1. Charles Darwin (1809-1882). The Diversity Paradox.

In the Pirandello play, "Six Characters in Search of an Author", the six characters are actors who arrive at a theater to begin rehearsing a play. The theater manager apologetically informs them that there has been a misunderstanding and he has no play for them to rehearse. He begs the actors to go home. But the leading actor refuses to leave and starts improvising a play, making up the story as he goes along. One by one, the other actors join in, each of them pushing the story in a different unexpected direction. At the end of the performance, all the actors are fully engaged, and together they bring the story to a dramatic climax. I have borrowed Pirandello's title, and used his characters, as a metaphor for the pioneers in our understanding of the concept of evolution over the last two centuries.

Until recently, evolution was considered to be a biological process, driven by the slowly acting forces of speciation and extinction. Speciation is the birth of new species by splitting of an existing species into genetically isolated populations that do not interbreed. Extinction is the disappearance of a species that dies out without leaving descendants. Our first character, Charles Darwin, published his great work, The Origin of Species, in 1859. He demonstrated, with a wealth of evidence, from observations of species in the wild and from the effects of selective breeding of plants and animals, that natural selection is [a] powerful force driving evolution. His book made a stronger statement, that natural selection is [the] cause of evolution. The difference between [a] and [the] was hardly noticed by the readers of his book. His theory triumphed and became for a hundred years the view of evolution accepted by almost all biologists and by the majority of educated people.

Darwin himself was well aware that nature contains many mysteries that his theory does not easily explain. There is a mismatch between the real world, with its amazing richness of diverse species, many of them obviously burdened with superfluous flowers and feathers, and the theoretical world of Darwinian evolution in which only the fittest should survive. Naively, we should expect Darwinian evolution to result in a world with a much smaller number of species, each selected by superior fitness to be a winner in the game of survival. All through his life, Darwin was puzzled by the abundance of weird and wonderful species that look like losers but still survive. I call this abundance the diversity paradox.   If only the fittest survive, we should expect to find a few hundred superbly fit species adapted to live in various habitats. Darwin looked at the real world and found an extravagant display of species, with a great diversity of superficial differences. He saw elaborate structures that are expensive to maintain. The theory of evolution by natural selection should tend to keep creatures plain and simple, but nature appears to prefer structures that are elegant and complicated.

Darwin understood that sexual reproduction is a powerful cause of diversity of species. For a sexual species to exist and survive, it is advantageous to have distinctive ornamentation of one sex, usually the male, and a strong preference of the other sex for a mate with that particular ornament. The mating system causes the population possessing it to be genetically isolated from other related populations. The mating system becomes a genetic barrier, creating a new species and maintaining its identity. A species like the bird of paradise with an elaborate mating system may derive enough advantage from the uniqueness of the system to pay for the cost of the feathers. Another cause of diversity of species is symbiosis, enabling two or more species to help each other to survive or to reproduce. A conspicuous example of symbiosis leading to diversity is the simultaneous evolution of flowering plants and insects. Another example is the coral reef and the reef-fish. Darwin concluded that sexual selection and symbiotic coevolution would explain the overall diversity of natural ecologies. But he had no hard evidence to justify this conclusion. We now know that he was mistaken. Another cause of diversity, of which he had no conception, also plays a dominant role in natural evolution.

Darwin knew nothing of genes. He was unaware of the work of Gregor Mendel, the Austrian monk who worked in his monastery garden and did experiments on the inheritance of pod-color in peas. Mendel discovered that heritable traits such as pod-color are inherited in discrete packages which he called genes. Any act of sexual reproduction of two parents with different genes results in offspring with a random distribution of the parental genes. Heredity in any population is a random process, resulting in a redistribution of genes between parents and offspring. The numbers of genes of various types are maintained on the average from generation to generation, but the numbers in each individual offspring are random. Mendel made this discovery and published it in the journal of the Brünn Natural History Society, only seven years after Darwin published The Origin of Species. Mendel had read Darwin's book, but Darwin never read Mendel's paper. In 1866, the year when Mendel's paper was published, Darwin did a very similar experiment, using snap-dragons instead of peas, and testing the inheritance of flower-shape instead of pod-color. Like Mendel, he bred three generations of plants, and observed the ratio of normal-shaped to star-shaped flowers in the third generation. Unlike Mendel, he had no understanding of the mathematics of statistical variations. He used only 125 third-generation plants and obtained a value of 2.4 for the ratio of normal to star-shaped offspring. This result did not suggest any clear picture of the way flower-shapes are inherited. He stopped the experiment and explored the question no further. Darwin did not understand that he would need a much larger sample to obtain a statistically significant result. Mendel understood statistics. His sample was sixty-four times larger than Darwin's, so that his statistical uncertainty was eight times smaller. He used 8023 plants.

