This book is mostly about the flood of recent DNA evidence in support of the single-origin hypothesis for the human race. What seems to be true now is that anatomically modern humans lived in sub-Sahara Africa as far back as 200,000 years or so and started to emigrate out about 60,000 to 70,000 years ago, eventually filling up the world. The message of the book is that every human in the world can trace his or her ancestry back to sub-Sahara Africa.
During the migration, as time passed and people moved into different environments, evolutionary forces (random mutations in their DNA) changed them. The book discusses several of these variations, sometimes even showing ancestral and recent DNA sequences.
One of the most prominent changes the book discusses is pigmentation of eyes, skin, and hair. In areas with intense sunlight, high pigmentation is beneficial by preventing UV degradation of folate – essential for fetal development. But when people reached northern Europe about 20,000 years ago high pigmentation hindered vitamin D production, resulting in rickets, especially affecting pregnant women. The DNA variant for light pigmentation became more beneficial.
The story of human migrations and genetic changes is really an epic.