Evolution of lactation: when, how and why.

Mammary glands are unique to mammals. Still unclear is whether lactation appeared suddenly, in line with the punctuated-equilibrium theory, or gradually, through progressive acquisitions, more in accordance with the classical Darwinian model. The mammary gland seems deriving from an ancestral apocrine gland associated with a hair follicle. Such associations with a vestigial type of hair are present in monotremes, and also in juvenile, developing marsupials. Monotremes have dense mammo-hair-sebacean units connected with nipple-free areas. Perhaps these units derive from archaic glandular concentrations that evolved to keep moist and provide other components to the synapsid parchment-shelled egg. In contrast to the sauropsid rigid-shelled egg, which has a semi-permeable wall that permits air to be absorbed, the parchment-shelled egg is permeable enough to absorb and evaporate fluids. Proto-mammary excretions, far before milk was ever devised, may have evolved to provide water and nutrients to eggs. These fluids contained also anti-microbial components that passed to the developing embryos. The lineage of amniotes that led to mammals first appeared about 305 Ma. Mammal-like reptiles radiated during the next 100 Ma. Basing on osteological evidence alone, paleontologists classify as mammals synapsids that possess a dentary-squamosal jaw articulation and dental occlusion with a transverse component to the movement. The conclusions of genomics analyses and physiological studies suggest that paleontologists should perhaps focus on other skeletal changes (i.e., smaller size, secondary palate, diphyodont dentition, and epipubic bones) that seem to be more or less directly correlated with the acquisition of mammary glands and higher metabolic rates.