close

Вход

Забыли?

вход по аккаунту

?

Technical note The midline and endocranial volume of the Taung endocast.

код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 146:319–322 (2011)
Technical Note: The Midline and Endocranial Volume
of the Taung Endocast
Ralph L. Holloway1 and Douglas C. Broadfield2
1
2
Department of Anthropology, Columbia University, New York, NY 10027
Department of Anthropology, Florida Atlantic University, Boca Raton, FL 33431
KEY WORDS
Taung; endocast; symmetry; midsagittal plane; brain evolution
ABSTRACT
The Taung endocast is one of the bestpreserved and most important known in paleoanthropology. Although the endocast is undistorted and preserves distinctive landmarks, Taung has proved a difficult endocast, because it is only about 60% complete. To
reconstruct Taung it is necessary to first use the available anatomical landmarks to define the midline of the
endocast. It is only with a proper description of the
midline that it is possible to reconstruct the endocast
and obtain an accurate measurement of Taung’s endocranial volume. Holloway (Science 168 (1970) 966–968)
determined a conservative estimate for Taung of
404 ml. More recently this estimate has been revised
downward by Falk and Clarke (Am J Phys Anthropol
134 (2007) 529–534) to 382 ml, giving Taung the smallest endocast for A. africanus. Certain challenges exist
with the reconstruction of any endocast, particularly a
hemi-endocast such as Taung. A virtual reconstruction
of Taung must assume perfect symmetry, a feature
called into question here in Taung’s most recent reconstruction by Falk and Clarke (2007). Holloway’s (1970)
reconstruction of Taung provides a guidepost for a conservative approach to endocast reconstructions, and
the most reliable measurement of Taung’s true endocranial volume. Am J Phys Anthropol 146:319–322,
2011. V 2011 Wiley-Liss, Inc.
As new technologies enable the examination of fossils
beyond the limits of external measurements it is necessary to also consider the limitations of these techniques.
For example, recently Falk and Clarke (2007) used laser
scan technology to arrive at a different volume estimate
than Holloway (1970) provided. They allege that since
this earlier estimate (404 ml) was done prior to the
availability of scanning technologies, the newer technologies are somehow capable of more accuracy, and that
their volume determination of 382 ml, based on the generation of a ‘‘virtual endocast,’’ is a more accurate estimate. While all previous measurements should be revisited, we believe that their methodology over-extends the
application of imaging technologies to fossil material.
First, the further data are removed from the original
specimen, the more opportunity there is for the introduction of significant error regardless of a technology’s
sophistication (Holloway et al., 2002). In the new reconstruction of Taung (Falk and Clarke, 2007) some nine
steps, using different casts, partial reconstructions, digital reconstructions and stereolithographs are employed
before a volume estimate is made. This potentially
increases the chance for error through distortion, shrinkage and remodeling. Second, they overlook the essential
fact that the midline of a hemi-endocast such as Taung
must be defined such as to be reproducible. Third, the
final analysis and brain size are derived from a cast,
calling into question the need for the additional steps
taken to achieve the final product. Last, the authors rely
heavily on mirror imaging to produce the final endocast,
but the reconstruction displays a visible lack of symmetry between right and left sides.
There are no indications whatsoever that the Taung
natural brain endocast was distorted (e.g., Figs. 1–3), or
that some undefined growth torque that normally contributes to cerebral asymmetry would invalidate the
choice of a midsagittal plane. Obviously, if there is only
one half of an endocast, the matter of cerebral asymmetry must remain moot, and all the CT and laser scanning
in the world cannot produce the true nature of the
totally missing side. One can reproduce an accurate
hemi-endocast, determine its volume and multiply by 2
(as did Holloway, 1970) or one can replicate the available
side and mirror it on the missing side as long as an
accurate midline has been determined, and symmetry is
perfect. These latter steps are missing in Falk and
Clarke’s (2007) article.
Since a plane is determined by three points in space,
it is simply a matter of identifying three points on the
Taung endocast that are in the midline, and this is
exactly what Holloway (1970) did. The dorsal surface of
the endocast is without distortion, and the gyral configuration and curvature into a smaller portion of the left
cerebral endocast make it possible to accurately pick
three points on the dorsal surface. This is particularly
the case as the sagittal suture is clearly present on the
original specimen. The presence of both left and right orbital plates of the frontal bone provides a curved contour
