Race involves a lot more than DNA. As a result, the proportion of blacks in the United States is rising. In 1997, about thirteen per cent of Americans perceived themselves as black and, over the past two decades the country's black population has increased;it twice the rate of the white. Most of this has nothing t<> do with genes, but is a matter of identify. Thirty years.1^,0 anyone of mixed ancestry would do their best to cl.issily themselves as white. Now, with the rise of black self-esteem, many find themselves more at home as blacks. As a result, any genetic measure of admixture then and now will give different results, as a reminder that race is constructed by society as much as by DNA.

Seventeenth- and eighteenth-century England, too, had a substantial black population. It disappeared; not because it died out, but because it was assimilated. Part of its heritage is, without doubt, still around in the streets of modern Britain. Dr Johnson himself had a black servant, Francis Barber, to whom he left enough money to set up in trade. Many people around Lichfield are proud to trace their descent from him, although their skins are as fair as those of their neighbours. White Britons contain other exotic genes as well. After all, the first slaves to cross the Atlantic were the Caribbean Indians sent to Spain by Columbus in 1495 and there was a sixteenth-century fashion for bringing newly discovered peoples back to Europe. The English explorer Frobisher brought back some Eskimos in 1577 and more than a thousand American Indians (including a Brazilian king) were transported to Europe. Many of the unwilling migrants died, but some brought up families. Their legacy persists, no doubt, today; but they have been absorbed so fully into the local population that only a genetic test — or provision of a dependable pedigree — can say who bears it.

Genes have taken us back for hundred of years — for fifteen generations or so where black Americans are concerned. But they bear messages from earlier in history. Sometimes, the evidence is direct, more often indirect: but in every case it links the present with the past.

For good historical reasons, a great deal is known about the genetics of Hiroshima and Nagasaki. The Americans spent many years on a survey of whether the atom bombs had increased the mutation rate. No effect was found, but a mass of information on the genes of the two cities was gathered. Each has a cluster of rare variants not present in the other. They are relics of an ancient history. Hiroshima and Nagasaki were each founded by the amalgamation of different warring clans that lived in the region eight thousand years ago. Like tribal peoples today, they had diverged in their DNAs. The slight differences between the ancient tribes persist in the modern towns. Nagasaki was one of the few ports open to the outside world during Japan's self-imposed isolation, but has no more sign of an influx of a foreign heritage than does Hiroshima. The voices of remote ancestors echo more loudly through the two cities than do those of more recent invaders.

Because genes copy themselves, there is no need to go back to the source to find an ancestor; but, sometimes, the source has been preserved. The Egyptian pharaoh Tutank-hamun was buried at about the same time as another mummy, Smenkhare. Their blood groups can still be identified and show them to have been brothers. The first piece of human fossil DNA was found in the dried corpse of an Egyptian child, buried in the sands. It had survived for two and a half thousand years. Since then, many pieces of ancient DNA have turned up {although their analysis is confused by a tendency for contamination with modern material).

It has, nevertheless, become possible to read ancestral genes directly. Some ancient DNA, like that of the Easter Islanders, whose civilization was destroyed by constant warfare and ecological vandalism, has no equivalent in the modern world and remains, like their enigmatic statues, as the sole evidence of a people who left no posterity. Sometimes, it adds to the clues of the present. Agriculture began in Japan with the Jonion people, about ten thousand years ago, but they also spent much of their time as hunters. Farming did not take oft as a way of life, with rice as a staple diet, until the Yayoi tribes who followed them, thousands of years later. Rice was brought by the Chinese, and the Japanese argue about how many of their genes entered the country with the crop. Many believe that the immigrants drove out most of the natives; that people moved, rather than ideas. However, DNA extracted from a two-thousand-year-old Chinese burial site links its inhabitants with modern Chinese, but not with the fossil DNA of the extinct Japanese. It proves that few mainland-ers made the journey. Instead, the locals of two millennia ago, much like their modern descendants, picked up and used a new technology invented in a foreign land. Modern Japan, on the other hand, does have biological links with the Chinese, so that a movement from the mainland had an impact much later.

Some ancestral voices are particularly fluent in telling the story of the past. Mitochondria are small energy-producing structures in the cell. Each has its own piece of DNA, a closed circle of about sixteen thousand DNA bases, quite distinct from that in the cell nucleus. Eggs are full of mitochondria but those in sperm are killed off as they enter the egg. As a result, such genes are inherited almost exclusively through females. Like Jewishness, they pass from mothers to daughters and sons, but daughters alone pass them on to the next generation.

Kvery family, every nation and every continent can trace descent from its mitochondrial Eve, a woman (needless to say, one of many alive at the same time) upon whom all their female lineages converge. Sometimes she lived not long ago: in New Zealand, lor iiist.iin.i-, iifiirly all Maoris share the same mitochondrial identity, hulling that just a few women founded their nation a thousand years ago. A world family tree based on mitochondria finds its roots in Africa, with more diversity in that continent than anywhere else. To track more recent paths of migration shows that mitochondria are an accurate record of history: thus, in the New World, native mitochondria have a tie with those of Siberia, confirming an ancient pattern of migration.

Shared genes link New Zealand, Siberia and the rest of the world to an African ancestor. The first modern human appeared in Africa over a hundred thousand years ago, in the continent that gave rise to most of our pre-human kin and of the apes to whom we claim affinity. A few of these African relatives from a deeper branch of the tree are alive today. One, the chimpanzee, has always seemed a near neighbour; and Koko (an inhabitant of the Gombe Stream Reserve) was the first animal to have an obituary in The Times.

As any literate teenager knows, Tarzan of the Apes was proved to be the son of Lord Greystoke by virtue of the inky fingermarks in a childhood notebook. Galton had shown that chimpanzees have fingerprints that look much like those of a human being. Chimps and men, they prove,share genes. A joint heritage goes beyond the fingertips. A distinguished geneticist of the 1940s once tested whether chimps share our variation in the ability to taste the bitter chemical PROP by feeding it to three of the inhabitants of London Zoo. Two swallowed the drink with every sign of delight, but the third spat the liquid all over the famous professor as further evidence of common ancestry.