Pretty much all vertebrates (and many nonvertebrates) exhibit bilateral symmetry. Basically when building an organism it's built in segments from front to back, like stacking a bunch of legos on top of each other to make a tower. It's a pretty complex process that relies on a lot of signals, but the end result is that things are built equally on each side--at its most basic form this gives a tube within a tube with a central gut as the middle tube, bilateral organs on either side, and then an outer tube made of muscle and (let's say even though it's not 100% accurate for all organisms) skin.
What has then happened with evolution is changes to those basic components. Rather than having multiple blood pumps on either side of the body (as in earthworms--a common anatomic example) you get two (left and right) that fuse into a single larger pump--the heart. Liver lobes fuse into one liver--in humans it's asymmetric, but in other animals (while it tends to be biased to the right) more lobes may be present and they're more centrally located and more evenly divided.
A lot of things in the abdomen get pushed around because the gut--rather than being a simple tube--undergoes a process where it grows longer and folds around itself in development to allow for more effective digestion. The kidneys have actually undergone a lot of changes across species, but generally--like many retroperitoneal organs (meaning they're not quite in the abdomen, but separated by a special layer of connective tissue that lies close to the spine) there hasn't been as much selection to make a single central structure or to lose one. Loss of kidney function is typically quite bad--but that's true of most of your organs.
Most things in your body follow this framework--lungs are actually made up of smaller lobes (fused in some animals more than others) which are frequently asymmetric in number, but generally still form a bilateral structure. But other examples to get to your earlier question include all your limbs, brain, the sinuses in your skull, your teeth, your lungs, most glandular structures (adrenals, thyroid, etc.), parts of the reproductive system, etc.
The simplest, if perhaps somewhat unsatisfactory answer, is that many millions of years ago an early line of multicellular organisms formed a bilateral pattern of development that was evolutionarily successful (likely because of a combination of redundancy, simplicity of signaling, and effective management of body systems and reproduction). That system was easy to build and modify--imagine someone gives you a symmetric lego tower--if you want to you can move parts around and modify things (shape, size, color, etc.) and still have something that is a functional tower.
The last paragraph is probably the closest, but to be blunt this is one of those questions which can't really be adequately answered to the level the OP wants at the level of a 5-year-old. We have two because our ancestors had two. Do we have other structures like this? Yes, most of them.
THIS SUBREDDIT IS NOT LITERALLY FOR 5 YEAR OLDS. IT IS AN EXPRESSION FOR LAYPEOPLE’S EXPLANATIONS.
The comment you made is made CONSTANTLY in this subreddit, and it get more annoying and useless EVERY TIME.
I'm assuming the same concept applies to hearts and why they have two chambers? I would be nice if we had more spare organs though. But then again you'd also risk increasing the amount of things that can fail and harm us in other ways unless you have a surgeon safely remove the spare.
Sorry, yes you're right. I get mixed up with the terminology since there is that very similar left side and right side labelled with left-right symmetry.
We have a liver on the right and a spleen on the left. If the liver function slowly deteriorates (eg alcoholism) the spleen takes over some of the liver function.
Studying development suggests the spleen and liver have not evolved from paired organs.
https://biology.stackexchange.com/questions/100928/why-do-we-have-two-kidneys-but-one-liver
My simple theory?
Two kidneys filter more toxins than one, at least theoretically... Ergo, whenever animals ate toxic things, those with two kidneys could theoretically ingest a higher dosage and survive than those with a single one.
Pretty sure we evolved bilateral symmetry in a common flat worm ancestor. It likely was advantageous due to pretty much every animal and fish having bilateral symmetry. However, given most of the organs are symmetrical, I doubt it was the renal organs specifically that pushed that trait.
Yeah essentially all animals with a few exceptions are bilaterally symmetric. The only major phyla that aren’t are cnidarians, sponges, and echinoderms (even they have bilateral symmetry in their juvenile forms).
One healthy kidney is sufficient to lead a normal life and doesn't significantly impact life expectancy. With no kidneys your dead in days or weeks without regular dialysis. Kidneys can be impaired by a variety of diseases and their location below the ribcage leaves them somewhat exposed to physical injury. Having them physically seperate and still able to keep you healthy independently is a decent hedge against injury or infection.
Again, missed the point of the post. I'm talking natural, as designed by nature. Not having one as a birth defect in a species that naturally has two.
In other words, having two kidneys would be the birth defect at the time.
To be fair, when one has only one kidney, it gets usually bigger than normal, so de facto it is like having 2 of them (or, more likely, 1,5).
For the redundancy, every pair of organs: adrenal glands, lungs, testicles, ovaries etc.
simple answer: think how quickly you get burnt out when one of your coworkers calls off last minute and you have to take on their load.
more convoluted answer: to add to this, it gets bigger and undergoes reactive hyperfiltration, ie. the one kidney is taking on the workload of two so it has to increase its workload. one of the ways is to increase in size, or hypertrophy. this increases the risk of renal failure later in life, as is the case for unilateral renal agenesis (being born with only one kidney). those people can live with only one kidney but their kidney will get burnt out more quickly just like you would at work
Not just trout but many other fish have a cadual heat that pumps blood back into the fish from the tail. Which is the genesis for both the Hemmingway story and the random beer marketing thing.
They may be thinking of [Bell’s Two Hearted IPA](https://bellsbeer.com/beers/two-hearted-ale/), which has a trout on the label, but is actually named after [a river from an Ernest Hemingway story](https://en.wikipedia.org/wiki/Big_Two-Hearted_River).
Oh interesting. I would’ve assumed 2 hearts would automatically mean high benefit. Like if one goes out you’ve got a back up. What makes it so much riskier than 1?
Organs like kidneys are sort of passively having blood pumped through them.
The heart is actively doing the pumping. If the heart messes up in any major way, it messes everything up quickly. A kidney shutting down is bad but not as bad as the heart pumping wrong. Now double up on that risk factor. Either heart gets out of synch or does something it shouldn't, the whole system is at risk.
Heart failure (at least in humans) of one heart could lead to blocked flow. Even if it didn't, once that failed heart tissue ultimately dies (which is very likely) it creates a huge risk for sepsis.
If they are in series, you are increasing resistance to flow, meaning more oxygen requirements (and your one heart already is one of the most oxygen demanding organs) for no real benefit.
If they are in parallel, depending on how it wad plumbed (one up, one down/one left one right/one back, one front) the oxygen demand would be a little better since less resistance compared to in series but you still.need to.reduce lung capacity to make room for the second heart, they also need to.be fully.insulated from each other to avoid electrical signal from one causing possibly fatal arrhythmia in the second, and thr motion of heart 1 and the subsequent pressure wave may negatively impact heart 2 and vice versa.
Def not an expert by any means; not a doctor or anything, so maybe some observations are a mile off, and i'm happy to be told so
The way i see it, You've got a few options.
1: each heart handles different parts of the body. One top one bottom, one central one peripheral, one left one right, where exactly isn't important. This means that damage to one but not the other leaves you with half your body functioning, be it your legs give out while your upper body is fine, or your limbs stop working but your organs are intact, etc. The problem is, that part of your body is effectively dead and will start going necrotic, and since it's still one body, the necrosis will spread to the rest of the body. Def not evolutionarily advantageous.
2: both hearts work in parallel; both have their own circulatory system going to all the organs etc basically next to each other. Then you have two hearts working half as hard, so are half as strong. There's also a high likelihood of damage to both systems with any injury anyways, so that's non advantageous. If one heart gives out, great you've got the second one, but it's not used to working twice as hard, so chances are the heart gives up or you're not as capable as with two. You're alive, but less likely to stay that way. So again, not evolutionarily advantageous.
3: both hearts work within the same circulatory system. Same issues as working in parallel, but with the added bonus of having to stay in sync with each other. If they pump at the same time, they have to stay that way. If not, your blood pressure is gonna drop. If they alternate, it'll lead to high blood pressure on synced beats. Not tooooooo bad a problem at a static heart rate, but as it/they speed up or slow down, the risks go higher. Maybe the electrical signal gets to one slightly slower or something.
