This past week I've been reading a very interesting book - Who We Are And How We Got Here: Ancient DNA and the Science of the Human Past, by David Reich. It examines the history of the spread of mankind using information not from bone morphology or artifact similarities, but from the arguably more rigorous basis of DNA sequences. Human migration is charted out of Africa (and back into it), up to Eurasia, and across to the Americas. Correlations are drawn with languages and artifacts, but it's the DNA that overturns Clovis First and other older schema. There is a lot of hard science, much of it way over my head, but it was clearly worth the browse, even just for the insight on Sally Hemings and the progeny of Genghis Khan. The final chapters discuss the question of "what is race?" - a more complex question than most people realize.
The most interesting insight for me is reflected in the diagram embedded above (via), and this paragraph from the opening chapter:
’The Bible and the chronicles of royal families record who begat whom over dozens of generations. Yet even if the genealogies are accurate, Queen Elizabeth II of England almost certainly inherited no DNA from William of Normandy, who conquered England in 1066 and who is believed to be her ancestor twenty-four generations back in time. This does not mean that Queen Elizabeth II did not inherit DNA from ancestors that far back, just that it is expected that only about 1,751 of her 16,777,216 twenty-fourth-degree genealogical ancestors contributed DNA to her. This is such a small fraction that the only way William could plausibly be her genetic ancestor is if he was her genealogical ancestor in thousands of different lineage paths, which seems unlikely even considering the high level of inbreeding in the British Royal family.’So, to oversimplify it for myself: As you go back through the generations of your ancestry, the number of ancestors you have begins to increase exponentially. For the first 6 generations back (to your great-great-great-great grandparents) there is "room" in your genome for some DNA from each of them. But once you are back a dozen generations, with 4,096 ancestors (barring consaguinity), only a minority of them will have any sequences reflected in your genome.
Interesting stuff. A hard read, but a good browse. I think the book is available fulltext online here.