Bilateral Symmetry: Why are some organs singular?

[Sorcerer]

Before I look into embryology further. I was wondering why are some organs are singular; the heart, liver, pancreas, gall bladder, appendix and maybe the cecum, bladder and stomach, (although part of the bilatterally symetric gut, are not symmetrical themselves).

Do they form on the axis of symmetry and migrate to their positions? Most seem to be quite symmentrical if you rotate them so they line up with the axis.

All of these seem to be related to the uptake, transport and regulation of nutrients and so are linked to the gut. The kidneys and lungs stand out as odd here in relation.

What is the explanation for these apparent anomalies?

Edited July 4, 2015 by Sorcerer

[fiveworlds]

All your organs aren't symmetrical.

[Roamer]

Do you want an explanation of how a body is formed or can i just blame/praise evolution ?

[Prometheus]

'Evolution did it', isn't a particularly satisfying answer. Surely the more interesting question is why did selective pressures at the time favour two kidneys, say, rather than one or three. Presumably there is a cost associated with having 2 kidneys over 1 so what benefit did having 2 confer, and when did this selection occur?

[fiveworlds]

Presumably there is a cost associated with having 2 kidneys over 1 so what benefitdid having 2 confer, and when did this selection occur?

Nature is weird did you know that the laryngeal nerve of this giraffe goes from one spot in the throat to another but to get there it goes the whole way down the giraffe's neck and back up again.

 

[swansont]

when did this selection occur?

 

AFAIK, amphibians have one kidney, as do fish. The split where amphibians evolves was probably the split (~400 MYa)

[Asimov]

Sorcerer:The development answer follows.The embryo has a midline tube from mouth to anus that becomes our gut. Many organs bud out from this tube.If the bud stays single there is one organ. If it divides there are two of the same organs.

 

OK, why does THIS happen? Most animals and all invertebrates have bilateral symmetry. Would you believe this evolved about 600 million years ago. Why? Because it worked. At some point some ancestor did best with the arrangement and number of organs we still have.Two of every organ might now be better or worse as either plan has advantages and disadvantages but evolution is selfish and short sighted and does what works best at that time.

Two brains, no thanks. one is confusing enough.

Given enough time we could adopt any number of body plans. Lots of time.The Octopus has 2 extra hearts.Some dinosaurs got so big they needed an ancillary "brain" to control the length of its body.We are getting taller unbelievably fast, will we some day need an ancillary brain. Will we sit on it? Will they argue?

[Acme]

...Two brains, no thanks. one is confusing enough.

...

Given enough time we could adopt any number of body plans. Lots of time. The Octopus has 2 extra hearts. Some dinosaurs got so big they needed an ancillary "brain" to control the length of its body. We are getting taller unbelievably fast, will we some day need an ancillary brain. Will we sit on it? Will they argue?

Arguably we do have two brains. The bilateral physiological symmetry of some parts is obvious and there is good evidence that each cerebral hemisphere performs different functions to some degree.

 

Symmetry and asymmetry in the human brain

...Language and the left side of the brain

It has been known for almost 150 years that the left hemispheres subserve language functions, while more recent research has pointed towards the right hemisphere as being specialized for processing of spatial relations and for emotional control. ...

[MonDie]

Perhaps the brain is a case in point that symmetry isn't always ideal. Lateralization means specialization by one hemisphere, for example the one controlling the dominant hand. Laterization is strikingly demonstrated by the polarities observed in split-brain patients.

The nervous system forms from the ectoderm along with the tooth enamel and skin. I mention this because the triploblasty, the additional mesoderm, of bilaterians would suggest that the gastrula was still evolving.

Someone who's taken anatomy could probably pick away at my reasoning.

https://en.m.wikipedia.org/wiki/Lateralization_of_brain_function

[Roamer]

Perhaps the brain is a case in point that symmetry isn't always ideal.

 

I think we only have double(-ish) organs etc. because it means we only have to evolve them once.

[MonDie]

 

I think we only have double(-ish) organs etc. because it means we only have to evolve them once.

Moving from proximate to ultimate, wouldn't it be better to have two of your most important organs? Imagine having an extra, back-up heart!

[Delta1212]

Moving from proximate to ultimate, wouldn't it be better to have two of your most important organs? Imagine having an extra, back-up heart!

But then you have to evolve it in such a way that it fits into your circulatory system without disrupting it and wouldn't just kill you anyway if one of them got damaged.

 

Which is not to say you couldn't, but it's not just a matter of one heart good, two hearts better. It's one heart good, two hearts completely different circulatory system.

[Sorcerer]

Most organs we consider singular exhibit bi lateral symmetry if we look at them in low detail. On the topic of the brain it has 2 hemispheres with a division down the middle, without going into great detail on the folding, it is roughly a bitlaterally symetric organ.

The same applies for the heart, it has 4 compartments, which can be divided down the middle, it has migrated off center slightly and each side has specialised.

If we divide the livers 4 lobes into 2 lobes through the thin plane, it's roughly symmetrical.

If we go into huge detail about bilateral symmetry, it becomes evident it is actually a gross oversimplification. Even our skeletal structure is slightly asymetrical. We have different finger prints on each hand. One boob, or one nut is slightly bigger than the other etc.

The basic plan of deuterostomes is that of bilatteral symetry though.

I think the main question I wanted answered was: "Do singular organs form on the axis of symmetry and migrate to their positions?"

Edited August 8, 2015 by Sorcerer

[MonDie]

The basic plan of deuterostomes is that of bilatteral symetry though

Bilaterians are triploblasts and triploblasts are bilaterians. Bilateria subdivides into deuterostomes and protostomes, or in molecular-based phylogeny there are two protostome clades: lophotrochozoa (contains mollusca) and ecdysozoa (contains arthropoda), but they are all triploblastic bilateria. The basal bilaterian Acoela might be an exception, I do not know.

Edited August 8, 2015 by MonDie

[Asimov]

MonDie: There may be many configuerations that might be better for us now. But evolution does what is best for our ancestor that adopted the body plan at that time. There was no regard for what might be best for us now. Still we are evolving. Those tails we lost would have made tailoring awkward and been hazardous on a bike.

[Roamer]

I think the main question I wanted answered was: "Do singular organs form on the axis of symmetry and migrate to their positions?"

 

Organs form on the position they more-or-less stay.

 

 

Also, your questioning is a bit confusing;

axis suggests that organs would revolve around this axis/centre, this doesn't happen(except when you do a pirouette)

[Endy0816]

I read one explanation that some start out doubled but end up being reabsorbed. Not sure on accuracy of that information however.

[Sorcerer]

 

Organs form on the position they more-or-less stay.

 

 

Also, your questioning is a bit confusing;

axis suggests that organs would revolve around this axis/centre, this doesn't happen(except when you do a pirouette)

English and its semantics

 

https://www.google.co.nz/#q=axis+of+symmetry+definition

https://www.google.co.nz/#q=bilateral+symmetry

Bilateral Symmetry:

Symmetrical arrangement of an organism or part of an organism along a central axis, so that the organism or part can be divided into two equal halves. Bilateral symmetry is a characteristic of animals that are capable of moving freely through their environments. Compare radial symmetry.

 

Axis of Symmetry:

Axis of Symmetry. more ... A line through a shape so that each side is a mirror image. When the shape is folded in half along the axis of symmetry, then the two halves match up. See: Symmetry.