The air was likely frigid as the hunter lit a small fire. The caribou would come in the morning—forced through the narrow strip of marshland where he camped. There was nowhere else to go. The land was flanked by water on both sides, and large stones had been laid out in slanting lines to funnel the animals into this bottleneck. The hunter struck his weapon to sharpen its edge in anticipation. In that moment, two glassy flakes splintered away from the point of impact and fell to his feet. They would be buried there for nearly 10,000 years.In 2013 those...