for the rostral bec (or rostrum of the frontal lobe) that
permits the identification of another point that must be
assumed to be in the midline. When three such described
points were aligned in a single plane, it was a simple
matter to scribe that plane on the surface of a first generation plaster replica of the original natural endocast,
C 2011
V
WILEY-LISS, INC.
C
*Correspondence to: Ralph L. Holloway, Department of Anthropology, Columbia University, New York, NY 10027.
E-mail: rlh2@columbia.edu
Received 23 December 2010; accepted 11 May 2011
DOI 10.1002/ajpa.21570
Published online 8 August 2011 in Wiley Online Library
(wileyonlinelibrary.com).
320
R.L. HOLLOWAY AND D.C. BROADFIELD
Fig. 1. Dorsal view of the Taung endocast, frontal portion
facing right, showing a line representing the midsagittal plane,
determined by using three points. SL, sagittal line; B, bregma.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
and by sanding away the left incomplete portion, arrive
at an accurate hemi-endocast, which then required minimal reconstruction to effect an accurate hemi-endocranial volume determination. This was all fully described
in Holloway (1970).
Such careful determination and description of the midline is important, since a slight miscalculation can result
in a significant increase or reduction in volume. Indeed,
Holloway (1970:967) calculated what a 1 mm variation
in midline choice would amount to, volumetrically, and
that was 7 ml. There is no possibility that Holloway’s
choice of a midsagittal plane varied by more than 1 mm.
If Falk and Clarke (2007) felt that Holloway (1970) had
erred in his choice of a midsagittal plane, they should
have provided the necessary critique, and chosen one
that they could demonstrate as being more accurate. It
could very well be the case that a CT or laser scan would
provide a more objective selection of three points, but after all, it will still be the human eye which makes the
selection from the scan images, and a trained anatomist
can surely do as well without the aid of a scanned
image, particularly if the original object is readily available, and undamaged or undistorted, as is the case with
the Taung endocast. Consequently, we do not believe the
original estimate of 404 ml by Holloway (1970) was in
error or ‘‘inflated’’. As for the final adult volume, we do
not necessarily accept that chimpanzee juvenile stages
based on dentition are the best method, as the question
of chimp vs. human dental stages and percentage of
growth is controversial (e.g., see Mann, 1988; Mann et
al., 1990).
In particular, the selection of the proper brain growth
trajectory weighs heavily on the position of Taung within
A. africanus, as well as A. africanus within Homininae.
While the difference between the proposed endocranial
volumes for Taung (Holloway 1970; Falk and Clarke,
2007), are only 5–6%, the impact of this difference is
magnified in light of the predicted adult values. Holloway (1970) predicted an adult value of 440 ml following
Tobias’ (1967) proposal that development was 92% complete in Taung. Falk and Clarke (2007) revised this
American Journal of Physical Anthropology
Fig. 2. Frontal view of Taung endocast (the prefrontal portion
is missing: it is still in cranial portion). Note in particular the line
representing the midsagittal plane passing through the center of
the inferior rostral bec portion of the endocast. S, superior frontal
gyrus; SL, sagittal line; OP, orbital portion; ‘‘RB’’, portion leading
to the rostral bec. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
growth trajectory following Ashton and Spence (1958),
assuming that development was 94% complete in Taung.
More recent and larger data sets suggest that the brain
develops more quickly in Pan troglodytes, the extant
hominoid species most often compared to Taung
(Herndon et al., 1999; Leigh, 2004). In particular, data
provided by Herndon et al. (1999) indicate that chimpanzee brain development may be 98–100% complete by age
three to five years greatly exceeding dental development.
The result is that it is likely that, especially during early
human evolution, the predicted adult brain size of Taung
is more constrained to actual brain size rather than a
superfluous figure that is less reflective of adult size. If
Taung’s predicted adult brain size is closer to 389.6 ml
based on a 98% comparison to adult capacity as Falk
and Clarke (2007) report rather than even the low estimate of 412 ml derived by Holloway (1970) then the cranial capacity of A. africanus is within the range and
near the average chimpanzee cranial capacity of 384 ml
reported in recent studies (Hendon et al., 1999; DeSilva,
2011). More importantly, acceptance of a reduced cranial
capacity for Taung would indicate that there was a
reduction of brain size within Sterkfontein hominins, the
group with which Taung is most closely aligned
(McNulty et al., 2006). This would suggest an evolutionary reversal for brain size in A. africanus, and an apparent rejection of this taxon as the ancestor to any later
