How mammals lost their egg yolks

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. Image: Velo Steve

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