Evolution mystery: Why human breasts are permanent unlike other mammals
Sydney Sweeney's fame meets ancient DNA to solve a 6-million-year puzzle...
A detailed post on LessWrong tackles the evolutionary mystery of permanent breasts in human females — a trait absent in all other mammals, where mammary tissue only swells during pregnancy and recedes after weaning. The author, rba, outlines four classical hypotheses, ranging from mate-signaling of nutritional health to a side effect of fat storage pathways, citing a 2021 review by Pawłowski and Zelazniewicz that even suggests origins in Homo ergaster.
Using ancient DNA from Neanderthals and comparisons with chimpanzee genomes, the post proposes a comparative genomic study to determine whether breast permanence evolved before or after the human-Neanderthal split (~500-700k years ago). Key references include the Venus of Willendorf figurine (30k years ago) and modern celebrity Sydney Sweeney as visual aids. The ultimate goal is to pin down the timing and selective pressures behind this uniquely human trait.
- Human breast permanence includes two separate traits: early puberty development and arrested involution after weaning.
- No fossil evidence exists for breasts, but Upper Paleolithic figurines like Venus of Willendorf (~30k years ago) suggest prominence.
- A proposed study will compare Neanderthal and modern human genomes to date when the trait evolved, potentially 500k+ years ago.
Why It Matters
Understanding unique human traits like breast permanence reveals how evolution shaped our biology and health, including cancer risk.