NAIROBI — On the eve of an unimaginably long walk — one that starts in Africa, winds through the Middle East, across Asia, hops over to Alaska, goes down the western United States, then Central and South America and ends in Chile — a question nagged journalist Paul Salopek: Should he take his house keys?

Salopek departed a small Ethiopian village Thursday for a planned 7-year, 21,000-mile walk across some 30 borders, where he will encounter dozens of languages and scores of ethnic groups. The 50-year-old’s quest is to retrace man’s first migration from Africa across the world in a go-slow journey so he can tell a global mosaic of people stories.