to inventory its inhabitants, they will unhesitatingly adopt the new classification.

Which particular genes are the ones that differ between humans and chimps? Before we can consider this question, we need first to understand what it is that DNA, our genetic material, does.

Much or most of our DNA has no function and may just constitute 'molecular junk': that is, DNA molecules that have become duplicated or have lost former functions, and that natural selection has not eliminated from us because they do us no harm. Of our DNA that does have known functions, the main ones have to do with the long chains of amino acids called proteins. Certain proteins make up much of our body's structure (such as the proteins keratin, of hair, or collagen, of connective tissue), while other proteins, termed enzymes, synthesize and break down most of our body's remaining molecules. The sequences of the component small molecules (nucleotide bases) in DNA specify the sequence of amino acids in our proteins. Other parts of our functional DNA regulate protein synthesis. Those of our observable features that are easiest to understand genetically are ones arising from single proteins and single genes. For instance, our blood's oxygen-carrying protein haemoglobin, already mentioned, consists of two amino acid chains, each specified by a single chunk of DNA (a single 'gene'). These two genes have no observable \ effects except through specifying the structure of haemoglobin, which is confined to our red blood cells. Conversely, haemoglobin's structure is totally specified by those genes. What you eat or how much you exercise may affect how much haemoglobin you make, but not the details of its structure.

That is the simplest situation, but there are also genes influencing many observable traits. For example, the fatal genetic disorder known as Tay-Sachs disease involves many behavioural as well as anatomical anomalies: excessive drooling, rigid posture, yellowish skin, abnormal head growth, and other changes. We know in this case that all these observable effects result somehow from changes in a single enzyme specified by the Tay-Sachs gene, but we do not know exactly how. Since that enzyme occurs in many tissues of our bodies and breaks down a widespread cellular constituent, changes in that one enzyme have wide-ranging and ultimately fatal consequences. Conversely, some traits, such as your height as an adult, are influenced simultaneously by many genes and also by environmental factors (for example, your nutrition as a child).

While scientists understand well the function of numerous genes that specify known individual proteins, we know much less about the function of genes involved in more complex determinations of traits, such as most behavioural features. It would be absurd to think that human hallmarks such as art, language, or aggression depend on a single gene. Behavioural differences among individual humans are obviously subject to enormous environmental influences, and what role genes play in such individual differences is a controversial question. However, for those consistent behavioural differences between chimps and humans, genetic differences are likely to be involved in those species' differences, even though we cannot yet specify the genes responsible. For instance, the ability of humans but not chimps to speak surely depends on differences in genes specifying the anatomy of the voice box and the wiring of the brain. A young chimpanzee brought up in a psychologist's home along with the psychologist's human baby of the same age still continued to look like a chimp and did not learn to talk or walk erect. But whether an individual human grows up to be fluent in English or Korean is independent of genes and dependent solely on its childhood linguistic environment, as proved by the linguistic attainments of Korean infants adopted by English-speaking parents.

With this as background, what can we say about the 1.6 % of our DNA that differs from chimp DNA? We know that the genes for our principal haemoglobin do not differ, and that certain other genes do exhibit minor differences. In the nine protein chains studied to date in both humans and common chimps, only five out of a total of 1,271 amino acids differ: one amino acid in a muscle protein called myoglobin, one in a minor haemoglobin chain called the delta chain, and three in an enzyme called carbonic anhydrase. But we do not yet know which chunks of our DNA are responsible for the functionally significant differences between humans and chimps to be discussed in Chapters Two to Seven: the differences in brain size, anatomy of the pelvis and voice box and genitalia, amount of body hair, female menstrual cycle, menopause, and other traits. Those important changes certainly do not arise from the five amino acid differences detected to date. At present, all we can say with confidence is this: much of our DNA is junk; at least some of the 1.6 % that differs between us and chimps is already known to be junk; and the functionally significant differences must be confined to some as-yet-unidentified small fraction of 1.6 %.

While we do not know which particular genes are the crucial ones, there are numerous precedents for one or a few genes having a big impact. I just mentioned the many large and visible differences between Tay-Sachs patients and normal people, all somehow arising from a single change in one enzyme. That is an example of differences among individuals of the same species. As for differences between related species, a good example is provided by the cichlid fishes of Africa's Lake Victoria. Cichlids are popular aquarium species, of which about two hundred are confined to that one lake, where they evolved from a single ancestor within perhaps the last 200,000 years. Those two hundred species differ among themselves in their food habits as much as do tigers and cows. Some graze on algae, others catch other fish, and still others variously crush snails, feed on plankton, catch insects, nibble the scales off other fish, or specialize in grabbing fish embryos from brooding mother fish. Yet all those Lake Victoria cichlids differ from each other on the average by only about 0.4 % of their DNA studied. Thus it took even fewer genetic mutations to change a snail-crusher into a specialized baby-killer than it took to produce us from an ape.

Do the new results about our genetic distance from chimps have any broader implications, besides technical questions of taxonomic names? Probably the most important implications concern how we think about the place of humans and apes in the universe. Names are not just technical details but express and create attitudes. (To convince yourself, try greeting your spouse this evening either as 'my darling' or as 'you swine', using the same expression and tone of voice.) The new results do not specify how we should think about humans and apes, but, just as did Darwin's On the Origin of Species, they will probably influence how we do \ think, and it will probably take us many years to readjust our attitudes. I shall mention just one example of a controversial area that might be affected: our use of apes. At present we make a fundamental distinction between animals (including apes) and humans, and this distinction guides our ethical code and actions. For instance, as I noted at the start of this chapter, it is considered acceptable to exhibit caged apes in zoos, but it is not acceptable to do the same with humans. I wonder how the public will feel when the identifying label on the chimp cage in the zoo reads 'Homo troglodytes'. Yet if it were not for the sympathetic interest in apes that many people gain at zoos, there might be much less public financial support for conservationists' efforts to protect apes in the wild.

I also noted earlier that it is considered acceptable to subject apes, but not humans, without their consent to lethal experiments for purposes of medical research. The motive for doing so is precisely because apes are so similar to us genetically. They can be infected with many of the same diseases as we can, and their bodies respond similarly to the disease organisms. Thus, experiments on apes offer a far better way to devise improved medical treatments for humans than would experiments on any other animals.

This ethical choice poses an even more difficult problem than does caging apes in zoos. After all, we regularly cage millions of human criminals under worse conditions than zoo apes, but there is no socially accepted human analogue of medical research on animals, even though lethal experiments on humans would provide medical scientists with far more valuable information than do lethal experiments on chimps. Yet the human experiments performed by Nazi concentration camp physicians are widely viewed as one of the most abominable of all the Nazis' abominations. Why is it all right to perform such experiments on chimps?

Somewhere along the scale from bacteria to humans, we have to decide where killing becomes murder, and where eating becomes cannibalism. Most people draw those lines between humans and all other species. However, quite a few people are vegetarians, unwilling to eat any animal (yet willing to eat plants). An increasingly vocal minority, belonging to the animal rights movement, objects to medical experiments on animals—or at least on certain animals. That movement is especially indignant at research on cats and dogs and primates, less concerned about mice, and generally silent about insects and bacteria.

