The Tally Stick

Up on the highway, the only evidence that the Chamberlains had ever been there was two smeared tire tracks in the mud leading into an almost undamaged screen of bushes and trees. No other cars passed that way until after dawn. By that time the tracks had been washed away by the heavy rain. After being in New Zealand for only five days, the English Chamberlain family had vanished into thin air. The date was 4 April 1978. In 2010 the remains of the eldest Chamberlain child are discovered in a remote part of the West Coast, showing he lived for four years after the family disappeared. Found alongside him are his father's watch and what turns out to be a tally stick, a piece of scored wood marking items of debt. How had he survived and then died? Where is the rest of the family? And what is the meaning of the tally stick?