How and why our right and left sides differ
This essay was written by Jonathan Cooke and was first published in the 1999 Mill Hill Essays.
Although most of us would be surprised to see ourselves exactly as others see us, rather than the reversed image we normally encounter in mirrors, we do look externally like basically left-right symmetrical beings. Yet ‘handedness’ – the preference of each individual to use one or other hand for most skilled manipulations – is an everyday but intriguing feature of human life. If some nine out of ten individuals were not right-handed in this sense, so that we arrange our environment and cultural conventions to accomodate this, it might be difficult for people to grow up reliably conceptualising and labelling the two sides of their body and its surroundings as right and left. In fact, at least two other kinds of profound left-right differences occur internally in our bodies, and though I shall return to hand-use at the end of this essay, I will first discuss these other ‘handed’ asymmetries. They are more consistent, in the sense that only one individual in hundreds or thousands develops with mirror-reversal or confusion, in relation to the ‘normal’ direction, and they are at least as important as differential hand use for a human being to function.
Our large internal organs, the heart and gut which are basically coiled tubes, and the liver and lungs with their asymmetrical lobed structures, are packed into our bodies one particular way round. They seem to make best possible use of the space enclosed by our externally streamlined shape. Though we share this asymmetry with our vertebrate cousins the mammals, frogs, fish and so on, it is usually of human concern just to surgeons. Yet mirror-reversal of the internal organs is compatible with normal life only in its very rare, exact form. Confusion and disharmony among the major organ asymmetries, still rare but less so, usually involves the individual in life-threatening medical problems. The human type of functional brain asymmetry is even more interesting. It occurs only in ourselves, with perhaps its beginnings detectable in apes, our nearest relatives. Substantial parts of the brain, though scarcely detectably different on right and left sides to the anatomist or microscopist, make profoundly different left-right contributions to our understanding of the world about us. These subtle divisions of labour are pivotal to language, to visual memory and, probably, to music appreciation. People with ‘unusual’ patterns of left-right brain function, while much less common than left-handers, are altogether more numerous than those with major irregularities in the plumbing of their internal organs. They are also much less dramatically, if at all, disadvantaged in life, yet the actual huge preponderance of one particular left-right pattern does strongly indicate what evolutionary biologists call a ‘functional significance’ for it. It is there because it helps our human brains do what they distinctively can do.
Left-right partitioning of brain function presumably is based on information, already residing in the embryo’s body, that controls the disposition of the major organs; this information evolved with our vertebrate ancestors. But what about the origin of the information itself? What were the historical origins, in evolution, of the massive and consistent internal asymmetries within the outwardly left-right symmetrical body of all animals with backbones? There are two main lines of thought about this particular one, which might be termed the argument from design and the argument from origins.
‘Design’ is a shorthand term for biology’s recognition that evolution, by natural selection of inherited variations across the generations, simulates a rational engineering process in optimising the performance or function of living organisms in relation to their lifestyles. Thus the vertebrates, the class of backboned animals to which we belong, clearly first evolved as unprecedentedly active organisms, seeking out their living requirements by a highly efficient form of locomotion. Old-hat as it may seem now, they rose to dominance by swimming in ancient seas with externally bilaterally symmetrical, muscle-filled bodies. But the more active they became, the more their tubelike internal organs required to expand their total surface area for digestion, gas-exchange and the pumping of blood to service this increased rate of metabolism. This necessitated their close packing into the body, without wasted space and yet in a way that would cause, if anything, co-operative rather than interfering effects between their continuous pumping activities. Coiling a mass of elongating tubes into a compact outer cylinder almost necessarily involves departures from internal symmetry, and any mechanism that could co-ordinate all the asymmetries in a standard way, rather than leaving them to develop randomly, would surely be advantageous. This could be brought about if a system of information arose in the early embryo’s tissue, recording which was the right and which the left side. The growth patterns within each organ system, that resulted in their asymmetrically packed shapes, could all then co-ordinate by, so to speak, independently but consistently paying attention to this global left-right information.
The argument from origins, in effect, passes the buck back further into the past, in order not to have to explain the evolutionary ‘invention’ of the embryonic left-right information. On this view we, the vertebrates, already possessed the information at our evolutionary origins, because contrary to modern appearances we came from fundamentally unsymmetrical animals; in fact, from animals with no symmetry at all. The idea is less bizarre than might at first seem. A survey of all the multicellular animals on Earth shows that more than half have an early embryonic body that lacks any plane of bilateral symmetry. Such a body plan or first group of cells in the embryo necessarily has definable, different right and left sides, once an upper and lower surface and a front-to-back axis (or even, say, a mouth) have been defined. This lack of symmetry in a primordial multicell body is not so surprising. Protein molecules, the building blocks of life, are often ‘handed’ in shape like this, the technical term is chiral.. Some of the most strikingly handed-shaped large assemblies of protein molecules may control the architecture of cell division and cell locomotion. When such embryos go on to develop into apparently bilaterally symmetrical adults, they do so by modifying their original plans as they grow, and the driving force for this is directional locomotion. Primordial relationships among the fundamental animal types are shrouded in mystery, but to the extent that we have any clues, our particular type either arose from, or included in its evolutionary history, a non-symmetrical form.
Of course, the arguments from design and from origins are not exclusive alternatives, but both contribute to what is perhaps the most likely scenario. If early backboned animals, having become externally symmetrical for engineering reasons, needed access to embryonic left-right information in order to co-ordinate internal asymmetries and optimise their design for living, they could well have possessed such information as an evolutionary relic rather than having to invent it.
Finally, what of the tradition of innuendo, within many cultures, that left-handers are ‘outsiders’, ‘death-prone’, subtly socially disruptive? In brief, it is poppycock; people whose left-handedness has the basis described above carry no inherent burden in life, except perhaps that imposed by our right-handed man-made environment. There is however one way in which such myths might have arisen. A small number, about one in one hundred babies suffer sufficient brain damage around birth that from that point they are irreversibly people with effectively only the right or left part of the brain, called the cerebral cortex, working as the basis for development. The remaining damaged part is largely unable to make its expected distinctive contribution to abilities, though such a brain can appear to perform remarkably well. But there tends to be a sharp cost in terms of the overall levels of ability attainable. Each half of our cerebral cortex basically controls skilled movement in the opposite half of the body. Thus, say one in a hundred people will be more or less subtly brain damaged in a way that means they have also had their handedness determined not by the natural developmental system I have been outlining, but instead by default; one particular side must control their skills since the other cannot. Say this bad luck happens equally often at random on right and left sides, and consider the ‘statistical’ consequences. Arithmetic will show that such people will constitute only an insignificant 5 in 896 of all right-handers but a noticeable 5 in 104 of all the left-handed members of the population. Such unfortunate people could have been the prototypes for the ‘sinister’, ‘gauche’ lefty, and may in fact underlie the finding, emerging in some surveys for some types of ability, of a very slight deficit in the performance of left-handed people averaged overall. As a left-hander who often refers to himself as ambisinistrous, but is ajudged near-ambidextrous by his friends, these speculations do not worry me. Nor should they worry you. Leftism is good for piano-playing anyway, because for obvious reasons only ‘impressive’ piano music is written with anything at all clever for the left hand to do!