When John R.

Johnson, by then the curator of anthropology at the Santa Barbara Museum of Natural History and Orr’s eventual successor in that role, and colleagues including the geochronologist Thomas Stafford Jr. turned their attention to the Arlington Springs femora in the late 1980s and early 1990s, they faced a chemical puzzle.

Ancient bone is not simply frozen organic material.

Over thousands of years in the burial environment, the original bone undergoes chemical changes.

Proteins are lost — some more quickly than others, depending on their specific chemical properties and the conditions of the burial environment.