Darwin's Island (39 page)

The fate of Charles Darwin’s experimental subjects is part of a larger litany of decline. The Red List of Threatened Species has sixteen thousand names inscribed upon its pages and, every year, a spectacular creature - or several - is declared to be extinct. Many more pass almost unremarked and unmourned. If weeds and crops are to spread, others must pay the price. Most disappear as their homes are destroyed. Almost half the rain forest has gone, and mangrove swamps and Mediterranean landscapes face the same disaster with less publicity.

Extinction is part of evolution. Around half a million species of bird have lived since the group evolved more than a hundred million years ago, but not many more than ten thousand lived at a particular time - and that is the figure today (although twelve hundred of them are threatened). Even so, nobody can deny that we live in a time of rapid change, in which as some species thrive, many more are doomed.

One small and specialised group of mammals - until not long ago a vigorous part of the fauna of Europe, Africa and Asia - is under particular threat. The globe, once filled with our hairy relatives, will soon be a Planet of the Ape. Almost half the world’s six hundred or so species of monkeys and apes face disaster. Habitat destruction is the main threat, and hunters and disease also do a lot to kill them off. Every large primate, except for one, is close to the end of its evolutionary road. Just two hundred thousand chimps are left in the wild. Gorillas have gone even faster and in some places have died in multitudes from the Ebola virus, caught from humans. Many populations of the orang-utan have been broken up as the forest is destroyed and are already too small to sustain themselves.

The genes show that the fortunes of the various great apes have suffered a real reversal over the past several hundred thousand years. The amount of variation hidden in DNA says a lot about the abundance of any creatures in ancient times, for populations that stay small for many generations lose genes through the accidents of reproduction, while abundant animals can maintain a pool of diversity that persists for long periods even if they suffer a later collapse in numbers. The double helix hints that, for most of that period, orangs, chimps and gorillas flourished while
Homo sapiens
and his immediate ancestors struggled to survive. Chimps, threatened as they now are, are three times more different at the molecular level, each from the next, than are humans. Even within the past seventy thousand years,
Homo sapiens
went through a bottleneck of just two or three thousand individuals during a long age of drought. There have been plenty of local crises since then as man filled new continents and set foot on remote islands. For much of history we were an endangered primate while our relatives boomed. Now the shoe is on the other foot.

The world has six billion people and the figure will rise by half when the population peaks at about the time of
The Origin
’s bicentennial.
Homo sapiens
has, like the kudzu vine, the earthworm and the giant barnacle, begun to move and to multiply. As a result, its own future, like that of every other inhabitant of the planet, has been transformed. We may not have noticed it, but human evolution is in the middle of a shift as great as that of the species that surround us. We have long been the weediest primate of all and, in the past few years, have become far weedier than we were. History has always been made in bed, but the beds are closer together than they have ever been.

Evolution takes place in space as much as in time. As Darwin saw on the Galapagos - and as is manifest in the geography of human genes - populations isolated from each other diverge, either in response to local forces of natural selection or by random change. Other genetic trends have emerged over just the past few thousand years through the accidents of settlement, with reduced levels of variation in newly settled places such as the far Pacific, or even the New World as a whole. Ancient patterns, the remnants of the migration of peoples from the Levant to the British Isles, have been preserved only through the preference of most people to stay at home. That history will soon be lost.

How far was your birthplace from that of your partner and how far apart were your mother and father, and your grandmother and grandfather on each side, born? In almost every case, the distance has increased over the generations and continues to do so (my wife and I first saw the light five thousand kilometres apart, my mother and father about five; as my students say, it shows). Even in the Middle East, that centre of sexual conservatism, education, affluence and the chance to travel mean that DNA is on the move. A series of Israeli Arab villages experienced a drop in the incidence of cousin marriages from about a quarter in the 1980s to a sixth at the millennium. The same is true in Jordan, Lebanon and Palestine.

