IT IS a chicken and egg question – did mammals evolve nutritional milk before or after they abandoned yolky eggs?
“Milk was originally for egg wetting,” says Henrik Kaessman at the University of Lausanne in Switzerland. Instead of a hard shell, the first mammalian eggs had a parchment-like covering which mothers rolled in milk to prevent them drying out, he says.
Today, placental and marsupial mammals nourish their newborn young with milk containing a calcium-packed protein called casein. It was suspected that even though the platypus lays eggs, its milk would also have casein-like proteins, which Kaessman’s team confirmed through genetic…
