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.

Why do we grow up?

We are born from a single cell that duplicates itself many times until we reach a volume and mass so many times bigger than our original self that one could very well wonder if we keep any link to that tiny part of us.

One could argue that development is needed in order to achieve a level of specialization of tissues where our organs will work properly and the inumerous tasks to be performed by our organism will be fulfilled. However, even after our organs are fully developed and in their right place, we countinue to grow up.

Let´s arbitrarily say that a 10-year-old child is completely functional - its teeth are all there, lungs, heart and intestines are all working and so is its brain and mind- then why does it keep growing, getting denser (more cells per cubic centimeter) and reach a point where many of its main functions will begin to decay (ageing)?

One hypothesis could be that a human being must grow until an optimal point for its survival and then push the envelop a little further so its offspring will be favored. A bigger mother can certainly bear more children and during more time (longer pregnancy, more developed children) but on the other hand this oversize will certainly claim its share and the life expectancy of the mother will be shortened.

Anoter reason for being born smaller than our maximum size is because an adult cannot fit inside another one, it´s mandatory that a life be born more simple than it will be at is apex. This rule can only be bent in cases such as unicelular organisms but even in this case the 'mother' cell must double its volume so when it splits into two 'daughter cells' the two newborns will be approximately the same size as an adult cell.

Development is as necessary for pluricelular organisms as the simplification of mechanisms is for life itself. Life is only possible if we are capable of breaking its functions into smaller more simple ones that will be themselves broken into more simple ones and so on.

One interesting question would be if death by ageing (and ageing-related diseases) is somehow linked to this dynamics and also if there is a correlation between the number of final cells of the organisms, its development time before birth, and the life expectancy of the indiviual.