In the past, hurdles in the mind kept people apart. If the few remnants of hunter-gatherer tribes still left are any guide, any attempt to join another group was likely to be met by death. European frontiers, too, are marked by genetic steps, with a deeper difference in identity between the bluff beer-drinkers of the north and the vapid wine-bibbers of Spain and Italy. The continent’s long history of staying at home is manifest in its surnames - those windows into sexual history - as much as its genes. A pedigree of names used to fit well with national boundaries with the Camerons more or less confined to Scotland and the Zapateros to Spain. Spain itself has the most localised patterns and can also boast of the commonest surname in Europe - García. In terms of names and genes, Paris is the most diverse city in Europe.

Names - and the DNA that accompanies them - are on the move. In 1881, Darwin’s last full year, and the date of a British census, his surname was borne by about one Briton in fifty thousand. Its origin is Welsh, from ‘derwen’, or oak; a name transferred to the River Darwen in Lancashire. In his day its headquarters was in Sheffield and its surrounds, where the tag was eight times more common than in Britain as a whole. Almost all those who bore it lived within eighty kilometres of the city (the Joneses - the group with the second most common British surname after Smith - were still, in those happy days, more or less confined to Wales, where in some villages they formed a majority). By 1998, the naturalist’s name had spread northwards to find a new centre in Durham, with secondary centres across the north from the Mersey to the Tyne and a minor outbreak in Herefordshire. The Joneses, too, had migrated, with a grand smear across the Welsh borders, north-west England and as far south as London. No longer must a Darwin or a Jones, or anyone else, marry - as so many once did - someone from their own family for lack of choice. Instead they come into contact with a diverse set of potential partners. The proportion of shared names in the marriage records of a typical English village has gone down by around 2 per cent a year even since the mid-1970s and by even more since the publication of
The Origin of Species
. Sheffield, once its author’s nominal capital, now has scores of new names from across the world.

The United States has gone further down the road to homogeneity. Its telephone directories contain a million different surnames. Some remnants of history remain, with Wisconsin full of Scandinavians while the phone books of New Mexico, Colorado and Texas reveal the presence of many Spanish immigrants. Even so, the general picture is - unlike Europe - one of national uniformity. One in eight Americans is foreign-born (and in California twice as many), and Americans move home, on average, a dozen times in a lifetime, with ten million each year shifting from one state to another. Such frantic migration soon mixes up names, and, in time, genes. The power of the train or plane as an agent of evolution will soon even out yet more of man’s genetic differences.

Almost everywhere, biological frontiers are becoming porous. An era of uniformity is at hand as vast hordes of people move in search of work or sunshine and, in the end, sex. In Britain, the proportion of those born abroad has doubled in the past fifty years and now represents a tenth of the population. Man, like the ecosystem he lives in, is in the midst of a grand averaging.

Intermarriage has been around for a long time. Many white Britons can trace a black ancestor from the small African population that lived in England several centuries ago. About half the men of a certain Yorkshire kindred, the family Revis, share a Y-chromosome type otherwise found only in West Africa. There have been Africans in Britain since the Romans and by the eighteenth century these islands held ten thousand black people. Since then the proportion of Britons who claim recent full or partial descent from Africa has gone up by twenty times and continues to climb. The popular view of the British as a nation walled into a series of ghettos is wrong. In 1991, one electoral ward in ten (a ward is the smallest parliamentary subdivision, with around eight thousand wards in England) had more than a tenth of its citizens from an ethnic minority. Ten years later, the figure was one in eight and by 2011 will rise to one in five. The growth comes in the main not from immigration, but from the movement of people within Britain and from the simple fact that young people, many of them migrants, have more children than do old.

All geneticists are firm believers in the healing power of lust; in the ability of desire to overcome social or geographic barriers. In 2001, about one British marriage in fifty - a quarter of a million in total, with many more couples cohabiting - was between partners from different ethnic groups. Hundreds of thousands of children have one parent from Britain and the other from the Caribbean and almost as many are the progeny of white and Asian parents. British Afro-Caribbean males are half again as likely to marry a white female than are black women to find a white husband, but for the Chinese those preferences are reversed. Such relationships are not, as often believed, found just among the poor for more than half of those involved live in the suburbs and are richer and more educated than average.

