The sex of human and all mammalian babies may be determined by a simple modification of a virus that insinuated itself into the mammalian genome as recently as 1.5 million years ago, a new study has found.

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The sex of human and all mammalian babies may be determined by a simple modification of a virus that insinuated itself into the mammalian genome as recently as 1.5 million years ago, a new study has found.