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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.