The mouth of the incoming Porcupine River gaped wide among scrawny black spruce. As we passed, we stared along its passage and wondered what was up there, what other worlds we were passing by. We had no idea that 250 miles up the Porcupine is a cluster of high limestone grottos known as the Bluefish Caves. Raised atop a hill, they look across rolling, hummocky country, similar to what this region looked like when the first people reached North America. Inside these caves, human-related deposits start showing up 23,000 years ago, putting one of the oldest pins into the map of the Americas.

We were entering one of North America's great historical crossroads. All around the northern bend of the Yukon and its fat, ox-bowed tributaries, paleontologists have scratched and scrabbled at the riverbanks, discovering rich deposits of what appear to be human-worked mammoth bones. This part of Beringia may have been a massive butchering ground. If this anomalous layer of mammoth bones was created by people, as some believe, then human presence here in Eastern Beringia may go back as far as 40,000 years ago.

Regardless of how many zeroes there are in the number, this is a lot of history to confront when you are a stranger, your generations in North America fewer than you can count on your hands. But I was still in my early 20s, and I had never questioned my ancestry or my right to be here. I was on the Yukon to explore, to feel the shape of the land.

The river opened wide as it passed the Arctic Circle. An anatomizing labyrinth, the Yukon braided until it was 20 miles wide. Backwater channels increased and became a challenge to avoid, some dammed by logjams from spring runoff. Spinning in eddies half a mile wide under a circle of endless sky, we were bedazzled, the sun looping around us like a slow hula-hoop.

A couple days into the flats, we saw someone waving from the point of a distant bar. I glassed him from a hundred yards out: He looked Asian, a lonely shirtless figure in cutoffs, his canoe pulled up on the cobbles. We turned ourselves with our paddles and started in his direction.

As we drew near, we saw how excited he was to be found.  A solo paddler, a Japanese guy our age, he grabbed our bow and helped drag us in.

"Little English," he said, wiping the back of his cutoffs as if he didn't know what to do with his hands. My partner, a shirtless Anglo man, asked if he was OK, and in some of the most nonsensical English I'd ever heard, he explained that he was lost. He'd been paddling alone in sloughs for days, climbing logjams only to see erratic driftwood blockades too big to portage alone. He didn't have a map. He hadn't thought you needed one; the river would just carry you.

Why was he here, we asked? Of all places in the world?

"Adventure is here," he said.

We pulled out our neatly folded maps and showed him different ways back to the river's main stem, tracing our fingers around blue-printed bends and the gray scrabble of gravel bars.

We asked how far he planned to go.

"To the waves of the sea," he said.

That was another thousand miles. At least he now grasped how to get back to the central channel so he wouldn't spend the rest of the summer paddling around in a maze. When we pulled away, he could hardly stop waving at us. He receded as the wide horizon turned, and we never saw or heard from him again.

I come from a restless, wandering culture – a generation or two here or there, rarely staying long in any place. As a kid, I moved every year or two or three, crossing the Southwest as the only child of a single mother, as if we were mariners bound for the promised land. The urge to journey to the farthest edges was in my blood, and obviously in the blood of this Japanese man. We reflected the yearning to explore new territory, the instinct that probably lured people across the land bridge in the first place.

A paleontologist who excavated some of the older human occupation sites in the Americas once told me he believed that people originally traveled great distances not merely in search of resources, but out of curiosity, inspired by a sense of adventure. That, he thought, is why the American continents were occupied so soon after the first human beings appeared. Within several thousand years, they had wandered as far south as Patagonia. If they saw an unknown mountain range, they traveled to it. Sooner or later, they would have discovered the mountains had another side, another landscape stretching even farther. Horizon after horizon, generation after generation, they crossed this ground.