There is no easy explanation for why the heart of most vertebrates is usually
found on the left. Ditto for why most people are right-handed. Ditto for why
the left hemisphere of the brain is dominant in language.
The British psychologist Chris McManus launches into an epic search for an explanation. He begins by quoting Kant: it is impossible to define right and left in purely verbal manners. The way we teach right and left and by showing, and, even so, children have trouble mastering the concept.
McManus then delves into biology, looking for asymmetry in the very building blocks of out bodies. The instructions stored in DNA are not enough to tell the fetus that the heart has to be placed on the left. Genes alone cannot do that. The propensity to asymmetry lies in the very building blocks of life: aminoacids are "left-handed", or, better, most proteins are built from left-handed aminoacids ("left handed" referring to the way the molecule organizes itself around the carbon atom). Molecules of living organisms are asymmetric. Something similar happens with sugars: our bodies are mainly made of "right handed" sugars.
Aminoacids and sugars exhibit a property that has intrigued Physicists ever since William Thomson named it "chirality" in 1904: some objects are such that they cannot superimpose with their mirror image, no matter how you rotate them. The most familiar case is the "chirality" of our hands: the two hands appear to be "identical" (one the mirror image of the other) and, still, no matter how you rotate one hand, you will never get them to superimpose perfectly well. That's why the right glove doesn't fit on the left hand and viceversa.
DNA is made of left-handed aminoacids, which means that RNA transfers left-handed aminoacids, which means that proteins are made of left-handed aminoacids. My body is made of left-handed aminoacids (and of right-handed sugars). This is not true of all animals: amphibian skin is a notable exception in the realm of vertebrate animals.
Two main conjectures give a plausible explanation of why nature prefers left-handed aminoacids when building vertebrates. One is that it just happened randomly, and then it stuck: the chances of having right-handed or left-handed aminoacids to start with are the same, but, once you pick one kind, you cannot mix them, so you end up with a world mainly made of the one that was accidentally picked first.
The other theory is that biological asymmetry is somewhat related to physical asymmetry. In 1956 Chen Ning Yang and Tsung-Dao Lee proved that "parity" is not conserved in the weak interaction. Parity is a Physicist's way to describe the right-left symmetry of the laws of nature. The laws of nature look the same if you look at the mirror image of the universe. Parity was assumed to be conserved in all processes until Yang and Lee proved their 1956 theorem: there is at least one law of nature that does not look the same if you look at the mirror image of the universe.
Statistics show that left-handedness has increased dramatically over the last century, but it could be simply due to cultural factors (more tolerance for left handedness). The fact remains that the vast majority of human beings are right-handed. Homo Abilis (two million years ago) was right-handed too. Cats, instead, are equally divided between right-handed and left-handed (or, better, right-pawed and left-pawed). That is also true of our closest living relatives, the chimps, although laboratory experiment show a slight preference for the right hand. Humans are the only species that shows such a strong asymmetry in the limbs we used.
The other major right-left asymmetry in human beings is found in the brain: the left and right hemispheres specialized in different cognitive tasks. The correlation between right/left-handed people and right/left-handed language hemispheres is not obvious. Just like right-handedness tends to be more widespread, so left-hemisphere language dominance tends to be more widespread, regardless of whether you are right or left handed.
The book is full of historical trivia, and the trivia quickly become the main story. There is no answer to the puzzle: we still don't know why most people are right-handed, why the heart is usually on the left and why the cerebral hemispheres have different functions.
Notably missing is the story of how the embryo develops asymmetrically. The fetus starts out perfectly symmetric but after just a few weeks visible signs of right-left asymmetry already appear. The heart, the stomach, and the liver start out along the midline of the embryo. Then the heart starts bending to the left and the stomach and the liver start moving clockwise. The origin of the asymmetrical development may lie with the fluid that surrounds the embryo: that fluid needs to flow, and a flow inevitably has a direction, usually from right to left. When scientists reversed that flow in embryos of mice, the mice developed reversed organs.
TM, ®, Copyright © 2012 Piero Scaruffi