If our ethical code makes a purely arbitrary distinction between humans and all other species, then we have a code based on naked selfishness devoid of any higher principle. If our code instead makes distinctions based on our superior intelligence, social relationships, and capacity for feeling pain, then it becomes difficult to defend an all-or-nothing code that draws a line between all humans and all animals.

Instead, different ethical constraints should apply to research on different species. Perhaps it is just our naked selfishness, re-emerging in a new disguise, that would advocate granting special rights to those animal species genetically closest to us. But an objective case, based on the considerations I have just mentioned (intelligence, social relationships, etc.), can be made that chimps and gorillas qualify for preferred ethical consideration over insects and bacteria. If there is any animal species currently used in medical research for which a total ban on medical experimentation can be justified, that species is surely the chimpanzee. The ethical dilemma posed by animal experiments is compounded for chimps by the fact that they are endangered as a species. In this case, medical research not only kills individuals but threatens to kill the species itself. That is not to say that demands for research have been the sole threat to wild chimp populations—habitat destruction and capture for zoos have also been major threats—but it is enough that research demands have been a significant threat. The ethical dilemma is further compounded by other considerations: that on the average several wild chimps are killed in the process of capturing one (often a young animal with its mother) and delivering it to a medical research laboratory; that medical scientists have played little role in the struggle to protect wild chimp populations, despite their obvious self-interest in doing so; and that chimps used for research are often caged under cruel conditions. The first chimp that I saw being used for medical research had been injected with a slow-acting lethal virus and was being kept alone, for the several years until it died, in a small, empty, indoor cage at the US National Institutes of Health. Breeding chimps in captivity for research use avoids objections based on depleting wild chimp populations, but that still does not get around the basic dilemma, any more than enslaving children of US-born blacks after abolition of the African slave trade made black slavery in the nineteenth-century US acceptable. Why is it all right to experiment on Homo troglodytes, but not on Homo sapiens'? Conversely, how should we explain to parents, whose children are at risk of dying from diseases now being studied in captive chimps, that their children are less important than chimps? Ultimately, we the public, not just scientists, will have to make these terrible choices. All that is certain is that our view of man and apes will determine our decision. Finally, changes in our attitudes about apes may be crucial in determining whether apes will survive at all in the wild. At present, their populations are threatened especially by destruction of their rainforest habitats in Africa and Asia, and by legal and illegal capture and killing. If present trends continue, the mountain gorilla, orangutan, pileated gibbon, Kloss's gibbon, and possibly some other apes as well will exist only in zoos by the time that this year's crop of human babies enters college. It is not enough for us to preach to the governments of Uganda, Zaire, and Indonesia about their moral obligation to protect their wild apes. These are impoverished countries, and national parks are expensive to create and maintain. If we as the third chimpanzee decide that the other two chimpanzees are worth saving, those of us in the richer countries will have to bear most of the expense. From the point of view of the apes themselves, the most important effect of what we have recently learned about the Tale of Three Chimps will be on how we feel about footing that bill.

TWO THE GREAT LEAP FORWARD

What happened at that magic moment in evolution around 40,000 years ago, when we suddenly became human?

As we saw in Chapter One, our lineage diverged from that of apes millions of years ago. For most of the time since then, we have remained little more than glorified chimpanzees in the ways we have made our living. As recently as/40,000 years ago, Western Europe was still occupied by Neanderthals, primitive beings for whom art and progress scarcely existed. Then there was an abrupt change, as anatomically modern people appeared in Europe, bringing with them art, musical instruments, lamps, trade, and progress. Within a short time, the Neanderthals were gone. That Great Leap Forward in Europe was probably the result of a similar leap that had occurred over the course of the preceding few tens of thousands of years in the Near East and Africa. Even a few dozen millenia, though, is a trivial fraction (less than one per cent) of our millions of years of history separate from that of the apes. Insofar as there was any single point in time when we could be said to have become human, it was at the time of that leap. Only a few more dozen millenia were needed for us to domesticate animals, develop agriculture and metallurgy, and invent writing. It was then but a short further step to those monuments of civilization that distinguish humans from animals acros's what used to seem an unbridgeable gulf- monuments such as the 'Mona Lisa' and the Eroica Symphony, the Eiffel Tower and Sputnik, Dachau's ovens and the bombing of Dresden.

This chapter will confront the questions posed by our abrupt rise to humanity. What made it possible, and why was it so sudden? What held back the Neanderthals, and what was their fate? Did Neanderthals and modern peoples ever meet, and if so, how did they behave towards each other?

Understanding the Great Leap Forward is not easy, and writing about it is not easy either. The immediate evidence conies from technical details of preserved bones and stone tools. Archaeologists' reports are full of terms obscure to the rest of us, such as 'transverse occipital torus', 'receding zygomatic arches', and 'Chatelperronian backed knives'. What we really want to understand—the way of life and the humanity of our various ancestors—is not directly preserved but only inferred from those technical details of bones and tools. Much of the evidence is missing, and archaeologists often disagree over the meaning of such evidence as has survived. Since the books and articles listed on pages 334-5 will slake the interest of readers curious to learn more about receding zygomatic arches, I shall emphasize instead the inferences from bones and tools.

Our ancestors were confined to Africa for millions of years, where, as we have already discussed, they diverged from the ancestors of chimps and gorillas between about six and ten million years ago. For comparison, life originated on Earth several billion years ago, and the dinosaurs became extinct around sixty-five million years ago. (Science-fiction films that depict cavemen fleeing from dinosaurs are just that, science fiction.) Initially, our ancestors would have been classified as merely another species of ape, but a sequence of three changes launched us in the direction of modern humans.

The first of these changes had occurred by around four million years ago, when the structure of fossilized limb bones shows that our ancestors were habitually walking upright on the two hindlimbs. In contrast, gorillas and chimps walk upright only occasionally, and usually proceed on all fours. The upright posture freed our ancestors' forelimbs to do other things, among which tool-making proved the most important.

The second change occurred around three million years ago, when our lineage split into at least two distinct species. Recall that members of two animal species living in the same area must fill different ecological roles and do not normally interbreed with each other. For example, coyotes and wolves are obviously closely related and (until wolves were exterminated in most of the US) lived in many of the same areas of North America. However, wolves are larger, mainly hunt big mammals like deer and moose, and often live in large packs, whereas coyotes are smaller, mainly hunt small mammals like rabbits and mice, and usually live in pairs or small groups. Similarly, Europe's wildcat and lynx are closely related and overlap widely in range but differ ecologically and do not interbreed.

Every human population living today has interbred with every other human population with which it has had extensive contact. Ecological differences among existing humans are entirely a product of childhood education: it is not the case that some of us are born with sharp teeth and equipped to hunt deer, while others are born with grinding teeth, gather berries, and do not marry the deerhunters. Therefore all modern humans belong to the same species.