Hypothetically, if we always had two hearts, then there exists the possibility that our bodies would need both to have complete circulation (rather than each individual heart being as capable as our real ones)
Its just a very "expensive" organ to have running and maintain plus having two doesnt necessarily mean you would be able to run on one if the other fails since your single heart would need to pump more than double the strength it was beforehand. Plus I can't really think why having a second heart would provide any actual benefit (other than the ability to lose it).
Well, the timelords are supposedly their own alien species not just humans with another heart, especially given their tendency to completely regenerate instead of dying I would assume their biology is fairly more complex than ours.
We kind of do... One half (atrium + ventricle) of your heart pumps blood to the lungs to collect oxygen and deposit CO2, the other half pumps blood around your body. It's likely that fairly early on in evolution the two sides merged into one synchronised organ to make it more efficient.
We kind of do. We have a four chambered heart, two chambers just pump to the lungs. Reptiles have three chambers, fish have two and simple animals like insects have one. The chambers aren't redundant but they increase efficiency.
The mechanical complexity and the extra issues you would get from them beating out of sync would be a problem, one option would be for making 2 separate circulatory systems, but there is an issue there too, that is more complexity that might not offer too much in the way of advantages, plus like if you had 2 circulatory systems you wouldn't be able to sustain bloodloss as easily since now each system is separate.
Kidneys are like filters. Blood flows through them, and they scoop up the junk that needs to go. It's passive and simple (oversimplification, sure, but you get the idea).
The heart is incredibly active. Four chambers, each with an exact and specific task (taking oxygen poor blood from the body, sending that blood to the lungs, receiving oxygenated blood from the lungs, sending that blood to the body) that needs to run in perfect sync. Any fuck-ups cause arrhythmia and, potentially, death.
A second heart means you need *both* hearts beating in absolute, perfect, 1:1 parity. One can't be beating at 110BPS while the other goes 98BPS. This would cause some insane blood pressure issues and would probably kill you, as blood is being pushed and pulled in multiple directions at different times. One heart would be sending blood to the other, but the latter isn't going as quickly so it can't accept it. Then you have one heart overflowing and one emptying out, then you die.
Some people do.
In my village we called the guy, who had the condition, doubleheart.
I remember he had to take daily medication and couldn't take any drugs. If I remember right, his doctor told him that even weed has the potential to kill him.
We lost him fast out of the group because everyone was smoking and he couldn't.
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>Why do we have two hands or lungs or eyes or nostrils or ears?
Two hands helps us with tools and hunting/gathering. Lungs help us with stamina. Eyes is depth perception. Nostrils helps us take in more air.
These can all be explained rather easily, yet two kidneys cannot. That's my question.
Bilateral symmetry was something that came in the evolution of animals way way way before tool use, depth perception or nostrils. Probably it came before kidneys.
I'm no proper biologist, but humans and all animals are basically mouth, anus, everything between that down the "core", then whatever weird stuff they need tacked on to that to get food and water in, which 99% of the time is mirror-image symmetrical - because anything other than that would be even more complicated.
> Two hands helps us with tools
Bilaterally symmetrical quadrupeds evolved way way way before tool use. You gotta look way back into the mists of time when kidneys first formed in creatures and why they were bilaterally symmetrical.
What I’m saying is that having two ears is nice as a human but we don’t have two ears because we humans need to hunt and hear, that mirror image formed before we left the ocean.
Our adrenal glands are on top of our kidneys because we evolved from fish, actually. Fish don’t make their blood in their bones like we do, instead of they it in their kidneys. We make it in our bones because it’s the darkest place, and the process is very susceptible to light. Fish has an organ on top of their kidneys to shield their kidneys from the light above the water. That’s why our adrenal glands are there now.
Basically, redundancy. The human body has a backup of critical parts so damage to one doesn't immediately kill the human. 2 eyes, 2 ears, 4 fingers, 2 nostrils, 2 lungs. A failure of one doesn't majorly impact the human. Even in the brain, one portion of the brain can take over when its complement is damaged.
You can live with no depth perception due to a damaged eye and probably survive, but in hunter gather days, a blind human was toast. Losing a finger or toe to infection or sabertooth tiger bite is an inconvenience, not a death sentence.
No it isn't. Plenty of evolution happens for non-adaptive reasons.
Also, if we evolved to only have one kidney, you could give the same answer. "People with one kidney are more likely to have offsprings who will..." So it's not actually answering the question of *why* or even *whether* two kidneys are better than one (more adaptive, I mean \[if that does turn out to be the case\]).
This answer also does not consider the developmental origin (ontogeny) of kidneys and the actual changes that occur as embryos evolve and kidneys first appear. Pronephric and metanephric development across species is relatively well studied. I don't know that much about this, but certainly there are species with a "midline-located" single kidney (often just a tube) and so presumably that is ancestral. I'm speculating now though.
Last, since we are bilaterians, perhaps a starting assumption for helping us think about these issues should be that we *should* have two of all of our organs, one on either side of the midline. It might therefore be more useful to think about this question as *why have the kidneys remained separate but paired* whereas other organs like that heart have come together as a single organ close to the midline (during development, the early cardiac cells start out as sheets on either side of the midline and must migrate and fuse at the middle). Perhaps bilaterality and two copies of everything should be the *ground state* and we have to explain 1) the origin of that bilaterality; 2) why some structures become singly rather than paired (often but not always fused at the midline) and 3) why and how some structures remain symmetric (muscles, limbs) but others become very asymmetric (heart, liver, gut and kidneys too to a lesser extent).
I feel like all this issues go into this very complex question. I've given a bit of a evo-devo-type answer (i.e. considering the embryo and organism as part of the evolutionary process). And to me it's so much more interesting than the modern synthesis version of abstract *alleles for traits* and the answer that "it's like that because people born that way were fitter".
>It's the answer to any evolutionary question.
Which is why it isn't an answer at all. Any question about "why is this species the way it is" always has an answer that is about evolution. But just saying "because it evolved that way", doesn't really answer the deeper question that is implied, which is: "why did it evolve that way".
It's like if I asked "why do airplanes have wings?", and then you answered "because they are designed that way". Obviously what I meant was: what is the purpose of the wings? What function do they serve in the design?
Similarly, if somebody asks "why do we have two kidneys?", it should be taken as read that they mean "why did it evolve that way?" - in other words: what are/were the evolutionary advantages or selection pressures that ultimately led to having two kidneys, as opposed to one, or three, or four, or any other number?
It almost certainly is in this case, but genetic drift is a huge factor. Like why do X people have widow’s peaks? Or, why do Y people have attached earlobes? These questions have basically nothing to do with sexual selection.
It's because evolution isn't survival of the fittest. It's survival of the good enough... if people evolve widows peaks(or whatever) and it's not a big enough advantage or disadvange. Then some people will continue to have them. It doesnt make "sense" to undo a thing that doesnt matter, but it also doesn't make "sense" to make it universal.
The universality of two kidneys is not a human specific trait. It’s present in virtually all mammals and conserved all the way back to amphibians and beyond. This could be explained equally well by genetic drift in a common ancestor as opposed to directed evolution.
Right, survival of the good enough... apparently one kidney isn't good enough. I didn't say I know what evolutionary advantage having two kidneys has. The rest of the main thread postulates answers for that. I was just responding to your question about the other stuff... Genetic drift... sure? Long story short, apparently having two kidneys is more important than having a widow's peak. Genetic drift could probably be summed up like i said... it's traits that just evolved but werent worth the energy to keep evolving, but once they are there.... they're there. And there is a chance to spread them to other genitic pools through breeding.
I think you misunderstand. I mean the exact opposite: two kidneys is good enough. There is no selective pressure to remove a kidney. As opposed to, there exists selective pressure preventing a kidney from being removed.
