Letting genes tell our tale of evolution
LUIGI Luca Cavalli-Sforza is a historian with a difference. The materials he works with are not dusty records or ancient artefacts but human genes. Cavalli-Sforza is the world’s most renowned population geneticist who, over the past half century, has helped transform his subject.
But he has also helped shape our understanding of human history arguably as much as any historian or archaeologist.
Born in Genoa in 1922, Cavalli-Sforza initially worked on bacterial genetics.
In the Fifties, however, he switched his attention to humans. Humans, he observes dryly, have more personality than bacteria. “I began wondering,” he writes, “whether it was possible to reconstruct the history of human evolution using genetic data from living populations.” From that simple question came a lifetime of study and an entirely new way of studying human evolution. Genes, Peoples and Languages is a summary of a lifetime’s work – and of the history of our species.
Cavalli-Sforza realised that human history is a history of migrations. And every time people move from one location to another they take their genes with them. Those genes will not be exactly the same as those of the population the migrants left behind. Studying differences in genes between populations, Cavalli-Sforza came to see, would therefore be a way of reconstructing the migrations that make up human history, and hence to reconstruct human history itself. Over the decades Cavalli-Sforza has built up an immense database of the distribution of more than 100 genetic traits across nearly 2,000 different populations. In 1994 he published, together with Paolo Menozzi and Alberta Piazza, the monumental History and Geography of Human Genes, the first genetic atlas of the world. The book reconstructed the origins of Homo sapiens and the paths by which they spread throughout the world.
A good illustration of the power of Cavalli-Sforza’s method can seen in his uncovering of the origins of farming in Europe.
Agriculture originated in the Middle East some 10,000 years ago, replacing hunting and gathering as the predominant way of life. From the Middle East the farming way of life soon spread across Europe, in a great wave beginning in the east around 9,000 years ago and reaching the Atlantic seaboard some 3,000 years later. But did it spread by existing hunter-gathers taking up farming, or did a new population overrun Europe, displacing existing hunter-gatherers and bringing agricultural technology with them?
Archaeologists had fruitlessly debated this issue for decades.
Cavalli-Sforza realised that gene patterns could resolve the dispute. If the farmers moved, they would have taken their genes with them; but if the farming habit alone was copied, it would have had no genetic consequences.
Using a sophisticated mathematical technique he revealed a gradient of genes across Europe, fanning out from the Middle East. The gene map almost exactly matched the archaeological map of the spread of wheat. It is now clear that the movement of people, rather than the copying of a practice, caused the spread of farming.
Perhaps the most fascinating aspect of Cavalli-Sforza’s work is the way he is able to link genetic data to archaeological and linguistic evidence. In an intriguing study, he showed how the “family tree” of languages almost exactly matches the human genetic family tree. In other words, the human history traced through language corresponds wonderfully to human history told through our genes.
There is another way too in which Cavalli-Sforza’s work has been immensely important: in undermining the idea of race. The 1950s, when Cavalli-Sforza began his work, were a particularly troubling time for the science of human genetics. The experience of Nazism and the Holocaust had thrown prewar genetics – with its rootedness in eugenics and racial science – into disrepute. Cavalli-Sforza played a huge role in demonstrating the falsity of racial science by demonstrating the difference between the genetic diversity of human groups and the idea of race. There are certainly differences between populations, but distribution of one physical or genetic characteristic – say skin colour – is not necessarily the same as that of another – such as blood group. The great majority of our genes are a mishmash and do not fall into any discrete racial category.
For more than 50 years, Cavalli-Sforza has been at the forefront of thinking about human genetics, human diversity and human history.
Genes, Peoples and Languages provides a wonderful introduction both to the work and the man.
P Kenan Malik’s Man, Beast and Zombie: What Science Can and Cannot Tell Us About Human Nature, will be published by Weidenfeld this month.