At the center of everything — of all the radiocarbon dates and stable isotope analyses and ancient-DNA sequences and population genetic models — is the physical reality of preserved human bone in a cave on Prince of Wales Island, Alaska.

Bone is mineral and protein.

The mineral — hydroxyapatite, the calcium phosphate ceramic that gives bone its hardness — is relatively stable over geological time in favorable chemical environments.

The protein — primarily collagen, the fibrous connective tissue that forms the organic scaffold of bone — is less stable but can survive for thousands of years under the right conditions.

In the alkaline environment of a limestone cave on a cool, northern island, both the mineral and the protein components of Shuká Káa’s bones survived long enough for the twenty-first century to read them.

The survival is not guaranteed.