1

What is the evolutionary advantage with two lungs (kidneys)? Most living beings only have one heart, one stomach. Most internal organs are not doubled and if one lung fails it is not exactly quite transparent due to having an extra lung. Before modern health care one failed lung would kill the individual.

The same can probably be said for kidneys.

d-b
  • 365
  • 1
  • 10
  • 3
    Evolution does not have a purpose. As for why things evolved that way, most animals are essentially tubes. It would seem to take very little evolutionary change - basically a few mutations to the HOX genes - to create bilateral symmetry. – jamesqf Mar 06 '21 at 03:59
  • 3
    The heart is two-sided, each side doing pretty much the same thing, much like the lungs and kidneys. Could easily be separate, though wouldn't work as well together. Also, you should cite sources for your claims. – anongoodnurse Mar 06 '21 at 05:47
  • 3
    Does this answer your question? Why do some bad traits evolve, and good ones don't? – kmm Mar 06 '21 at 14:02
  • most organs come in pairs in vertebrates, (the heart starts as two separate organs embryonically) the digestive tract organs are the exceptions, you may want to reframe your question about that. – John May 29 '21 at 04:47

3 Answers3

3

Humans and most organisms have bilateral symmetry. Anything that is on the left side of the body will be on the right also. Not everything can be in the middle, so ... you end up with two. This is evolutionarily labile - for example, fish generally have one dorsal swim bladder, though it is derived from the lungs. In a similar manner, the heart is formed by the fusion of primordia on either side during development - see UNSW for a cute video.

Mike Serfas
  • 2,455
  • 4
  • 10
  • -1. Your thesis does not stand. The heart and other organs like spleen does not have bilateral symmetry. – Hans Mar 07 '21 at 09:20
  • Bilateral symmetry is of course an approximation - biology doesn't know theory. But it is a good one. For example, the spleen begins from the dorsal mesogastrium (the mesentery connected to the greater curvature of the stomach), making it, conceptually, an unpaired midline organ that merely reacts to surrounding circumstances. In some humans the situation is more bilateral – Mike Serfas Mar 07 '21 at 14:11
  • Bilateral symmetry goes much further that the internal organs, of course, so bilateralism must be deeply embedded in the genes. (Even structures like vertebrae are symmetric.) So the question really should be why some few organs AREN'T paired or symmetric. – jamesqf Mar 07 '21 at 17:45
  • As I said, "other organs" in plural, with spleen being but one example, which do not have bilateral symmetry. What is the mechanism of all the other non-bilateral symmetric organs? With respect to the spleen in particular, it just begs the question: what makes the stomach non-bilateral symmetric? You can say they just compete for the same spot and randomly eking out a space and deforming each other. Why do all of them not have two or more which can all be arranged bilaterally? Another question is why we do not have more equal distribution of people having their hearts and spleen situated – Hans Mar 07 '21 at 18:36
  • randomly on either sides of the spine. – Hans Mar 07 '21 at 18:40
  • The nodal signalling pathway is clearly involved, which can be traced back to the clockwise motion of cilia in some species. But I have not kept up with recent developments in this area; you should find more with a little research. – Mike Serfas Mar 07 '21 at 22:48
  • @Hans the heart actually starts as two sperate organs embryonically. the only organs that do not start as pairs are the digestive organs (stomach, liver, pancreas, spleen) because the digestive system starts as a single tube. – John May 29 '21 at 04:50
  • @John: Wow, really? Does the heart start out as two bilaterally symmetrically embryonically? If so, my previous other question still remains: why do we not have equal number of people having left situated and right situated single-organ heart and bilaterally symmetrically situated two-organ heart? – Hans May 31 '21 at 23:27
  • 1
    @Hans the heart starts as two unconnected blood vessels. this may help https://open.oregonstate.education/aandp/chapter/19-5-development-of-the-heart/ I have a whole answer in the unclosed question about this. https://biology.stackexchange.com/questions/100928/why-do-we-have-two-kidneys-but-one-liver/100935#100935 Your questions about bilateral symmetry are great questions I suggest asking them as a stack questions (look for identical questions first) but in short is comes down to shoving 30ft of digestive tract in 2 feet of torso and the fact we are no longer laterally narrow fish. – John Jun 01 '21 at 00:01
1