On perhaps two occasions in the past, however, the human lineage split into separate species, as distinct as wolves and coyotes. The most recent such occasion, which I shall describe later, may have been at the time of the Great Leap Forward. The earlier occasion was around three million years ago, when our lineage split into two: a man-ape with a robust skull and very big cheek teeth, assumed to eat coarse plant food, and often referred to as Australopithecus robustus (meaning 'the robust southern ape'); and a man-ape with a more lightly built skull and smaller teeth, assumed to have an omnivorous diet, and known as Australopithecus africanus ('the southern ape of Africa') >The latter man-ape evolved into a larger-brained form termed Homo habilis ('man the handyman'). However, fossil bones often attributed to male and female Homo habilis differ so much in skull size and tooth size that they may actually imply another fork in our lineage yielding two distinct kahilis-like species: Homo habilis himself, and a mysterious 'Third Man'. Thus, two million years ago there were at least two, and possibly three, proto-human species. The third and last of the big changes that began to make our ancestors more human and less apelike was the regular use of stone tools. This is a human hallmark with clear animal precedents: woodpecker finches, Egyptian vultures, and sea otters are among the other animal species that evolved independently to employ tools in capturing or processing food, though none of these species is as heavily dependent on implements as we are now. Common chimpanzees also use tools, occasionally of stone, but not in numbers sufficient to litter the landscape. But by around two-and-a-half million years ago, very crude stone tools appear in numbers in areas of East Africa occupied by the proto-humans. Since there were two or three proto-human species, who made the tools? Probably the light-skulled species, since both it and the tools persisted and evolved. With only one human species surviving today but two or three a few million years ago, it is clear that one or two species must have become extinct. Who was our ancestor; which species ended up instead as a discard in the rubbish-heap of evolution; and when did this shakedown occur? The winner was the light-skulled* Homo habilis, who went on to increase in brain size and body size. By around 1,700,000 years ago the differences were sufficient that anthropologists give our lineage a new name, Homo erectus, meaning 'the man that walks upright'. (Homo erectus fossils were discovered before all the earlier fossils I have been discussing, so anthropologists did not realize that Homo erectus was not the first proto-human to walk upright.) The robust man-ape disappeared around 1,200,000 years ago, and the 'Third Man' (if he ever existed) must have disappeared by then also. As for why Homo erectus survived and the robust man-ape didn't, we can only speculate. A plausible guess is that the robust man-ape could no longer compete, since Homo erectus ate both meat and plant food, and since tools and a larger brain made Homo erectus more efficient at getting even the plant food on which his robust sibling depended. It is also possible that Homo erectus gave his sibling a direct push into oblivion, by killing him for meat.

All the developments that I have been discussing so far were played out within the continent of Africa, to which our closest living relatives (the chimps and gorilla) are still confined. The shakedown had left Homo erectus as the sole proto-human on the African stage. Around one million years ago Homo erectus expanded his horizons. His stone tools and bones show that he reached the Near East, then the Far East (where he is represented by the famous fossils known as Peking Man and Java Man) and Europe. He continued to evolve in our direction by an increase in brain size and in skull roundness. By around 500,000 years ago, some of our ancestors looked sufficiently like us, and different enough from earlier Homo erectus, to be classified as our own species (Homo sapiens, meaning 'the wise man'), though they still had thicker skulls and brow ridges than we do today.

Readers unfamiliar with details of our evolution might be forgiven for assuming that the appearance of Homo sapiens constituted the Great Leap Forward. Was our meteoric ascent to sapiens status half-a-million years ago the brilliant climax of Earth's history, when art and sophisticated technology finally burst upon our previously dull planet? Not at all: the appearance of Homo sapiens was a non-event. Cave paintings, houses, and bows and arrows still lay hundreds of thousands of years off in the future. Stone tools continued to be the crude ones that Homo erectus had been making for nearly a million years. The extra brain size of those early Homo sapiens had no dramatic effect on our way of life. That whole long tenure of Homo erectus and early Homo sapiens outside Africa was a period of infinitesimally slow cultural change. In fact, the sole candidate for a major advance was possibly the control of fire, of which caves occupied by Peking Man provide one of the earliest indications in the form of ash, charcoal, and burnt bones. Even that advance—if those cave fires really were man-lit rather than natural—would belong to Homo erectus, not Homo sapiens.

Thus, the emergence of Homo sapiens illustrates the paradox discussed in Chapter One, that our rise to humanity was not directly proportional to the changes in our genes. Early Homo sapiens had progressed much further in anatomy than in cultural attainments along the road up from chimpanzeehood. Some crucial ingredients still had to be added before the Third Chimpanzee could conceive of painting the Sistine Chapel.

How did our ancestors make their living during the one-and-a-half million years that spanned the emergence of Homo erectus and Homo sapiens'?

The only surviving tools from this period are stone tools that can charitably be described as very crude, in comparison with the beautiful, polished stone tools made until recently by Polynesians, American Indians, and other modern stone-age peoples. Early stone tools vary in size and shape, and archaeologists have used those differences to give the tools different names, such as 'hand-axe', 'chopper', and 'cleaver'. These names conceal the fact that none of those early tools had a sufficiently consistent or distinctive shape to suggest any specific function, as do the obvious needles and spear-points left by the much later Cro-Magnons. Wear-marks on the tools show that they were variously used to cut meat, bone, hides, wood, and non-woody parts of plants, but any size or shape of tool seems to have been used to cut any of those things, and the tool names applied by archaeologists may be little more than arbitrary divisions of a continuum of stone forms.

Negative evidence is also significant here. Many advances in tools that appear after the Great Leap Forward were unknown to Homo erectus and early Homo sapiens. There were no bone tools, no ropes to make nets, and no fishhooks. All the early stone tools may have been held directly in the hand; they show no signs of being mounted on other materials for increased leverage, as we mount steel axe-blades on wooden handles.

What food did our early ancestors get with those crude tools, and how did they get it? At this point, anthropology textbooks usually insert a long chapter entitled something like 'Man the Hunter'. The point here is that baboons, chimps, and some other primates occasionally prey on small vertebrates, but recently surviving stone-age people (like Bushmen) did a lot of big-game hunting. So did Cro-Magnons, according to abundant archaeological evidence. There is no doubt that our early ancestors also ate some meat, as shown by marks of their stone tools on animal bones and by wear-marks on their stone tools caused by cutting meat. The real question is: how much big-game hunting did our early ancestors do? Did big-game hunting skills improve gradually over the past one-and-a-half million years, or was it only since the Great Leap Forward that they made a large contribution to our diet?

