Who Got Breasts First and How We Got Them

It really is Sydney Sweeney’s world, and we’re all just living in it. Human female breasts are an evolutionary mystery along several dimensions. First, breast permanence is unique to humans. All other mammals develop breast prominence during pregnancy or nursing, and the mammary tissue recedes after weaning. This process is called “involution”. In contrast, humans develop breast tissue at puberty before first pregnancies and maintain it permanently after last pregnancies. Second, breasts are costly, both metabolically and potentially from a fitness perspective.