Apples, oranges, and the lunate sulcus

Holloway (1984) used a method of direct tape-arc measurements on chimpanzee brain casts to reject the hypothesis that the lunate sulcus is located in an anterior position in the Taung endocast. However, Holloway neglected to measure the occipital pole-lunate sulcus (OP-LS) arc directly on the Taung endocast as he did on chimpanzee brain casts (a crucial part of his methodology); instead, he determined the relative position of Taung's lunate sulcus on the basis of a calculation that confounds direct measurements and measurements from photographs. When arc OP-LS is measured directly on Taung according to Holloway's methods, the feature that has been identified as the medial end of the lunate sulcus is shown to be located within the range that Holloway determined for chimpanzees. Thus Holloway's methodology and data support rather than refute the claim that the lunate sulcus is located in a pongid-like position in australopithecines.     

Ape-like endocast of “ape-man” Taung

I have identified and illustrated a spherical “dimple” or “depression” on the Taung endocast as indicating the most likely position of the medial end of the lunate sulcus but have not drawn an actual lunate sulcus on Taung because one is not visible. In a recent paper, R.L. Holloway (Am. J. Phys. Anthropol. 77:27–33, 1988) drew a lunate sulcus on his copy of the Taung endocast, incorrectly attributed this sulcus to me, and used it to obtain a ratio of 0.254 to describe “Falk's” position of the lunate sulcus. My published ratio of 0.242 for Taung (Falk: Am. J. Phys. Anthropol. 67:313–315, 1985a) was not considered, although the focus of Holloway's paper was my assessment of the position of the lunate sulcus. Holloway also excluded published ratios for a chimpanzee in my collection from his statistical analysis but, even so, my published ratio for Taung is still only 1.5 standard deviations from his chimpanzee mean. If my chimpanzee brain is included in the sample, the ratio for Taung is 1.2 standard deviations from the mean. Furthermore, one of Holloway's own chimpanzees (B60–7) has a ratio of 0.241, just 0.001 below my ratio for Taung. There is no sulcus where Holloway has drawn one on Taung, his “F(LS)” is not mine, his 2 mm error is not mine, and the correct ratio for my measurement of Tuang is the one that I published, not the one that Holloway attributes to me. Assessment of Holloway's chimpanzee data supports my claim that the dimple on the Taung endocast is within the chimpanzee range for the medial end of the lunate sulcus.     

The Taung endocast: a reply to Holloway

Indices of rostrality (ir, ir') are developed to assess the extent to which the medial end of the lunate sulcus (L) is rostrally positioned in photographs and figures of lateral views of primate brains and endocasts, and indices are determined for chimpanzees, SK 1585 and the Taung endocast. Ir quantifies the extent of rostrality as it has traditionally been viewed (in A-P projections) while ir' takes dorsal curvature into account. The ir of the feature that I have identified as the lunate sulcus of Taung is within one standard deviation of the mean ir for Pan and its ir' is within 1.5 standard deviations from the mean ir' for Pan. Both findings are compatible with my earlier statement that the medial end of the lunate sulcus of the Taung endocast is in a pongid-like position. Use of stereoplotting to transfer the position of L from chimpanzee endocasts and brains to australopithecine endocasts is critically assessed: Holloway stereoplotted five chimpanzee brains and then transferred their mean coordinates that describe the lunate sulcus to the Taung endocast. If stereoplotting successfully transfers the extent to which L is rostrally located, one would expect the mean L of Pan and its transferred counterpart in Taung to have identical index values of rostrality. However, the ir of the lunate sulcus that Holloway located on Taung is over two standard deviations lower than the mean ir for the five chimpanzees he stereoplotted to determine its angular coordinates, and Holloway's ir' for Taung is one standard deviation lower than the five chimpanzees' mean ir'. These discrepancies are shown to be due to shape differences, and it is concluded that stereoplotting should not be used to transfer sulci between differently shaped endocasts without correcting for these differences. I also reply to Holloway's criticisms of my use of L/H indices, palpation, techniques for sampling endocasts, and illustration of the Taung endocast. It is shown that there is room on the Taung specimen for the lateral end of L, and the pongid-like sulcal pattern of Taung is reaffirmed. Thus, we do not yet know when human-like sulcal patterns first appeared in the hominid fossil record.     

