HUMAN MIGRATION: OUT OF AFRICA THEORY

As far as theories go,

“Out of Africa” has a fairly recent origin. In 1970, “no palaeoanthropologist of whom I am aware held the view that Africa was the evolutionary home of modern humans,” says anthropologist Chris Stringer. While Louis and

Mary Leakey had unearthed the iconic toolmaker Homo habilis, he stood just over a metre high with long arms and a small brain. When it came to ancestors further along the family tree, all the action appeared to be in

Europe and Asia. This was the home of the tall, largebrained Neanderthals, of

Java man and Peking man (varieties of Homo erectus) and Homo heidelbergensis.

One of the reigning theories was “multiregional evolution” – that we moderns arose through mingling. Hominins, notoriously footloose and fancy-free, kept the genes flowing freely and eventually produced the single melting-pot variety that is us. Chris Stringer’s PHD thesis explored that theory. Travelling around Europe in 1971, he measured Neanderthal skulls to examine whether they were the likely ancestors for stone-age modern humans. He concluded they were not, and that African fossils like Omo 1, discovered in Ethiopia in 1967, were better candidates. It was an argument Stringer would continue to build as more and more African skeletons with reliable dates were unearthed in the next decades.

Stringer found a surprising comrade-in-arms in New Zealand geneticist Allan Wilson, based at the University of California, Berkeley. Wilson pioneered the use of the “molecular clock” to measure the evolutionary distance between species. For instance, by measuring changes in the blood protein albumin, he estimated chimps and humans had branched off from a common ancestor around five million years ago.

In the 1980s, Wilson began testing a DNA clock. A new technique had made it possible to follow the variation in the code, not yet by reading it letter by letter, but a method called “restriction fragment length polymorphism”. This involves breaking the DNA in a consistent way and checking the pieces, sort of like breaking a standard chocolate bar into 60 squares. But imagine the factory mould is not perfect: some bars yield 58 unit squares and a double; others 57 units and a triple, and so on.

Wilson employed that DNA fragmentation approach to a small string of DNA code that resides within the cell’s energy-producing mitochondria rather than within the secure vault of the nucleus, where the main code is kept. Mitochondrial DNA provides a particularly sensitive clock because it mutates faster than the main code. And because it is inherited exclusively via the female egg, it traces a line of maternal descent.

When Wilson compared different populations, he found Africans carried the greatest diversity in their mitochondrial DNA. All other populations carried a single subset of that diversity.

The interpretation, published in a landmark paper in Nature in 1987, was that if the human race is represented as a family tree, its root was Africa. Here our species had sprouted into diverse branches over time.

The rest of the world was the product of just a single twig. Wilson used the DNA clock to date the root of the African tree at 200,000 years ago.

And because he had used mitochondrial DNA, all modern women could trace their maternal ancestry to an African “Eve” – the lucky woman whose offspring kept on having daughters who had daughters.