Anthropologists routinely reply that we have been successful big-game hunters for a long time. The supposed evidence comes mainly from three archaeological sites occupied around 500,000 years ago: a cave at Zhoukoudian near Beijing, containing bones and tools of Homo erectus ('Peking Man') and bones of many animals; and two non-cave (open-air) sites at Torralba and Ambrona in Spain, with stone tools and bones of elephants and other large animals. It is usually assumed that the people who left the tools killed the animals, brought their carcasses to the site, and ate them there, but all three sites also have bones and faecal remains of hyenas, which could equally well have been the hunters. The bones of the Spanish sites in particular look like they came from a collection of scavenged, water-washed, trampled carcasses such as one can find around African water-holes today, rather than from a human hunters' camp. Thus, while early humans ate some meat, we do not know how much meat they ate, nor whether they got the meat by hunting or scavenging. It is not until much later, around 100,000 years ago, that we have good evidence about human hunting skills, and it is clear that humans then were still very ineffective big-game hunters. Human hunters of 500,000 years ago and earlier must have been even more ineffective.

The mystique of Man the Hunter is now so rooted in us that it is hard to abandon our belief in its long-standing importance. Today, shooting a big animal is regarded as an ultimate expression of macho masculinity. Trapped in this mystique, male anthropologists like to stress the key role of big-game hunting in human evolution. Supposedly, big-game hunting was what induced proto-human males to cooperate with each other, develop language and big brains, join into bands, and share food. Even women were supposedly moulded by men's big-game hunting: women suppressed the external signs of monthly ovulation that are so conspicuous in chimps, so as not to drive men into a frenzy of sexual competition and thereby spoil men's cooperation at hunting. As an example of the purple prose spawned by this men's locker-room mentality, consider the following account of human evolution by Robert Ardrey in his book African Genesis: In some scrawny troop of beleagured not-yet-men on some scrawny forgotten plain a radian particle from an unknown source fractured a never-to-be-forgotten gene, and a primate carnivore was born. For better or worse, for tragedy or for triumph, for ultimate glory or ultimate damnation, intelligence made alliance with the way of the killer, and Cain with his sticks and his stones and his quickly running feet emerged on the high savannah. What pure fantasy!

Western male writers and anthropologists are not the only men with an exaggerated view of hunting. In New Guinea I have lived with real hunters, men who recently emerged from the Stone Age. Conversations at campfires go on for hours over each species of game animal, its habits, and how best to hunt it. To listen to my New Guinea friends, you would think that they eat fresh kangaroo for dinner every night and do little each day except hunt. In fact, when pressed for details, most New Guinea hunters admit that they have bagged only a few kangaroos in their whole life.

I still recall my first morning in the New Guinea highlands, when I set out with a group of a dozen men, armed with bows and arrows. As we passed a fallen tree, there was suddenly much excited shouting, men surrounded the tree, some spanned their bows, and others pressed forward into the brushpile. Convinced that an enraged boar or kangaroo was about to come out fighting, I looked for a tree that I could climb to a perch of safety. Then I heard triumphant shrieks, and out of the brushpile came two mighty hunters holding aloft their prey: two baby wrens, not quite able to fly, weighing about a third of an ounce each, and promptly plucked, roasted, and eaten. The rest of that day's catch consisted of a few frogs and many mushrooms.

Studies of most modern hunter-gatherers with far more effective weapons than early Homo sapiens show that most of a family's calories come from plant food gathered by women. Men catch rabbits and other small game never mentioned in the heroic campfire stories. Occasionally the men do bag a large animal, which does indeed contribute significantly to protein intake. But it is only in the Arctic, where little plant food is available, that big-game hunting becomes the dominant food source, and humans did not reach the Arctic until within the last few dozen millenia. Thus I would guess that big-game hunting contributed only modestly to our food intake until after we had evolved fully modern anatomy and behaviour. I doubt the usual view that hunting was the driving force behind our uniquely human brain and societies. For most of our history we were not mighty hunters but skilled chimps, using stone tools to acquire and prepare plant food and small animals. Occasionally, men did bag a large animal, and then retold the story of that rare event incessantly.

In the period just before the Great Leap Forward, at least three distinct human populations occupied different parts of the Old World. These were the last truly primitive humans, supplanted by fully modern people at the time of the Great Leap. Let's consider those among the last primitives whose anatomy is best known and who have become a metaphor for brutish subhumans: the Neanderthals.

Where and when did they live? Their geographic range extended from Western Europe, through southern European Russia and the Near East, to Uzbekhistan in Central Asia near the border of Afghanistan. (The name 'Neanderthal' comes from Germany's Neander Valley (valley = Thai in German), where one of the first skeletons was discovered.) The time of their origin is a matter of definition, since some old skulls have characteristics anticipating later full-blown Neanderthals. The earliest 'full-blown' examples date to around 130,000 years ago, and most specimens postdate 74,000 years ago. While their start is thus arbitrary, their end is abrupt: the last Neanderthals died around 40,000 years ago. During the time that Neanderthals flourished, Europe and Asia were in the grip of the last Ice Age. Neanderthals must have been a cold-adapted people—but only within limits. They got no further north than southern Britain, northern Germany, Kiev, and the Caspian Sea. The first penetration of Siberia and the Arctic was left to later, fully modern humans.

Neanderthals' head anatomy was so distinctive that, even if a Neanderthal dressed in a business suit or a designer dress were to walk down the streets of New York or London today, everybody else (all the homines sapientes) on the street would be staring in shock. Imagine converting a modern face to soft clay, gripping the middle of the face from the bridge of the nose to the jaws in a vice, pulling the whole mid-face forward, and letting it harden again. You will then have some idea of a Neanderthal's appearance. Their eyebrows rested on prominently-bulging bony ridges, and their nose and jaws and teeth protruded far forward. Their eyes lay in deep sockets, sunk behind the protruding nose and brow ridges. Their foreheads were low and sloping, unlike our high vertical modern foreheads, and their lower jaws sloped back without a chin. Despite these startlingly primitive features, Neanderthals' brain size was nearly ten per cent greater than ours! A dentist who examined a Neanderthal's teeth would have been in for a further shock. In adult Neanderthals, the incisors (front teeth) were worn down on the outer-facing surface, in a way found in no modern people. Evidently, this peculiar wear-pattern somehow resulted from a use of their teeth as tools, but what exactly was that function? As one possibility, they may have routinely used their teeth as a vice to grip objects, like my baby sons, who gripped their milk bottle in their teeth and ran around with their hands free. Alternatively, Neanderthals may have bitten hides with their teeth to make leather, or bitten wood to make wooden tools. While a Neanderthal in a business suit or dress would attract attention today, one in shorts or a bikini would have drawn gasps. Neanderthals were more heavily muscled, especially in their shoulders and neck, than all but the most avid modern bodybuilders. Their limb-bones, which took the force of those big muscles when they were contracting, had to be considerably thicker than ours to withstand the stress. Their arms and legs would have looked stubby to us, because the lower leg and forearm were relatively shorter than ours. Even their hands were much more powerful than ours; a Neanderthal's handshake would have been literally bone-crushing. While their average height was only around 5 feet 4 inches, their weight would have been at least 20 pounds more than that of a modern person of that height, and this excess was mostly in the form of lean muscle. One other possible anatomical difference is intriguing, though its reality as well as its interpretation are quite uncertain. A Neanderthal woman's birth canal may have been wider than a modern woman's, permitting her baby to grow inside her to a bigger size before birth. If so, a Neanderthal pregnancy might have lasted a year, instead of our nine months. Besides their bones, our other main source of information about Neanderthals is their stone tools. Like the earlier human tools, Neanderthal tools may have been simple hand-held stones not mounted on separate parts such as handles. The tools do not fall into distinct types with unique functions. There were no standardized bone tools, no bows and arrows. Some of the stone tools were undoubtedly used to make wooden tools, which rarely survive. One notable exception is a wooden thrusting spear 8 feet long, found in the ribs of a long-extinct species of elephant at an archaeological site in Germany. Despite that (lucky?) success, Neanderthals were probably not very good at big-game hunting, because Neanderthal numbers (to judge from the number of their sites) were much lower than those of later Cro-Magnons, and because (as I will explain later) even anatomically more modern people living in Africa at the same time as the Neanderthals were undistinguished as hunters.

