Editor(s): Pearay L. Ogra, Allan Walker, Bo Lonnerdal.
Key Note Lecture: The Evolution of Lactation in Mammalian Species
The evolution of the mammary gland and milk has been the subject of speculation since the time of Charles Darwin. Olav T. Oftedal (Smithsonian Environmental Research Center) described the latest revelations on the paleobiology and evolution of this fascinating aspect of mammalian reproduction.
The synapsid ancestors of mammals diverged from the sauropsids (ancestral to crocodiles, lizards, and birds) around 300 million years ago during the Carboniferous period. Lactation may have first evolved to provide a source of moisture and anti-microbial compounds for parchment-shelled eggs.
Milk contains a species-specific mixture of lactose and a variety of neutral and acidic oligosaccharides. Lactose is dominant in most eutherian milks, but oligosaccharides dominate in monotremes, marsupials and caniform carnivores. The distribution of oligosaccharides across the mammalian taxa suggests that oligosaccharides may be ancestral. Compared to the milk of other mammals, human milk has the greatest oligosaccharide diversity.
The milk constituents of evolutionary interest include the casein micelles, milk fat globule membrane, components of the milk sugar synthetic pathways and whey proteins. Many of these are unique to the mammary gland and can be found across all three existing mammalian taxa. All the mammalian milks studied so far contain the four primary types of caseins (αs1-, αs2-, β- and κ-caseins), suggesting that these originated from a pre-mammalian ancestor. Caseins are members of the secretory calcium-binding phosphoproteins (SCPP). Based on related SCPP genes, caseins are thought to have originated from a proto-lacteal secretion that delivered calcium to eggs.
The milk fat globule is another mammary-specific component. Interestingly, some of the key proteins in the milk fat globule membrane participate in immune function in other tissues. These observations suggest that the ability to defend against microbial invasion was of great functional importance during the evolution of milk components.
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