Calcium homeostasis and human evolution

The maintenance of calcium homeostasis is critical to survival. The systems dependent upon the availability of calcium include the musculoskeletal, nervous, and immune systems, all of which utilize the ionic properties of divalent calcium to perform critical functions. The widespread utilization of calcium in essential biological processes across a wide range of phyla argues strongly for its adoption early in the evolution of life. In the broad context of evolution, the use of calcium in the structure of the skeleton is a relatively late, or derived trait. It is not surprising, therefore, that when conditions demand the resorption of bone to sustain normal blood calcium levels, resorption can lead to serious impairment of function. Nowhere is the force of natural selection greater than when traits essential to successful reproduction are directly involved. Thus, when the requirements of pregnancy and lactation demand it, major shifts in the endocrine regulation of calcium turnover occur. While beneficial to reproductive success, elements of these changes can have deleterious effects later in life. However, selective pressure on such traits diminishes with the years of remaining reproductive life. The genetic determinants of traits that only manifest their deleterious aspects after reproduction has ceased altogether are thus able to remain and even increase in frequency in the population's gene pool. Such traits have been designated antagonistic pleiotropies. The following discussion explores the possibilities of Type I osteoporosis, a condition experienced by many postmenopausal women being an example of antagonistic pleiotropy.