Making it Look Easy

When skaters take to the ice during the Olympics, we all watch in amazement as they move through their routines involving various jumps, spins, spirals and lifts. Part of our amazement is not just what they accomplish, but how they make it look easy––– almost effortless. We know from all the background stories we are given about the skaters, however, that the ability to perform in this manner came with a tremendous amount of effort on the part of the skaters, trainers, coaches and parents.

In the presentation of evolutionary change, a similar thing seems to happen. The presenters make changes in animal morphology look easy. For example, to show the evolution of the heart the National Science Foundation provides us with an image which leads us to believe that going from a three-chambered heart to a four-chambered heart was an easy transition – all you have to do is increase the size of the septum between the sides of the heart. This image makes it easy for us to imagine that a small genetic mutation could produce such a result. But as with the skaters, it is just not that easy.

Image source: https://www.nsf.gov/news/mmg/media/images/reptile_hearts_h.jpg

The advantage of having a four-chambered heart is intuitive: it provides a more efficient delivery of oxygen to the body than a three-chambered heart which is constantly mixing the oxygenated blood from the lungs and the de-oxygenated blood coming from the body. If such an advantage could be had with a simple gene mutation, then one should ask why all amphibians and reptiles don’t have a four chambered heart. Simply dividing the ventricle, though, would not confer an advantage­­­ – it would effectively decrease the blood flow to the body.

To compensate for the division of the ventricles, the four chambered heart has a larger muscle surrounding the left ventricle than the right ventricle. The larger muscle increases the amount of force behind the blood going out to the body than the blood only being delivered to the lungs. Such differentiation would not be an advantage without the septum, so both structural changes would need to be in place at the same time to confer an advantage.

The explanation for the four-chambered heart resides in the move to a homeothermic metabolism (crudely: being warm-blooded) found in birds and mammals. What may seem advantageous actually comes with a great cost, and would not be possible if other morphological changes were not in place first. Homeothermic metabolism first requires a change in the biochemistry of cells. When little food is available and temperatures are too cold, amphibians and reptiles can enter periods of estivation which involve little or no metabolism of nutrients. For birds and mammals, however, there is a constant breakdown of sugars and fats required to generate body heat. This demands a much larger acquisition of food on a year-round basis.

In order to obtain more energy, birds and mammals must have an overall different body structure which enables them to more effectively acquire and consume greater amounts of food. This would demand significant modifications to the skeleton, skeletal muscles and the digestive system. Such changes would not be an advantage to the poikilothermic (cold-blooded) animal because their usefulness is predicated on access to greater amounts of energy.

Increased metabolism not only demands more food, it also demands more oxygen. To accomplish this, a much larger and more elaborate set of lungs must be developed. Furthermore, a more robust set of muscles in the diaphragm and between the ribs must develop to generate stronger pressure differences to allow for more rapid exchange of gases in the lungs. Dependence on a regular oxygen supply also demands changes in the nervous system which can detect depletion of oxygen in the blood stream and therefore alter the breathing rate and heart rate to accommodate regular changes in oxygen demand throughout the day. The development of these structures and systems would be equally useless to the poikilotherm, and yet homeothermic metabolism would not be possible without them.

So, as with our Olympic skaters, what seems at first to be an accomplishment which comes with ease, we realize that it is truly one which requires great effort, foresight and planning. It is only reasonable to presume that this irreducibly complex set of structures, organs and systems which underlies the presence of the four-chambered heart did not come by random genetic mutations, but was more likely obtained by intelligent agency.

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