Two of the duplicated organs work better (e.g.: also kidney) or even enable something new (e.g.: depth perception for eyes). Source

tsttst
  • 1,597
  • 9
  • 25
  • Two hearts? Two brains? – d-b Mar 06 '21 at 11:00
  • Same applies to animals with multiple hearts or brains. https://www.mentalfloss.com/article/52337/3-creatures-more-one-heart And https://medium.com/illumination/the-animal-with-many-brains-that-makes-us-rethink-intelligence-1fc5e1325bcb – tsttst Mar 06 '21 at 13:23
  • 2
    @d-b: Vertebrates do basically have two brains, separate left and right hemispheres with interconnections between them. – jamesqf Mar 06 '21 at 17:32
  • -1 for the falsehood. 1. Why is 2 the optimal rather than 3 or 4 or more like the number of teeth? 2. It is not true for other organs like the heart and spleen. – Hans Mar 07 '21 at 09:24
  • My answer addresses OP’s question (what are advantages of multiple organs). The question is not, why different animals all are not the same / why they occupy a given niche in the evolutionary landscape (and hence experience different tradeoffs). Additionally Mike Serfas great answer well explains the developmental possibility of two organs. – tsttst Mar 07 '21 at 14:31
  • @jamesqf they also have two hearts that fuse into a single one during early embryonic development. – John May 29 '21 at 04:44
-1

Eels have two lungs and so do lungfish and all vertebrates, so the design is at least 375 million years old, although vertebrate lungs have developed different air flow systems, i.e. birds and iguanas. No vertebrates have a very small and big lung as far as I know.

  • Rib punctures can affect one lung which fills with fluid.
  • Stick and tooth punctures can actually cause air to escape the lung through the chest, a condition that can sometimes be survived in th wild, but less likely with only one lung.
  • Pulmonary infections can cause a lung to fill with water, so having two lungs gives some extra protection.
  • Breathed in objects like twigs can sit inside a lung for many years and cause weakness, for example a man coughed up a twig from his lung 14 years after having breathed it in while playing in a hedge with friends.
  • Two lungs may also bring higher surface area and higher performance than one lung.
bandybabboon
  • 10,397
  • 22
  • 39
  • Most of your points are deadly in a non-civilization (i.e., for all animals except humans living in a society that offers some surplus food etc) context, even if you have a spare lung. Re the surface are, that is why lungs are "wrinkled", and that works fine with a single lung. – d-b Mar 07 '21 at 09:07
  • 1
    -1. None of your points answer the question. Regarding the last point, the fractal-like structure of each lung already increases the surface. It is not necessary to have two to increase the surface. – Hans Mar 07 '21 at 09:28
  • Actually, if your lung fills with water from a stick/rib/pneumonia, you will die irrespective of the fractal structure, which is not part of the question, but having two lungs will stop the liquid spilling into the second lung and choking the vertebrate in question, and two hemispherical spheres have a surface area of 3.14+1.4, i.e. 30% bigger. – bandybabboon Mar 07 '21 at 16:17
  • @d-b Humans have only lived as a civilization for 10,000 years, previous to that they were hunter gatherers with very low populations because the surplus-giving crops did not exist, and they inherited their lungs from other primates and the primates from basal placental animals.... Two hemispheres have 30% more surface area to place the wrinkles on in the first place, so a 5 liter double-hemisphere lung is equivalent to a 7.5 liter single spherical lung, approximately. – bandybabboon Mar 07 '21 at 16:25
  • Given the total volume and a geometric shape the more isolated domain of the same size one makes the more surface area one gets in the form of $n^{1/3}$ where $n$ denotes the number of isolated geometry. So for $n=2$ there should be $2^{1/3}-1~26%$ more area. It is not clear what you are computing and where 3.14+1.4 comes from. If you split a sphere into two hemispheres, the area increases by 50%, not 30%. More importantly, it is not the outer shape and area we are looking at. It is the ultimate (inner) surface shapes, i.e, alveola sacs we are talking about, – Hans Mar 07 '21 at 18:28
  • not the exterior shape which has nothing to do with respiratory gas exchanges. One needs only more and smaller alveolar sacs. So there is no need to separate into two largely isolated domains if our objective is just to increase the gas exchange surface. You do have a point in asserting the risk diversification. That though begs more questions. Why we do not have more than 2 lungs? Why do the other organs such as the heart and spleen have only one isolated unit? – Hans Mar 07 '21 at 18:34
  • The lungs and the windpipe both lead straight into the middle of the lungs and provide better air and blood distribution than if it was a sphere. https://cdn.britannica.com/s:690x388,c:crop/20/125820-050-F1FD8A7E/veins-arteries-human.jpg Do you also disagree about the protective effects outlined in the first 4 bullet points, and disagree with only the last one? What about the puncture, pathogen and fluid protection afforded by two lungs? The heart and spleen can't absorb pathogens and twigs and get pierced by ribs as easily as the lungs can. – bandybabboon Mar 07 '21 at 21:40
  • Let us continue this discussion in chat. – Hans Mar 08 '21 at 04:23
  • Please check out my response to your last comment in the chat. – Hans Mar 08 '21 at 17:15