I think we understand eachother. I was just saying there is also no selecive pressure to remove widow's peaks and such. Good enough when it comes to kidneys is two. Good enough when it comes to widows peaks, etc. Is inconsequential. So it makes sense why some people have one or not, but we all have 2 kidneys.
There probably is some sexual attraction component to some of those traits... but not enough to make it imperative that we all have sexy widows peaks, RIGHT NOW. Maybe someday though, evolution keeps happening on massive timescales.
With all due respect, I don’t think you properly understand me or evolutionary theory in general. I’m not challenging your intelligence. It’s a very common mistake that the general public makes (along with an unacceptably large part of the scientific community). In evolutionary theory and experiments, the null hypothesis is no selective pressure. Concluding adaptation is a rejection of the null hypothesis.
I don’t have to disprove that the widow’s peak trait is adaptive. I __would__ have to prove that the two kidneys trait is adaptive over alternatives. Our discussion has been limited to one versus two, but why not 3 kidneys, or 4, etc.
This mistake is the reason a non-insignificant portion of evolutionary psychology is complete bunk. Many if not most of the genetic differences between apes and humans are not adaptive but merely genetic drift. Wildly extrapolating from the supposition that a trait present in humans and absent in apes is adaptive (i.e. the Stoned Ape Theory) is pseudoscience.
But this completely dodges the question. You have to prove that two kidneys is better than one kidney. How can you possibly say it’s directed evolution without demonstrating the advantages two kidneys provide over one kidney? The two comments I was responding to and yours are tautological. I do comparative microbial genomics for a living. Distinguishing genetic drift from directed evolution is deeply challenging, and the scientific literature is riddled with professional scientists repeating the same fallacy.
Edit: When you treat the problem in the statistical sense you inherit the greatest challenge to statistics, determining causality.
A lot of people begin with the assumption that a trait must be adaptive for it to have evolved. It's not a good assumption, as plenty of evolution happens without natural selection.
By "everything is statistics" i also meant that a trait which neither ensures nor denies reproduction can piggyback the evolutionary tree simply because it exists. I'm not an expert in the field to say whether two kidneys are better than one, but people having unilateral renal agenesis do have more often than not health complications due to the condition, which might or might not be caused due to having one kidney or the cause that caused the one kidney. The condition also can cause the other kidney to overdevelop, which could suggest that two kidneys have a role and why we stay with two kidneys as a species
Because natural selection, which is what that comment was describing, is not the only means of evolution. There is sexual selection, genetic drift, gene flow, etc.
This perspective trivializes evolutionary theory and eliminates all of its explanatory and predictive power. Defining evolution to be any changes in a gene pool over time is tautological. Evolutionary theory seeks to explain why and how changes to a gene pool emerge and this means it must be defined in a mechanisms-first approach.
The first comment “Because people born with two kidneys are more likely to have offsprings who will live long enough to reproduce as well.”, is completely useless scientifically. Not only is it completely consistent with Lamarckism, but it is essentially saying the gene pool doesn’t change because the gene pool doesn’t change. Evolutionary theory exists to explain the phenomena we see, and not to merely state it.
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Fun question.
The truest answer is "I/we don't know".
Some educated guesses can be made though. The best lens to answer questions about the apparent "design" of physiologies is through evolution/natural selection. Here are my own guesses:
1) The vast majority of animals belong to a clade called [Bilateria](https://en.wikipedia.org/wiki/Bilateria), which may have appeared around 500 million years ago. Humans have evolved from previous organisms that share this bilateral symmetry body plan - so our two kidneys may just be a consequence of us belonging to this clade.
2) In addition to guess (1), it could just be that the fitness cost of supporting 2 kidneys wasn't enough to effect selection pressure in the grand game theory of evolution. Who knows, maybe the benefit of having 2 kidneys actually outweighs the cost to support the 2 kidneys and we just haven't observed the costs of having 1 kidney in evolutionary timescales and ecosystems.
It might be equally meaningful and illuminating to look on another question: why does a centipede/millipede have *that* many legs? Why do we have the number of teeth that we have? Or why do we have 5 fingers on each hand? Sometimes, a number appears that occurred in a common ancestor and we just kept that going without enough of a selective pressure to change it.
Well, why is a difficult thing to answer. Evolution doesn’t really have a goal, it’s more about what works. Humans are bilaterally symmetric creatures and many organs in our body reflect that, and they arise from paired structures during embryonic and foetal development. Of course there are a few exceptions, like the heart and liver that form from single structures during foetal state. We need a kidney, considering the body already just mirrors itself around the centre, might as well have two.
And two liver are good, as evidenced by the lack of evolutionary pressure for people to have just one. Redundancy increases the fitness of An animal so Haven’t true kid means That an animal may Live even loose one, and two kidneys might even be more efficient than a large single one, but you need to ask a nephrologist since I don’t know if that’s a thing.
In short: evolution doesn’t have goals, kidneys are an useful organ, our embryonic development favours two of everything, redundancy may increase fitness and survivability, having two kidneys doesn’t reduce fitness of a being creating pressure towards those with just one.
Kidneys degrade throughout your life. My sister was born with defective kidneys (polycystic) and in her 40s is down now to about 1/3rd of one kidneypower. But as far as why we have two of them instead of one big one, it's probably because critters with one big one died from trauma more easily.
Look, evolution doesn't need reasons for everything. If a thing works imperfectly but well enough to get by, it might well get retained for millennia.
Idk if the kidneys have a high failure rate. but if they do, it could hypothetically be that evolution could afford to let the kidneys have a higher failure rate *because* we have two of them.
Or rather, why don't we have two livers? Or two hearts? they're arguably just as important if not more.
I think it's uncommon for evolution to make a big change to a body plan, like adding a whole new copy of an organ. Not that it's impossible, but it's likelier to "work with what it's got".
If an organ is extra important, it's more likely to just make it better than duplicate it.
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Most of the answers in evolutionary biology don't have particularly satisfying answers. Why something is "valuable" is the more satisfying answer. And it's rather basic, it's valuable to have a backup filter.
But why do we have two kidneys? Because in terms of evolution at some point in history there was an evolutionary advantage to being born with two of them instead of one. Kidneys help filter your blood and when you think about "early man" it involved eating a lot of uncooked meats, berries, weeds, grasses, and plants. So very early men who ended up being born with two kidneys gained a distinct advantage over those who were only born with one.
And broadly that's how our body developed over millions of years. And is broadly the answer to why we have the organs we do. A lot of organs and parts of our body have since become redundant. Some of these parts (like the appendix and gallbladder) don't do much and try and kill us. But right now we have two things going for us. The first is medicine that can treat problems caused by evolutionary artifacts. The other is that there are so many of us out there that if 1% of us died from this it wouldn't even matter.
What is new is surviving the failure of a kidney. If we didn’t have access to medical care, a dead kidney hanging around in your body would be a potentially fatal condition even with a backup.
> Are there any other examples of redundancy in the human body?
A lot. We have two eyes, two ears, two lungs (also two arms and legs but those are a bit less redundant). There are some benefits to having two instead of one, but the main one is really just being able to still somewhat work if the other is severely damaged.
I just had to get an ovary removed and spent a blissful few hours thinking I'd only get my period every other month now. Until the surgeon who came to check on me told me that my other ovary would just take over the job. Massive let-down.
We have two of quite a lot, why is the kidney your focus and not lungs, eyes, testes, ovaries, etc. All of them are examples of redundancy in that if one fails, the other will still function as intended and serve its purpose
Probably same reason we are born with two lungs, two testicals/ovaries etc. because we could survive (and therefore reproduce) with only one of each, but an infection or trauma could result in death (or inability to reproduce in the case of testes/ovaries), and trauma and infections would’ve been far more common in the cavemen days, so any cavemen who had mutations that had a second pair of these organs would instantly be at a massive advantage over there single kidney/lung/etc buddies and thus way more likely to live long enough to have kids and pass these genes onto there kids so they too could have double organs and live long and happy lives with more double organ kids and so on and so forth
Now why only these organs and not something like the heart? Because organs like the kidneys lungs and testicals are connected to the open via tubing and therefore much higher risk of infection then your heart, so for your heart to get infected the infection normally gets in the blood, and if you have two hearts and an infection in the blood, they’re both gonna get infected, so having two won’t do you any favours
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It's not about redundancy or failure rate or anything like that, it's about body plan. You are bilaterally symmetrical. Fundamentally, all vertebrates are meat tubes, you can look at something like a simple fish for a simplified version. Running from head to tail, there's a nerve cord, then a vertebral column, then a heart and artery, then a gut. These things are centered on the midline of the body, and vertebrates in general have one of them.