Mendel's essential insight was to see that sexual reproduction is a system for introducing randomness into inheritance. In peas as in humans, inheritance is carried by genes that are handed down from parents to offspring. His simple theory of inheritance carried by genes predicted a ratio of three between green and yellow pods in the third generation. He found a ratio of 3.01 with the big sample. This gave him confidence that the theory was correct. His experiment required immense patience, continuing for eight years with meticulous attention to detail. Every plant was carefully isolated to prevent any intruding bee from causing an unintended fertilization. A monastery garden was an ideal place for such experiments. Unfortunately, his experiments ended when his monastic order promoted him to the rank of abbot. Obedient to his vows, he ceased to be an explorer and became an administrator. His life-work lay hidden in an obscure German-language journal in Brünn, the city that later became Brno and is now in the Czech Republic.

The idea of genes remained generally unknown to biologists until twenty years after Darwin's death. Darwin imagined various ways of mixing inherited traits of parents and distributing them to offspring, but he never imagined genes. Without the concept of genes, it was impossible for him to calculate correctly the rates of speciation and extinction in any natural population. He never attempted such calculations. If he had made such calculations with a model of inheritance based on mixing, he would have got drastically wrong answers. He had the good sense not to make such calculations without a verified model of inheritance. Without experimental knowledge of the statistics of inheritance, he had no way to guess reliably how effective natural selection could be in creating new species and exterminating old ones.

2. Motoo Kimura (1924-1994). Smaller Populations Evolve Faster.

At this point in the play, our second character enters, Motoo Kimura, author of the book, The Neutral Theory of Molecular Evolution, published in 1983, more than a hundred years after Darwin's masterpiece. Kimura was a Japanese geneticist who came as a student to work with Sewall Wright at the University of Wisconsin. Sewall Wright was one of the biologists who explored the evolutionary implications of Mendel's discovery after Mendel's paper was rediscovered in 1900. I was lucky to meet Sewall Wright accidentally at the faculty club at the University of Wisconsin in 1987. I was visiting the University and went to the faculty club for lunch. Sitting alone at a small table was a lively old man who turned out to be Sewall Wright, then 98 years old but still in full possession of his wits. He gave me a first-hand account of how he read Mendel's paper and decided to devote his life to understanding the consequences of Mendel's ideas. Wright understood that the inheritance of genes would cause a fundamental randomness in all evolutionary processes. The phenomenon of randomness in evolution was called Genetic Drift. Kimura came to Wisconsin to learn about Genetic Drift, and then returned to Japan. He built Genetic Drift into a mathematical theory which he called the Neutral Theory of Molecular Evolution.

After the discovery of the structure of DNA molecules by Crick and Watson in 1953, Kimura knew that genes are molecules, carrying genetic information in a simple code. His theory applied only to evolution driven by the statistical inheritance of molecules. He called it the Neutral Theory because it introduced Genetic Drift as a driving force of evolution independent of natural selection. I never met Kimura, but he was still alive when I began to study his work, and I was delighted to receive a personal message of encouragement from him before he died in 1994.

Kimura did not prove that Darwin's theory was wrong. He proved that Darwin's theory was incomplete. Darwin missed Genetic Drift, which was sometimes important and sometimes unimportant. The evolutionary effects of natural selection are generally independent of the size of the evolving population, while the effects of genetic drift depend strongly on population size. Other things being equal, the rate of genetic drift is proportional to the inverse square-root of population size. The inverse square-root is a simple consequence of the statistics of independent random variables. The average of any N independent random variables varies at a rate inversely proportional to the square-root of N. For any firmly established species with a population measured in millions or billions, genetic drift is negligible, natural selection is dominant, and the Darwin theory is accurately valid. For a newly emerging or endangered species with population measured in tens or hundreds, genetic drift dominates, selection is relatively unimportant, and the Kimura theory is valid. The random jumps of genes in a small population produce evolutionary change much faster than the gentle push of natural selection. Kimura understood that genetic drift is the main driving force in the quick jumps when species are created or extinguished.

