"External testicles—which swing delicately outside the abdominal cavity in an exposed, thin-skinned sack—are sensitive, finicky, and make a glaringly obvious target for any enemies of men...."

"So it’s no surprise that the evolution of descended testicles has both baffled and bemused biologists for decades. A study published today in PLOS Biology offers an answer to one of the mystery’s biggest debates: did our earliest ancestors wear them up, or down? The researchers conclude that the first mammals already had this bewildering trait, with millennia of descendants thereafter inheriting the family jewels on full display. Yet strangely, it appears that since then, internal testes have evolved at least four separate times.... Scientists have known for years that one of the most important benefits of scrota is ventilation: mammalian sperm matures and stores better at temperatures 2.5 to 3 degrees Celsius lower than the rest of the body, and jettisoning these organs keeps them cool. But we are far less sure if this is the reason that scrota evolved. It’s a classic rooster-and-egg dilemma: testes may have fled the abdomen because temperatures got too toasty, or sperm may have adapted to love the chill because they had already been ousted for some other reason."

From "The Earliest Mammals Kept Their Cool With Descended Testicles/But if free-swinging sperm sacs are the norm, then why did undescended ones evolve four separate times?" (Smithsonian).