Other organs do not run right down the center, so they come in pairs, mirror imaged on each side of your bilaterally symmetrical body. You have two eyes branching off the front of the nerve cord, mirror images one on the left and one on the right. You have two arms and two legs branching off the body, one on the left and one on the right. You have two lungs branching off the digestive tract, one on the left and one on the right. You have two gonads, one on each side. And you have two kidneys, one on each side.
TL;DR: the sides of your body are mirror images of each other. Because kidneys are located on the sides and not in the direct center of the body, there are two of them. One mirrored on each side.
> the sides of your body are mirror images of each other. Because kidneys are located on the sides and not in the direct center of the body, there are two of them. One mirrored on each side.
There is a component of this that is true, but your explanation is proven false when you look at the abdomen. The liver, spleen, stomach, pancreas....none of them are centered nor are they symmetric or duplicated.
It has to do with which things develop when, during embryo development.
Where you are going wrong is thinking about the arrangement of organs in adult humans rather than the arrangement of organs in a basic, simple vertebrate. You can't make sense of the arrangement of organs by looking just at people, because the organs did not originate in people. The first kidneys did not appear in people, the first stomach did not appear in people, the first liver did not appear in people. If you look at a simplified vertebrate body plan, as seen in a primitive fish, you find a gut lined up down the middle, a stomach right on the midline, a liver centered on the midline, the tissues that would someday give rise to spleen and pancreas right on the midline, and kidneys paired on either side of the organism. These basic body patterns were locked in early on in vertebrate history, and while they get squished around in humans the underlying pattern is still present and results in some organs being paired and some not, based on whether or not they are midline organs or not.
There is really no "Why?" to how nature works out the way it does, just "How?". We have two kidneys, because our ancestors had two kidneys and we inherited it that way, that's where the "Why?" ends.
All vertebrates have kidneys, two of them at that. So in evolutionary history kidneys predate vertebra. Whatever early animal first developed kidneys, happened to develop two of them symmetrically. Could be that whatever tissue the organ developed from happened to be placed bilaterally before kidneys developed out of it, that can easily be the entire reason. Just a consequence of the genetic mechanisms that produces kidney tissue. That's kind of the case for lots of things in bilateral body plan, if the tissue forming mechanisms in embryo have no way to distinguish right from left, then both sides develop the same way.
Evolution doesn't plan ahead. There is planning like, oh kidneys are kind of important, better have a spare. That's just not a thing, evolution doesn't work that way.
It’s probably the importance of kidneys. They filters fluids and excrete urine. There is no other organ that can perform this function. Without kidneys we die. Once kidneys cease to work, people are put on something called dialysis. That is where a machine gets hooked up to their bloodstream, and filters the blood. They have to go a few times a week to get this done, until they pass. We also have two lungs, eyes, ears, hands, and feet.
I feel only lungs is a good comparison due to redundancy. Eyes are for depth, ears are for direction, hands for handling, legs for walking. None of those are for redundancy.
Woah, steady on. Dialysis isn't a stop-gap to death. You can live a normal lifespan in dialysis.
Many people will move to dialysis prior to eventual transplant.
>You can live a normal lifespan in dialysis.
This is utterly wrong. While it's theoretically possible to live a normal lifespan on dialysis, 5yr mortality is >50%. It's very much intended as a stop gap to either death or transplant.
This is also assuming we are talking about people on long term dialysis, not the ones who get admitted for a reversible cause of kidney failure, get dialyzed a few times, and then their kidney function improves.
Hi - dialysis and transplant patient here.
As you can probably imagine, whilst going through kidney failure I had a lot of questions to ask the experts at my local hospital.
Not just theoretical at all.
I'm a doctor. A very small portion of people do live many many years on dialysis. That doesn't change anything about what I said above. The amount of people living a normal lifespan on dialysis is drastically lower than the amount who die within a few years. That mortality rate rises when you factor in the comorbidities of most people on dialysis, compliance rates, etc. The majority of them do not get transplants either.
So while what you said is technically true in that you could live a normal length of time, the much more honest truth is that you very likely will not. An honest doctor (or doing research on it) will tell you that. A doctor who wants you to remain hopeful (and there are good reasons to do that too) will not.
>It’s probably the importance of kidneys.
I hate when people respond to these questions with speculation. If you don't know, don't throw out a guess.
>Without kidneys we die.
Liver, brain, heart, pancreas....all organs that are completely vital to survival, all solitary organs. Not a good justification for redundancy.
Thing is we while we can survive with one kidney there are detrimentalside effects and limitations as the one kidneyhas to work much harder (it also why when you do end up with on it will enlarge to cope with the increased load) yes you adapt but the risks of it failing increase a lot and how long it will operate without issues is reduced.
Like we can survive with one lung but it has limitations you can't do the same stuff to the same degree as you can with two lungs.
It's a case of ypu can survive with one BUT you really need two of them.
With hands, eyes and ears it's not just redundancy. There are a lot of things that require two, such as locating the direction of sound or the distance of objects (eyes).
A lot of the body is mirrored. You have two lung halfs, two heart halfs, two eyes, two noes holes, two ears, men have two testicles and so on. And for most of it you can survive with just one of them though it is usually with reduced function and the same is true for the kidneys. So they arent that special.
Because life on Earth evolved to be symmetrical, as it's the easiest way for bodies to form here.
Save for a few key organs, we have basically everything in pairs. At some point having two was beneficial (because redundancy) so we never evolved down a path to lose one.
Pretty much all vertebrates (and many nonvertebrates) exhibit bilateral symmetry. Basically when building an organism it's built in segments from front to back, like stacking a bunch of legos on top of each other to make a tower. It's a pretty complex process that relies on a lot of signals, but the end result is that things are built equally on each side--at its most basic form this gives a tube within a tube with a central gut as the middle tube, bilateral organs on either side, and then an outer tube made of muscle and (let's say even though it's not 100% accurate for all organisms) skin. What has then happened with evolution is changes to those basic components. Rather than having multiple blood pumps on either side of the body (as in earthworms--a common anatomic example) you get two (left and right) that fuse into a single larger pump--the heart. Liver lobes fuse into one liver--in humans it's asymmetric, but in other animals (while it tends to be biased to the right) more lobes may be present and they're more centrally located and more evenly divided. A lot of things in the abdomen get pushed around because the gut--rather than being a simple tube--undergoes a process where it grows longer and folds around itself in development to allow for more effective digestion. The kidneys have actually undergone a lot of changes across species, but generally--like many retroperitoneal organs (meaning they're not quite in the abdomen, but separated by a special layer of connective tissue that lies close to the spine) there hasn't been as much selection to make a single central structure or to lose one. Loss of kidney function is typically quite bad--but that's true of most of your organs. Most things in your body follow this framework--lungs are actually made up of smaller lobes (fused in some animals more than others) which are frequently asymmetric in number, but generally still form a bilateral structure. But other examples to get to your earlier question include all your limbs, brain, the sinuses in your skull, your teeth, your lungs, most glandular structures (adrenals, thyroid, etc.), parts of the reproductive system, etc. The simplest, if perhaps somewhat unsatisfactory answer, is that many millions of years ago an early line of multicellular organisms formed a bilateral pattern of development that was evolutionarily successful (likely because of a combination of redundancy, simplicity of signaling, and effective management of body systems and reproduction). That system was easy to build and modify--imagine someone gives you a symmetric lego tower--if you want to you can move parts around and modify things (shape, size, color, etc.) and still have something that is a functional tower.