Kimura's theory explains the diversity paradox that puzzled Darwin. Why are we surrounded by such an astonishing diversity of birds and insects and microbes? From the point of view of Darwin, a small number of dominant species would have been sufficient. Kimura explains the mystery by invoking the power of genetic drift, which becomes suddenly rapid and effective just when it is needed, when small populations can vary fast enough to become genetically isolated and form new species.

Genetic drift in local enclaves gives to every large established species the power to diversify into a family of new species. At the ragged edges of small populations, where random jumps prevail, speciation is driven by Kimura's neutral theory. Darwin's theory is still true away from the edges, where selection has time to operate on big populations.

3. Ursula Goodenough (1943- ). Nature Plays a High-Risk Game.

After Kimura, our third character enters the play. She is Ursula Goodenough, a biologist born in 1943 and still active at Washington University in St. Louis. Like Darwin and unlike Kimura, she is an observer and experimenter. She gave us another important insight into the mystery of diversity. She analyzed published reports on the rate of random genetic mutation in genes of various kinds in many different sexually reproducing species, from algae to mammals. She and others noted that in a large number of species there are two families of genes that have mutation-rates much higher than average genes. The two families both have specialized functions. One family is genes involved with the immune system. There is an obvious reason for immune-function genes to mutate rapidly, since they must respond rapidly with production of fresh antibodies to detect and kill invading microbes.

The other rapidly mutating family of genes is involved with sexual mating systems. Goodenough observed a systematic tendency for genes active in the rituals of mating to mutate fast. The reason for this accelerated variation of mating genes is not obvious. Nature is forcing genetic drift to move faster in mating systems than in other bodily functions. If this is generally true, as Goodenough observes, it means that genetic drift in mating systems must have a special importance as a driving force of evolution. She proposes a general theory to explain the facts. In the big picture of life evolving over billions of years, established species with large populations evolve slowly and have a mainly conservative effect on the balance of Nature. The big jumps in evolution occur when established species become extinct and new species with small populations diversify. The big jumps, made by new species, are driven by genetic drift of small populations. For small populations to form new species, they must become genetically isolated. Rapid change of mating systems is a quick road to genetic isolation. Goodenough concludes that the rapid mutation of mating-system genes is Nature's way of achieving big jumps in large-scale evolution. Rapidly evolving mating systems gave us the diversity of species that astonished Darwin. Twenty years ago, Goodenough wrote a paper with the title, "Rapid Evolution of Sex-related Genes", describing her observations and conclusions. I consider this paper a great piece of work, a classic contribution to science, comparable with the books of Darwin and Kimura.

The picture of Nature revealed by Kimura and Goodenough is new and striking. Nature loves to gamble. Nature thrives by taking risks. She scrambles mating-system genes so as to increase the risk that individual parents will fail to find mates. The increase of risk of sterility of individuals is a part of Nature's plan. She imposes the increased risk on the whole population, so that a rare event will occur with greater probability, when a pair of lucky parents, whose names might happen to be Adam and Eve, are born with matching mating-system mutations. That rare event gives a pair of parents a chance to give birth to a new species. Nature knows how to play the odds. By putting her thumb on the mating-system mutation scale, she increases the risk of sterility of all parents, and increases the chance that a lucky pair will start a new species. Nature knows that, in the long run, established species are expendable and new species are essential. That is why Nature is ruthless to the individual parent and generous to the emerging species. Risk-taking is the key to long-term survival and is also the mother of diversity.