If you say 'Neanderthal' to friends and ask for their first association, you will probably get back the answer 'caveman'. While most excavated Neanderthal remains do come from caves, that is surely an artifact of preservation-, since open-air sites would be eroded much more quickly. Among my hundreds of campsites in New Guinea, one was in a cave, and that is the only site where future archaeologists are likely to find my pile of discarded tin cans intact. So archaeologists will also be deceived into considering me a caveman. Neanderthals must have constructed some type of shelter against the cold climate in which they lived, but those shelters must have been crude. All that remains are a few piles of stones and a pesthole, compared to the elaborate remains of houses built by the later Cro-Magnons.

The list of other quintessentially modern human things that Neanderthals lacked is a long one. They left no unequivocal art objects. They must have worn some clothing in their cold environment, but it had to be crude, as they lacked needles and other evidence of sewing. They evidently lacked boats, as no Neanderthal remains are known from Mediterranean islands nor even from North Africa, just eight miles across the Straits of Gibraltar from Neanderthal-populated Spain. There was no longdistance overland trade: Neanderthal tools are made of stones available within a few miles of the site.

Today we take cultural differences among people inhabiting different areas for granted. Every human population alive today has its characteristic house-style, implements, and art. If you were shown chopsticks, a Guinness beer bottle, and a blowgun and asked to associate one object each with China, Ireland, and Borneo, you would have no trouble giving the right answers. No such cultural variation is apparent for Neanderthals, whose tools look much the same whether they come from France or Russia.

We also take cultural progress with time for granted. The wares from a Roman villa, medieval castle, and 1990 New York apartment differ obviously. In the year 2000 my sons will look with astonishment at the slide rule I used for calculations throughout the 1950s: 'Daddy, are you really that old? But Neanderthal tools from 100,000 and 40,000 years ago look essentially the same. In short, Neanderthal tools had no variation in either time or space to suggest that most human of characteristics, innovation. As one archaeologist put it, Neanderthals had 'beautiful tools stupidly made'. Despite Neanderthals' big brains, something was still missing. Grandparenting, and what we consider old age, must also have been rare among Neanderthals. Their skeletons make clear that adults might live to their thirties or early forties, but not beyond forty-five. If we lacked writing and if none of us lived past forty-five, just think how the ability of our society to accumulate and transmit information would suffer. I have had to mention all these subhuman qualities of Neanderthals, but there are three respects in which we can relate to their humanity. First, virtually all well-preserved Neanderthal caves have small areas of ash and charcoal indicating a simple fireplace. Hence, although Peking Man may have already used fire hundreds of thousands of years earlier, Neanderthals were the first people to leave undisputed evidence of the regular use of fire. Neanderthals may also have been the first people who regularly buried their dead, but that is disputed, and whether it would imply religion is a matter of pure speculation. Finally, they regularly took care of their sick and aged. Most skeletons of older Neanderthals show signs of severe impairment, such as withered arms, healed but incapacitating broken bones, tooth loss, and severe osteoarthritis. Only care by young Neanderthals could have enabled such older Neanderthals to stay alive to the point of such incapacitation. After my long litany of what Neanderthals lacked, we have finally found something that lets us feel a spark of kindred spirit in these strange creatures of the last Ice Age—nearly human in form, and yet not really human in spirit.

Did Neanderthals belong to the same species as we do? That depends on whether we could and would have mated and reared a child with a Neanderthal man or woman, given the opportunity.

Science-fiction novels love to imagine the scenario. You may remember the blurb on many a back cover:

A team of explorers stumbles on a steep-walled valley in the centre of deepest Africa, a valley that time forgot. Here they find a tribe of incredibly primitive people, living in ways that our stone-age ancestors discarded thousands of years ago. Do they belong to the same species as we do? There's only one way to find out, but who among the intrepid explorers can bring himself [male explorers, of course] to make the test?

At this point one of the bone-chewing cavewomen suddenly is described as beautiful and sexy in a primitively erotic way, so that modern novel readers will find the brave explorer's dilemma believable: does he or doesn't he have sex with her?

Believe it or not, something like that experiment actually took place. As we shall now see, it happened repeatedly around 40,000 years ago, at the time of the Great Leap Forward.

I mentioned that the Neanderthals of Europe and Western Asia were just one of at least three human populations occupying different parts of the Old World around 100,000 years ago. A few fossils from Eastern Asia suffice to show that people there differed from Neanderthals as well as from us moderns, but too few bones have been found to describe these Asians in more detail. The best characterized contemporaries of the Neanderthals are those from Africa, some of whom were virtually modern in their skull anatomy. Does this mean that, 100,000 years ago in Africa, we have at last arrived at the watershed of human cultural development?

Surprisingly, the answer is still 'no'. The stone tools of these modern-looking Africans were very similar to those of the decidedly unmodern-looking Neanderthals, hence we refer to them as

'Middle Stone Age Africans'. They still lacked standardized bone tools, bows and arrows, nets, fishhooks, art, and cultural variation in tools from place to place.

Despite their almost modern bodies, these Africans were still missing that vital something necessary to endow them with full humanity. Once again, we face the paradox that almost modern bones, and presumably almost modern genes, are not enough by themselves to produce modern behaviour.