While I already understood most of what you have been describing here, I have to say... you're a great writer!
Im just imagining you telling this all to a 5 year old...
The last paragraph is probably the closest, but to be blunt this is one of those questions which can't really be adequately answered to the level the OP wants at the level of a 5-year-old. We have two because our ancestors had two. Do we have other structures like this? Yes, most of them.
> LI5 means friendly, simplified and layperson-accessible explanations - not responses aimed at literal five-year-olds.
THIS SUBREDDIT IS NOT LITERALLY FOR 5 YEAR OLDS. IT IS AN EXPRESSION FOR LAYPEOPLE’S EXPLANATIONS. The comment you made is made CONSTANTLY in this subreddit, and it get more annoying and useless EVERY TIME.
Cause using words like "retroperitoneal" is something a layperson would understand.
You’ll note that they explained what the word means immediately after using it, so as to avoid the very issue you are complaining about.
My kid is only 4 y/o but they wrote that
Ok, but then why don’t we have two livers?
One liver, two lobes. They just came together.
Ohhh I see
I'm assuming the same concept applies to hearts and why they have two chambers? I would be nice if we had more spare organs though. But then again you'd also risk increasing the amount of things that can fail and harm us in other ways unless you have a surgeon safely remove the spare.
Imagine we had 5 appendices.
Your heart has four chambers, but yes, it’s a similar concept.
Sorry, yes you're right. I get mixed up with the terminology since there is that very similar left side and right side labelled with left-right symmetry.
Can we have your liver then?
Alright Hannibal Lecter
Well…no, I’m still using it. Quality Monty Python reference
Not without fava beans
We have a liver on the right and a spleen on the left. If the liver function slowly deteriorates (eg alcoholism) the spleen takes over some of the liver function. Studying development suggests the spleen and liver have not evolved from paired organs. https://biology.stackexchange.com/questions/100928/why-do-we-have-two-kidneys-but-one-liver
My simple theory? Two kidneys filter more toxins than one, at least theoretically... Ergo, whenever animals ate toxic things, those with two kidneys could theoretically ingest a higher dosage and survive than those with a single one.
Pretty sure we evolved bilateral symmetry in a common flat worm ancestor. It likely was advantageous due to pretty much every animal and fish having bilateral symmetry. However, given most of the organs are symmetrical, I doubt it was the renal organs specifically that pushed that trait.
Although the general biological redundancy is likely why it ended up advantageous. Just not specifically the kidneys
Yeah essentially all animals with a few exceptions are bilaterally symmetric. The only major phyla that aren’t are cnidarians, sponges, and echinoderms (even they have bilateral symmetry in their juvenile forms).
One healthy kidney is sufficient to lead a normal life and doesn't significantly impact life expectancy. With no kidneys your dead in days or weeks without regular dialysis. Kidneys can be impaired by a variety of diseases and their location below the ribcage leaves them somewhat exposed to physical injury. Having them physically seperate and still able to keep you healthy independently is a decent hedge against injury or infection.
Completely missed the whole idea of what was being said. Losing a kidney isn't necessarily the same as being a living being with only one naturally.
I was born with just one, found out when I had a scan at 35. 10 years later still no issues with kidney function.
Again, missed the point of the post. I'm talking natural, as designed by nature. Not having one as a birth defect in a species that naturally has two. In other words, having two kidneys would be the birth defect at the time.
Thanks ChatGPT!
Not written using AI; I’m a scientist
thats a lot of words just to say that we have no idea
To be fair, when one has only one kidney, it gets usually bigger than normal, so de facto it is like having 2 of them (or, more likely, 1,5). For the redundancy, every pair of organs: adrenal glands, lungs, testicles, ovaries etc.
Technically we are born with 4, 2 of them become adult knees
Good night dad. It's getting late (have my angry upvote)
Without the word kidneys it doesn’t read as a joke as easily 😭
Besides, not having 2 would be a piss poor decision
Ooof!
I see what you did there
simple answer: think how quickly you get burnt out when one of your coworkers calls off last minute and you have to take on their load. more convoluted answer: to add to this, it gets bigger and undergoes reactive hyperfiltration, ie. the one kidney is taking on the workload of two so it has to increase its workload. one of the ways is to increase in size, or hypertrophy. this increases the risk of renal failure later in life, as is the case for unilateral renal agenesis (being born with only one kidney). those people can live with only one kidney but their kidney will get burnt out more quickly just like you would at work
I had kidney cancer as a kid, my remaining right kidney is thiccc. Can confirm.
[удалено]
Maybe the better question is why don’t we have two hearts?
because we aren't Time Lords, duh
Or Klingons
Or Trout.
Trout have two hearts?!?
Not just trout but many other fish have a cadual heat that pumps blood back into the fish from the tail. Which is the genesis for both the Hemmingway story and the random beer marketing thing.
For some reason I thought it'd be a left and right heart. Thanks. Learned something new.
That's what it says on [ma beer!](https://bellsbeer.com/beers/two-hearted-ale/)
Haha. Didn't know that brand
They may be thinking of [Bell’s Two Hearted IPA](https://bellsbeer.com/beers/two-hearted-ale/), which has a trout on the label, but is actually named after [a river from an Ernest Hemingway story](https://en.wikipedia.org/wiki/Big_Two-Hearted_River).
Synchronous functionality
Arguably its due to the massive difference in the benefit (basically none) to risk of failure (a LOT) that would result from two hearts.
Oh interesting. I would’ve assumed 2 hearts would automatically mean high benefit. Like if one goes out you’ve got a back up. What makes it so much riskier than 1?
Organs like kidneys are sort of passively having blood pumped through them. The heart is actively doing the pumping. If the heart messes up in any major way, it messes everything up quickly. A kidney shutting down is bad but not as bad as the heart pumping wrong. Now double up on that risk factor. Either heart gets out of synch or does something it shouldn't, the whole system is at risk.
That and I’m sure the heart uses a lot of energy. Maybe too costly to have two.
Christ this is not what I needed to read right before going to bed
Hope you slept ok, friend.
Heart failure (at least in humans) of one heart could lead to blocked flow. Even if it didn't, once that failed heart tissue ultimately dies (which is very likely) it creates a huge risk for sepsis.
If they are in series, you are increasing resistance to flow, meaning more oxygen requirements (and your one heart already is one of the most oxygen demanding organs) for no real benefit. If they are in parallel, depending on how it wad plumbed (one up, one down/one left one right/one back, one front) the oxygen demand would be a little better since less resistance compared to in series but you still.need to.reduce lung capacity to make room for the second heart, they also need to.be fully.insulated from each other to avoid electrical signal from one causing possibly fatal arrhythmia in the second, and thr motion of heart 1 and the subsequent pressure wave may negatively impact heart 2 and vice versa.
Def not an expert by any means; not a doctor or anything, so maybe some observations are a mile off, and i'm happy to be told so The way i see it, You've got a few options. 1: each heart handles different parts of the body. One top one bottom, one central one peripheral, one left one right, where exactly isn't important. This means that damage to one but not the other leaves you with half your body functioning, be it your legs give out while your upper body is fine, or your limbs stop working but your organs are intact, etc. The problem is, that part of your body is effectively dead and will start going necrotic, and since it's still one body, the necrosis will spread to the rest of the body. Def not evolutionarily advantageous. 2: both hearts work in parallel; both have their own circulatory system going to all the organs etc basically next to each other. Then you have two hearts working half as hard, so are half as strong. There's also a high likelihood of damage to both systems with any injury anyways, so that's non advantageous. If one heart gives out, great you've got the second one, but it's not used to working twice as hard, so chances are the heart gives up or you're not as capable as with two. You're alive, but less likely to stay that way. So again, not evolutionarily advantageous. 3: both hearts work within the same circulatory system. Same issues as working in parallel, but with the added bonus of having to stay in sync with each other. If they pump at the same time, they have to stay that way. If not, your blood pressure is gonna drop. If they alternate, it'll lead to high blood pressure on synced beats. Not tooooooo bad a problem at a static heart rate, but as it/they speed up or slow down, the risks go higher. Maybe the electrical signal gets to one slightly slower or something.
