The Evolution of the Human Head
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In one sense, human heads function much like those of other mammals. We use them to chew, smell, swallow, think, hear, and so on. But, in other respects, the human head is quite unusual. Unlike other animals, even our great ape cousins, our heads are short and wide, very big brained, snoutless, largely furless, and perched on a short, nearly vertical neck. Daniel E. Lieberman sets out to explain how the human head works, and why our heads evolved in this peculiarly human way.
Exhaustively researched and years in the making, this innovative book documents how the many components of the head function, how they evolved since we diverged from the apes, and how they interact in diverse ways both functionally and developmentally, causing them to be highly integrated. This integration not only permits the head’s many units to accommodate each other as they grow and work, but also facilitates evolutionary change. Lieberman shows how, when, and why the major transformations evident in the evolution of the human head occurred. The special way the head is integrated, Lieberman argues, made it possible for a few developmental shifts to have had widespread effects on craniofacial growth, yet still permit the head to function exquisitely.
This is the first book to explore in depth what happened in human evolution by integrating principles of development and functional morphology with the hominin fossil record. The Evolution of the Human Head will permanently change the study of human evolution and has widespread ramifications for thinking about other branches of evolutionary biology.
the three cell lines go from an ovoid trilaminar disk to generate an entire embryo? As outlined in Chapter 2, they do so by a combination of patterning, morphogenesis, and growth. All of these processes involve sequential, inductive interactions between adjacent tissues. First, the ectoderm. Beginning in the third week, the ectoderm differentiates into several portions, including a central neural plate. This plate of cells (the neuroectoderm) runs along the long axis of the embryo above the
is in contact with ectoderm on the surface of the embryo at the dorsal, anterior edge of the ethmoid. This contact persists for some time, causing a temporary, tubelike projection of the membrane to the top of the face (at the frontonasal suture). Later, as the face grows around this projection, it gets sealed off, forming the foramen caecum at the end of the cranial base. Incomplete closure of this dural projection can lead to brain hernias (Sperber, 2001). Temporals The temporals, which form
morphological integration is their importance from an evolutionary perspective. As Chapter 1 proposed, there is good reason to believe that heads work effectively and are highly evolvable in part because of numerous interactions among modules during development. My dog’s malocclusion appears to have no negative effect on her ability to chew, possibly because her skull has compensated during growth in many ways for whatever minor genetic deficiencies or incompatibilities she inherited. In
measurements and angles on a skull, you would not be able to reconstruct what the actually skull looks like. Fortunately, the last few years have seen a revolution in a method known as geometric morphometrics (GM), which uses new technologies to measure the coordinates of landmarks instead of measuring distances between landmarks. Rather than measure 190 distances for a set of 20 landmarks, one needs to record only the coordinates of the 20 landmarks (often using a CT scan or a machine that
for adjusting to one another. In other words, tinkering takes advantage of modularity and leads to integration (and probably vice versa to some extent, too). It should be evident that the head provides countless opportunities for tinkering because it offers many elements with which to tinker and many functions that potentially confer a selective advantage. Moreover, because the head is highly integrated both functionally and developmentally, such changes to the head are reasonably likely to work