4. Herbert Wells (1866-1946). Varieties of Human Experience.

With three characters on stage, it would appear that our play is coming close to an end. Then a fourth character bursts in, jumping back a hundred years into the past and telling a different story. His name is Herbert Wells, born in 1866, educated as a biologist but using his knowledge to give us a fresh view of evolution. The first three characters thought of evolution as a biological process, governed by the rules of inheritance from parent to offspring. Wells knew that biological evolution is only half of a bigger story. The other half of the story is cultural evolution, the story of changes in the life of our planet caused by the spread of ideas rather than by the spread of genes. Cultural evolution had its beginnings as soon as animals with brains evolved, using their brains to store information and using patterns of behavior to share information with their offspring. Social species of insects and mammals were molded by cultural as well as biological evolution. But cultural evolution only became dominant when a single species invented spoken language. Spoken language is incomparably nimbler than the language of the genes.

Wells saw that we happen to live soon after a massive shift in the history of the planet, caused by the emergence of our own species. The shift was completed about ten thousand years ago, when we invented agriculture and started to domesticate animals. Before the shift, evolution was mostly biological. After the shift, evolution was mostly cultural. Biological evolution is usually slow, when big populations endure for thousands or millions of generations before changes become noticeable. Cultural evolution can be a thousand times faster, with major changes occurring in two or three generations. It has taken about two hundred thousand years for our species to evolve biologically from its origin in Africa until today. It has taken only about two hundred years of cultural evolution to convert us from farmers to city-dwellers, and to convert a large part of North America from forest to farmland.

Besides his expert knowledge of biology, Wells had a deep interest in the lives of ordinary human beings, with their destiny governed by ancient human emotions of love and hate, fear and greed. He began his professional life as a novelist, telling stories and bringing his characters vividly to life. His view of the human condition can be seen more clearly in his novels than in his biology. One of his novels is Tono-Bungay, written in 1912. The narrator is George Ponderevo, a young and capable crook who is at home in the chaotic world of twentieth century capitalism. The chief character is uncle Teddy Ponderevo, an amiable swindler who invented the wonder-drug Tono-Bungay, guaranteed to cure all diseases and to bring us health and happiness. George knows how to keep the cash flowing, with raucous advertising campaigns and sales of shares in fraudulent companies.

For a while, the Tono-Bungay bubble makes them rich. Then the bubble bursts, and they are hunted criminals. Uncle Teddy dies in the crash of a home-made air-ship. George escapes in a private war-ship that he happens to own. The last chapter is entitled, "Night and the Open Sea", with George's ship swiftly and silently slicing through the dark waves. Wells is writing with a premonition of the horrors of World War One, which broke out two years later, destroying millions of people who would sacrifice their lives to the tribal gods of Empire and Country. The owners of war-ships would survive to find new victims.

Another novel, The Time Machine, is concerned directly with evolution. The Time Traveler finds himself in the future, eight hundred thousand years from now. Wells draws one of the bleakest pictures of the future ever imagined. Humans have evolved downhill into two degenerate species, predators and prey, with diminished bodies and minds. The predators are the Morlocks, living like rats in the cellars of ruined buildings. The prey are the Eloi, living aimless lives on the surface in beautiful surroundings, tended like cattle by the Morlocks as a source of meat. The Time Traveler befriends an Eloi girl who gives him two flowers to take home with him. The story ends with the Time Traveler vanished on another trip into the future, leaving behind the two withered flowers. The flowers are our proof that, even after the spark of reason has been extinguished, friendship and gratitude can live on in the human soul.

Wells's biggest work is Outline of History, published in 1920, a picture of cultural evolution as the main theme of history since the emergence of our species. He begins with a proud claim: "This Outline of History is an attempt to tell, truly and clearly, in one continuous narrative, the whole story of life and mankind so far as it is known today." The next fifty pages describe biological evolution up to the rise of two human species, Modern Man and Neanderthal Man. A famous picture by the illustrator John Horrabin shows Wells's literary rival George Bernard Shaw as a Neanderthal emerging from a cave, with the caption, "Our Neanderthal Ancestor, Not a Neanderthal Man but a Parallel Species". The recent discovery of a substantial fraction of Neanderthal genes in modern Europeans shows that Wells's joke came close to the truth. After the Neanderthals come the cave-painters in France and Spain. Cultural evolution has begun and dominates the story from that time onward.

