THE EVOLUTION OF BONE

Abstract

Vertebrates are practically unique among the Metazoa in their possession of a skeleton made from calcium phosphate rather than calcium carbonate. Interpretation of the origin of a phosphatic skeleton in early vertebrates has previously centered primarily on systemic requirements for phosphate and/or calcium storage or excretion. These interpretations afford no anatomical or physiological advantage(s) that would not have been equally valuable to many invertebrates. We suggest the calcium phosphate skeleton is distinctly advantageous to vertebrates because of their relatively unusual and ancient pattern of activity metabolism: intense bursts of activity supported primarily by rapid intramuscular formation of lactic acid. Bursts of intense activity by vertebrates are followed by often protracted periods of marked systemic acidosis. This postactive acidosis apparently generates slight skeletal dissolution, associated with simultaneous vascular hypercalcemia. A variety of apparently unrelated histological features of the skeleton in a number of vertebrates may minimize this postactive hypercalcemia. We present new data that suggest that postactive skeletal dissolution would be significantly exacerbated if bone were composed of calcium carbonate rather than calcium phosphate. The former is far less stable both in vivo and in vitro than is calcium hydroxyapatite, under both resting and postactive physiological conditions.