How does natural selection account for the symmetry of the human body?
In a typical human , the following similarities may be quickly discerned:
The corresponding fingers of each hand (on the OPPOSITE side of the body) are roughly identical in length and shape; the same relationship exists concerning the size and shape of the hands, arms, shoulders, ribs, legs, knees, ankles, feet, toes, eyes, ears, and nostrils.
Additionally, the eyes and ears are located in the same relative positions on both sides of the face. Also, the corresponding teeth in the upper and lower jaw are roughly identical in structure.
What accounts for this symmetry?
One Response to “How does natural selection account for the symmetry of the human body?”
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secretsauce on June 20th, 2009
Well, I hope you notice that this symmetry doesn't just apply to the human body … but practically *all* animals … all primates, all mammals, all terrestrial vertebrates, even fish.
In other words, it is all explainable by common ancestry tracing back to an early ancestor that had a symmetrical body plan. So natural selection obviously has some role in preserving this symmetry throughout evolution.
The natural selection explanation is that bilateral symmetry provides some benefit … and so there has never been much pressure to change it. It is useful because it is economical during development of the embryo. The same genetic instructions for building the right hand, will also build a left hand. The instructions for one eye can build the other. Instructions for building the left-side of the rib cage, also builds the right.
A more interesting question is why almost all animals have this left-right (bilateral) symmetry, but don't have dorsal-ventral (belly vs. back) symmetry, or anterior-posterior (mouth vs. rear) symmetry. The answer is that these other kinds of non-symmetry are themselves beneficial.
Dorsal-ventral non-symmetry (the fact that the belly-side of most animals is different from the back-side) traces back to our ancestors that either swam or crawled with belly-side down and back-side up … so and the dangers from above are different from the dangers from below. Incidentally, there is absolutely no reason this has to be true for we humans who walk upright … there's no reason our belly is soft and back is hard … except that this has some benefit for animals that walk on all fours.
Anterior-posterior non-symmetry (the fact that the front of the animal is different from the rear) traces back to our aquatic ancestors, where one end had a mouth, and the other end was for excretion … so there was obvious benefits to keeping them at opposite ends of the animal.
However, there have never been benefits to left-right non-symmetry … animals are just as likely to get attacked from the left as from the right … or to find food on the left more than the right. So the benefits of economy in embryology won out.
Hope that helps.