In species with penile spines, Kingsley said, females tend to mate with multiple males. Penile spines may have evolved to clear out a competitor’s sperm – or to abrade the female’s vagina, making her less likely to mate with others. Either way, Bejerano said, “the loss of the spines is most often seen in species that have gone more the monogamous way.”

Mice whose androgen receptor gene is disrupted don’t develop penile spines, Kingsley said. The same may be true of humans, who have lost 60,000 base pairs of DNA right next to that gene. (A base pair consists of two nucleotide molecules that sit opposite one another on complementary strands of DNA.)

“Humans have thrown away the molecular switch from a key gene that’s required to form the spine,” Kingsley said.