Half-way through the history comes the birth of the great world religions, Buddhism, Judaism, Christianity, Islam. Wells tells the story of these religions with a sympathetic eye, recognizing their crucial importance to cultural evolution in the last two thousand years. He gives an evocative account of the life and death of Jesus, with a memorable Horrabin illustration, three crosses on the hill of Golgotha in evening twilight. The caption reads; "The darkness closed upon the hill; the distant city set about its preparation for the Passover; scarcely anyone but that knot of mourners on the way to their homes troubled whether Jesus of Nazareth was still dying or already dead". From Golgotha the story continues with empires rising and falling, wars and pestilences raging, wealth and industry growing, and always quietly in the background the great religions with their holy books preserving the words of the prophets, raising the hopes of powerless people with visions of a better world.

The history ends with the catastrophe of World War One, and with the abortive attempt, still in progress while Wells was writing, to establish a League of Nations with effective power to keep the world at peace. Here is the message of the Outline of History as Wells saw it. "Life begins perpetually. Gathered together at last under the leadership of man, the student-teacher of the universe, unified, disciplined, armed with the secret powers of the atom and with knowledge as yet beyond dreaming, life, forever dying to be born afresh, forever young and eager, will presently stand upon this earth as upon a footstool, and stretch out its realm amid the stars."

As a result of cultural evolution, a single species now dominates the ecology of our planet, and cultural evolution will dominate the future of life so long as any species with a living culture survives. When we look ahead to imagine possible futures for our descendants, cultural evolution must be our dominant concern. But biological evolution has not stopped and will not stop. As cultural evolution races ahead like a hare, biological evolution will continue its slow tortoise crawl to shape our destiny.

We have detailed knowledge of our cultural evolution only for the last few thousand years in Europe and Asia from which written records survive. I am ignorant of Chinese history and literature, so I discuss only the Western part of the story. In Western culture we see a series of creative events occurring in small urban communities: Jerusalem around 1000 BC inventing monotheistic religion, Athens around 500 BC inventing philosophy and drama and democratic government, Florence around 1500 AD inventing modern art and science, Manchester around 1750 AD inventing modern industry. In each case, a small population produced a star-burst of pioneers who permanently changed our way of thinking. Genius erupted in groups as well as in individuals. It seems likely that these bursts of creative change were driven by a combination of cultural with biological evolution. Cultural evolution was constantly spreading ideas and skills from one community to another, stirring up conservative societies with imported novelties. At the same time, biological evolution acting on small genetically isolated populations was causing genetic drift, so that the average intellectual endowment of isolated communities was rising and falling by random chance.

Over the last few thousand years, genetic drift caused occasional star-bursts to occur, when small populations rose to outstandingly high levels of average ability. The combination of imported new ideas with peaks of genetic drift would enable local communities to change the world.

The big uncertainty in this picture of genetic drift as a driving force of human progress is the genetic isolation of small communities. We have no reliable information about the mating habits of the populations in Jerusalem and Athens and Florence and Manchester during the centuries before they became creative. They were to some extent isolated geographically, but they were also divided into tribes and hereditary classes that were isolated socially. Class prejudice and snobbery were probably the most powerful causes of genetic isolation, and these are not measurable quantities. The contribution of genetic drift to cultural evolution remains a speculative hypothesis.

When we look to the future of evolution, it is convenient to divide the future into near and far. The near future is the next century, for which we can make some reliable predictions. The far future is everything beyond the next century. In the near future, we can be sure that genetic drift is fading rapidly as a driving force of change. All over the world, humans are moving from villages into big cities where genetic drift is negligible. In the populations that are still geographically isolated, humans are becoming less socially isolated by barriers of race and class. It is unlikely that any small town in the next century can be another Athens or another Jerusalem. Wells ended his Outline with a glimpse of the far future, where nothing is certain, and all predictions are guesswork. In the far future, it is likely that humans and other forms of life will be spread out to great distances in the universe, as Wells imagined. If our destiny takes us to the stars, our descendants will again be genetically isolated, and genetic drift will resume its ancient power to mold life into new patterns of diversity.

