Journal of Heredity Advance Access originally published online on March 23, 2005
Journal of Heredity 2005 96(4):471; doi:10.1093/jhered/esi048
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Book Review |
Right Hand, Left Hand: The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures
Chris McManus.Harvard University Press, Cambridge, Massachusetts. 2002.
This book is devoted to providing a better understanding of the various aspects of asymmetry in biological and chemical systems. But I was disappointed that it did not give me the answer to my question; in fact, there is not even a consideration of my personal interest—just why it is that the human umbilical cord is nearly always spiraled to the left, that is, having chirality. But there are an awful lot of nuggets to be found in this book, this being only one of numerous such books that attempt to answer the big question for the reason of "handedness." However, when all is said and done, the real scientific understanding for handedness still escapes our knowledge. Moreover, it is also not easy to see how handedness might be connected to the lack of symmetry found in most biological systems or to asymmetric brain development.
The book is replete with superb references and, among other notions, it dispels the commonly held belief that mirror imagery in monozygotic twins is frequent, or that the rare individuals with situs inversus are opposite-handed as well. Numerous studies to explain situs, to provide access to statistics on handedness, to the origin of the usage of chirality (Lord Kelvin), to famous lefthanders, to mirror imagery—indeed to Alice's Looking Glass—are provided by Chris McManus, who is obviously widely read and has had a burning interest in understanding asymmetry. When he arrives at his final conclusion that the "D gene was the principal factor in separating humans from other apes ... and ... language and motor control in right-handers are controlled by the left hemisphere because the D gene is probably a mutation of the situs gene," this is where I part company with the putative cause of left-handedness. But later McManus even admits that "this argument may not be correct."
Nevertheless, the interested reader will find many fascinating aspects of biology and culture explained in this book, considerations that may not have been so apparent before. Why are the "lefties" the rebellious people? Why do the Democrats sit in Congress where they sit? Why do we write from left to right, and when was this writing style first invented and why? What reasons are there for driving on different sides of the road in different countries? And so on; there is much to educate the reader. McManus proffers a model of the prevalence of the D and C genes to interpret the familial nature of handedness. And while it may very well fit the observations of the frequencies of opposite-handedness and their familial nature, the model does not truly explain the ultimate cause of the exceptional left-handedness. We still do not know and are humbled by its frequency and the lack of our understanding.
Department of Pathology, UCSD Medical Center, San Diego, CA 92103
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