Why are human beings symmetric along our length?

[pears]

Humans would look practically the same if you put a mirror down the centre of us (as would many other creatures). Why is that? I assume it's something to do with the way the enbryo develops, like clusters of cell for producing a particular organ form and then kind of maybe split into two along an axis resulting in two eyes, two kidneys, two lungs etc. Presumably our two kidneys didn't evolve independently of each other.

[swansont]

Symmetry is pretty easy to "code" in terms of the mount of extra instruction required, and the organ redundancy from the symmetry allows for enhanced survivability.

[pears]

You mean coded in DNA? Do the pairs then evolve together? (i.e. an advantage in one kidney is reproduced in both) Is the symmetry there in the DNA that tells the embryo how to develop? (Is that right? Does DNA tell an embryo how to develop?) Does the advantage appear in the DNA and then the two kidneys are replicated from the same piece of DNA?

 

Sorry that's a lot of questions and as you can see my background knowledge is a bit poor.

[Euler's Identity]

Humans would look practically the same if you put a mirror down the centre of us (as would many other creatures). Why is that? I assume it's something to do with the way the enbryo develops, like clusters of cell for producing a particular organ form and then kind of maybe split into two along an axis resulting in two eyes, two kidneys, two lungs etc. Presumably our two kidneys didn't evolve independently of each other.

 

From an evolutionary POV its known that humans (and many, many other species) find symmetry more attractive (as you may know lol) therefore those with symmetrical body structures would have better reproductive success. Of course sexual selection wouldn't be the end of it, Natural selection selects for those with symmetrical body structures I assume because of ability to compete for whatever reason. But I'm by no stretch of the imagination an expert so my answer is educated speculation at best.

[CharonY]

Or it is simply due to evolutionary history. Our ancestor happened to be bilateral symmetric and that build plan was quite successful among the animalia (i.e. more phyla than in porifera and radiata). From a developmental viewpoint I would think once the general development was comitted towards bilateralism there was little in the way of alternatives. However, a evolutionary biologist or someone from evo-devo is likely to have a better answer to the question.

Ultimately it is obviously not something that we can look for in one species (e.g. humans) but we would have to look way back into our evolutionary history.

Edited September 17, 2013 by CharonY

[arc]

It's basic engineering that natural selection would prefer. As an example, would anyone try to build a suspension bridge like the Golden gate or any of the other examples in asymmetrical design. Unlikely, the additional design considerations would be costly and difficult to overcome. And would most likely lead to structural compromise. Nature will always avoid excessive cost overruns of any type on its construction sites.

[pears]

I think I'm asking more of the how than the why. I can accept that symmetry is favoured by natural selection etc and I can accept that external symmetry is favoured by attraction (though not symmetry of internal organs) but I just wondered how it happens, i.e. the mechanism that gives you two almost identical organs in symmetry. In my mind I can't help thinking of something splitting, either in the enbryo or in the DNA. I suspect the actual answer might be too technical for a simple post though?

Edited September 17, 2013 by pears

[john5746]

I think I'm asking more of the how than the why. I can accept that symmetry is favoured by natural selection etc and I can accept that external symmetry is favoured by attraction (though not symmetry of internal organs) but I just wondered how it happens, i.e. the mechanism that gives you two almost identical organs in symmetry. In my mind I can't help thinking of something splitting, either in the enbryo or in the DNA. I suspect the actual answer might be too technical for a simple post though?

Interesting question and one I never really thought about.

 

Here is a link to an article I found referenced in a wiki, hopefully you are able to download the pdf. In the beginning, they mention that basically the cell splits into two daughter cells with identical states, but in mirror orientation.

[Euler's Identity]

I think I'm asking more of the how than the why. I can accept that symmetry is favoured by natural selection etc and I can accept that external symmetry is favoured by attraction (though not symmetry of internal organs) but I just wondered how it happens, i.e. the mechanism that gives you two almost identical organs in symmetry. In my mind I can't help thinking of something splitting, either in the enbryo or in the DNA. I suspect the actual answer might be too technical for a simple post though?

 

Well in embryological terms, Everything starts as one general purpose cell (stem cell) and goes through mitosis consecutively to form two cells, as this continues the cells will become more specialized (e.g Internals, Externals, Intermediaries to specific systems) but the cells on each side from the original cell mitosis should be multiplying usually the same ways at about the same rate with the exception of some internals which aren't perfectly symmetrical, but again i'm no expert

[pears]

Thanks John, that paper looks awesome. I look forward to reading it.

 

And thanks everyone else too, for your answers.

Edited September 17, 2013 by pears