Why do we have two of some organs, but not all?

[Drakkith]

Just as the title says. Why do some organs come in pairs, but others come singly? What happens during the development of an embryo that causes two or one organ to develop?

 

[phinds]

The evolution of two eyes seems very reasonable since depth perception is useful for both predator and pray, but I can't think of any reason why two mouths would be all that useful and two sex organs would just be damned confusing.

As to why the embryo develops that way, that just genetics. It's evolution that causes the genetics to be that way.

 

[Drakkith]

The evolution of two eyes seems very reasonable since depth perception is useful for both predator and pray, but I can't think of any reason why two mouths would be all that useful and two sex organs would just be damned confusing.

Perhaps, but the mouth and sex organs have bi-lateral symmetry. Just like the body does with those organs we have two of.

As to why the embryo develops that way, that just genetics. It's evolution that causes the genetics to be that way.

Well, obviously, but I was hoping for a little more detail you mangy mutt, you! *smacks Phinds with a newspaper* Off to the bath with you!

 

[256bits]

Here is a brief summary of the cell division of the zygote.
https://www.boundless.com/biology/animal-reproduction-and-development/fertilization-and-early-embryonic-development/cleavage-the-blastula-stage-and-gastrulation/

Rapid cell division form a blastula, or a round ball shape. The next stage is the differentiation of the cells called gastrulation, forming layers of cells - ectoderm, the mesoderm, and the endoderm. It is here where you want to look to find out how the three layers become the organs and tissues of your body.

Note that the digestive system of an animal is continuous from the mouth to the anus, so you in fact have an inner layer of the digestive tract, stomach, esophagus, etc and an outer layer called your skin. There is not really any chance that these organs would be doubled up, just due to the way they are formed.

Other organs such as the lungs, kidneys, ovaries, testicles, etc, from bilateral symmetry, which again is a result of the germ layer responsible for these organ developments.

You might want to look at the development of the fetus especially at the early stages, to which organs develop first and how cleavages among the cells will differentiate into special types of cells and how.

 

[phinds]

Arf, arf, arf, *bites Drakkith on the ankle*

 

[.Scott]

It's Darwinian engineering.
For locomotion, it's best to have a front and back. And since we're moving through a gravitation field, a different top and bottom has its advantages. But once you've got a good left, mirror image it and you've got a good right as well.

In some cases, a backup makes sense. We could have two spines carrying redundant data - but there would still have to a single decision point in the brain feeding both spines and muscles would have to decide while spine to listen to. According to the Darwinian experiments, one well-protected spine provides a better pay-off than redundancy.
Portions of the brain are duplicated, so a sudden minor injury would take us completely out immediately. But the parts that decide when we're too hot, too cold, or too hungry, or control out sleep or blood pressure have more tightly integrated forms of redundancy.
Two kidneys make sense - each with its own tap into the blood stream. But the liver's function doesn't require separation for redundancy. If you need more liver, and your liver is healthy, you'll grow more liver. Similarly, it would be difficult to arrange the plumbing so that two hearts would be better then one. If either heart failed, it could sabotage the efforts of the other.

 

[Drakkith]

Got any references for that, Scott?

 

[AlephZero]

Similarly, it would be difficult to arrange the plumbing so that two hearts would be better then one.

Hmm... earthworms have ten. Evolution doesn't have follow human logic to work.

 

[.Scott]

Got any references for that, Scott?

By "Darwinian experiments", I was referring to the results of evolution. So my citation is you, in the corporal sense.

It's too late to edit that post, but I do notice that I used "would" when I meant "wouldn't" referring to effects of brain injury. As far as citation for the brain injury statements, I don't think I was claiming anything uncommon. Examples of animals and people sustain brain injury without immediate incapacitation are in many studies. Here's one from yesterday: http://www.eurekalert.org/pub_releases/2014-07/aaon-emt071014.php

Other than the evolutionary result, there really is no reference for the heart thing - at least not for a human model. How far back in human evolution would you have to go before you found an opportunity to add in a backup heart? Would it be better to arrange it in parallel or series? Actually, with those four chambers, it's already two pumps in series. Does that really serve as a backup though?

 

[.Scott]

Hmm... earthworms have ten. Evolution doesn't have follow human logic to work.

Apparently worms are a significantly different engineering problem. I'm not surprised.

 

[Drakkith]

By "Darwinian experiments", I was referring to the results of evolution. So my citation is you, in the corporal sense.

That's not how it works here at PF. Do you have any actual references that support your explanation? Something that elaborates on what you've said? I'm just looking for something more detailed than general explanations, not contesting your post.

 

[DiracPool]

But the parts that decide when we're too hot, too cold, or too hungry, or control out sleep or blood pressure have more tightly integrated forms of redundancy.

Do you mean the hypothalamic-pituitary complex? How does this region have any more of a tightly integrated form of redunancy than any other region? You need to provide references if you're going to make assertions like that.

We could have two spines carrying redundant data - but there would still have to a single decision point in the brain feeding both spines and muscles would have to decide while spine to listen to.

That's an unsubstantiated hypothetical claim as far as I can tell. If we were to duplicate our already paired spinal columns, that would confer a measure of signal traffic confusion that would be unworkable. Nature would have selected that out immediately.

According to the Darwinian experiments, one well-protected spine provides a better pay-off than redundancy.

The entire neuraxis is a paired structure. It's not as if we have a paired left brain hemisphere and right brain hemisphere, and a single spinal column. The spinal cord also has a paired division, just like the brain. While motorically one hemisphere of the brain-spinal cord controls the contralateral half of the body, there is also a smaller measure of ipsilateral control which may serve a role of redundancy, but that is a matter of some speculation.

 

[.Scott]

If we were to duplicate our already paired spinal columns, that would confer a measure of signal traffic confusion that would be unworkable. Nature would have selected that out immediately.

That was my point, nature did select it out immediately - or quickly enough.

 

[Evo]

Please, let's post supporting peer reviewed studies instead of guesses, or this thread will be closed.

 

[256bits]

How far back in human evolution would you have to go before you found an opportunity ...

For bilateral symmetry as far back as
http://en.wikipedia.org/wiki/Bilateria
Humans, by the way, belong to Chordata

 

[Chronos]

You might want to see this http://www.thenakedscientists.com/HTML/questions/qotw/question/3091/. Another issue: is the energy required to support 'spare' parts worth the benefit?