The authors’ thesis is that things changed people in the last few thousand years. The invention of agriculture, with its increased human and population densities, increased the disease load and selected for disease resistance: not immunity, but relative resistance. When Europeans met isolated tribes (e.g. the Americas) the difference in disease resistance was illustrated dramatically. On the other hand, sub-Saharan Africa has an even vaster disease load, and most Europeans succumbed pretty quickly before modern medicines.
They note that racial differences are not just skin deep (the pros can tell the race of a skeleton with pretty decent accuracy), and that these differences have to have happened in the fairly recent past, implying fairly rapid evolution.
They go into detail about the Ashkenazi with their genetic diseases and higher IQ (IQ measurement seems, at least in recent years, to be pretty reliable) and their possible connection. I’m not sure they have adequate historical documentation—I recall reading a few years ago that most Jews in England about the time of Shaftesbury were poor: not middle-class or well-off lenders with lots of children (selecting for smarter people). So I'm a bit dubious about their model. But, there it is: an endogenous population that started out not obviously smarter than anybody else in that era, and now they provide a disproportionate number of Nobel winners and other geniuses.
The book is informative, though I have a few nits to pick: Southern AmerIndians developed agriculture (and the implied agricultural science/technology) quite early—why didn’t they have commensurate disease loads? Because of a lack of animals? Other things I know to be a little oversimplified, though that may be for lack of space.
The painful part was the “might have” and “could have” throughout. Still, if you're not familiar with human genetics, give it a read.