Origin and evolution of the major constituents of milk

Abstract

Accumulating evidence indicates that the evolutionary novelty known as milk, and its constituents, is of ancient origin, dating back 300+ million years. Milk may derive from the secretion of apocrine-like skin glands that produced water- and antimicrobial-rich fluids of benefit to attended eggs. The casein micelle evolved from much simpler secretory calcium-binding phosphoproteins. The milk fat globule evolved by co-opting membrane (butyrophilin), cytosol (xanthine oxidoreductase), and intracellular lipid droplet (adipophilin) proteins of secretory cells into new or expanded functions. Milk sugar secretion required structural and functional modification of a secreted antibacterial protein into a Golgi regulatory protein (α-lactalbumin). Other whey proteins (β-lactoglobulin, whey acidic protein) apparently lost prior functions in becoming nutrient sources for suckling young.