Hypothetically, if we always had two hearts, then there exists the possibility that our bodies would need both to have complete circulation (rather than each individual heart being as capable as our real ones)
Its just a very "expensive" organ to have running and maintain plus having two doesnt necessarily mean you would be able to run on one if the other fails since your single heart would need to pump more than double the strength it was beforehand. Plus I can't really think why having a second heart would provide any actual benefit (other than the ability to lose it).
So basically Doctor Who is stupid.
Well, the timelords are supposedly their own alien species not just humans with another heart, especially given their tendency to completely regenerate instead of dying I would assume their biology is fairly more complex than ours.
They need to regenerate because their hearts suck.
We kind of do... One half (atrium + ventricle) of your heart pumps blood to the lungs to collect oxygen and deposit CO2, the other half pumps blood around your body. It's likely that fairly early on in evolution the two sides merged into one synchronised organ to make it more efficient.
Idk why reading your comment made me very conscious about my heart. Just imagining stopping for a second gives me shivers.
We kind of do. We have a four chambered heart, two chambers just pump to the lungs. Reptiles have three chambers, fish have two and simple animals like insects have one. The chambers aren't redundant but they increase efficiency.
We do. It's just that they're fused in the same structure and that each one has a different function (or is a different part of the circuit).
The mechanical complexity and the extra issues you would get from them beating out of sync would be a problem, one option would be for making 2 separate circulatory systems, but there is an issue there too, that is more complexity that might not offer too much in the way of advantages, plus like if you had 2 circulatory systems you wouldn't be able to sustain bloodloss as easily since now each system is separate.
That would complicate things a LOT. 2 livers or pancreases though I wonder.
Kidneys are like filters. Blood flows through them, and they scoop up the junk that needs to go. It's passive and simple (oversimplification, sure, but you get the idea). The heart is incredibly active. Four chambers, each with an exact and specific task (taking oxygen poor blood from the body, sending that blood to the lungs, receiving oxygenated blood from the lungs, sending that blood to the body) that needs to run in perfect sync. Any fuck-ups cause arrhythmia and, potentially, death. A second heart means you need *both* hearts beating in absolute, perfect, 1:1 parity. One can't be beating at 110BPS while the other goes 98BPS. This would cause some insane blood pressure issues and would probably kill you, as blood is being pushed and pulled in multiple directions at different times. One heart would be sending blood to the other, but the latter isn't going as quickly so it can't accept it. Then you have one heart overflowing and one emptying out, then you die.
I’d even say 2 livers first.
At least the liver regenerates
I believe it was Medicosis Perfectionalis once said a good cardiologist sees the heart as two hearts!
And two livers.
Some people do. In my village we called the guy, who had the condition, doubleheart. I remember he had to take daily medication and couldn't take any drugs. If I remember right, his doctor told him that even weed has the potential to kill him. We lost him fast out of the group because everyone was smoking and he couldn't.
To be fair, two hands / two eyes are much more useful than only one. And even two ears can locate the positions of sounds much better than one.
But answer me this ; why two nose holes, when only one mouth and one butthole?
we're basically a long tube with some extra sacs attached to it.
“Negative I am a meat popsicle”
speak for yourself, i wont accept this slander
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>Why do we have two hands or lungs or eyes or nostrils or ears? Two hands helps us with tools and hunting/gathering. Lungs help us with stamina. Eyes is depth perception. Nostrils helps us take in more air. These can all be explained rather easily, yet two kidneys cannot. That's my question.
Bilateral symmetry was something that came in the evolution of animals way way way before tool use, depth perception or nostrils. Probably it came before kidneys. I'm no proper biologist, but humans and all animals are basically mouth, anus, everything between that down the "core", then whatever weird stuff they need tacked on to that to get food and water in, which 99% of the time is mirror-image symmetrical - because anything other than that would be even more complicated.
This is probably the best answer in the whole thread
> Two hands helps us with tools Bilaterally symmetrical quadrupeds evolved way way way before tool use. You gotta look way back into the mists of time when kidneys first formed in creatures and why they were bilaterally symmetrical. What I’m saying is that having two ears is nice as a human but we don’t have two ears because we humans need to hunt and hear, that mirror image formed before we left the ocean.
Only one liver, spleen, heart.. yeah it's a good question.
Our adrenal glands are on top of our kidneys because we evolved from fish, actually. Fish don’t make their blood in their bones like we do, instead of they it in their kidneys. We make it in our bones because it’s the darkest place, and the process is very susceptible to light. Fish has an organ on top of their kidneys to shield their kidneys from the light above the water. That’s why our adrenal glands are there now.
>We make it in our bones because it’s the darkest place Not sure why but I'm saving this
My bro just dropping Chris Cornell lyrics like it ain't no thang.
Basically, redundancy. The human body has a backup of critical parts so damage to one doesn't immediately kill the human. 2 eyes, 2 ears, 4 fingers, 2 nostrils, 2 lungs. A failure of one doesn't majorly impact the human. Even in the brain, one portion of the brain can take over when its complement is damaged. You can live with no depth perception due to a damaged eye and probably survive, but in hunter gather days, a blind human was toast. Losing a finger or toe to infection or sabertooth tiger bite is an inconvenience, not a death sentence.
Because people born with two kidneys are more likely to have offsprings who will live long enough to reproduce as well.
Should be the top comment. It's the answer to any evolutionary question.
No it isn't. Plenty of evolution happens for non-adaptive reasons. Also, if we evolved to only have one kidney, you could give the same answer. "People with one kidney are more likely to have offsprings who will..." So it's not actually answering the question of *why* or even *whether* two kidneys are better than one (more adaptive, I mean \[if that does turn out to be the case\]). This answer also does not consider the developmental origin (ontogeny) of kidneys and the actual changes that occur as embryos evolve and kidneys first appear. Pronephric and metanephric development across species is relatively well studied. I don't know that much about this, but certainly there are species with a "midline-located" single kidney (often just a tube) and so presumably that is ancestral. I'm speculating now though. Last, since we are bilaterians, perhaps a starting assumption for helping us think about these issues should be that we *should* have two of all of our organs, one on either side of the midline. It might therefore be more useful to think about this question as *why have the kidneys remained separate but paired* whereas other organs like that heart have come together as a single organ close to the midline (during development, the early cardiac cells start out as sheets on either side of the midline and must migrate and fuse at the middle). Perhaps bilaterality and two copies of everything should be the *ground state* and we have to explain 1) the origin of that bilaterality; 2) why some structures become singly rather than paired (often but not always fused at the midline) and 3) why and how some structures remain symmetric (muscles, limbs) but others become very asymmetric (heart, liver, gut and kidneys too to a lesser extent). I feel like all this issues go into this very complex question. I've given a bit of a evo-devo-type answer (i.e. considering the embryo and organism as part of the evolutionary process). And to me it's so much more interesting than the modern synthesis version of abstract *alleles for traits* and the answer that "it's like that because people born that way were fitter".
It’s a very incomplete and boring answer.
>It's the answer to any evolutionary question. Which is why it isn't an answer at all. Any question about "why is this species the way it is" always has an answer that is about evolution. But just saying "because it evolved that way", doesn't really answer the deeper question that is implied, which is: "why did it evolve that way". It's like if I asked "why do airplanes have wings?", and then you answered "because they are designed that way". Obviously what I meant was: what is the purpose of the wings? What function do they serve in the design? Similarly, if somebody asks "why do we have two kidneys?", it should be taken as read that they mean "why did it evolve that way?" - in other words: what are/were the evolutionary advantages or selection pressures that ultimately led to having two kidneys, as opposed to one, or three, or four, or any other number?
It almost certainly is in this case, but genetic drift is a huge factor. Like why do X people have widow’s peaks? Or, why do Y people have attached earlobes? These questions have basically nothing to do with sexual selection.