Some South African caves occupied around 100,000 years ago provide us—for the first time in human evolution—with detailed information about what people actually were eating. Our confidence stems from the fact that the African caves are full of stone tools, animal bones with cut-marks from stone tools, and human bones, but few or no bones of carnivores like hyenas. Thus, it is clear that people, not hyenas, brought the bones to the caves. Among the bones are many of seals and penguins, as well as shellfish such as limpets. Hence Middle Stone Age Africans are the first people for whom there is even a hint that they exploited the seashore. However, the caves contain very few remains offish or flying seabirds, undoubtedly because people still lacked the fishhooks and nets needed to catch fish and birds. The mammal bones from the caves include those of quite a few medium-sized species, among which those of eland, an antelope, predominate by far. Eland bones in the caves represent eland of all ages, as if people had somehow managed to capture a whole herd and kill every individual. At first, the relative abundance of eland among hunters' prey is surprising, since the caves' environment 100,000 years ago was much as it is today and since eland is now one of the least common large animals in the area. The secret to the hunters' success with eland probably lay in the fact that eland are rather tame, not dangerous, and easy to drive in herds. This suggests that hunters occasionally managed to drive a whole herd over a cliff, explaining why the distribution of eland age groups among the cave kills is like that in a living herd. In contrast, remains of more dangerous prey such as Cape buffalo, pigs, elephants, and rhinos yield a very different picture. Buffalo bones in the caves are mainly of very young or very old individuals, while pigs, elephants, and rhinos are virtually unrepresented.

Middle Stone Age Africans can be considered big-game hunters, but only barely. They either avoided dangerous species entirely or confined themselves to old, weak animals or babies. Those choices reflect sound prudence on the hunters' part, since their weapons were still spears, for thrusting, rather than bows and arrows. Along with drinking a strychnine cocktail, poking an adult rhinoceros or Cape buffalo with a spear ranks as one of the most effective means of suicide that I know. Nor could the hunters have succeeded often at driving eland herds over a cliff, since elands were not exterminated but continued to coexist with hunters. As with earlier peoples and modern stone-age hunters, I suspect that plants and small game made up most of the diets of these not-so-great Middle Stone Age hunters. They were definitely more effective than chimpanzees, but not up to the skill of modern Bushmen and Pygmies. Thus, the scene that the human world presented from around 100,000 to somewhat before 50,000 years ago was this. Northern Europe, Siberia, Australia, the oceanic islands, and the whole New World were still empty of people. In Europe and Western Asia lived the Neanderthals; in Africa, people increasingly like us moderns in their anatomy; and in Eastern Asia, people unlike either the Neanderthals or Africans but known from only a few bones. All three of these populations were, at least initially, still primitive in their tools, behaviour, and limited innovativeness. The stage was set for the Great Leap Forward. Which among these three contemporary populations would take that leap?

The evidence for an abrupt rise is clearest in France and Spain, in the Late Ice Age around 40,000 years ago. Where there had previously been Neanderthals, anatomically fully modern people (often known as Cro-Magnons, from the French site where their bones were first identified) now appear. Had one of those gentlemen or ladies strolled down the Champs Elysees in modern attire, he or she would not have stood out from the Parisian crowds in any way. As dramatic to archaeologists as the Cro-Magnons' skeletons are their tools, which are far more diverse in form and obvious in function than any in the earlier archaeological record. The tools suggest that modern anatomy had at last been joined by modern innovative behaviour. Many of the tools continued to be of stone, but they were now made from thin blades struck off a larger stone, thereby yielding ten times more cutting edge from a given quantity of raw stone than previously obtainable. Standardized bone and antler tools appeared for the first time. So did unequivocal compound tools of several parts tied or glued together, such as spear points set in shafts or axe-heads fitted on to wooden handles. Tools fall into many distinct categories whose function is often obvious, such as needles, awls, mortars and pestles, fishhooks, net-sinkers, and rope. The rope (used in nets or snares) accounts for the frequent bones of foxes, weasels, and rabbits at Cro-Magnon sites, while the rope, fishhooks, and net-sinkers explain the bones offish and flying birds at contemporary South African sites.

Sophisticated weapons for safely killing dangerous large animals at a distance now appear—weapons such as barbed harpoons, darts, spear-throwers, and bows and arrows. South African caves occupied by people now yield bones of such vicious prey as adult Cape buffalo and pigs, while European caves were full of bones of bison, elk, reindeer, horse, and ibex. Even today, hunters armed with high-powered telescopic rifles find it hard to bag some of these species, which must have required highly skilled communal hunting methods based on detailed knowledge of each species' behaviour.

Several types of evidence testify to the effectiveness of Late Ice Age people as big-game hunters. Their sites are much more numerous than those of earlier Neanderthals or Middle Stone Age Africans, implying more success at obtaining food. Numerous species of big animals that had survived many previous ice ages became extinct towards the end of the last Ice Age, suggesting that they were exterminated by human hunters' new skills. These likely victims include the mammoths of North America (Chapter Eighteen), Europe's woolly rhino and giant deer, southern Africa's giant buffalo and giant Cape horse, and Australia's giant kangaroos (Chapter Nineteen). Thus, the most brilliant moment of our rise already contained the seeds of what may yet prove a cause of our fall.

Improved technology now allowed humans to occupy new environments, as well as to multiply in previously occupied areas of Eurasia and Africa. Australia was first reached by humans around 50,000 years ago, implying watercraft capable of crossing stretches of water as much as sixty miles wide between eastern Indonesia and Australia. The occupation of northern Russia and Siberia by at least 20,000 years ago depended on many advances: tailored clothing, whose existence is reflected in eyed needles, cave paintings of parkas, and grave ornaments marking outlines of shirts and trousers; warm furs, indicated by fox and wolf skeletons minus the paws (removed in skinning and found in a separate pile); elaborate houses (marked by pestholes, pavements, and walls of mammoth bones), with elaborate fireplaces; and stone lamps to hold animal fat and light the long Arctic nights. The occupation of Siberia and Alaska in turn led to the occupation of North America and South America around 11,000 years ago (Chapter Eighteen). Whereas Neanderthals obtained their raw materials within a few miles of home, Cro-Magnons and their contemporaries throughout Europe practised long-distance trade, not only for raw materials for tools but also for 'useless' ornaments. Tools of high-quality stone such as obsidian, jasper, and flint are found hundreds of miles from where those stones were quarried. Baltic amber reached southeastern Europe, while Mediterranean shells were carried to inland parts of France, Spain, and the Ukraine. I saw very similar patterns in modern stone-age New Guinea, where cowry shells prized as decorations were traded up to the highlands from the coast, bird-of-paradise plumes were traded back down to the coast, and obsidian for stone axes was traded out from a few highly valued quarries.

The evident aesthetic sense reflected in the Late Ice Age trade in ornaments relates to the achievements for which we most admire the Cro-Magnons—their art. Best known, of course, are the rock paintings from caves like Lascaux, with stunning polychrome depictions of now-extinct animals, but equally impressive are the bas-reliefs, necklaces and pendants, fired-clay ceramic sculptures, Venus figurines of women with enormous breasts and buttocks, and musical instruments ranging from flutes to rattles.

