The darkness in the deep zone is still total — the kind of darkness that exists nowhere in the modern lived environment, the darkness of a place that has never seen artificial lights except the few that have been brought in by humans over thousands of years.

In the passages leading toward the place where the bones were found, the torch marks are still on the walls.

Their carbon has been there since before the pyramids of Egypt were built, since before the invention of writing, since a time when the human occupation of this hemisphere was still young and the world the Hourglass Cave man knew was already ancient by any human measure.

Modern science — radiocarbon dating, osteological analysis, ancient DNA extraction and sequencing, population genetic comparison — extracted from those bones and those cave surfaces a story that reaches from a specific cold passage in the Colorado Rockies to the broader question of who the first Americans were and how they spread across two continents.