Secular decline in cranioskeletal size over two millennia of interior central California prehistory: Relation to calcium deficit in the reconstructed diet and demographic stress

An osteometric study of residual rickets (RR) skeletal plasticity has been made of a time‐ordered sequence of fifteen series of adult skeleton sets ( n =359) from the Great Valley of central California (GV), spanning the three archaeological horizons of California Indian prehistory in this region: Early (EH), Middle (MH) and Late (LH). By least‐squares linear regression analysis, a clear and continuous downward trend obtained in cranioskeletal size in both sexes, proceeding from EH to LH in the GV sequence, pari passu with a steady upward trend in the RR‐15 score. To ascertain if dietary calcium deficit (CADEF), a known cause of active rickets, can explain the observed secular increase in RR, the aboriginal diet was reconstructed by the diet grid method: CADEF was found to rise from borderline in the EH, to moderate in the MH, to severe in the LH, as the subsistence base shifted gradually away from hunted prey, and storable seed foods—mainly leached acorn meal—became the dominant source of energy. Further regression analysis has shown that the secular upward trend in CADEF is highly correlated with the temporal clines of the GV sequence, directly with the RR‐15 score, and inversely with cranioskeletal size: in round numbers, one grade of CADEF brings about a loss of 120 cm 3 or 4 per cent in partial skeleton volume. Two biocultural influences—demographic stress (DS) in females, and the less common ‘male nutritional advantage interaction co‐factor’ (MNAIC)—modulate the primary bone antitrophic CADEF effect: DS enhancing, MNAIC ameliorating.