1 There are several ways to view faces. Figure 1-1 shows two fish.14 If two eyes and a mouth define a face, then fish have faces. They may have one or two nostrils, which are not connected to the mouth. However, water entering the nostrils does bathe the olfactory mucosa. Fish lack an ear canal, but the inner ear is present. Weber’s bones connect the swim bladder to the inner ear, and sound is transmitted.10 Another definition requires evolutionary transformation of the skull, in which a face is recognizable in mammals, but not in fish, amphibians, or reptiles (Fig. 1-2). The jaw is suspended from the braincase in reptiles. Mammals, however, have three ear ossicles, a secondary palate separating the airway passage from the mouth, and vertical positioning of the dentary.35 All animals communicate with each other in various ways by tactile, chemical, visual, and acoustical signaling. Insects have heads, but do they have faces? Paper wasps (Polistes dominulus) signal their status to each other by the number of black splotches on their yellow faces (Fig. 1-3).41 Figure 1-3 Wasp faces signal their status to each other by the number of black splotches on their yellow faces. The more blotches, the higher the status. Copyright 2004 Elizabeth Tibbetts. Modified by M. Michael Cohen Jr. Some define faces by the presence and use of facial muscles, which do not exist in fish, amphibians, reptiles, or birds. In contrast, mammals can suckle and later chew, supported by a muscular tongue and movable lips and cheeks (Fig. 1-4). Often, the muscles in the ears can change positions to aid in hearing; a movable nose for smelling and touching, and facial hair, the vibrissae, are associated with musculature and serve as tactile organs.10 Figure 1-4 Mammals can suckle and later chew, supported by a muscular tongue and movable lips and cheeks. For some, a behavioral definition must include facial expressions, which are found in various primates (Fig. 1-5), including humans (Fig. 1-6).10 Figure 1-5 Rhesus monkey facial expressions. From left to right, a bared-teeth display, a scream, an open-mouth threat, and a relaxed, open-mouth face (play face). From Parr and Heintz, 2009. Box 1-1 summarizes all possible perspectives from which the face can be described; Box 1-2 lists all the stages in the life cycle; and Box 1-3 lists the origin of some craniofacial components. Figure 1-9 shows embryonic facial development at approximately 42 to 44 days.25 Figure 1-10 shows skull molding in a newborn. Figure 1-11 shows the face of a small child, and Figure 1-12 shows a painting, The Stages of Human Life, by Hans Baldung Grien. In this allegory, a young woman, an old woman, and a dead woman are linked by their hands and arms.4 The dead woman holds an hourglass timer, indicating that life is over. Figure 1-9 Embryo of about 42 to 44 days. The developing face has a frontal area with bulging cerebral hemispheres; a nasodorsal center with nasoseptal and nasozygomatic portions; nasal pits delineated by nasal ridges with premaxillary, medionasal, and lateronasal portions; maxillary primordia; interpremaxillary depression; a premaxillary–maxillary junction; and a lower jaw. Courtesy of Jan E. Jirásek, Prague. Figure 1-10 Molding of the head caused by compression during passage through the birth canal. From Cohen, 2006. Figure 1-11 Baby’s face with relatively large head, prominent eyes, and a diminutive face. From Cohen, 2006. The evolution of the mammalian brain is based on the development of the neocortex (Fig. 1-13), which also resulted in a cranial shape that was different than that of the dinosaurs. The coexistence of dinosaurs and mammals and their competition during the Mesozoic era made mammalian neocortical development possible.5 Figure 1-13 Stages in the evolution of the brain. A comparison of the brain size of the fish, reptile, rabbit, and human. As the cerebral cortex increases in size, the cranium becomes larger and more rounded. Based on Campbell, 1970. The gene Microcephalin (MCPH1) was first identified in its mutant form in which it causes primary microcephaly, but the normal gene was then adaptively found to be important in regulating brain size, and it continues to evolve in humans.18 Homozygous ASPM mutations also cause microcephaly. The normal gene may regulate neural stem cell proliferation and/or differentiation, possibly by mediating spindle-cell assembly during cell division.31 SIGLEC11, a gene involved in sialic acid biology, is expressed in high concentration in microglial cells in the human brain, but only occasionally in the cells of chimpanzees.24 The Ret finger protein-like 1,2,3 (RFPL 1,2,3) genes on chromosome 22 are evolutionary forces that play a role in neocortical development.3 GTF2IRD1—a gene that, when mutated, causes craniofacial anomalies—has been shown in its normal form to be a regulatory determinant of craniofacial development.39 Figure 1-14 shows the orientation of the foramen magnum and the anterior cranial base in a rodent and a human. The foramen magnum is posteriorly placed in the rodent and vertically placed in the human. The anterior cranial floor and the cribriform plate face forward in the rodent and are vertically placed in the human.16 Figure 1-15 shows a dog’s skull compared to a Chevrolet Corvette in contrast to a human skull compared to a camper, indicating an expanded, upright forehead above the face.17 Figure 1-14 Orientation of the foramen magnum and the anterior cranial base in a human and a rodent. The foramen magnum is vertically placed in the human (A) and posteriorly placed in the rodent (C). The anterior cranial floor and the cribriform plate face downward in the human (B) and forward in the rodent (D). Based on Enlow, 1968.
The New Perspectives on the Face+
What Constitutes a Face?
Structural Definitions
Behavioral Definitions
Facial Perspectives and Stages in the Life Cycle
Evolutionary Considerations
Evolution of the Mammalian Brain
Molecular Components in Primate Brain and Craniofacial Evolution
Comparison of Different Skulls
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The New Perspectives on the Face
