Homo sapiens, an unlikely but effective model in Evo-Devo-a review of Quirks of Human Anatomy: an Evo-Devo Look at the Human Body, by Lewis I. Held, Jr.

Abstract

What can we expect from an Evo-Devo look at the human body? The subject speciesFman rather than the worm or the fruitflyFis one toward which a lot of potential readers will be attracted. But is an Evo-Devo approach to our species really promising? How good canHomo sapiens be as a model species for Evo-Devo? The simple fact of restricting focus to only one species is likely to reduce to a very minimum any hope for a compelling evolutionary argument. In addition, the previous publication record of the author deals solely with pattern formation in Drosophila melanogaster: a model species quite distant from humans. A potential reader might envision a possibly popular but eventually less than academic exercise. All these quibbles, however, are destined to rapidly fade as soon as we start reading from this fascinating book. Held’s prose is brilliant, very often informal but always accurate, and the message it conveys is always clear and well documented, sometimes incredibly so. This is not the first time this author provides a list of references filling roughly one half of the whole book (cf. Held 1992, 2002). In Quirks of Human Anatomy, 152 pages of text and illustrations are followed by 86 pages of references, with 2924 entries, followed in turn by a 22-pages index. Moving into new territory, Held has obviously taken care to gather a very broad and sound background of published information. What, then, of the suitability of Homo sapiens as model species in Evo-Devo? If one is effective, as Held is, in mining the literature for potentially relevant data, there is indeed a lot of evidence to be found there, particularly from the perspective of comparative developmental genetics. To be sure, this comparative evidence differs qualitatively from the body of knowledge we are gathering with flies or worms, that is animals amenable to experimental manipulation. In the case of humans, the comparative dimension is obtained by studying intraspecific variation already present in human populations, in all kinds of pathological (if not monstrous) deviation from the ‘‘normal’’ phenotype. Held’s Evo-Devo approach to the anatomy of the human body, and its many oddities, is nonetheless rooted in a search for genes and gene expression cascades such as those involved, for example, in establishing the characteristic asymmetry of our viscera, the odd crossing of the body midline by optic nerves, or the more or less obvious morphological differences between males and females. A search for genes, of course, because body parts can diverge developmentallyFthat is, ‘‘dissociate’’Fto any great extent only if they have acquired distinct genetic identities. Held admirably also maintains an open eye toward those features, like our fingerprints, which are beyond the control of our genome. This adventurous path amidst the oddities of the human body includes a couple of pages on the presence of nipples in men, or men’s privilege, so to speak, to have a chance of getting bald. Held’s welljustified interest in monsters, for example humans affected by cyclopia, or with completely furry skin, inclusive of hands and feet, opens the door to a related, very intriguing question. How do we determine whether a given kind of monstrosity never observed in humans, but which is nonetheless imaginable, can ever evolve in our species? More precisely, why are some morphologies forbidden in humans, whereas EVOLUTION & DEVELOPMENT 12:6, 647–648 (2010)