Assimilation is well under way in modern Britain, among the most sexually open nations in the world. In today’s England, mate choice is made as much by level of education as by skin colour. Many other countries too, whether they like it or not, have opened up their gene pools.
Homo sapiens -
already the most tedious of all mammals in its geography - will soon, like the worms and the insects, be even more uniform than it was.

 

In that great global coalescence,
Homo sapiens
has evolved in just the same way as have other weeds. In other ways, man is quite unique: for he is the only animal that has escaped - or almost so - from the reach of evolution’s pitiless laws of life and death. Natural selection has long been at work on our species, even if our ingenuity has mitigated its power, with far less response in our own line than in that of the chimpanzee. Now, in the nation in which the idea was invented - and, more and more, in the world as a whole - the process has slowed down and may soon stop.

I depress my first-year students with the statement that two out of three of them will die for reasons connected to the genes they carry (a vague claim, but good enough for undergraduates). Then I try to cheer them up by pointing out that had I given the lecture in Shakespeare’s time, two out of three of them would, at the age of eighteen or so, be dead already. Even in the year of Darwin’s birth, about half of all British newborns died before they reached their majority. Life in these islands has seen a real change for the better. An English baby born in the year of the millennium had a 99 per cent chance of surviving until 2021 and that figure continues to improve. Japan does even better, and the United States rather worse, but most of the developed world has seen a revolution in the prospects of the young. In most countries, even those as poor as Ecuador, the death rate of children is less than, or no more than twice as high as, the British figure. Africa is, alas, a real exception. The death rate for under-fives is one in four in Sierra Leone and is almost as high in Angola and Liberia, while Swaziland has an overall life expectancy of just forty years, half that of Japan. As a result, differences in childhood mortality still account for most of the globe’s variation in life expectancy but, outside the continent where
Homo sapiens
began, those differences have withered away.

Natural selection is very interested in the death of young people, for they have not yet passed on their genes. It cares far less about the fate of the old - those over forty or so - for their sexual lives are over and their relevance to evolution at an end. As a result, the great agent of change has lost a good part of its fuel. For most of history, the Grim Reaper was the master of man’s biological fate. Now, he is taking a rest, and inherited differences in the ability to withstand cold, starvation, vitamin deficiency or disease no longer do much to power the evolutionary machine. Plenty of people still die for those reasons, but they do so when they are elderly and evolution no longer notices them.

The Darwinian examination has two parts. In the developed world, most people pass the first paper: they stay alive until they are old enough to have children. The second section is just as inexorable but has a wider range of marks, for any candidate for evolutionary success has to find a mate and reproduce. The more children they have, the better the prospects for their genes.

Females are limited in the number of offspring they can produce by the mechanics of pregnancy and child care, while males are free to spread their sperm to a multitude of partners, even if a certain amount of persuasion is needed first. As a result, males compete for the attention of females, while females must decide which males should be allowed in. Sexual selection depends on the same logic as selection on the ability to survive: on inherited differences, not in the chances of life or death, but in the number of young. The rule applies to humans just as it does to birds and flowers.

Both humans and peacocks have more variation in male sexual success than in female. Until not long ago, many societies contained a few satisfied libertines, outnumbered by a huge mass of frustrated men. The powerful have always taken their amatory chances when they arise. Henry VIII was a minor player in the marital stakes. The emperor in the Ch’I dynasty of China maintained a palace with several thousand women available for his amusement, while in tribal societies men with high status still have many more mates than do their humble (and often celibate) fellows. Mohammed bin Laden, father of Osama of that ilk, had twenty-two wives and fifty-three children (and in the year of Osama’s birth he had six). His best known son had, last time they were counted, five wives and twenty-two children. Given the equal numbers of men and women at birth, plenty of his henchmen will be obliged to die, naturally or otherwise, without issue.