In his previous book, Harcourt (Emeritus, Anthropology/Univ. of California, Davis) wrote a definitive text on his specialty: Human Biogeography (2012). This book, directed at a popular audience, is a dense and often politically incorrect but lucid summary of everything you would want to know about human diversity.
Homo sapiens originated in Africa about 200,000 years ago. The exodus, which was really a slow trickle, began 60,000 years ago, but these few hundred homogenous emigrants produced 7 billion descendants with wildly dissimilar cultures, appearances, physiologies, and even DNA. Evolution works slowly, but we can see its action over the life of our species. All human babies once lost the ability to digest milk as they matured. Herders who began keeping cattle and goats gradually regained it. People who live at high altitudes—for example, in Tibet or Bolivia—develop enlarged lungs and increased blood oxygen transport. Small, thin bodies lose heat faster, so humans in cold climates grew larger and huskier. The dark skin of Africans protects them from the sun’s harmful ultraviolet rays. Northwest Europeans are pale because they need some ultraviolet to produce vitamin D. In regions where travel is difficult and tribes quarrelsome, cultures multiply. New Guinea has over 800 indigenous languages; Britain, 12. Evolution is not the only factor. “Humans are extraordinarily good at picking trivial differences to separate ‘us’ from ‘them,’ ” writes Harcourt. “Ironically, this irrationality is the key to preserving the world’s rich cultural diversity because technology, prosperity, and easy travel are a relentless force for homogenization.” Unlike in his previous, academic book, the author makes all of this information comprehensible for general readers.
Homogenization is inevitable, but we are an extraordinarily varied species today, and Harcourt delivers an opinionated but always science-based account of how we got that way.