This article never answered a very basic question: so evolution exiled man's balls from the body because sperm are temperature sensitive - but why didn't sperm just evolve to be less vulnerable to high temperatures? Anyone have a clue on this one?
With evolution, there's no "why", unless you count "it was evolutionarily beneficial, so it was selected for". But even that doesn't explain everything, like genetic drift.
From Wikipedia: Teleology (Greek: telos: end, purpose) is the philosophical study of design and purpose.
What happens is that ideas are often conveyed using teliological phrases, such as
"the sparrows migrate to africa in the winter month so as to find abundant food sources and rear the next generation..." or "testicles reside outside of the body major in order to benefit from a reduced temperature...".
The problem is that these make sense to us, although they are _very_ incorrect. The teleological concepts of "so as" or "in order to" are the whys and they are very tricky beasts. So, for utmost correctness one would prefer to say:
"the sparrows migrate to africa in the winter month finding the abundant food sources needed to rear the next generation..." or "testicles reside outside of the body major benefitting from a reduced temperature..."
It's a very subtle issue, and one that is compounded by the popularity and ease with which we talk about biology and evolution in particular. Ernst Mayr wrote some spectacular essays on the subject, he's one of my heros so forgive me for the blatent plug! He claims that a formal understanding of the errors of telology must permeate the sciences, and most especially biology (he was a philosopher of biology), so as to clear up the ambiguities that can lead one to think there is an end or purpos (telos) to evolution. Even now it is still quite common for people to think of it in terms of progress and constant betterment instead of just simple and blind adaptation.
Another reason that people may react harshly when you use teleological language is that biology (unlike physics) is constantly under attack by people with all sorts of crazy theories about a supernatural designer - (talk about a teleology!) so I can understand why such language would be somewhat taboo.
Another reason might be that maintaining the testicles at body temperature increases the risk of infection. There are very few purely sexually transmitted diseases, likely due to this temperature difference. Those same sexually transmitted diseases rarely cause harm beyond the genitals, and in fact these diseases are far more harmful to the female reproductive system than the male (a woman with undiagnosed gonorrhoea or chlamydia is essentially guaranteed to become sterile, however in men it either takes longer, or they may never become sterile - note that a male can lose a testicle to injury or disease without becoming sterile or losing fertility).
It's likely that there were more advantages to any male with dangling gonads over retracted. The reason why our testicles require constant temperature control (and independent control) is probably a more intriguing question, which IMO likely has a lot to do with our method of motility. A dog's testicles are unlikely to be affronted by the sun, however any naked male human has a serious problem when walking toward the sun. This likely wasn't a problem when we were semi-erect (in the evolutionary sense, not the other sense) and had hairy gonads, however why did modern humans develop wholly unprotected gonads? Think about it, our testicles don't even have protection against the sun! Dense hair provides protection against heat stroke, so did humans already have protection for our genitals before this became a problem? IE did our invention of clothing happen early enough that it actually changed the development of our sexual organs?
(nature tends to work this way; it's essentially a hack right)
From what I know about biology/evolution, this answer is just as likely to have a single clean reason as it is that you killed the one and only cockroach living in your apartment.
One thing the article did get me wondering, and if any hacker here has gone through this and be willing to admit it, i'd love to know:
If you have to ask a doctor for help get pregnant... i.e. if your little fella isn't up to the job- i wonder if there's a course of therapeutic treatment simply around better managing the temperature of the scrotum? I'd imagine it'd be hanging around in loose fitting shorts in a temperature controlled setting until such time as your partner would be ovulating? :)
With ~4 million sperm cells per... egg, there is enormous selective pressure that gets exerted by competition among individual spermatozoa. So the sperm cells that are the most temperature-sensitive (hence best able to take advantage of the metabolic boost due to increased ambient temperature, hence on balance faster-moving) would be heavily favored.
I feel like there needs to be a Your Mom joke in here somewhere...
The sperm are all produced by the same individual, and so you might expect them to be essentially identical aside from the genes they carry, in which case there's no Darwinian selection between them.
However, if genes in the sperm are actually expressed (which would be normal for most other cells) then due to the process of meiosis, different sperm will have different genes and so selection between sperm from the same man could indeed occur.
Sadly I don't know the details of gene expression in sperm... the question really is whether they get their proteins from the parent cells (all of which are genetically identical to the man) or whether they make some of their own proteins.
Ah, I see from Google that there is indeed some gene expression in sperm: http://8e.devbio.com/article.php?ch=19&id=193
(In any case, there could of course be selection at the level of individual human males, with those producing better sperm having higher reproductive success.)