DNA Lines Trace Ancestry to Our Door

Cape Times (South Africa), November 12, 2012 | Go to article overview

DNA Lines Trace Ancestry to Our Door


BYLINE: Himla Soodyall

Genetics is a useful tool to reconstruct humanity's ancient past, and there's some fascinating new science coming out in this arena.

Trying to reconstruct history is the subject of many disciplines. Each has its particular toolkit, which converges back into the past in its own way. By using a multi- disciplinary approach, we can improve on the view back into the past, like looking out of a room with windows on each wall giving us glimpses from each side. Some windows will allow us to look further back into the past than others.

Linguists use language as a tool to see how populations have engaged in the past and how people speaking particular languages, for instance the Bantu-language speakers, spread across sub-Saharan Africa in the past 3 000 years. So language serves as a useful tool in reviewing the southern African story.

We also have the archaeological record. As people lived in particular regions, they left records of their settlement patterns, the types of food they ate, the tools they used.

Each of these disciplines has strengths and limitations and should to be used in the right context. For instance, if you were to ask what it was like 10 000 years ago in terms of people living on the African continent, DNA would not be the method of choice. The archaeological record might be able to provide more convincing ideas about what landscape the people occupied, how they hunted and so on. Asking the right questions, coupled with the right data, is particularly important in this scenario. We understand these tools, assumptions, limitations and strengths. And if we use information from various types of sources, we get a magnified view of the past, of the history that we're trying to reconstruct.

I've spent the past 25 years in the field of genetics and have been fascinated by the history of the people of southern Africa.

In 1987, scientists using mitochondrial DNA showed that all humans living today are descended from a population that came out of Africa. Prior to this, when classical genetic markers (for instance, blood groups or serum proteins) were used, the geographical region of origins were assigned to Europe and/or Asia.

Mitochondrial DNA studies advanced the "out of Africa" theory of modern human evolution and dated the point of the common origin back to a pool of humans living in Africa to about 200 000 years ago.

Since then, we've been trying to test, refine and refute these ideas concerning modern human origins. Genetics is an important tool to help do this, since it is an unbiased record of our past. Many other systems that are adopted can be changed in a single generation. For example, I can change the language I choose to speak, I could choose to change the way I dress which is part of our cultural and ethnic imprinting. But as much as I try I can't change what's in my biological make-up.

This brings us to the question of who, living today, still carries these most ancient DNA signatures. Of course, it's the Khoe and the San.

The only two markers that allow us to look at how people move in a geographic landscape are mitochondrial DNA and the Y chromosome. These are "uni-parental" markers. The mitochondrial DNA is passed from mother to children, both sons and daughters, but only daughters pass it on. The Y chromosome is only passed from fathers to sons. These two types of genetic material don't mix or recombine, so they serve as useful tools to trace inheritance strictly along maternal and paternal lines.

Much of my research has focused on making use of these two systems and tracing how the DNA in living peoples can be traced back to the common ancestor who lived here in Africa.

If we look at the tree showing the relationships of the various branches on the mitochondrial DNA tree, most of the variation is found in branches located in Africa compared with those branches found in the rest of the world. …

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