Chris McManus

RIGHT HAND, LEFT HAND

The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures

Probably most of us think we know a fair amount about left and right handedness, and probably much of what we think we know is wrong. This is one of the main messages I get from this really fascinating book. Every conceivable aspect of the subject is covered, as well as quite a few that might not have been expected. And, if that is not enough, McManus has a good website containing "hypernotes" to the text and some on-screeen tests.

The book starts with a discussion of situs inversus, a rare condition in which the normal position of the internal organs is reversed, so that the heart is on the right and the liver on the left, for example. What should surprise us about this is not the occasional reversal of the normal order but rather the fact that, in the vast majority of cases, only one pattern is seen. Why do we not find the population approximately equally divided between left-heart and right-heart types? Little progress was made in answering this question until the 1990s, when research in molecular biology provided an explanation, at least for mouse embryos. Whether the same applies to other species, including humans, is still unclear. McManus describes this complex story with commendable clarity.

Other deep questions about left and right still await a solution. Indeed, even philosophers are involved, for Immanuel Kant raised fundamental issues which are still being debated today. And left and right matter at the most fundamental biological level, for the basic building blocks of living organisms, amino acids and sugars, can exist either in "left-handed" (L) or "right-handed" (D) forms. Living organisms use left-handed amino acids and right-handed sugar molecules almost exclusively, though there are some intriguing exceptions.

Why has this come about? It may simply be a matter of chance. Perhaps the earliest organisms on earth, from which we are all descended, just happened to use L-amino acids and D-sugars and we have inherited this propensity. Possibly, however, there is a deeper reason. Could the preference for one form over another reflect some basic disparity in the universe or in the nature of matter, leading to a preponderance of L-amino acids in the early Earth? There are theoretical grounds for this suggestion but it is difficult to prove. However, certain rare meteorites contain amino acids of extra-terrestrial origin and analysis of these may help to decide the matter. It may also soon become possible to know whether the amino acids in giant interstellar clouds are predominantly of one form.

Reverting to the biological level, McManus considers the human brain. As we have been told in countless popular books and articles, the two halves of the brain are different not only in structure but also in function. Speech, for example, is usually lateralized to the left side, though there are exceptions. McManus makes a fine job of demonstrating that much of the popular writing on this topic is considerably over-simplified and indeed misleading. He provides a good selection of intriguing clinical cases to illustrate the extraordinary complexity of the subject. There are, as usual, many surprises. For example, swallowing is one of the many functions that are lateralized, left and right being equally represented in different individuals. Other lateralized functions include appreciation of smells and flavours, interest in fine foods, appreciation of melodies (right) and rhythm (left), and even sexual arousal (both hemispheres participating in this).

There are many popular misunderstandings or urban legends connected with handedness, and these receive a chapter to themselves. McManus does not accept that left-handers die younger or are more likely to suffer from immune disorders, even though some studies have indicated these things. This will reassure left-handers, but they will be less pleased to discover that they are not more creative or intelligent than average. A number of well-known people, including Picasso, were reputedly left-handed but McManus dismisses most of these claims, which in at least some cases were based on nothing more than that their photographs were printed the wrong way round. I was, however, disappointed to find no discussion of something that has long intrigued me: is there a tendency for left-handers to be over-represented among actors? I seem to see an unusually large number of people writing left-handedly in television dramas, or perhaps I just notice them more.

There is much more in this book than I have touched on so far. McManus considers, among other things, why different countries drive on one or other side of the road, how handedness is referred to in languages, and ("that hoary old chestnut") why mirrors appear to reverse from side to side but not up and down. But enough said. I found this to be a really absorbing read, popular science of the first order. I cannot remember when I have learned so many new facts from a book on a subject about which I should have thought I was already fairly well informed.