The primate cranial base: ontogeny, function and - Harvard University

156 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 43, 2000
base in humans occurs prior to the eruption
of the first permanent molars and then remains
stable, but that the external cranial
base extends gradually in all nonhuman primates
throughout the period of facial
growth. 8 Moreover, the patterns of external
and internal cranial base angulation mirror
each other. The internal cranial base (measured
using both Ba-S-FC and Landzert’s
angle) flexes in humans rapidly prior to 2
postnatal years and then remains stable,
but extends in nonhuman primates gradually
throughout the period of facial growth
(Fig. 7) (Lieberman and McCarthy, 1999).
Cranial base shape and facial
projection in Homo
Although the effects of cranial base angulation
on the angle of the face relative to the
rest of the skull (facial kyphosis) have long
been the subject of much research (see
above), there has been recent interest in the
role of the cranial base on facial projection.
Facial projection (which is a more general
term for neuro-orbital disjunction) is defined
here as the extent to which the nonrostral
portion of the face is positioned anteriorly
relative to the foramen caecum, the
most antero-inferior point on the cranial
base (note that facial projection and prognathism
are different). Variation in facial
projection, along with an understanding of
their developmental bases, may be important
for testing hypotheses about recent
hominin evolution. In particular, Lieberman
(1995, 1998, 2000) has argued that
variation in facial projection accounts for
many of the major differences in overall
craniofacial form between H. sapiens and
other closely-related “archaic” Homo taxa,
including the Neanderthals. Whereas all
nonextant hominins have projecting faces,
“anatomically modern” H. sapiens is
uniquely characterized by a retracted facial
profile in which the majority of the face lies
beneath the braincase (Weidenreich, 1941;
Moss and Young, 1960; Vinyard, 1994;
Lieberman, 1995, 1998; Vinyard and Smith,
1997; May and Sheffer, 1999; Ravosa et al.,
2000b). As a consequence, H. sapiens also
has a more vertical frontal profile, less projecting
browridges, a rounder overall cranial
shape, and a relatively shorter oropharyngeal
space between the back of the hard
palate and the foramen magnum—virtually
all of the supposed autapomorphies of “anatomically
modern” H. sapiens.
What is the role of the cranial base in
facial projection? Lieberman (1998, 2000)
proposed that four independent factors account
for variation in facial projection: 1)
antero-posterior facial length, 2) anterior
cranial base length, 3) cranial base angle,
and 4) the antero-posterior length of the
middle cranial fossa from sella to PM
plane. 9 Each of these variables has a different
growth pattern, but combine to influence
the position of the face relative to the
basicranium and neurocranium. For example,
facial projection can occur through having
a long face relative to a short anterior
cranial fossa, a long middle cranial fossa
relative to the length of the anterior cranial
fossa, and/or a more extended cranial base.
Partial correlation analyses of cross-sectional
samples of Homo sapiens and Pan
troglodytes indicate that each contributes
significantly to the ontogeny of facial projection
in humans and apes when the associations
between these variables and with
overall cranial length and endocranial volume
as well as other cranial dimensions are
held constant (Lieberman, 2000). In other
words, chimpanzees and humans with relatively
longer faces, shorter anterior cranial
bases, less flexed cranial bases, and/or
longer middle cranial fossae tend to have
relatively more projecting faces.
In an analysis of radiographs of fossil
hominins, Lieberman (1998) argued that
the major cause for facial retraction and its
resulting effects on modern human cranial
shape was a change in the cranial base
rather than the face itself. Specifically, middle
cranial fossa length (termed ASL) was
estimated to be approximately 25% shorter
in anatomically modern humans, both re-
8 May and Sheffer (1999) did not find evidence for postnatal
extension of the cranial base in Pan, largely because of insufficient
sample sizes that were divided into overly large ontogenetic
stages.
9 This dimension was originally termed anterior sphenoid
length (ASL), but it is really a measure of the midline prechordal
length of the middle cranial fossa (Lieberman, 2000).