Before we can embark on grand voyages to the stars, we must navigate the mundane hazards of the twenty-first century. The most important achievement of the twenty-first century is likely to be the emergence of China as a rich country and a world power. The rise of China is a return to the political patterns of the past, when China was a great empire ruled by a conservative Confucian bureaucracy. The intervening five hundred years, when China was left behind and impoverished by aggressively expanding Western cultures, were an unfortunate departure from the older stable equilibrium. The rise of China in this century will be a restoration of traditionally organized society after centuries of turmoil. The big problem for Western societies will be to learn how to coexist peacefully with the new Celestial Kingdom. Fortunately, we will have the powerful force of cultural evolution erasing differences between East and West. Cultural evolution must battle against the divisive forces of nation and race and political ideology.

The strongest driving force of cultural evolution today is science. Science is the international enterprise that brings us together most powerfully in a common purpose, requiring us to share ideas and tools, economic resources and material benefits. The task of East and West in this century will be to work together as friends in science and technology, while respecting our differences in politics and culture.

When we look to the future beyond one or two centuries, expansion of the domain of life into the universe will be inevitable and also desirable for many reasons. Inevitable because biotechnology and space technology will provide the means for life to make the big jump. Desirable because the cultural evolution of creative new societies requires more elbow-room than a single planet can provide. Creative new societies need room to take risks and make mistakes, far enough away to be effectively isolated from their neighbors. Life must spread far afield to continue the processes of genetic drift and diversification of species that drove evolution in the past. The restless wandering that pulled our species out of Africa to explore the Earth will continue to pull us beyond the Earth, as far as our technology can reach.

5. Richard Dawkins (1941- ). Genes and Memes.

Wells has been monopolizing the stage for far too long, and it is time for our fifth actor, Richard Dawkins, to have his turn. Wells was at heart a novelist, portraying history as a story of human beings with ideas and emotions as well as neurons and genes. Dawkins is a biologist who began his career with a study of animal behavior, only later transferring his attention to humans. Dawkins published his great work, The Selfish Gene, in 1976, He is interested in general patterns of behavior rather than in individual humans. His book portrays human society as a mechanical system of agents with behavior governed by genes, similar to a collection of machines with behavior governed by computer programs. The selfish gene is a device with a single purpose, to achieve its own survival and replication. It is not concerned with our welfare or with our human needs. Dawkins caused a revolution in our thinking about genes with his insight that the selfish behavior of genes can explain the unselfish behavior of humans. His book is a classic because he makes a convincing case for a paradoxical conclusion, that selfish genes can orchestrate the evolution of cooperation, generosity and self-sacrifice in humans. He succeeds brilliantly in reducing our high moral principles and our ethical beliefs to the action of unthinking and uncaring molecules of DNA.

In the final chapter of his book, Dawkins turns his attention away from biological evolution to cultural evolution and introduces another innovation to our thinking about human behavior. The new idea is the meme, the cultural analog to the gene. A meme is a unit of cultural behavior, just as a gene is a unit of biological behavior. Examples of memes are ideas, customs, slogans, fashions in dress or in hair-style, skills, tools, laws, religious beliefs and political institutions. Memes spread through human populations by social contact far more rapidly than genes spread by sexual contact. Just as our behavior at the individual level is controlled by selfish genes, our behavior at the social level is controlled by selfish memes.

Dawkins's vision of human society, as the visible face of an invisible network of selfish genes and memes, is to a large extent true. His book gave us a new understanding of the evolution of morality and religion. Like Darwin's view of nature, Dawkins’s vision may be incomplete. It is reasonable to accept his view of genes and memes as powerful agents of human behavior, but to reject his view that they explain everything.

6. Svante Pääbo (1955- ). Cousins in the Cave.

Our sixth and last actor, Svante Pääbo, born in 1955 and now a world leader in the study of human genomes, comes to the stage with startling news. After long struggles, his team of paleontologists and chemists have developed the technology for sequencing ancient DNA degraded and contaminated with modern DNA. They have succeeded in sequencing accurately the genomes of our Neanderthal cousins who lived in Europe about fifty thousand years ago. They also sequenced genomes of our own species who lived in Europe around the same time, and genomes of a third species, called Denisovans because they were found in Denisova cave in Siberia. He published the story of the sequencing and the surprising results in his book, Neanderthal Man: In Search of Lost Genomes, in 2014.

