Darwinian transformation of a 'scarcely nutritious fluid' into milk

In an early challenge to an aspect of Darwin's theory of natural selection, Jackson Mivart contended that milk could not have evolved 'from a scarcely nutritious fluid from an accidentally hypertrophied cutaneous gland'. The evolutionary change from a gland secretion to milk involves an increase in calcium and protein concentrations by up to 100- and 1000-fold, respectively. Even so, the challenge, we suggest, is not just a problem of scale. An increase in the concentrations of calcium and phosphate brings an increased risk of calcification of the secretory gland because calcium phosphate is highly insoluble. In addition, two of the four constituent milk casein proteins (κ and α _S2) aggregate to produce toxic amyloid fibrils. It is proposed that both problems were solved through the cosecretion of ancestral β- and κ-caseins to form a stable amorphous aggregate of both proteins with sequestered amorphous calcium phosphate, that is, a primordial casein micelle. Evolutionarily, a gradual increase in the concentration of casein micelles could therefore produce progressively more nutritious fluids for the neonate without endangering the reproductive potential of the mother.