Focus on causation: Human migration

Again and again, when a single wave of colonists spread out over diverse environments, their descendants developed in separate ways, depending on those environmental differences.

For instance, we have seen that South Chinese developed indigenous food production and technology, received writing and still more technology and political structures from North China, and went on to colonize tropical Southeast Asia (...). Within Southeast Asia, among the descendants or relatives of those food-producing South Chinese colonists (...) in the mountain rain forests of northeastern Thailand and Laos reverted to living as hunter-gatherers, while the[ir] close relatives the Vietnamese remained food producers in the rich Red Delta and established a vast metal-based empire.

(...) Austronesian emigrant farmers (...) in the rain forests of Borneo were forced to turn back to the hunter-gatherer lifestyle, while their relatives living on Java’s rich volcanic soils remained food producers, founded a kingdom under the influence of India, adopted writing, and built the great Buddhist monument at Borobudur. The Austronesians who went on to colonize Polynesia became isolated from East Asian metallurgy and writing and hence remained without writing or metal.