The drift of humankind
Obituary: Luca Cavalli-Sforza
The population geneticist died on August 31st, aged 96
FOR a man who spent his career illuminating the vast, dim migrations of people in prehistory, Luca Cavalli-Sforza’s life was remarkably circular. He first became interested in his major field, genetics, in the house of the geneticist Adriano Buzzati at Belluno, in the hills north of Venice. There he helped to collect thousands of flies in search of mutant Y chromosomes; and though he subsequently travelled the world to study the makeup of its tribes and populations, it was in Belluno that he died.
He had vaguely meant to be a doctor, but found his training useless in wartime hospitals. Instead, he took up the genetic study of E. coli bacteria, shutting himself away in the laboratory. But human beings still fascinated him: not least because he increasingly realised that the human genome contained not only the instructions for building future members of the species, but also its entire past. Migration, conquest and isolation had also left their traces in living populations. He set out to find them.
The technology available was rudimentary. He pored over names in parish registers and learned to do statistics, convinced that with enough patience he could measure anything. For one early paper he pulled surnames from Italian phone directories as proxies for father-son transmission. By plotting the distribution of names in 91 provinces, he showed that the Apennines—the backbone of Italy—had formed a natural barrier to migration for millennia, resulting in genetically distinct populations on either side. And because he could also measure variation in blood proteins, such as that between A, B and O blood types, he also collected blood samples from townsfolk all over the Parma valley to show that marriage between related individuals had led to many of the genetic differences between those towns.
For two decades from 1970, when he took up a professorship at Stanford, he devoted himself to this subject. By the 1990s he was able to study variation in DNA itself. When he and his colleagues instructed a computer to sort around 1,000 people from across the globe into five clusters by similarity of DNA, the clusters matched the labels by which humans had long grouped themselves intuitively: West Eurasians, East Asians, Native Americans, New Guineans and Africans. He soon concluded, though, that race was not a scientifically valid way to classify them. Europeans, for example, were about two-thirds Asian and one-third African, but after millennia of mixing there was no such thing as pure Asian or pure African either. Skin colour, or the shape of a nose, were just superficial adaptations to climate and place.
He represented his genetic data as “trees” branching over time: simplistic diagrams, but beautiful in their simplicity. They often agreed, as he had hoped, with the findings of linguists and archaeologists: suggesting, for example, that humanity arose in Africa, where it stayed for a long time before moving outwards, somewhere between 100,000 and 50,000 years ago. The earliest African migrants probably reached Asia first, moving on to Oceania, Europe and America, in that order.
He particularly liked to tell his story of farmers. By checking variations between individuals based on their blood groups, he discerned a gradient in that variation that stretched south-east-to-north-west across Europe. This he saw as the genetic signature of farming after its invention around 9,000 years ago in the Fertile Crescent. As early farmers radiated out of the Near East, he argued, they bred with indigenous hunter-gatherers, until by the time they reached colder climates their genes were a good mix of both.
This theory stood until the 2000s, when new technology reversed it. He had often carped about the public’s interest in fossils, so much less informative—to him—than genetic studies of the living. Once DNA could be extracted from ancient bones, however, it showed that although agriculture had indeed reached Europe from the Middle East, a wave of immigration from the north-east, starting about 5,000 years ago, had diluted those first farmers with tall herdsmen from the Pontic Steppe. People had moved and mixed in prehistory more than he thought; and not all ancient events, as he supposed, had left their mark in modern populations.
Breaking down walls
Even more controversial was his Human Genome Diversity Project, which he set up in the 1990s. He wanted to study isolated populations in order to understand where all the others came from; to measure the background of drift, or genetic change, that takes place without marauding or migration. Some people thought this racist, and a more worldly man might have realised that. A few critics even brought up his membership of Benito Mussolini’s fascist youth organisation, compulsory before the war. All this dismayed him, as he had done so much to strike down “scientific” studies based on race. The project died in its original form, though it was resurrected as the Genographic Project, which goes on.
From Belluno he witnessed the overturning of many other conclusions. Every evolutionary story he had touched became more complex by the day. Yet he could comfort himself that without his original vision for the study of human history through its genes, much of that great debate would not have happened at all.
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