Mental note: keep eyes focused above waist level if I run into CookingWithGas.
Lefty loosey, righty tighty.
I know a no-testicled man that has a child that looks like someone else. Does that help?
One is the lonliest number.
We have to be careful of delving into speculative evolutionary “just-so” stories here. It’s easy to look at the situation and think up all sorts of reasons why it is the way it is, but without hard evidence, we can’t say we know that any given benefit is the reason that drove things that way.
IOW, we can look down and go “oh, right, redundancy!” and go about our day. However, what evidence is there for this? As someone pointed out, there’s only one penis. Our internal organs are mixed - some we have one of, some we have two of. There’s no obvious reason why redundancy, as a driving force, shouldn’t have been there for all of those.
The fact is, the basic vertebrate body plan predates any adaptation to specific shapes. Dual gonads came along long before they were dangling out in the air, open to damage.
I’m not an evo/devo guy, but I think the only real answer is that when bilateral symmetry came along, the gonads ended up in the “let’s make two of these” camp. Evolution being evolution, we’ve been using that same plan ever since. Without some clear evidence of other driving considerations - and perhaps more importantly, mutations that give viable alternatives - I don’t think we can really say more than that.
Yes, but so are both kidneys and both lungs.
I think Smeghead has the best answer so far: we really don’t know.
True. I believe it reinforces the theory that evolution does not have a path. Changes are random, sometimes they persist because they are beneficial, sometimes they persist because they are simply benign.
Sometimes an advantageous change just doesn’t happen, I mean how useful would eyes in the back of your head be?!
It might be mentioned that in some organisms organs that are normally doubled have been reduced to single ones. Most birds only have a single ovary (as do some burrowing snakes, and most snakes have only one functional lung.
Offhand, however, I can’t think of an animal that normally only has a single testis.
Snakes and lizards have two functional penises (called hemipenes). Most marsupials have a bifurcate penis (two-pronged dong, as we referred to it in vertebrate anatomy).
Strangest of all, female marsupials not only have two separate uteri, they have three, count 'em, three, vaginas. (Two are for reception of the penis(es), while the third is actually a birth canal that develops after conception.)
Yes. I have a cousin who lost a ball (to cancer), married, had a daughter, and had a son, in that order.
Colibri, do I understand you correctly that in birds with one ovary, the males still have two testes? How does that work embryologically? Is the number of gonads fixed from the start of development, or do they start with two and one atrophies in females after the point of sexual differentiation, or do they start with one and then males grow another one?
Males have two testes. It’s the easiest way to sex birds internally, by noting whether they have one gonad or two.
Female embryos develop paired ovaries and oviducts, but after hatching the right ovary and oviduct degenerate (in most species).
This situation seems to have evolved at least 125 million years ago.
I know the jury is still out on this one, but a single organ, in the centre, would also be bilaterally symmetric.
Good username and post combo!
I used to work in an office where 4 out of 6 guys had only one testicle. That was one boring day when that subject came up! 3 of those 4 have had both male and female children who look vaguely like them. One of them had 2 children via egg donation and ivf as his wife had issues as well. So that’s almost guaranteed to be his sperm that was used for both kids.
(Just for the record, I was one of the 2 that had an above average number of balls)
IANA biologist or even a serious student of evolution, although I understand the principles. Therefore I do not know what would constitute hard evidence. What do scientists consider evidence of specific evolutionary adaptations? What would be considered evidence for the evolution of duplicate organs for the sake of redundant backups?
There has also been mention in the thread of symmetry. How does one explain all the organs that are not symmetrically duplicated?
This is a tricky one to test. Ideally, you’d want to have two identical organisms (or populations of organisms), except one had the organ in question duplicated, and one would have it just singular. Then you’d have to experiment and measure the level of evolutionary fitness. Perhaps mix them together and see which type fathered more children. That sort of thing.
Alternately, you might be able to look for signatures of selection in the genome, but that would involve knowing exactly which genes were mutated in the change from “one teste” to “two testes”. And since that happened so incredibly long ago in evolutionary time, I doubt any such signature would still be around. And if the mutation in question duplicated other organs - not just testes - then it would be unclear which duplication was being selected for.
Reminds me of a prank that a friend of mine with one testicle (actually two, but the other had failed to descend into his scrotum) pulled on a young lady he was dating.
One evening early in their relationship they were having fun in bed, and her attention turned southwards. He said he could feel her down there apparently counting his tackle, and getting puzzled when instead of “One, two!” she kept coming up with “One…one…one!?”
Her head popped up and she blurted, “You only have one ball!”
He started to laugh. “Hah! That’s a good one. I suppose you’re going to try to tell me other guys have more!”
Humans have two hearts, in a manner of speaking. It’s just that the right side pumps blood to the pulmonary system to be oxygenated and the left side pumps the oxygenated blood to the other tissues. In theory, we could have one ventricle and have our blood oxygenated on the way. We just didn’t develop that way.
As has been noted, the reason we don’t have two hearts or brains is that they require tremendous amounts of energy. In humans, 25% of our metabolism is devoted solely to supporting the one brain we do have.
I vaguely recall reading (Colibri?) that flying animals invariably have less brain mass. Not because brains are heavy (though they are), but because the amount of energy required to fly (versus locomotion) leaves too little to support “normal” brain function.
Would one with 10 kids suffice? No, it wasn’t that the second one was undescended, or so I’m told. His widow jokes that if he’d had both balls they would have been able to repopulate Sahara.
Although you have to remember that once something evolves, it doesn’t have to remain there for the same reasons. If redundancy, for example, wasn’t the original advantage to two testes, it could still be part of the reason we have still two now.
I’ve never heard that, and a priori I would expect the opposite. Flying, especially in complex environments like forests, is a much more complicated task than walking or running. For one thing, you have to navigate in three dimensions.