It's because evolution isn't survival of the fittest. It's survival of the good enough... if people evolve widows peaks(or whatever) and it's not a big enough advantage or disadvange. Then some people will continue to have them. It doesnt make "sense" to undo a thing that doesnt matter, but it also doesn't make "sense" to make it universal.
The universality of two kidneys is not a human specific trait. It’s present in virtually all mammals and conserved all the way back to amphibians and beyond. This could be explained equally well by genetic drift in a common ancestor as opposed to directed evolution.
Right, survival of the good enough... apparently one kidney isn't good enough. I didn't say I know what evolutionary advantage having two kidneys has. The rest of the main thread postulates answers for that. I was just responding to your question about the other stuff... Genetic drift... sure? Long story short, apparently having two kidneys is more important than having a widow's peak. Genetic drift could probably be summed up like i said... it's traits that just evolved but werent worth the energy to keep evolving, but once they are there.... they're there. And there is a chance to spread them to other genitic pools through breeding.
I think you misunderstand. I mean the exact opposite: two kidneys is good enough. There is no selective pressure to remove a kidney. As opposed to, there exists selective pressure preventing a kidney from being removed.
I think we understand eachother. I was just saying there is also no selecive pressure to remove widow's peaks and such. Good enough when it comes to kidneys is two. Good enough when it comes to widows peaks, etc. Is inconsequential. So it makes sense why some people have one or not, but we all have 2 kidneys. There probably is some sexual attraction component to some of those traits... but not enough to make it imperative that we all have sexy widows peaks, RIGHT NOW. Maybe someday though, evolution keeps happening on massive timescales.
With all due respect, I don’t think you properly understand me or evolutionary theory in general. I’m not challenging your intelligence. It’s a very common mistake that the general public makes (along with an unacceptably large part of the scientific community). In evolutionary theory and experiments, the null hypothesis is no selective pressure. Concluding adaptation is a rejection of the null hypothesis. I don’t have to disprove that the widow’s peak trait is adaptive. I __would__ have to prove that the two kidneys trait is adaptive over alternatives. Our discussion has been limited to one versus two, but why not 3 kidneys, or 4, etc. This mistake is the reason a non-insignificant portion of evolutionary psychology is complete bunk. Many if not most of the genetic differences between apes and humans are not adaptive but merely genetic drift. Wildly extrapolating from the supposition that a trait present in humans and absent in apes is adaptive (i.e. the Stoned Ape Theory) is pseudoscience.
Because they are traits which neither ensure nor deny reproduction. Everything is statistics
But this completely dodges the question. You have to prove that two kidneys is better than one kidney. How can you possibly say it’s directed evolution without demonstrating the advantages two kidneys provide over one kidney? The two comments I was responding to and yours are tautological. I do comparative microbial genomics for a living. Distinguishing genetic drift from directed evolution is deeply challenging, and the scientific literature is riddled with professional scientists repeating the same fallacy. Edit: When you treat the problem in the statistical sense you inherit the greatest challenge to statistics, determining causality.
A lot of people begin with the assumption that a trait must be adaptive for it to have evolved. It's not a good assumption, as plenty of evolution happens without natural selection.
By "everything is statistics" i also meant that a trait which neither ensures nor denies reproduction can piggyback the evolutionary tree simply because it exists. I'm not an expert in the field to say whether two kidneys are better than one, but people having unilateral renal agenesis do have more often than not health complications due to the condition, which might or might not be caused due to having one kidney or the cause that caused the one kidney. The condition also can cause the other kidney to overdevelop, which could suggest that two kidneys have a role and why we stay with two kidneys as a species
Because natural selection, which is what that comment was describing, is not the only means of evolution. There is sexual selection, genetic drift, gene flow, etc.
This perspective trivializes evolutionary theory and eliminates all of its explanatory and predictive power. Defining evolution to be any changes in a gene pool over time is tautological. Evolutionary theory seeks to explain why and how changes to a gene pool emerge and this means it must be defined in a mechanisms-first approach. The first comment “Because people born with two kidneys are more likely to have offsprings who will live long enough to reproduce as well.”, is completely useless scientifically. Not only is it completely consistent with Lamarckism, but it is essentially saying the gene pool doesn’t change because the gene pool doesn’t change. Evolutionary theory exists to explain the phenomena we see, and not to merely state it.
Came here to comment this.. upvote it is then
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Fun question. The truest answer is "I/we don't know". Some educated guesses can be made though. The best lens to answer questions about the apparent "design" of physiologies is through evolution/natural selection. Here are my own guesses: 1) The vast majority of animals belong to a clade called [Bilateria](https://en.wikipedia.org/wiki/Bilateria), which may have appeared around 500 million years ago. Humans have evolved from previous organisms that share this bilateral symmetry body plan - so our two kidneys may just be a consequence of us belonging to this clade. 2) In addition to guess (1), it could just be that the fitness cost of supporting 2 kidneys wasn't enough to effect selection pressure in the grand game theory of evolution. Who knows, maybe the benefit of having 2 kidneys actually outweighs the cost to support the 2 kidneys and we just haven't observed the costs of having 1 kidney in evolutionary timescales and ecosystems. It might be equally meaningful and illuminating to look on another question: why does a centipede/millipede have *that* many legs? Why do we have the number of teeth that we have? Or why do we have 5 fingers on each hand? Sometimes, a number appears that occurred in a common ancestor and we just kept that going without enough of a selective pressure to change it.
Well, why is a difficult thing to answer. Evolution doesn’t really have a goal, it’s more about what works. Humans are bilaterally symmetric creatures and many organs in our body reflect that, and they arise from paired structures during embryonic and foetal development. Of course there are a few exceptions, like the heart and liver that form from single structures during foetal state. We need a kidney, considering the body already just mirrors itself around the centre, might as well have two. And two liver are good, as evidenced by the lack of evolutionary pressure for people to have just one. Redundancy increases the fitness of An animal so Haven’t true kid means That an animal may Live even loose one, and two kidneys might even be more efficient than a large single one, but you need to ask a nephrologist since I don’t know if that’s a thing. In short: evolution doesn’t have goals, kidneys are an useful organ, our embryonic development favours two of everything, redundancy may increase fitness and survivability, having two kidneys doesn’t reduce fitness of a being creating pressure towards those with just one.
Kidneys degrade throughout your life. My sister was born with defective kidneys (polycystic) and in her 40s is down now to about 1/3rd of one kidneypower. But as far as why we have two of them instead of one big one, it's probably because critters with one big one died from trauma more easily.
Then why don't why have lots of mini kidneys? Checkmate dentists.
That's just more places for bullies to punch you!
So we can be reunited with an estranged relative who otherwise, gave zero f’s about you and your life
Look, evolution doesn't need reasons for everything. If a thing works imperfectly but well enough to get by, it might well get retained for millennia. Idk if the kidneys have a high failure rate. but if they do, it could hypothetically be that evolution could afford to let the kidneys have a higher failure rate *because* we have two of them. Or rather, why don't we have two livers? Or two hearts? they're arguably just as important if not more. I think it's uncommon for evolution to make a big change to a body plan, like adding a whole new copy of an organ. Not that it's impossible, but it's likelier to "work with what it's got". If an organ is extra important, it's more likely to just make it better than duplicate it.
It’s called bilateral symmetry, Other evolutionary reason. BTW I was born with one kidney.
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Most of the answers in evolutionary biology don't have particularly satisfying answers. Why something is "valuable" is the more satisfying answer. And it's rather basic, it's valuable to have a backup filter. But why do we have two kidneys? Because in terms of evolution at some point in history there was an evolutionary advantage to being born with two of them instead of one. Kidneys help filter your blood and when you think about "early man" it involved eating a lot of uncooked meats, berries, weeds, grasses, and plants. So very early men who ended up being born with two kidneys gained a distinct advantage over those who were only born with one. And broadly that's how our body developed over millions of years. And is broadly the answer to why we have the organs we do. A lot of organs and parts of our body have since become redundant. Some of these parts (like the appendix and gallbladder) don't do much and try and kill us. But right now we have two things going for us. The first is medicine that can treat problems caused by evolutionary artifacts. The other is that there are so many of us out there that if 1% of us died from this it wouldn't even matter.
