Why groovy hearts lean left

Stories resulting from deep scientific inquiry are among the most intricate and fascinating tales that have ever emerged for our edification and entertainment, with the added punch that they are about something we did not invent or imagine, which we call reality. Chris McManus' book Right Hand, Left Hand is full of such tales, richly elaborated on, many of which arise from a theme connected with the following question: how could you describe the difference between left and right to a being from a different planet only by telling, not by showing? This question has exercised a number of sophisticated minds in our culture, including Kant and Newton, many mathematicians and scientists, and the consensus is that it cannot be done. Right and left are distinguished by our direct experience of living in our particular three-dimensional world. It is embodied and experienced, not describable, knowledge.

This conclusion presents us with a problem. Our bodies distinguish between left and right, with the heart usually pointing toward the left and our liver displaced to the right. But genes are involved in producing these left-right asymmetries, and genes do the equivalent of telling by directing the production of molecules that inform the developing embryo what to do at particular times and places during the emergence of the body.

Are genes doing what the pundits have concluded is impossible? Not at all.

The developing embryo also has to experience the reality of space and the laws of physics to create a difference between left and right, but it does so in its own unique way. At the earliest stage in the development of the body axis in the mouse, for instance, when head and tail, front and back have been determined in the embryo, a triangular groove arises from the movements of cells in the mid-body line, the broad end of the triangle facing headward and the point tailward. Lining this groove are cells with cilia, little organelles that protrude from the cells and rotate clockwise when looking down on them. This motion sweeps fluid around in a vortex in the groove. But because of the shape of the groove, fluid moves more effectively in the direction of rotation at the wide end than at the narrow end of the triangle, so there is net fluid flow from the embryo's right to left across the groove. This fluid carries a transient signal, released by all cells on the edges of the groove, which accumulates on the left side.

This signal tells the embryo where to make the heart. This explanation is provisional but it does account for a great diversity of experimental results on the positions of the internal organs that are asymmetrical.

The next bit of this tale is even deeper, linking the embryo with the physics of the cosmos. Why do the cilia rotate in a clockwise direction? Because they are made of amino acids that are themselves handed, conventionally referred to as L-amino acids because of the way they rotate polarised light to the left. Why are the amino acids in living organisms of the L-form? Here the leap to quantum mechanics and cosmology is dramatic and plausible, but still tentative. One of the fundamental forces of physics, the so-called weak force, was shown in 1957 not to obey parity so that it produces an excess of left-handed over right-handed electrons. This results in D-amino acids being less stable than the L-form throughout the cosmos. But the effect is small and some amplifier is needed to produce the observed overwhelming predominance of L-amino acids in organisms. What this is remains to be discovered.

The other strand of this book is complementary to scientific tales, involving cultural stories and myths blended with biological evidence. Why is right-handedness favoured in virtually all human cultures ? Is there some intrinsic advantage to being right-handed, so that it is favoured by natural selection? Left-handers would disagree, and they are right. There is no selective advantage for the majority. But there is an advantage to being asymmetrical, though it can be either right or left. Nature seems to abhor perfect symmetry. However, people can have their hearts and livers on the "wrong" side with everything working perfectly, and such people can be right or left-handed.

Having a dominant hand and asymmetrical distribution of various specialised functions in the brain, such as language and fine motor control, definitely works best. So why does right-handedness predominate in human cultures? Again, genes are involved, but their effect is amplified strongly by cultural stories about right being good and left being bad. Studies of these by anthropologists and sociologists are well described by McManus, who explores attempts at explanation that lead in the direction of tolerance of diversity within and between cultures, and the recognition that humans always seek meaning. This is what stories serve to provide, despite their often-bizarre structure.

There is a remarkable amount of detail in this book, which is a veritable compendium of everything you might want to know about handedness. By the end of McManus' book, I felt pretty well integrated with my body, the cosmos, human culture and its diversity. Would that this feeling could prevail in our shattering world.

Brian Goodwin is professor of biology, Schumacher College, Dartington.