Unlike Neanderthals, few of whom lived past the age of forty, some Cro-Magno" n skeletons indicate survival to sixty years of age. Many Cro-Magnons, but few Neanderthals, lived to enjoy their grandchildren. Those of us accustomed to getting our information from the printed page or television will find it hard to appreciate how important even just one or two elderly people are in a pre-literate society. In New Guinea villages it often happens that younger men lead me to the oldest person in the village when I stump them with a question about some uncommon bird or fruit. For example, when I visited Rennell Island in the Solomons in 1976, many islanders told me what wild fruits were good to eat, but only one old man could tell me.what other wild fruits could be eaten in an emergency to avoid starvation. He remembered that information from a cyclone that had hit Rennell in his childhood (around 1905), destroying gardens and reducing his people to a state of desperation. One such person in a pre-literate society can thus spell the difference between death and survival for the whole society. Hence the fact that some Cro-Magnons survived twenty years longer than any Neanderthal probably played a big role in Cro-Magnon success. As we shall see in Chapter Seven, living to an older age required not only improved survival skills but also some biological changes, possibly including the evolution of human female menopause.

I have described the Great Leap Forward as if all those advances in tools and art appeared simultaneously 40,000 years ago. In fact, different innovations appeared at different times. Spear-throwers appeared before harpoons or bows and arrows, while beads and pendants appeared before cave paintings. I have also described the changes as if they were the same everywhere, but they were not. 'Among Late Ice Age Africans, Ukrainians, and French, only the Africans made beads out of ostrich eggs, only the Ukrainians built houses out of mammoth bones, and only the French painted woolly rhinos on cave walls. These variations of culture in time and space are totally unlike the unchanging monolithic Neanderthal culture. They constitute the most important innovation that came with our rise to humanity: namely, the capacity for innovation itself. To us today, who cannot picture a world in which Nigerians and Latvians in 1991 have virtually the same possessions as each other and as the Romans in 50 BC, innovation is utterly natural. To Neanderthals, it was evidently unthinkable.

Despite our instant sympathy with Cro-Magnon art, their stone tools and hunter-gatherer lifestyle make it hard for us to view them as other than primitive. Stone tools evoke cartoons of club-waving cavemen uttering grunts as they drag a woman off to their cave. We can form a more accurate impression of Cro-Magnons if we imagine what future archaeologists will conclude after excavating a New Guinea village site from as recently as the 1950s. The archaeologists will find a few simple types of stone axes. Virtually all other material possessions were made of wood and will have perished. Nothing will remain of the multistorey houses, beautifully woven baskets, drums and flutes, outrigger canoes, and world-quality painted sculpture. There will be no trace of the village's complex language, songs, social relationships, and knowledge of the natural world.

New Guinea material culture was until recently 'primitive' (that is, stone-age) for historical reasons, but New Guineans are fully modern humans. New Guineans whose fathers lived in the Stone Age now pilot aeroplanes, operate computers, and govern a modern state. If we could carry ourselves back 40,000 years in a time machine, I expect that we Would find Cro-Magnons to be equally modern people, capable of learning to fly a jet plane. They made stone and bone tools only because no other tools had yet been invented; that is all that they had the opportunity to learn.

It used to be argued that Neanderthals evolved into Cro-Magnons within Europe. That possibility now seems increasingly unlikely. The last Neanderthal skeletons from around 40,000 years ago were still 'fullblown' Neanderthals, while the first Cro-Magnons appearing in Europe at the same time were already anatomically fully modern. Since anatomically modern people were already present in Africa and the Near East tens of thousands of years earlier, it seems much more likely that anatomically modern people invaded Europe from that direction than that they evolved within Europe.

What happened when invading Cro-Magnons met the resident Neanderthals? We can be certain only of the end result: within a short time, no more Neanderthals. The conclusion seems to me inescapable that Cro-Magnon arrival somehow caused Neanderthal extinction. Yet many archaeologists recoil at this conclusion and invoke environmental changes instead. For example, the Encyclopaedia Britannica's fifteenth edition concludes its entry for Neanderthals with the sentence, 'The disappearance of the Neanderthals, although it cannot yet be fixed in time, was probably the result of being creatures of an interglacial period unable to avoid the ravages of another Ice Age. In fact, Neanderthals thrived during the last Ice Age, and suddenly disappeared over 30,000 years after its start and an equal time before its end.

My guess is that events in Europe at the time of the Great Leap Forward were similar to events that have occurred repeatedly in the modern world, whenever a numerous people with more advanced technology invades the lands of a much less numerous people with less advanced technology. For instance, when European colonists invaded North America, most North American Indians proceeded to die of introduced epidemics; most of the survivors were killed outright or driven off their land (Chapter Sixteen); some of the survivors adopted European technology (horses and guns) and resisted for some time; and many of the remaining survivors were pushed on to lands that Europeans did not want, or else intermarried with Europeans (Chapter Fifteen). The displacement of Aboriginal Australians by European colonists, and of southern African San populations (Bushmen) by invading iron-age Bantu-speakers, followed a similar course. By analogy, I guess that Cro-Magnon diseases, murders, and dis-44-JUST ANOTHER SPECIES OF BIG MAMMAL placements did in the Neanderthals. If so, then the Cro-Magnon/ Neanderthal transition was a harbinger of what was to come, when the victors' descendants began squabbling among themselves. It may at first seem paradoxical that Cro-Magnons prevailed over the far more muscular Neanderthals, but weaponry rather than strength would have been decisive. Similarly, it's not gorillas that are now threatening to exterminate humans in Central Africa, but vice versa. People with huge muscles require lots of food, and they therefore gain no advantage if slimmer, smarter people can use tools to do the same work.

Like the Great Plains Indians of North America, some Neanderthals may have learned some Cro-Magnon ways and resisted for a while. This is the only sense I can make of a puzzling culture called the Chatel-perronian, which coexisted in Western Europe along with a typical Cro-Magnon culture (the so-called Aurignacian culture) for a short time after Cro-Magnons arrived. Chatelperronian stone tools are a mixture of typical Neanderthal and Cro-Magnon tools, but the bone tools and art typical of Cro-Magnons are usually lacking. The identity of the people who produced Chatelperronian culture was debated by archaeologists, until a skeleton unearthed with Chatelperronian artifacts at Saint-Cesaire in France proved to be Neanderthal. Perhaps, then, some Neanderthals managed to master some Cro-Magnon tools and hold out longer than their fellows. What remains unclear is the outcome of the interbreeding experiment posed in science-fiction novels. Did some invading Cro-Magnon men mate with some Neanderthal women? No skeletons that could reasonably be considered Neanderthal/Cro-Magnon hybrids are known. If Neanderthal behaviour was as relatively rudimentary, and Neanderthal anatomy as distinctive, as I suspect, few Cro-Magnons may have wanted to mate with Neanderthals. Similarly, although humans and chimps continue to coexist today, I am not aware of any matings. While Cro-Magnons and Neanderthals were not nearly as different, the differences may still have been a mutual turn-off. And if Neanderthal women were geared for a twelve-month pregnancy, a hybrid foetus might not have survived. My inclination is to take the negative evidence at face value, to accept that hybridization occurred rarely if ever, and to doubt that living people of European descent carry any Neanderthal genes.