The taung endocast and the lunate sulcus: A rejection of the hypothesis of its anterior position

Using an independent method of direct tape-arc measurements on six chimpanzee brain casts, it is shown that Falk's (1980, 1983) claims regarding an anterior pongidlike placement of a lunate sulcus on the Taung specimen remain unconfirmed. Thus Holloway's (1981) stereoplotting method of testing Falk's hypothesis is independently confirmed, using the actual specimens rather than photographs of them. Falk's (1980) placement of a lunate sulcus falls at least 2.5 standard deviations anterior to a position expected on the basis of a Pan location.     

Relationship of squamosal suture to asterion in pongids (Pan): relevance to early hominid brain evolution.

Based on 244 measurements of the relationship of the squamosal suture to the landmark asterion in 49 chimpanzee skulls, it is shown that in the normal lateral view the squamosal suture is very rarely inferior to asterion. In hominid crania, the squamosal suture is always well superior to asterion. Even in Pan, that part of the squamosal suture most homologous with the remnant found on the Hadar AL 162-28 Australopithecus afarensis hominid cranial fragment is very rarely inferior to asterion. Such variability suggests that Falk's (Nature 313:45-47, 1985) orientation of the Hadar specimen is incorrect; she places asterion superior to the position of the squamosal suture if projected endocranially. The implication for the brain endocast is that, however the fragment is oriented, the posterior aspect of the intraparietal (IP) sulcus is in a very posterior position relative to any chimpanzee brain. The distance from the posterior aspect of IP to occipital pole is twice as great in chimpanzee brain casts than on the Hadar AL 162-28 endocast, even though the chimpanzee brain casts are smaller in overall size. This suggests that brain reorganization, at least as exemplified as a reduction in primary visual striate cortex (area 17 of Brodmann), occurred early in hominid evolution, prior to any major brain expansion.     

The basal dendrites of Meynert cells in the striate cortex of the monkey

A study has been made of the basal dendrites of Meynert cells in the striate cortex of the macaque monkey in sections parallel to the pial surface impregnated by the Golgi technique. The longest basal dendrite observed extended up to 0.6 mm and the average length of the longest dendrite on each cell was about 0.28 mm. In general, the dendritic field was in the form of an ellipse with mean major and minor axes of 0.22 mm and 0.11 mm respectively, and encompassing an area of about 0.13 mm2. The directions of the major axes were perpendicular to the lunate sulcus in the sections adjacent to the lunate sulcus, and parallel to the horizontal meridian in sections taken from the region of the representation of the meridian, suggesting that the basal dendritic fields are orientated in parallel with the directions of the ocular dominance bands.     

Evidence for a dual pattern of cranial venous sinuses on the endocranial cast of Taung (Australopithecus africanus)

According to published accounts, an enlarged occipital-marginal sinus system is absent in Australopithecus africanus, although it occurs in high frequencies in A. robustus, A. Boisei, and Hadar hominids commonly designated A. afarensis. In this report, we describe, for the first time, an enlarged occipital-marginal sinus system on the endocranial cast of the Taung specimen, which is part of the holotype of A. africanus. In addition, well-developed right transverse and sigmoid sinuses are represented on the Taung endocast. The various components of the dual venous sinus system on the Taung endocast are measured, and the system is compared to those of other fossil hominids. The compresence of a lateral sinus system and enlarged occipital and marginal sinuses occurs in two Hadar specimens, 2 specimens of A. robustus crassidens, 1 A. boisei specimen, and several early H. sapiens crania. Hence, the presence of strong transverse sinus impressions in a fragmentary specimen may not be interpreted as an indication that an enlarged occipital-marginal sinus system was not present in the original specimen. Conversely, lack of transverse sinus grooves in a fragmentary specimen does provide indirect evidence than an enlarged occipital-marginal system would probably have been present in the whole specimen, as in 2 specimens of A. boisei. Including Taung, enlarged occipital and marginal sinuses occur in 1 out of 5, or 20%, of A. africanus specimens. This figure compares well with the range of mean frequencies in modern human cranial series (1.5 to 28%), but is much lower than are the frequencies for A. boisei, A. robustus, and the Hadar hominids.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lunate morphology

