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What is difficult, however, is nourishing unborn young the way mammals do.
A female mammal allows each embryo to burrow deep into the wall of her
womb, where it takes nutrients straight from her blood. This intimate
arrangement was long thought to have only evolved once, in mammals.
Not so. It now appears that it evolved at least twice: once in mammals,
and once in an obscure African lizard called Trachylepis ivensii.