People constructing family trees are typically investigating events from the past few centuries, while population genetics starts there and pushes further into the past. Most of us have a sense of our family history, but eventually we all hit a brick wall. Our DNA breaks through that wall, providing a unifying path that leads from the present into the realm of deep ancestry. (p. 13)
This book is effectively The Genographic Project for Dummies: no scientific background required. Spencer Wells is the director of the Genographic Project, which was founded by National Geographic and IBM as a non-profit genetic anthropology study, attempting to map historical human migration. Proceeds from the sale of DNA-testing kits go to 'help indigenous peoples around the world' (174). Unlike its predecessor, the Human Genome Diversity Project, it does not concern itself with patenting genetic data or preserving cell lines.
The story so far, grossly oversimplified: about sixty thousand years ago (two thousand generations), the ancestors of the human race began to migrate from Africa. As they spread out over the world, their DNA mutated ('genetic drift'). These mutations, which persist in the DNA of an individual's descendants, are genetic markers which can be used to identify migration patterns. Small groups -- and they were all small groups: "Genetic data suggest that the human population size crashed to as few as 2,000 people around [the time of the last great ice age]" (p. 139) -- could be cut off from other groups by mountains, glaciers, rivers. If an individual in that group had a particular genetic marker, all his or her descendants would also have that marker. (Male progeny will carry it on the Y chromosome; female in mitochondrial DNA.) So everyone with the marker would have a single ancestor. By mapping the geographical distribution of a marker, and subsequent mutations, it's possible to identify roughly when and where that single ancestor lived.
Wells explores the science and culture of genetic genealogy in a number of case studies: 'Odine', a descendant of Thomas Jefferson and Sally Hemings; 'Margaret', Wells' grandmother, whose DNA is a potted history of agriculture; 'Phil', whose Native American ancestry links him to Siberian populations; 'Virumandi', an Indian man who shares a rare genetic marker with some aborigines in Australia; and 'Julius', a Tanzanian hunter-gatherer whose DNA is used to illustrate the vast genetic diversity within Africa.
One thing I learnt as a result of reading this book is that there are actually only two types of people: those who think the Genographic Project is dead cool, and those who think it isn't. I am firmly in the former camp, not least because I know so little about my family history.
Well, I know a bit more now. I am (like 5-10% of the population of Great Britain) in haplogroup K, which means I have discovered a new cousin and that I share a common ancestor with a great many Ashkenazi Jews. Having opted for the basic, anonymous test, I was intrigued enough to sign up for Family Tree DNA, which might make it possible for me to trace my maternal grandmother's ancestry.
Deep Ancestry emphasises the is sometimes over-breezy, and sometimes poorly worded ("no two people look alike": er, you mean 'identical', don't you?) but it does what I've always loved in pop science books: it makes me want to know more, to learn, to understand.