American Scientist

THE EVOLUTION OF THE HUMAN HEAD. Daniel E. Lieberman. xii + 756 pp. The Belknap Press of Harvard University Press, 2011. $39.95.

It is said that we all love a mirror, so what could be more intriguing than a new 700-page study of the human head? The author of The Evolution of the Human Head, Daniel E. Lieberman, is a prolific scholar with primary interests in the evolution of skull form in humans and their closest relatives. His highly successful research program incorporates both comparative and experimental approaches and has recently expanded to consider questions relating to human locomotion. This impressive book contains the fruits of more than 15 years of scholarly research and teaching. Lieberman’s ardent supporters will likely echo the dust jacket’s claim that the book “will permanently change the study of human evolution.” Others may take a dimmer view and criticize the book for shining bright lights down well-worn paths; the discussion of the role of running in human evolution, for instance, recycles arguments first put forth decades ago by David Carrier and Dennis Bramble.

In any event, this hefty and well-written book offers a scholarly breadth and attention to detail that are certainly laudable. The book is quite unusual in that it includes a comprehensive review of the soft tissues associated with cranial features and discusses them within the context of evolutionary morphology and the fossil record of the human skull. I can think of no other volume that packages the anatomy of the human head in this fashion.

The organizational core of the book comprises a series of strong chapters focusing on primary regions of the head. These cover the brain and its relation to the skull; chewing structures and associated functions; pharyngeal/laryngeal structures and speech production; support and movement of the head; and finally the sensory systems and associated skeletal regions. This core is preceded by chapters discussing embryonic and fetal development and postnatal growth, and it is followed by a penultimate three-chapter overview of the evolution of the human head (really the skull, considering the realities of fossilization). The book’s opening and closing chapters attempt to contextualize the bony and soft-tissue anatomy within broader developmental and evolutionary themes.

Lieberman has done a masterful job of incorporating all sorts of relevant and interesting research on the functioning soft tissues housed within the various bony cavities of the head. In fact, this may be the strongest contribution of the book, and I see myself referring back to the text and references on these topics frequently in the future. The discussions of the anatomy of language production, sensory adaptations for running, and olfactory-respiratory structures are particularly strong. But here Lieberman is mostly collating the research of others, and this points to the inevitable question of what new theoretical orientations, methodological pathways or key critical perspectives emerge in this volume. On this front, the book is somewhat less compelling and probably doesn’t live up to the publisher’s aforementioned hyperbole. But in all fairness, which books would match this claim, and which reviewers (certainly not this one) fill those scholarly brogans?

One of the most disappointing aspects of the book for me is that nary a case of sexy jargon popularized within the field of evolutionary biology over the past several decades escapes serious lobbying here. We read of “tinkering,” “evo-devo,” “emergent properties,” “integration,” “modules,” “complexity,” “constraint,” “evolvability” and many other such. Lieberman trusts that we join him in wonder(ing)—“How is something as complex and vital as the head so evolvable”? It’s apparently all “modules” and/or “integration,” yet these concepts are not here directly grounded or translated into clearly stated testable hypotheses and specific predictions in the discussions of quantitative skull form. Moreover, it seems unlikely that comparing evolutionary change to a game of “Mr. Potato Head,” or saying that the basicranium shapes the brain the way tilted bread pans shape loaves of bread, will effectively generate specific research initiatives.

What I was searching for in the book were those critical testable hypotheses and the pathways by which our scientific understanding of head form will next be markedly advanced. Instead, we find Lieberman beckoning us to join him in drowning in the wondrous complexities of the evolutionary anatomy of the head: “Hypotheses abound, and none are necessarily mutually exclusive, given the way that heads are integrated.” Sounds a bit like anthropology, doesn’t it? He maintains that “it is useful to think about how the head functions and develops as an ensemble instead of focusing on particular parts and functions,” but I worry that such an approach may more likely yield muddleheadedness (and I also wonder why he organized the book around “parts and functions”).

