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Genes, ancestry, and where we came from

Johnjoe McFadden

DNA research can tell us a great deal about our ancestry _ but the jury is still out on its results.

IN SUMMER, we often head for "home." To many this isn't where we live but where we were brought up. There is something comforting about returning to the bosom of our homeland. But how deep are our roots? Family trees tend to wither into uncertainty beyond a few hundred years. To go back further we need tools that dig deeper. Remarkably, our DNA carries a record of our ancient past that has been used to examine the movement of people all over the world.

`Out of Africa'

In a sense we are all Africans: modern man is thought to have evolved in Africa between 100,000 and 200,000 years ago and then, between 50,000 and 100,000 years ago, spread from Africa to colonise the rest of the world (the "out of Africa" hypothesis). The first signs of Homo sapiens in Europe are the appearance of new tools and the paintings of hunting scenes in the caves of southern Europe. But are we modern Europeans descended from those stone-age hunters?

A few decades ago archaeologists would have mostly said no. Archaeology used to be dominated by "diffusionists" who saw all cultural innovations as the result of migrations. So the Neolithic revolution that originated about 10,000 years ago was thought to have been carried westward by the early farmers. The diffusionists imagined great migrations of Neolithic people who replaced the indigenous hunter-gatherer populations in Europe, either by out-breeding or by exterminating them.

The first chink in the diffusionist case came with the demonstration that the impressive Neolithic sites in the West, such as Stonehenge or Newgrange in Ireland, were contemporary with or even predated similar structures built by eastern civilisations. The builders of the megalithic passage grave in Newgrange could not have come from Egypt if Newgrange was built a thousand years before the pyramids. So diffusion gave way to "indigenism," which emphasised local invention together with diffusion of ideas, rather than people.

Then DNA entered the fray. Genetic fingerprinting finds gene signatures that characterise the DNA of Africans, Asians or Europeans. Our genes are inherited from our distant ancestors and thereby provide clues to where they lived.

Luigi Luca Cavalli-Sforza, of Stanford University, plotted gene frequency patterns across Europe and the Middle East and discovered that his genetic maps bore a striking similarity to those of the spread of agriculture. He proposed that the diffusionists were right after all. Farmers and their genes seemed to have travelled along with their crops.

The revival of diffusionism hasn't pleased everyone and most archaeologists still prefer a balance between local invention and diffusion of ideas with only limited population migration. And many question the validity of DNA typing of modern populations to infer ancient migrations. But a study published in Science by Wolfgang Haak and colleagues at the University of Gutenberg sought to overcome this objection by sequencing DNA from 7,500-year-old skeletons of the first European farmers. Surprisingly, the scientists found that a quarter of the skeletons yielded a very rare DNA type that is hardly found at all among modern Europeans. The authors proposed that although these early farmers left their agricultural implements and Neolithic culture to Europe, they didn't leave their genes. Professor Haak's team suggested that the ancestors of modern Europeans are not the farmers from the Middle East but the Palaeolithic populations of Europe. We were back to the spread of ideas rather than people.

But the diffusionists aren't giving up. A few weeks ago Science published a "comment" on Professor Haak's paper by Albert Ammerman and colleagues along with a "response" from Professor Haak's group. Dr. Ammerman claims several interpretational flaws in Professor Haak's analysis. For instance, Professor Haak examined only DNA (the mitochondrial DNA) from the female line. Dr. Ammerman says the early farmers may have taken wives from the local Mesolithic hunter-gatherer populations. This would have diluted out their female Neolithic inheritance. Professor Haak's team complains about Dr. Ammerman's use of the phrase "the first farmers' wives and daughters" implying that the first farmers were all male (whereas women tended the fields in many pre-industrial societies).

Both sets of authors agree that more data are needed to conclusively settle the issue one way or another. So if you do feel the need to return to the European homeland, you will have to wait a while to find out where to go. —

© Guardian Newspapers Limited 2006

(Johnjoe McFadden is professor of molecular genetics at the University of Surrey and editor of Human Nature: Fact and Fiction.)

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