I remember from High School Biology that Leopard Frogs, and perhaps all Amphibians, have a single kidney. How is it that all higher animals now have two kidneys, and at least humans can live perfectly well with just one?
Is there some reason why it was so advantageous to have two kidneys, and if redundant organs are so valuable why not two hearts, two spleens or two livers?
Because if we had 2 hearts we would be Timelords?
Other than that, I got nothing.
The Wikipedia entry on frogs says that they have two kidneys.
What makes you think there’s a “reason” why we have two kidneys instead of two hearts?
Bilateral symmetry.
It need not be advantageous – often it’s sufficient that a given trait just not be detrimental.
My guess is that this was some failed attempt to pretend he was asking for a religious reason. If he was, it’s still a useless comment, and he wasn’t. Scientifically speaking, there’s a LOT of reasons we have the former but not the latter.
Almost the entire human body is duplicated. Of the major organs, the most significant items we don’t have backups for are: the brain, heart, external genitals, and digestive system. In each case, duplication is largely unneccessary and/or would require massive alterations which would likely hinder the actual function, AND in some cases we have other forms of backup or regenerative capacity.
The brain,of course, does have two lobes, and if one side is damaged can be partly compensated for from the other. It’s also very well-protected and difficult to seriously damage without repeated shocks or a fairly powerful penetrating weapon. the external genitals could in theory be duplicated. But because of the location and internal plumbing involved, this wouldn’t really help anyone. The limiting factor on human reproduction isn’t really the raw physical limits of childbearing, and the gentials are situated such that any injury bad enough to damage one set would harm another. Note, of course, that males do have two testicles, but really only need one in any case. The digestive system is a huge energy drain in its own right, so duplication could be a problem for that reason. But even apart from, damage to the digestive system is usually recoverable provided that you don’t die from blood loss or sepsis. It could be a hypothetical aid in keeping you eating while wounded, but if you’re badly wounded enough to be laid up for that long in a pre-modern environment, you’re likely dead anyhow so it’s a pretty minor gain. The liver is pretty resiliant and can usually survive damage to one point; it actually has a lot of lobes and and doesn’t need all of them.
The heart is a bit like all of them. It’s a huge energy sink, very well protected, and likely to kill you if damaged anyway. Remember, too, that it’s a pump system. Pumps need to be insulated form each other somehow or they would interfere - you’d be giving yourself heart attacks.
In short, most of our duplicated organs are ones which we need to survive, but which are relatively at risk for damage or where having duplicates is a small investment. None of this means such an arrangement is impossible, but it’d be very difficult. You’d have to redesign the entire human body structure to support it, and there’d likely be enough downsides that you won’t “win” at the end of the day.
The more I think about it, the better the question is actually.
You could reasonably divide the body into to categories - single and duplicated organs. And the list doesn’t really make a lot of sense.
Single: heart, digestive tract, liver, pancreas, spleen, pituitary, pineal, thyroid,
Duplicate: lungs, kidneys, adrenals, testes/ovaries, eyes, ears, breasts.
Special case: brain - it is partially symmetric, but the lower parts aren’t really, and function is all over the place.
One notes the kidneys, adrenals and ovaries/testes develop together, and there is likely some link between this and their duplication.
But, whereas we can make a good case for some fitness advantage to the redundancy afforded by duplication of some organs, in many cases it isn’t huge. I would guess that the “reason” is going to be whether the point at which the cells that are the progenitors of the specific organs differentiate is before or after the embryo differentiates into its bilateral symmetry. If the differentiation occurs early, you get one organ, after, you get two. But that is going to be pretty simplistic.
I also suspect that at least some animals that are noted to have one kidney, may well simply have two functionally separate kidneys fused into one lump. Looking at the blood supply and ureter plumbing might be a reasonable thing to do to decide if there is really only one organ.
Our body plan is based on bilateral symmetry. Organs which during a critical phase of embryonal development are located in the plane of symmetry develop into a single, often itself bilateral symmetric organ, eg. the spinal cord, the heart, bladder or the gastrointestinal tube. Organs which develop at a distance of the plane of symmetry develop as paired organs: ovaries and testes, kidneys, lungs, as do the extremities: arms and legs.
The heart could also be said to be partly symmetrical as it is really 2 pumps fused together and I do believe there are conditions where one side is damaged but the other still can pump - or some such variations.
It appears like when a larger single system is more beneficial the 2 smaller systems a single larger system is what happens. The most obvious one is digestive, where we have the small intestine that loops back and forth and a long length is desirable to absorb as much as it can. Having 2 would make this system shorter (or narrower), and less efficient.
Another combined system is the reproductive system where the most beneficial way appears to be having a single large space for a child to develop inside the mother instead of 2 smaller spaces.
For kidneys I would say there was no real benefit to a single large one so there was never any pressure to produce it. Also due to there position it would seem to ideal for a duel system.
You have two hearts. They are just conjoined to form the single larger organ. Your whole body is this way. We have two distinct kidneys because they aren’t right next to each other. The obvious advantage is that you can lose one kidney and stay alive. Lose half your heart and the whole thing stops functioning.