The journey begins in Ethiopia at one of the world’s oldest human fossil sites, Herto Bouri, and unspools across the scalding Afar Triangle, in the Rift Valley. Along this pathway our restless forebears ventured forth toward the Gulf of Aden, where they first stepped out of the mother continent to explore the wider world. As Salopek bears witness, this ancient pathway remains a conduit of opportunity—and sometimes fatal tragedy—for migrants seeking a better life today.

The trek through the Middle East begins in the fabled deserts that guard the cradle of Islam—the Hejaz of Saudi Arabia, long forbidden to outsiders. Meetings with Bedouin nomads and echoes of the ghost of Lawrence of Arabia give way to encounters in the contested borderlands between Israelis and Palestinians. Crossing the West Bank, Salopek gets caught in a skirmish between these two peoples. Then, to circumvent the bloody civil war in Syria, he is forced to take cargo ships from Israel to Turkey.

In southern Turkey, one of the oldest farmed landscapes, Salopek meanders through pistachio orchards, Bronze Age ruins, and walled medieval cities. And he walks into one of the greatest humanitarian catastrophes of our time: tides of refugees from Syria’s civil war. Turning northeast, he passes through tense Kurdish villages en route to the Caucasus Mountains and a frigid crossing into Georgia—an oasis of stability in a turbulent region. From the capital, Tbilisi, Salopek sprints through Azerbaijan to the shore of the Caspian Sea. Central Asia and the ancient Silk Roads beckon.

Horses were domesticated in Kazakhstan, but finding a cargo animal today to span this country’s vast steppes proves no easy task. Salopek and his local guides make the first foot traverse in almost a century of the wild Ustyurt Plateau straddling Kazakhstan and Uzbekistan—a barren upland crisscrossed in earlier times by Neolithic hunters, Silk Road camel drivers, and armies of Scythians, Mongols, and Russians. The furnace of the Kyzyl Kum, or Red Desert, sears the route to the legendary oasis cities of Bukhara and Samarkand.

After scaling the snow-draped ramparts of Central Asia—the Pamirs, the Hindu Kush, the Karakoram—the Out of Eden Walk's global trail meanders down onto the immense, river-fed plains of South Asia. From the green Punjab plateau in Pakistan, Paul and his local walking partners ramble eastward across ancient pilgrim roads in India, through a busy, booming world of villages and megacities toward the monsoon-drenched Bay of Bengal. Then the green pathways of Bangladesh and Myanmar lead onward to the edge of China.