Fifty healthy volunteers were subjected to the CT examination of the wrist joint to provide normal database of the shape and size of the lunate. The various parameters of the lunate were measured taking help of the reformatted images in sagittal, coronal and axial planes. The mean maximum antero-dorsal diameter of the lunate measured on axial section was 16.96mm (SD 1.60) with the range of 13-19mm while the mean medio-lateral diameter of the lunate was 12.80mm (SD 1.37) with the range of 10-15mm. The mean axes of the scaphoid and the triquetral articular surfaces of the lunate were 11.83 degrees (SD 9.33) and 1.54 degrees (SD 9.70), respectively, while the mean axial index was 2.04 (SD 1.33).Lunate is reported to have shapes of three different types on plain radiographs. The CT measurements of most lunates failed to classify them into the described three shapes since many lunates showed dissimilar typing on the various chosen sagittal sections of the same lunate. The classical wedged lunate with its apex towards the dorsum has been described to have a tendency to extend under the capitate compressive force. However, in a study on plain radiography no correlation was reported between the radio lunate angle and the shape of the lunate measured in the direction of the lunate's axis. Our study confirmed the same on plain radiographs and on the CT also. We measured lunate's shape in the direction of the capitate's axis too, which demonstrated significant correlation with the RLA (p<0.001).     

Development and validation of a computed tomography-based methodology to measure carpal kinematics

Motion of the wrist bones is complicated and difficult to measure. Noninvasive measurement of carpal kinematics using medical images has become popular This technique is difficult and most investigators employ custom software. The objective of this paper is to describe a validated methodology for measuring carpal kinematics from computed tomography (CT) scans using commercial software. Four cadaveric wrists were CT imaged in neutral, full flexion, and full extension. A registration block was attached to the distal radius and used to align the data sets from each position. From the CT data, triangulated surface models of the radius, lunate, and capitate bones were generated using commercial software. The surface models from each wrist position were read into engineering design software that was used to calculate the centroid (position) and principal mass moments of inertia (orientation) of (1) the capitate and lunate relative to the fixed radius and (2) the capitate relative to the lunate. These data were used to calculate the helical axis kinematics for the motions from neutral to extension and neutral to flexion. The kinematics were plotted in three dimensions using a data visualization software package. The accuracy of the method was quantified in a separate set of experiments in which an isolated capitate bone was subjected to two different known rotation/translation motions for ten trials each. For comparison to in vivo techniques, the error in distal radius surface matching was determined using the block technique as a gold standard. The motion that the lunate and capitate underwent was half that of the overall wrist flexion-extension range of motion. Individually, the capitate relative to the lunate and the lunate relative to the radius generally flexed or extended about 30 deg, while the entire wrist (capitate relative to radius) typically flexed or extended about 60 deg. Helical axis translations were small, ranging from 0.6 mm to 1.8 mm across all motions. The accuracy of the method was found to be within 1.4 mm and 0.5 deg (95% confidence intervals). The mean error in distal radius surface matching was 2.4 mm and 1.2 deg compared to the use of a registration block. Carpal kinematics measured using the described methodology were accurate, reproducible, and similar to findings of previous investigators. The use of commercially available software should broaden the access of researchers interested in measuring carpal kinematics using medical imaging.     