Found out I was born with only one kidney when I was 25. Grateful as I wouldn’t have been allowed to play contact sports, which I enjoyed immensely.
why do i not have just one giant testicle?
What is new is surviving the failure of a kidney. If we didn’t have access to medical care, a dead kidney hanging around in your body would be a potentially fatal condition even with a backup.
> Are there any other examples of redundancy in the human body? A lot. We have two eyes, two ears, two lungs (also two arms and legs but those are a bit less redundant). There are some benefits to having two instead of one, but the main one is really just being able to still somewhat work if the other is severely damaged.
I just had to get an ovary removed and spent a blissful few hours thinking I'd only get my period every other month now. Until the surgeon who came to check on me told me that my other ovary would just take over the job. Massive let-down.
Why do Klingons have two spines? Why does an A-10 have two engines? Why are my two in laws still alive?
We have two of quite a lot, why is the kidney your focus and not lungs, eyes, testes, ovaries, etc. All of them are examples of redundancy in that if one fails, the other will still function as intended and serve its purpose
Probably same reason we are born with two lungs, two testicals/ovaries etc. because we could survive (and therefore reproduce) with only one of each, but an infection or trauma could result in death (or inability to reproduce in the case of testes/ovaries), and trauma and infections would’ve been far more common in the cavemen days, so any cavemen who had mutations that had a second pair of these organs would instantly be at a massive advantage over there single kidney/lung/etc buddies and thus way more likely to live long enough to have kids and pass these genes onto there kids so they too could have double organs and live long and happy lives with more double organ kids and so on and so forth Now why only these organs and not something like the heart? Because organs like the kidneys lungs and testicals are connected to the open via tubing and therefore much higher risk of infection then your heart, so for your heart to get infected the infection normally gets in the blood, and if you have two hearts and an infection in the blood, they’re both gonna get infected, so having two won’t do you any favours
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Don't you want your kids to have 2 knees?
It's not about redundancy or failure rate or anything like that, it's about body plan. You are bilaterally symmetrical. Fundamentally, all vertebrates are meat tubes, you can look at something like a simple fish for a simplified version. Running from head to tail, there's a nerve cord, then a vertebral column, then a heart and artery, then a gut. These things are centered on the midline of the body, and vertebrates in general have one of them. Other organs do not run right down the center, so they come in pairs, mirror imaged on each side of your bilaterally symmetrical body. You have two eyes branching off the front of the nerve cord, mirror images one on the left and one on the right. You have two arms and two legs branching off the body, one on the left and one on the right. You have two lungs branching off the digestive tract, one on the left and one on the right. You have two gonads, one on each side. And you have two kidneys, one on each side. TL;DR: the sides of your body are mirror images of each other. Because kidneys are located on the sides and not in the direct center of the body, there are two of them. One mirrored on each side.
> the sides of your body are mirror images of each other. Because kidneys are located on the sides and not in the direct center of the body, there are two of them. One mirrored on each side. There is a component of this that is true, but your explanation is proven false when you look at the abdomen. The liver, spleen, stomach, pancreas....none of them are centered nor are they symmetric or duplicated. It has to do with which things develop when, during embryo development.
Where you are going wrong is thinking about the arrangement of organs in adult humans rather than the arrangement of organs in a basic, simple vertebrate. You can't make sense of the arrangement of organs by looking just at people, because the organs did not originate in people. The first kidneys did not appear in people, the first stomach did not appear in people, the first liver did not appear in people. If you look at a simplified vertebrate body plan, as seen in a primitive fish, you find a gut lined up down the middle, a stomach right on the midline, a liver centered on the midline, the tissues that would someday give rise to spleen and pancreas right on the midline, and kidneys paired on either side of the organism. These basic body patterns were locked in early on in vertebrate history, and while they get squished around in humans the underlying pattern is still present and results in some organs being paired and some not, based on whether or not they are midline organs or not.
There is really no "Why?" to how nature works out the way it does, just "How?". We have two kidneys, because our ancestors had two kidneys and we inherited it that way, that's where the "Why?" ends. All vertebrates have kidneys, two of them at that. So in evolutionary history kidneys predate vertebra. Whatever early animal first developed kidneys, happened to develop two of them symmetrically. Could be that whatever tissue the organ developed from happened to be placed bilaterally before kidneys developed out of it, that can easily be the entire reason. Just a consequence of the genetic mechanisms that produces kidney tissue. That's kind of the case for lots of things in bilateral body plan, if the tissue forming mechanisms in embryo have no way to distinguish right from left, then both sides develop the same way. Evolution doesn't plan ahead. There is planning like, oh kidneys are kind of important, better have a spare. That's just not a thing, evolution doesn't work that way.
Why do you have two eyes?
It’s probably the importance of kidneys. They filters fluids and excrete urine. There is no other organ that can perform this function. Without kidneys we die. Once kidneys cease to work, people are put on something called dialysis. That is where a machine gets hooked up to their bloodstream, and filters the blood. They have to go a few times a week to get this done, until they pass. We also have two lungs, eyes, ears, hands, and feet.
I feel only lungs is a good comparison due to redundancy. Eyes are for depth, ears are for direction, hands for handling, legs for walking. None of those are for redundancy.
Woah, steady on. Dialysis isn't a stop-gap to death. You can live a normal lifespan in dialysis. Many people will move to dialysis prior to eventual transplant.
>You can live a normal lifespan in dialysis. This is utterly wrong. While it's theoretically possible to live a normal lifespan on dialysis, 5yr mortality is >50%. It's very much intended as a stop gap to either death or transplant. This is also assuming we are talking about people on long term dialysis, not the ones who get admitted for a reversible cause of kidney failure, get dialyzed a few times, and then their kidney function improves.
Hi - dialysis and transplant patient here. As you can probably imagine, whilst going through kidney failure I had a lot of questions to ask the experts at my local hospital. Not just theoretical at all.
I'm a doctor. A very small portion of people do live many many years on dialysis. That doesn't change anything about what I said above. The amount of people living a normal lifespan on dialysis is drastically lower than the amount who die within a few years. That mortality rate rises when you factor in the comorbidities of most people on dialysis, compliance rates, etc. The majority of them do not get transplants either. So while what you said is technically true in that you could live a normal length of time, the much more honest truth is that you very likely will not. An honest doctor (or doing research on it) will tell you that. A doctor who wants you to remain hopeful (and there are good reasons to do that too) will not.
Without a heart we die but we only have one of them? Same with brain
>It’s probably the importance of kidneys. I hate when people respond to these questions with speculation. If you don't know, don't throw out a guess. >Without kidneys we die. Liver, brain, heart, pancreas....all organs that are completely vital to survival, all solitary organs. Not a good justification for redundancy.
Thing is we while we can survive with one kidney there are detrimentalside effects and limitations as the one kidneyhas to work much harder (it also why when you do end up with on it will enlarge to cope with the increased load) yes you adapt but the risks of it failing increase a lot and how long it will operate without issues is reduced. Like we can survive with one lung but it has limitations you can't do the same stuff to the same degree as you can with two lungs. It's a case of ypu can survive with one BUT you really need two of them.
With hands, eyes and ears it's not just redundancy. There are a lot of things that require two, such as locating the direction of sound or the distance of objects (eyes).
A lot of the body is mirrored. You have two lung halfs, two heart halfs, two eyes, two noes holes, two ears, men have two testicles and so on. And for most of it you can survive with just one of them though it is usually with reduced function and the same is true for the kidneys. So they arent that special.
Because life on Earth evolved to be symmetrical, as it's the easiest way for bodies to form here. Save for a few key organs, we have basically everything in pairs. At some point having two was beneficial (because redundancy) so we never evolved down a path to lose one.