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

D.E. Lieberman et al.]
PRIMATE CRANIAL BASE 131
base angle among primates must also be
related to variation in facial growth, orbit
orientation, and relative orbit size (Ross and
Ravosa, 1993; Ravosa et al., 2000a). As
noted above, the ontogenetic pattern of prenatal
cranial base angulation in humans is
largely unrelated to the rate at which the
brain expands (Jeffery, 1999). In addition,
the nonhuman primate cranial base angle
(regardless of whether the cribriform plate
is included in the measurement) mostly extends
during the period of facial growth,
after the brain has ceased to expand
(Lieberman and McCarthy, 1999). Therefore,
we will next explore in greater depth
the relationship between cranial base angle,
brain size, relative orbit size and position,
facial orientation, and other factors such as
pharyngeal shape and facial projection.
ASSOCIATIONS BETWEEN CRANIAL
BASE AND BRAIN
Because of the close relationship between
the brain and the cranial base during development
(see above), the hypothesis that
brain size and shape influence basicranial
morphology is an old and persistent one.
The bones of the cranial cavity, including
the cranial base, are generally known to
conform to the shape of the brain, but the
specifics of this relationship and any reciprocal
effects of cranial base size and shape
on brain morphology remain unclear. For
example, the human basicranium is flexed
when it first appears in weeks 5 and 6 because
in the fourth week, the neural tube
bends ventrally at the cephalic flexure
(O’Rahilly and Müller, 1994). The parachordal
condensations caudal to the cephalic
flexure are therefore in a different
anatomical plane than the more rostral prechordal
condensations (which develop by
week 7). However, as noted above, it is difficult
to attribute many of the subsequent
changes in prenatal chondrocranial or basicranial
angulation (or other measures of the
base) as responses solely to changes in brain
morphology.
Here we review several key aspects of the
association between brain and cranial base
morphology, as derived from interspecific
analyses of adult specimens. Structural relationships
between the cranial base and
the face are discussed below.
Brain size and cranial base angle
Numerous anatomists have posited a relationship
between brain size and basicranial
angle (e.g., Virchow, 1857; Ranke, 1892;
Cameron, 1924; Bolk, 1926; Dabelow, 1929,
1931; Biegert, 1957, 1963; Delattre and Fenart,
1963; Hofer, 1969; Gould, 1977; Ross
and Ravosa, 1993; Ross and Henneberg,
1995; Spoor, 1997; Strait, 1999; Strait and
Ross, 1999; McCarthy, 2001). The most
widely accepted of these hypotheses is that
the angle of the midline cranial base in the
sagittal plane correlates with the volume of
the brain relative to basicranial length
(DuBrul and Laskin, 1961; Vogel, 1964;
Riesenfeld, 1969; Gould, 1977). This hypothesis
is supported by independent analyses of
different measures of basicranial flexion
across several interspecific samples of primates
(Ross and Ravosa, 1993; Spoor, 1997;
McCarthy, 2001) (Fig. 8): the adult midline
cranial base is significantly and predictably
more flexed in species with larger endocranial
volumes relative to basicranial length.
In particular, the analysis by Ross and Ravosa
(1993) of a broad interspecific sample
of primates found that the correlation coefficient
between relative encephalization
(IRE1, see below) and cranial base angle
(CBA4, see below) was 0.645 (P 0.001),
explaining approximately 40% of the variation
in cranial base angle.
Attempts to extend this relationship to
hominins have proved controversial. Ross
and Henneberg (1995) reported that Homo
sapiens have less flexed basicrania than
predicted by either haplorhine or primate
regressions. They posited that spatial constraints
limit the degree of flexion possible,
and that humans accommodate further
brain expansion relative to cranial base
length through means other than flexion,
such as superior, posterior, and lateral neurocranial
expansion. In contrast, Spoor
(1997), using different measures of flexion
and relative brain size taken on a different
sample, found H. sapiens to have the degree
of flexion expected for its relative brain size.
Spoor (1997) used the angle basion-sellaforamen
caecum (CBA1) to quantify basi-