symmetry and asymmetry in humans -- 7/24/19
Today's selection -- from Lucifer's Legacy by Frank Close. Symmetry and asymmetry in humans:
"Although people are symmetrical neither from front to back nor from head to toe, the human body from the outside appears to be broadly symmetric left to right. Apart from self-selected cosmetic asymmetry, such as the parting of one's hair or the greater muscle development on the right half of the natural right-hander, there is only one externally visible 'natural' asymmetry and that is in the tendency of a man's left testicle to hang lower than his right. This is clearly a case where symmetry is not desirable as two equally 'slung' testes would keep bumping into one another. In addition to the discomfort this would cause, this would be detrimental to the healthy generation of sperm. Here is a case where it is plausible that natural selection has led to the asymmetry -- though why it should have chosen to lower the left rather than the right is part of the mystery.
"Apart from these and other minor exceptions such as scars, vaccination marks, and freckles, humans superficially appear to be symmetrical. However, such symmetry is only skin deep. In all vertebrates, the heart and stomach are on the left, the liver and appendix on the right, though one person in ten thousand has their heart in the wrong place (namely the right instead of the left). If the heart is on the 'wrong' side, all the other organs tend to be also which can give surgeons a nasty surprise if they are not well prepared. The heart loops to the right (in 'normal' people) while the stomach loops to the left. In those rare individuals where the organs are in their mirror position relative to the norm, the directions of the loops are also inverted. Whatever determines the orientation for one organ, determines it for them all. The mirror inversion is absolute. Somehow the original microscopic spherical embryo becomes chiral, left-right asymmetric; the whole then choosing either the 'standard' or the mirror version.
"Our right lungs are larger than our left, while the left half of our brain is slightly larger than the right. Asymmetries in our brain and in the way that our nervous system is connected control much of our behavioural asymmetries. The nerves that connect our brain to our muscles cross over so that the left side of the brain controls the right side of our body and vice versa. The right-handed dominance is therefore equivalent to a greater control by the left half of the brain. The left side of the brain is concerned with language, reading, writing, speaking, and hearing, while the right controls our visual, artistic, and musical awareness. Thus, if a stroke damages the left side of the brain, speech impediment often occurs.
"Our right and left eyes are also controlled by our asymmetric brain such that we are all either left-eyed or right-eyed. You can test which you are by pointing at an object that is about two to three metres away, then cover the left eye and see if your finger is pointing at the object or is directed slightly away from it. Then repeat the exercise by covering the right eye rather than the left. If your finger was still pointing at the object when your left (right) eye was covered then you are right (left)-eye dominant.
"One of the most familiar examples of mirror symmetry is the fact that humans use one hand or the other, but not both, for skilled tasks such as writing. Are you left-handed? If so, you will be all too aware of the way that society discriminates against you. Left-handedness is not that uncommon -- one in ten people on average are -- and yet we are surrounded by artifacts designed with right-handers in mind. For example, it is easier for one's right hand to twist clockwise than anticlockwise, the left hand showing the opposite preference. In years gone by ropes were made by humans physically twisting hemp. When right-handed people did the twisting the result was a rope with a left-handed twist, lefthanders producing a right-handed thread. By looking at old ropes you can tell the handedness of whoever twisted it."