It is surely a strong commitment to some sort of holism and to ensemble thinking that inclines Lieberman to accept the dated claims of Donald H. Enlow and Miyuki Azuma that all mammalian skulls (and all stages of skull development!) tightly follow structural constraints expressed in a series of angular relationships that are supposedly invariant. Poorly supported to begin with, these claims have never been well corroborated and in fact do not hold for even narrow series of primates, let alone all mammals. Nevertheless Lieberman invokes them to reconstruct fossil hominid crania and account for highly specific and tightly constrained structural and developmental interactions among different regions of the skull. Enlow’s framework of constraint was in fact firmly based on the notion that the size and shape of the brain somehow inflexibly constrain the placement of the senses (orbits, olfactory bulbs, auditory apparatus), which in turn (somehow or other) then rigidly limits the direction and extent of growth and size of the various other regions of the skull and head. Thus, this series of purported deep structural constraints on mammalian skull form emerges from what is an almost mystical “anthropo-cephalo-centrism.” Lieberman has also argued, in a 1998 paper in Nature, that the sphenoid bone is an underappreciated “regulator” of myriad changes throughout the skull and head; interestingly, Enlow once claimed that the lacrimal bone would one day triumphantly receive its rightful place as the keystone in craniofacial development and evolution. Holism can have its drawbacks.

Lieberman also has a disturbing tendency to maintain a preferred view in spite of apparently contradictory evidence. For instance, he is committed to a model of “increased turbulence” associated with nasal anatomy in humans, despite the fact that no such effect was found in either of the recent studies he cites that directly measured such airflow patterns. And he appears to favor the view that humans exhibit increased flavor and odor perception associated with cooked food, in spite of the finding that we “probably cannot detect odors as well as chimpanzees.” Also, his argument for the localized effect of bone loading on limb remodeling, which cites his own experimental studies of pigs on treadmills, is juxtaposed to the observation that the skulls of the pigs also exhibited bone thickening, supporting a more general systemic influence. In addition, after first stressing that “it is not true that bones have no intrinsic genetic regulation,” he argues a few paragraphs later that “if bone morphology were highly genetic, then such a mutation would have to coevolve with other mutations” influencing neighboring bone shape. Then he assures us that the morphology of the bony cranial base directly influences the characteristic domed shape of the human brain, only to point out that Callum F. Ross and colleagues in fact found no support for this proposed association. Finally, we get repeated assurances that the dominant factor generating variation in cranial base angulation (CBA) is brain size (a favorite hypothesis in the book), only to then read an offhand reference to Lieberman’s own studies of modern human variation revealing that “brain volume is independent of CBA.” And if this is true of intraspecific patterning, then how do the suggested higher-level associative patterns evolve via known Darwinian mechanisms in the first place? I am also left contemplating the cloned Neanderthal with whom Lieberman fancifully envisions us all sharing a drink, “perhaps someday,” who gruffly wants to know why, given his very large brain, his basicranium isn’t more strongly flexed, and why he hasn’t evolved a sleek “runner’s head” in spite of all his dogged persistence in tracking and hunting.

Those outside the field of biological anthropology will be struck by the anthropocentrism perhaps inherent in such a detailed dissection of human head anatomy. Lieberman encourages resistant readers to appreciate that “to transform the ontogeny of a great ape’s head into a modern human’s may not be as difficult as one might imagine.” But only if we mirror-gaze and don’t get out much into the world of comparative anatomy does it require all that much imagination in the first place. Certainly equally or even more impressive are the gigantic eyes and inflated orbits of our relatives the tarsiers, the elongated and buttressed faces of some baboons, the hypertrophied laryngeal and basicranial morphology of howling monkeys, the bent and elongated skull of the walrus, the vertically oriented skull hafted to elongated tubular jaws (spanned by baleen plates) in right whales, the divergent heads of narwhals and Sirenian sea cows, the giant faces and tusks of hippopotamuses, the elaborate flanges on the face of the skull of the extinct glyptodonts, the inflated auditory bullae dominating the heads of tiny kangaroo rats and mice, the attenuated and toothless jaws and heads of anteaters, or the truly bizarre platypuslike spatulate lower face of the appropriately tagged Xenocranium (an extinct superburrowing paleanodont from the Oligocene). The list of course goes on and on, just among the mammals, and this degree of morphological diversity—using predominantly the same genes and biological processes driving our own head’s evolutionary history—is a far superior database for pondering general issues relating to the “evolvability” of head structures. This impressive diversity reflects the importance of localized structures and functions, and the power of natural selection to generate such dramatic transformations of the various components of head anatomy. We would never expect a central paradigm of “integration” to overly constrain such a diverse array of functioning and evolving structures as we find in the head—this is after all a region, not a system.

On the whole, Lieberman’s big book definitely moves us ahead in effectively synthesizing so much of what is currently understood about the structure, function and evolution of the human head. I just wish he had also more clearly shown us how to specifically apply our impressive brains to the task of facing down some of the mushiness, contradictions, and unproductive models and analogies that still persist as we attempt scientific contemplation of that reflection in the glass.

Brian T. Shea is a professor in the department of molecular and cell biology at Northwestern University Medical School. His research interests include primate development, morphology and evolution.