So much for the Great Leap Forward in Western Europe. The replacement of Neanderthals by modern people occurred somewhat earlier in Eastern Europe, and still earlier in the Near East, where possession of the same area apparently shifted back and forth between Neanderthals and modern people from 90,000 to 60,000 years ago. The slowness of the transition in the Near East, compared to its speed in western Europe, suggests that the anatomically modern people living around the Near East before 60,000 years ago had not yet developed the modern behaviour that ultimately let them drive out the Neanderthals.

Thus, we have a tentative picture of anatomically modern people arising in Africa over 100,000 years ago, but initially making the same tools as Neanderthals and having no advantage over them. By perhaps 60,000 years ago, some magic twist of behaviour had been added to the modern anatomy. That twist (of which more in a moment) produced innovative, fully modern people who proceeded to spread westward into Europe, quickly supplanting Europe's Neanderthals. Presumably, those modern people also spread east into Asia and Indonesia, supplanting the earlier people there of whom we know little. Some anthropologists think that skull remains of those earlier Asians and Indonesians show traits recognizable in modern Asians and Aboriginal Australians. If so, the invading moderns may not have exterminated the original Asians without issue, as they did the Neanderthals, but instead interbred with them.

Two million years ago, several proto-human lineages had coexisted side by side until a shake-up left only one. It now appears that a similar shake-up occurred within the last 60,000 years, and that all of us alive in the world today are descended from the winner of that upheaval. What was the last missing ingredient whose acquisition helped our ancestor to win?

The identity of that ingredient poses an archaeological puz/le without an accepted answer. To help focus our speculations, let me recapitulate the pieces of the puzzle.

Some groups of humans who lived in Africa and the Near East over 60,000 years ago were quite modern in their anatomy, as far as can be judged from their skeletons, but they were not modern in their behaviour. They continued to make Neanderthal-like tools and to lack innovation..The ingredient that produced the Great Leap Forward does not show up in fossil skeletons. There is another way to restate that puzzle. We share ninety-eight per cent of our genes with chimpanzees (Chapter One). The Africans making Neanderthal-like tools just before our sudden rise to humanity had covered almost all of the remaining genetic distance between us and chimps, to judge from their skeletons. Perhaps they shared 99.9 % of their genes with us. Their brains were as large as ours, and Neanderthals' brains were even slightly larger. The missing ingredient may have been a change in only 0.1 % of our genes. What tiny change in genes could have had such enormous consequences?

Like some other scientists who have speculated about this question, I can think of only one plausible answer: the anatomical basis for spoken complex language. Chimpanzees, gorillas, and even monkeys are capable of symbolic communication not dependent on spoken words. Both chimpanzees and gorillas have been taught to communicate by means of sign language, and chimpanzees have learned to communicate via the keys of a large computer-controlled console. Individual apes have thus mastered 'vocabularies' of hundreds of symbols. While scientists argue over the extent to which such communication resembles human language, there is little doubt that it constitutes a form of symbolic communication. That is, a particular sign or computer key symbolizes a particular something else.

Primates can use not only signs and computer keys, but also sounds, as symbols. For instance, wild vervet monkeys have a natural form of symbolic communication based on grunts, with slightly different grunts to mean leopard', 'eagle', and 'snake'. A month-old chimpanzee named Viki, adopted by a psychologist and his wife and reared virtually as their daughter, learned to 'say' approximations of four words: 'papa', 'mama', 'cup', and 'up'. (The chimp breathed rather than spoke those words.) Given this capability for symbolic communication using sounds, why have apes not gone on to develop much more complex natural languages of their own? The answer seems to involve the structure of the larynx, tongue, and associated muscles that give us fine control over spoken sounds. Like a Swiss watch, all of whose many parts have to be well-designed for the watch to keep time at all, our vocal tract depends on the precise functioning of many structures and muscles. Chimps are thought to be physically incapable of producing several of the commonest human vowels. If we too were limited to just a few vowels and consonants, our own vocabulary would be greatly reduced. For example, take this paragraph, convert all vowels other than 'a' or 'i' to either of those two, convert all consonants other than 'd' or 'm' or V to one of those three, and then see how much of the paragraph you can still understand. Therefore, the missing ingredient may have been some modifications of the proto-human vocal tract to give us finer control and permit formation of a much greater variety of sounds. Such fine modifications of muscles need not be detectable in fossil skulls.

It is easy to appreciate how a tiny change in anatomy resulting in capacity for speech would produce a huge change in behaviour. With language, it takes only a few seconds to communicate the message, 'Turn sharp right at the fourth tree and drive the male antelope towards the reddish boulder, where I'll hide to spear it. Without language, that message could be communicated only with difficulty, if at all. Without language, two proto-humans could not brainstorm together about how to devise a better tool, or about what a cave painting might mean. Without language, even one proto-human would have had difficulty thinking out for himself or herself how to devise a better tool.

I do not suggest that the Great Leap Forward began as soon as the mutations for altered tongue and larynx anatomy arose. Given the right anatomy, it must have taken humans thousands of years to perfect the structure of language as we know it—to arrive at the concepts of word order and case endings and tenses, and to develop vocabulary. In Chapter Eight I shall consider some possible stages by which our language might have become perfected. But if the missing ingredient did consist of changes in our vocal tract that permitted fine control of sounds, then the capacity for innovation would follow eventually. It was the spoken word that made us free. This interpretation seems to me to account for the lack of evidence for Neanderthal/Cro-Magnon hybrids. Speech is of overwhelming importance in the relations between men and women and their children. That is not to deny that mute or deaf people learn to function well in our culture, but they do so by learning to find alternatives for a spoken language that already exists. If Neanderthal language was much simpler than ours or non-existent, it is not surprising that Cro-Magnons did not choose to marry Neanderthals.

I have argued that we were fully modern in anatomy and behaviour and language by 40,000 years ago, and that a Cro-Magnon could have been taught to fly a jet aeroplane. If so, why did it take so long after the Great Leap Forward for us to invent writing and build the Parthenon? The answer may be similar to the explanation why the Romans, great engineers that they were, didn't build atomic bombs. To reach the point of building an A-bomb required two thousand years of technological advances beyond Roman levels, such as the invention of gunpowder and calculus, the development of atomic theory, and the isolation of uranium. Similarly, writing and the Parthenon depended on tens of thousands of years of cumulative developments after the arrival of Cro-Magnons—developments that included the bow and arrow, pottery, domestication of plants and animals, and many others.

Until the Great Leap Forward, human culture had developed at a snail's pace for millions of years. That pace was dictated by the slow rate of genetic change. After the Leap, cultural development no longer depended on genetic change. Despite negligible changes in our anatomy, there has been far more cultural evolution in the past 40,000 years than in the millions of years before. Had a visitor from outer space come to the Earth in Neanderthal times, humans would not have stood out as unique among the world's species. At most, the visitor might have mentioned humans along with beavers, bowerbirds, and army ants as examples of species with curious behaviour. Would the visitor have foreseen the change that would soon make us the first species, in the history of life on Earth, capable of destroying all life?