taxa, since the trend in human brain evolution both
prior to (e.g., A. afarensis) and after (e.g., Australopithecus spp., Paranthropus, and Homo) the time of Taung is
characterized by increased brain size (Holloway et al.,
2004).
A careful examination of Falk and Clarke’s (2007) Figure 2 (see Fig. 5), showing the four views of the virtual
endocast, indicates that a lack of symmetry exists
between left and right cerebral hemispheres. This is particularly apparent in their basal view, in which a small
left occipital petalia appears. Other issues of symmetry
DEFINING THE MIDLINE OF TAUNG
Fig. 3. Basal view of Taung endocast showing continuation
of midsagittal plane line, continuing line seen in Figure 2.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
321
Fig. 4. Dorsal views of Taung endocast (right) and reconstruction made by Holloway, 1970 (left), showing midsagittal
plane and 3 points two along sagittal suture, the third and frontal portion, continuous with frontal bec portion show in Figures
2 and 3. The prefrontal portion is in black and not visible
against the background. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
Fig. 5. Figure 2 of Falk and Clarke (2007), with arrows (A, right; B, left) pointing to the asymmetries. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
American Journal of Physical Anthropology
322
R.L. HOLLOWAY AND D.C. BROADFIELD
include: the notation of a marginal sinus on the right
side that is missing on the mirror-imaged left side; a left
prefrontal that is not an exact duplicate of the actual
right side; and a frontal reconstruction that interprets
the missing prefrontals as squared off, though no evidence is presented to support this interpretation.
In resolving the shape of the frontal lobes it is useful
to follow the morphology present in Sterkfontein hominins due to the affinities of Taung to A. africanus
(McNulty et al., 2006) rather than introducing an additional variable in WT 17400. However, even allowing for
a liberal reconstruction of the missing frontal lobes,
along with excision of the extraneous petrosal and temporal portions, and the assumption that the endocast
was not rotated off its perpendicular axis when reconstructed, the Falk and Clarke (2007) reconstruction of
Taung miscalculates the midsagittal plane, resulting in a
significant reduction in cranial capacity that may call
into question the taxonomic significance of the fossil.
Again, it is mandatory that the missing left half be
exactly the same as the present right half. Neither Figures 1, 2, or 3 of Falk and Clarke (2007) provide any
demonstration regarding where a true midsagittal plane
in Taung lies.
Modern technologies are imperative to the viability of
the field. However, as with any technology it is important to fully appreciate its applications and limitations.
When only a portion of a specimen is present it is important to approach an analysis in the most conservative
manner, considering the reproducibility for the data in
light of distinct morphological points as well as the taxonomic position of the fossil.
American Journal of Physical Anthropology
LITERATURE CITED
Ashton EH, Spence TF. 1958. Age changes in the cranial
capacity and foramen magnum of hominoids. Proc Zool Soc
Lond 130:169–181.
DeSilva JM. 2011. A shift toward birthing relatively large
infants early in human evolution. Proc Natl Acad Sci USA
108:1022–27.
Falk D, Clarke R. 2007. New reconstruction of the Taung endocast. Am J Phys Anthropol 134:529–534.
Herndon JG, Tigges J, Anderson DC, Klummp SA, McClure
HM. 1999. Brain weight throughout the life span of the chimpanzee. J Comp Neurol 409:567–572.
Holloway RL. 1970. Australopithecine endocast (Taung specimen
1924): a new volume determination. Science 168:966–968.
Holloway RL, Broadfield DC, Yuan MS. 2004. The human fossil
record, Vol. 3: Brain endocasts. New York: Wiley.
Holloway RL, Broadfield DC, Yuan MS, Degusta D, Richards
GD, Silvers A, Shapiro JS, White TD. 2002. Missing Omo
L338y-6 occipital-marginal sinus drainage pattern: ground
sectioning, computer tomography scanning and the original
fossil fail to show it. Anat Rec 266:249–257.
Leigh SR. 2004. Brain growth, life history, and cognition in primate and human evolution. Am J Primatol 62:139–164.
Mann A. 1988. The nature of Taung dental maturation. Nature
333:123.
Mann A, Lampl M, Monge J. 1990. Patterns of ontogeny in
human evolution: evidence from dental development. Am J
Phys Anthropol 33:111–150.
McNulty KP, Frost SR, Strait DS. 2006. Examining affinities of
the Taung child by developmental simulation. J Hum Evol
51:274–296.
Tobias PV. 1967. Olduvai Gorge. The cranium and maxillary
dentition of Australopithecus (Zinjanthropus) boisei, Vol. 2.
Cambridge: Cambridge University Press.
Документ
Категория
Без категории
Просмотров
5
Размер файла
309 Кб
Теги
volume, note, endocranial, taung, technical, midline, endocast
1/--страниц
Пожаловаться на содержимое документа