Monday, November 13, 2006

External Simmetry versus Internal Assimetry

Most mammals display a remarkable external simmetry (same number os limbs on both sides of body, same numbers of fingers on each hand/foot, eyes placed in symmetric positions, etc.) in spite of a considerable asimmetry of organs within the body.

First we can point some organs that are placed unevenly in the body such as the heart and organs whose size are different such as the two lungs. Other organs such as pancres, liver and appendix are placed in one side of the body and have a corresponding counterpart of completely different function on the respective position on the opposite position in the body.

One way of seeing the point is to state that the position of these organs does not matter so much as long as they work properly. This could be true if we found human beings with livers on either sides of the body and the same for pancreas as well as intestines winded clockwise and counterclockwise but the reality is that these organs's placements is quite conserved.

Why do we have two kidneys if one unique bigger kidney would do the same job? Many theorists state that Nature does not work with redundancy, meaning that Natural Selection would not creatures with two kidneys so they could survive a kidney failure: instead of that, natural selection would remove those that carry alleles that will lead to liver conditions. This said one could expect the second kidney to be either a mandatory step during development of a mammal or simply a 'bug' during the development program and this bug will be soon corrected.

A bug, however, shouldn´t have spreaded all over the mammalian kingdom meaning that the second kidney development must really have a mandatory presence.


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