When he compared the ancient genomes of the three species with modern human genomes, he saw abundant evidence of mixing. Modern humans originating in Europe and Asia carry several percent of Neanderthal genes. Modern humans in Papua New Guinea carry several percent of Denisovan genes. The ancient genomes come from times when the severe climate of the last ice age prevailed in Europe and Northern Asia. Humans and their cousins were precariously surviving in caves, where they probably sat huddled around the cave-fire to keep warm, cooking dinners and telling stories. It now appears that the three species frequently sat around the cave-fires together rather than separately. They mated and raised families together. Our species had the larger share of the populations and supplied most of the genes to the mixed offspring. But the Neanderthals and Denisovans never became extinct. They simply merged their genomes with ours. They survive as a part of our genetic inheritance.

The discoveries of Svante Pääbo show that as early as fifty thousand years ago the transition from biological to cultural evolution was already far advanced. Biological evolution, as demonstrated by Kimura and Goodenough, accelerated the birth of new species by favoring the genetic isolation of small populations. Cultural evolution had the opposite effect, erasing differences between related species and bringing them together. Cultural evolution happens when cousins learn each other's languages and share stories around the cave-fire. As a consequence of cultural evolution, biological differences become less important and cousins learn to live together in peace. Sharing of memes brings species together and sharing of genes is the unintended consequence.

In the long-range history of life, the transition from biological to cultural evolution was an event of transcendent importance. We became aware of its importance only recently, as a result of the discoveries of Svante Pääbo and his colleagues. The transition caused a reversal of the direction of evolution from diversification to unification, from the proliferation of diverging species to the union of species into a brotherhood of man. We see a small-scale example of this transition in the recent history of racism. Until recently, racism was a force of nature favoring the diversification of species. Humans traditionally hated and despised people of a different skin color. The natural evolutionary consequence would have been the division of our species into three new species, one pink, one black and one yellow. Only in the last few centuries, a strong reaction against racism has emerged, inter-racial marriage has become respectable, and the cultural unification of our species has pushed us toward biological unification. This is a small step in the long history of the transition of human societies from incessant warfare to brotherhood.

With our six actors all on stage, the play begins and my story ends. As an epilogue to the performance, I add some brief remarks about the practical lessons that we may learn from the story. Our species faces two great tasks in the next few centuries. Our first task is to make human brotherhood effective and permanent. Our second task is to preserve and enhance the rich diversity of Nature in the world around us. Our new understanding of biological and cultural evolution may help us to see more clearly what we have to do.

Nature's tool for the creation and support of a rich diversity of wildlife is the species produced in abundance by the rapid genetic drift of small populations according to Kimura, and in even greater abundance by the rapid mutation of mating-system genes according to Goodenough. In the near future, we will be in possession of genetic engineering technology which allows us to move genes precisely and massively from one species to another. Careless or commercially driven use of this technology could make the concept of species meaningless, mixing up populations and mating systems so that much of the individuality of species would be lost. Cultural evolution gave us the power to do this. To preserve our wildlife as nature evolved it, the machinery of biological evolution must be protected from the homogenizing effects of cultural evolution.

Unfortunately, the first of our two tasks, the nurture of a brotherhood of man, has been made possible only by the dominant role of cultural evolution in recent centuries. The cultural evolution that damages and endangers natural diversity is the same force that drives human brotherhood through the mutual understanding of diverse societies. Wells's vision of human history as an accumulation of cultures, Dawkins's vision of memes bringing us together by sharing our arts and sciences, Pääbo's vision of our cousins in the cave sharing our language and our genes, show us how cultural evolution has made us what we are. Cultural evolution will be the main force driving our future.

Our double task is now to preserve and foster both biological evolution as Nature designed it and cultural evolution as we invented it, trying to achieve the benefits of both, and exercising a wise restraint to limit the damage when they come into conflict. With biological evolution, we should continue playing the risky game that nature taught us to play. With cultural evolution, we should use our unique gifts of language and art and science to understand each other, and finally achieve a human society that is manageable if not always peaceful, with wildlife that is endlessly creative if not always permanent.