In 1902, in Paris, a horrible murder was solved by the great French detective Alfonse Bertillon. He used a piece of new technology which struck fear into the heart of the criminal community. Eight decades later two young girls were killed near the Leicestershire village of Narborough. Again, the murderer was found through a technical advance, although the machinery involved would have been beyond the comprehension of Bertillon. These events link the birth and the coming-of-age of human genetics.

The Parisian killer was trapped because he left a fingerprint at the scene of the crime. For the first time, this was used in evidence as a statement of identity. The idea came from ancient Japan, where a finger pressed into a clay pot identified its maker. The Leicestershire murderer was caught in the same way. A new test looked for individual differences in genetic material. This 'DNA fingerprint' was as much a statement of personal uniqueness as Bertillon's clue or the potter's mark. As usual, life was more complex than science. The killer, a baker called Colin Pitchfork, was caught only after DNA fingerprints had eliminated a young man who had made a false confession and after Pitchfork had persuaded a friend to give a fraudulent blood sample under his name.

The idea that fingerprints could be used to trace criminals came from Charles Darwin's cousin, Francis Galton. He founded the laboratory in which I work at University College London, the first human genetics institute in theworld. Every day I walk past a collection of relics of his life. They include some rows of seeds that show similarities between parents and offspring, an o!d copy of The Times and a brass counting gadget that can be hidden in the palm of the hand. Each is a reminder of Gallon. As well as his revolution in detective work Galton was the first person to publish a weather map and the only one to have made a beauty map of Britain, based on a secret ranking of the local women on a scale of one to five (the low point, incidentally, being in Aberdeen).

His biography reveals an unrelieved eccentricity, well illustrated by the titles of a dozen of his three hundred scientific papers: On spectacles for divers; Statistical inquiries into the efficacy ol" prayer; Nuts and men; The average flush of excitement; Visions of sane persons; Pedigree moths; Arithmetic by smell; Three generations of lunatic cats; Strawberry cure for gout; (hitting a round cake on scientific principles; Good and bad temper in English families; and The relative sensitivity nl' men and women at the nape of the neck. Galton travelled much in Africa, regarding the natives with some contempt and measuring the buttocks of the women using a sextant and the principles of surveying.

Galton's work led, indirectly, to today's explosion in human genetics. His particular interest was in the inheritance of genius (a class within which he placed himself). In his 1869 book Hereditary Genius, he investigated the ancestry of distinguished people and found a tendency for talent to crop up again and again in the same family. This, he suggested, showed that ability was inborn and not acquired. Hereditary Genius marked the first attempt to establish patterns of human inheritance with well-defined traits — such as becoming (or failing to become) a judge — rather than with mere speculation about vague qualities such as fecklessness.

Galton and his followers would be astonished at what biology can now do. It still does not understand attributes such as genius (and reputable scientists hardly concern themselves with them), but DNA is much involved in mental and physical illness. Half a million DNA samples have been taken by police in Britain since the test was invented, and the government has a scheme to follow the genes — and the ailments — of the same number of its citizens over two decades in the hope of finding the biological errors responsible for killers like cancer and heart disease. New tests mean that parents can sometimes choose whether to risk the birth of a child with.in inborn defect. Ten thousand such illnesses are known,uul il we include, as we should, all ailments with an inherited component, most people die because of the genes they carry.

Genetics does more than reveal fate. Humans share much of their heritage with other creatures. As Galton himself illustrated with the appropriate impression pasted near that made by Gladstone, the prime minister, chimpanzees have fingerprints. Now we know that much of their DNA is identical to our own (as indeed is that of bananas). All this suggests that humans and apes are close relatives.

Genetics is the key to the past. As every gene must have an ancestor, inherited diversity can be used to piece together a picture of history more complete than from any other source. Each segment of DNA is a message from our forebears and together they contain the whole story of human evolution. Everyone alive today is a living fossil and carries within themselves a record that revisits the birth of humankind. The Origin of Species expresses the hope that light will be thrown on the origin of man and his history'. Darwin's hint that humans share a common descent with all other creatures is now accepted by all scientists, because of the evidence of the genes.

Evolution, the appearance of new forms by the alteration of those already present, is no more than descent with modification. The same is true of language. As a boy, I was amused by the tale of the order going down the line of command to soldiers in the trenches. 'Send reinforcements, we're going to advance' changed to 'Send three and fourp-ence, we're going to a dance' as it passed from man to man. This simple tale illustrates how accidents, as an inherited message is copied, can lead to change. Because of mutation, life, too, is garbled during transmission.

This book is about inherit.nur: about the clues of our past, present and future that we.ill contain. The language of the genes has a simple.ilpluhct, with not twenty-six letters, but four; the DNA bases.ulenine, guanine, cytos-ine and thymine (A, G, (1 ami T for short). They are arranged in words of thin- letters such as CGA or TGG. Most code for different.imnio.u ids, which are themselves joined together to in.ike proteins, the building blocks of the body.

The economy of life's lanj'.ii.u'.r *..m be illustrated with an odd quotation from a book c.illeil (utiisby, written in 1939 by one Ernest Wright: 'I am going 10 show you how a bunch of bright young folks ditl find a chiimpion, a man with boys and girls of his own, a man ot so dominating and happy individuality that youth was dr.iwit 10 him as a fly to a sugar bowl.' This sounds somewhat peculiar, as does the rest of the fifty-thousand word book, 1111! it is. The quotation, and the whole work, lacks ihe letter 'e\ An English sentence can be written with twenty — five letters instead of twenty-six, but only just. Biology manages with a mere four.

Although its vocabulary is simple the genetic.il message is very long. Each cell in the body contains about six feet of DNA. There are so many cells that if all the DNA in a single human body were stretched out it would reach to the moon and back eight thousand times.  

Twenty years ago, the Human Genome Project set out to read the whole of its three thousand million letters, and to publish perhaps the most dreary volume ever written, the equivalent of a dozen or so copies of the Encyclopaedia Britannica. The task is now more or less complete. The sequencers followed a grand scientific tradition: the Admiralty, after all, sent the Beagle to South America with Darwin on board not because they were interested in evolution but because they knew that if they were to understand (and, with luck, control) the world, the first step was to map it. The chart of the genes, like that <>t the Americas, lias been expensive to make; but — like the theory of evolution itself — it may change our perception of ourselves.