I would like to go back to discussing how humans spread through the world.
Numerous books have appeared on this subject in recent years, stimulated by new techniques of genetic research. I find these interesting but I question their claimed definitiveness.
Imagine photocopying documents, where you photocopy the photocopies, perhaps making dozens of copies from each copy. In the course of this, blemishes will occasionally appear. When you photocopy the blemished copy, the same blemishes will be reproduced in all subsequent copies. If you look at two copies and see that they share the same blemish, you know they must "descend" from the same original, blemished copy. If you also see that the two copies have additional, different blemishes, and you know how often blemishes arise--say, once every thousand copies--you can work out how many copying generations it is since the copy from which they are descended. E.g. if they differ by three blemishes, it must be three thousand generations since the "ancestor" photocopy passed through the photocopier.
A similar principle has been used to determine the family relationships between human populations.
It does not work with ordinary DNA, which is mixed and matched at every generation (half from each parent) by sexual reproduction--as if the photocopies were cut in half and taped together in new combinations, so that blemishes are spread around and clear distinctions between lines are not maintained.
Instead, it relies on parts of the genome that do not undergo recombination. These are mitochondrial DNA, which people inherit only from their mother, and the Y-chromosome, which is passed down intact (or almost so) from father to son. When these genes are passed down, copying mistakes occasionally occur, so that people today show many variations deriving from ancient errors. As with photocopier blemishes, such variations make it possible to estimate how long ago any two people shared a common ancestor (female ancestor for mitochondrial DNA and male ancestor for the Y-chromosome).
Stephen Oppenheimer, in his book Out of Eden: the peopling of the world, describes how geneticists have used this principle to trace the ancestral connections between human populations and to work out how long ago they split off from one another.
The message he likes to convey is that archaeologists and historians were fumbling around in the dark until these new genetic techniques came to shine the clear light of day on their disciplines.
For instance, the geneticists have apparently instantly and conclusively resolved an issue that archaeologists might have been arguing about until kingdom come. While archaeologists once saw changes in the distribution of pottery as indicating the wholesale movement of the people who produced the pottery, more recently they have argued that people remained put and only styles and technology spread from one region to another. The genetic evidence seems to show that, indeed, most modern populations have been where they are for a long time and the formerly hypothesised migrations never took place.
Nevertheless, I suspect that, in the fullness of time, the geneticists' arguments will come to seem as problematic as those of the archaeologists' and historians', which themselves once seemed so obvious and compelling. It will be revealed that there are other ways of interpreting the results and doubt will creep into this young discipline where today certainty is supreme.
After all, according to Oppenheimer, the entire non-African population of the world is descended from a single group that left Africa via the Arabian peninsula about 80,000 years ago. Yet the notion that, over the entire lifetime of the human species, no other people had the idea of leaving Africa is hard to credit. Right there lies a reason for having doubts about the completeness of the genetic picture.
Even if the geneticists prove to be completely correct in their picture of ancient population movements, there remains a vital role for historians and archaeologists in advancing understanding of the past. The things that make, say, Britain what it is today are Magna Carta, the Elizabethan Age, the Civil War, the Industrial Revolution, and so on--i.e. sociological things that are not passed down in our DNA. History and society, the concerns of this website, are the subject of cultural, not genetic, transmission, and the connection between genes and culture is probably looser than people like Oppenheimer tend to assume.