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. Metabolically, because they are fat deposits requiring calories and fitness-wise, because the tissue easily lends itself to malignancy. Breast cancer is apparently rare in captive apes and is overwhelmingly a human disease, often striking women young enough to have children, and so subject to evolutionary selection.

Background

In Descent of Man, Darwin catalogs human secondary sexual characteristics, but he doesn’t seem to have noted human breast permanence as an issue of interest. Cant, 1981 seems to have been the first to speculate about this systematically and believed breast prominence and permanence might have evolved as a nutritional signal of health to mates indicating potential for maternal investment, a la Robert Trivers. Since then, quite a range of hypotheses have developed, and not surprisingly, none of them have any strong evidentiary basis. Pawłowski and Zelazniewicz, 2021, is a review of many of them, and they argue that breasts were a side effect of evolution in fat storage pathways; they even go out on a limb and suggest the trait originated in Homo Ergaster.

The truth is, we know practically nothing about the timing or evolution of breast permanence in humans. Here’s what we do know.

We know our chimpanzee relatives don’t have breast permanence, and our ancestors split from theirs 6-7M ya.
We know that Neanderthals’ ancestors split from ours 500-700K ya. Despite the arguments in the review paper above, we have no idea about breast permanence in any extinct archaic hominins. There are no boobs in the fossil record.
We know various Upper Paleolithic figurines like the Venus of Willendorf, originating around 30K ya show breast prominence, though it’s possible that depicts a lactating woman or is otherwise exaggerated as artistic liberty.
We know Sydney Sweeney is a famous person today.

The visual...

Perseus

Perseus