Definitely appreciate that you put design in quotes - we aren't really designed in the way that most people think about the word.

My understanding is more like things are tried until something sticks. Maybe internal testicles would have been optimal, but external ones were first and simply good enough to stick around.

It’s also possible that there was a different adaptation that happened at the same time that was more important and that external testicles weren’t necessarily good enough to stick around, but rather not bad enough to ditch!

to make a bad CS analogy, imagine 4.5 billion years of grad student code. Each generation does juuust enough work to graduate, makes liberal use of chicken wire and duct tape, JB weld if you're lucky. Source control is for suckers. Mutation is a great design pattern. Globals are obviously the best. Goto is fine. Dependency management is for lesser beings. The code was hard to write, so it should be hard to read. There are no comments and if there were they would lie.

And meta programming. Lots of meta programming.

It's also possible, in a roundabout way, that external testicles are more for display rather than practicality. Sexual selection has a fairly heavy weight on evolution, even if such a trait has some pretty big downsides (male peacocks being a prime example of this).

That would make total sense if the human mating ritual involved the male impressing the female with the size of his testicles.

How do you know it didn't? Culture changes faster than evolution.

http://mentalfloss.com/article/79897/why-arent-classical-sta...

“Ancient Greece was a highly masculinist culture,” she explained. “They favored ‘small and taut’ genitals, as opposed to big sex organs, to show male self-control in matters of sexuality."

exactly, most of culture changes fast and for there to be a sexually selective pressure on testicles for a long enough time to make a difference, it would surely persist into recorded history. Either human mating practices are cultural and transitive so they would not exert continuous evolutionary pressure, or they are somehow more ingrained and thus could exist long enough to trigger noticeable evolutionary changes.

> the pain associated with getting hit in these all-important reproductive organs is so severe that he will go to great lengths to avoid it and protect them

Too bad we're so soft-headed that we spend our formative years doing our best to hit each other in the testicles as much as possible!