Something like 99 per cent of human history occurred before the invention of writing and the start of our cultural heritage. For tens of thousands of years, our ancestors lived, gave birth and died during the long period when history went unrecorded. It is thanks to Luigi Luca Cavalli-Sforza, and the other pioneers of human genetics, that scientists now have a powerful tool with which to open a revealing window on our earliest existence.
Cavalli-Sforza, Professor Emeritus at Stanford University in California, was among the first to demonstrate that the early chapters in the story of mankind can be told by reading the messages written in the indelible language of our genes.
He is one of the founding fathers of population genetics, the mathematical analysis of how and why the frequencies of genes change over time. The science has been an important influence on what has become known as "biological anthropology", which attempts to understand human prehistory by analysing the DNA of present-day people as well as the preserved tissue of those who are long dead.
One of the earliest attempts to understand human evolution using population genetics was carried out by Cavalli-Sforza in Italy in the the early 1950s, after he had studied in Cambridge under the great RA Fisher, the British geneticist who established a statistical link between mutation and evolution. Cavalli-Sforza says he owes this early insight to help from the Roman Catholic Church, which owned a unique, 300-year archive of births, marriages and deaths in Italian villages, as well as records on the family relationships of those intending to get married.
"I had access to all these records and I decided to use them for a problem that had not been solved at the time," Cavalli-Sforza explained during a visit this week to London to mark the publication of his latest book (Genes, Peoples and Languages, Penguin, pounds 18.99). Fifty years ago, he wanted to know whether natural selection was really the sole factor driving human evolution or whether another, random element, called "genetic drift", was also important.
At the time, there was something of a schism among geneticists studying human evolution, with those on this side of the Atlantic, under Fisher's influence, suggesting that only natural selection was involved. Many in America, led by Sewall Wright, the other leading geneticist of the day, thought that the frequency of human genes could drift up and down within a population purely on chance alone.
Cavalli-Sforza had an opportunity to test both ideas using his privileged access to the Catholic Church's records of nearly 100 Italian villages spanning three centuries.
He first set about collecting samples of blood from each village to record the frequency of the three main blood groups - groups A, B and O. Since the early part of the 20th century, geneticists had known that different populations from around the world had different proportions of the three blood groups for no apparent or obvious reason. Cavalli-Sforza thought this difference might, at least in part, be the result of random drift, something that Fisher, in his brilliant mathematical analysis, had dismissed.
"The question was, `is drift really important to Man?' In England there was a tendency to believe that natural selection was the only thing that mattered," says Cavalli-Sforza. In trying to explain why human genes are what they are today, random genetic drift was always a theoretical possibility, but Fisher and others had calculated that human populations, even many thousands of years ago, were just too large for such a random event to have any importance. "I studied the blood groups of people so I could measure the genetic variation between villages. I could also make a prediction of what it should be from the records of the parish books, which allowed me to construct the …