It seems to me that it is not beneficial for the male human that the nutsack is outside the body. I’ve read of animals that have their nuts inside their bodies, and this setup seems greatly preferable to what men have. The downside of the nutsack being external is that it leaves the nuts dangling and exposed to danger. Obviously today we wear clothes that (usually) cover the groin, so some of the danger is reduced, but still, the nuts being outside of the body gives a man a really obvious weak spot - a good kick in the testes can bring even the strongest man to his knees, writhing in pain.
It seems like evolution should have favored men who were, for whatever reason, born with undescended balls. Why hasn’t it?
Apparently, sperm production in humans works better when the temperature is slightly below your normal body temperature. The advantages of having robust swimmers outweighs the disadvantage of having dangly nuts. The primary disadvantage, of course, being shrinkage.
My daughter when asked about the International Space Station stated that she swore it was located in Orlando. To this day we laugh about me asking her- "Out of a hundred million sperm you were the fastest and strongest?
The Wikipedia article suggests that alongside the efficiency of spermatogenesis (due to spermatogenic DNA polymerase beta and recombinase), basal boreoeutherian mammals may have provided humans with an irreversible adaption. It is hypothesised that these creatures may have existed in competitive sexual environment, had larger testes, and therefore developed them outside the body. The enzymes evolved at this temperature, and we’ve been stuck with them ever since. Notably however, other examples such as the kangaroo, belonging to a different mammalian clade, have also developed this adaption, perhaps indicating some other reason for its occurrence. This could be protection from abdominal cavity pressure changes due to jumping or leaping, which might otherwise expel scrotum contents into the reproductive tract, due to the lack of a sphincter (Chance 1995).
Certain mammalian species are able to retract the scrotum semi-permanently or in a reduced external form outside of their mating season. Humans, being tropical, don’t have a time-limited breeding pattern so maintain them externally all the time. They also don’t need to be as large, due to less competition. We do have some protection insofar as the cremasteric reflex can semi-retract the testes during danger. Other mammals have circulatory adaptions to keep them cool inside the body, or with birds, maintain them at normal body temperature.
I wouldn’t say that the risk of injury is that great. Sure, any male can attest that the injuries that do happen are pretty brutal, but in day-to-day living the testicles are fairly safely located between the legs. They’re especially protected given that humans have excellent instinctive reflexes to swivel the body slightly when danger’s coming our way, keeping the next generations intact. Plus, they’re naturally insulated by a tuft of hair, and they naturally retract closer to the body when cold, keeping the swimmers from freezing.
Also; there are mammals with internal testicles that keep them cool with an external “cooling system”. However; the thing about evolution is that it has no foresight, no plan, and no way of copying a superior design cross species. So unless just the right mutations just happen to randomly appear, evolution won’t push humanity in that direction because it has nothing to work with. Evolution is a reactive process.
Captain Awesome, good answer. Of course the real answer is:
There doesn’t HAVE to be a particular benefit to humans having them there. We inherited them from some other precursor animal that developed them there, and it hasn’t proven disadvantageous enough to the continuation of the species to disappear.
They also serve as an advertisement to maleness and male fitness. Some species of primates have very brightly colored nuts. If you ever see howler monkeys in Costa Rica, the males have very dark fur and bright, blue-white nuts. Having such a conspicuous sack might be an advertisement to females and/or a warning against other males.
Even if its not the best system, having everything internally isnt so great either. Since day 1 of caveman surgery, you’ve always had the option to remove the testes. A life threatening tumor can be cut off and if done before metastasizing can save a life (ignoring infection, bleeding to death, etc). Historically, a man has a 1 in 250 chance of getting testicular cancer.
A tumor in a woman’s ovaries doesnt have such a simple solution.