To be blunt they serve a purpose: They're for lactation to feed infants, and they're also fat repositories but that's just utilisation of en existing organ. They serve one purpose: food. They're a feature of every mammal, but I suspect due to loss of fur, a woman who had a propensity to carry subcutaneous fat due to genetic mutations passed down, and had decent repositories would have been more likely to survive cold and famine, and therefore was more likely to pass on her genes that coded for that.
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