I am on a journey. I am in pursuit of an idea, a story, a chimera, perhaps a folly. I am chasing ghosts. Starting in humanity’s birthplace in the Great Rift Valley of East Africa, I am retracing, on foot, the pathways of the ancestors who first discovered the Earth at least 60,000 years ago. This remains by far our greatest voyage. Not because it delivered us the planet. No. But because the early Homo sapiens who first roamed beyond the mother continent—these pioneer nomads numbered, in total, as few as a couple of hundred people—also bequeathed us the subtlest qualities we now associate with being fully human: complex language, abstract thinking, a compulsion to make art, a genius for technological innovation, and the continuum of today’s many races. We know so little about them. They straddled the strait called Bab el Mandeb—the “gate of grief” that cleaves Africa from Arabia—and then exploded, in just 2,500 generations, a geological heartbeat, to the remotest habitable fringe of the globe.

Millennia behind, I follow.

Using fossil evidence and the burgeoning science of “genography”—a field that sifts the DNA of living populations for mutations useful in tracking ancient diasporas—I will walk north from Africa into the Middle East. From there my antique route leads eastward across the vast gravel plains of Asia to China, then north again into the mint blue shadows of Siberia. From Russia I will hop a ship to Alaska and inch down the western coast of the New World to wind-smeared Tierra del Fuego, our species’ last new continental horizon. I will walk 21,000 miles.

via To Walk the World.