Intercontinental trade routes linked the four great cultures by the Christian era

Evidence

Toynbee, Civilization on Trial, p. 69. Cf. William H. McNeill, The Rise of the West, pp. 295–298, who emphasizes the extent to which by the advent of the Christian era, “Organized trade routes, both by land and by sea, … linked the four great cultures of the continent.” — Samuel P. Huntington, The Clash of Civilizations and the…, 942