Migration began with our origin as the human species and continues today. Each chapter of world history features distinct types of migration. The earliest migrations spread humans across the globe. Over the centuries, as our cultures, societies, and technologies evolved in different material environments, migrants conflicted, merged, and cohabited with each other, creating, entering, and leaving various city-states, kingdoms, empires, and nations. During the early modern period, migrations reconnected the continents, including through colonization and forced migrations of subject peoples, while political concepts like "citizen" and "alien" developed.