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 analysis. This finding suggests nutritional milk would have arisen in all mammals in a common ancestor, up to 310 million years ago.


Kaessman says mammals abandoned yolky eggs long after they began lactating, as confirmed by his genetic analysis of three genes for a protein called vitellogenin, which ferries nutrients into the yolky egg. All the genes are active in chickens. None are functioning in placental or marsupial mammals, but one still works in the platypus, which has small yolks in its eggs.

Journal reference: PLoS Biology (DOI: 10.1371/journal.pbio.0060063)