The effect of lunate position on range of motion after a four-corner arthrodesis: a biomechanical simulation study

A four-corner arthrodesis of the wrist is a salvage procedure for the treatment of specific wrist disorders, to achieve a movable, stable and pain free joint. However, a partial arthrodesis limits the postoperative range of motion (ROM). The goal of this study is to understand the mechanism of the reduction of the ROM and to evaluate the effect of the orientation of the lunate in the four-corner arthrodesis on the range of motion by using a biomechanical model, containing articular contacts and ligaments. Multi-body models of a normal wrist and a four-corner arthrodesis wrist with different orientation of the lunate were used for simulations of flexion-extension motion (FEM) and radial-ulnar deviation motion (RUD). The ROM of the postoperative wrist was reduced from 145° to 82° of the total arc of FEM and from 73° to 41.5° of the total arc of RUD. The model simulations show that the range of motion reduction is caused by overtension of the extrinsic wrist ligaments. Different positioning of the lunate changes the balance between the contact forces and ligament forces in the wrist. This explains the effect on the postoperative range of motion. The 20° flexed lunate did not give any gain in the extension motion of the wrist, caused joint luxation in flexion and limitation in RUD. The 30° extended lunate caused overtension of the extrinsic ligaments attached to the lunate. The ROM in this case is dramatically reduced. The model simulations suggest that the neutral position of the lunate seems to be most favorable for mobility of the wrist after a four-corner arthrodesis procedure.     

A Comparative Analysis of Regulatory Regions of the Transthyretin Gene in the Mouse, Human, and Chimpanzee Genomes

A full genome analysis of differences between the gene expression in the human and chimpanzee brains revealed that the gene for transthyretin, the carrier of thyroid hormones, is differently transcribed in the cerebella of these species. A 7-kbp DNA fragment of chimpanzee was sequenced to identify possible regulatory sequences responsible for the differences in expression. One hundred and thirteen substitutions were found in the chimpanzee sequence in comparison with the human sequence. About 40% of the substitutions were revealed within the repeating elements of the genome; their location and sizes did not differ from those in the corresponding fragments of the human genome, and the nucleotide sequences had a high degree of identity. A comparison of nucleotide sequences of the transthyretin region of human, chimpanzee, and mouse genes revealed substantial differences in the distribution of G + C content along the examined fragment in the human (chimpanzee) and mouse genes and allowed us to localize three sequence tracts with a higher degree of identity in the three species. One of these tracts was located in the promoter region of the gene, and the other two probably determine the specificity of transthyretin gene expression in the liver and brain. One of the conserved tracts of the chimpanzee genome was found to have a single and a triple nucleotide substitution. The triple substitution distinguishes chimpanzees from humans and mice, which have identical sequences of this site. It is likely that these substitutions are responsible for the differences in the expression levels of the transthyretin gene in the human and chimpanzee brains.     

A comparative analysis of regulatory regions of the transthyretin gene in the mouse, human, and chimpanzee genomes

A full genome analysis of differences between the gene expression in the human and chimpanzee brains revealed that the gene for transthyretin, the carrier of thyroid hormones, is differently transcribed in the cerebella of these species. A 7-kbp DNA fragment of chimpanzee was sequenced to identify possible regulatory sequences responsible for the differences in expression. One hundred and thirteen substitutions were found in the chimpanzee sequence in comparison with the human sequence. About 40% of the substitutions were revealed within the repeating elements of the genome; their location and sizes did not differ from those in the corresponding fragments of the human genome, and the nucleotide sequences had a high degree of identity. A comparison of nucleotide sequences of the transthyretin region of human, chimpanzee, and mouse genes revealed substantial differences in the distribution of G + C content along the examined fragment in the human (chimpanzee) and mouse genes and allowed us to localize three sequence tracts with a higher degree of identity in the three species. One of these tracts is located in the promoter region of the gene, and the other two probably determine the specificity of transthyretin gene expression in the liver and brain. One of the conserved tracts of the chimpanzee genome was found to have a single and a triple nucleotide substitution. The triple substitution distinguishes chimpanzees from humans and mice, which have identical sequences of this site. It is likely that these substitutions are responsible for the differences in the expression levels of the transthyretin gene in the human and chimpanzee brains.     

Relationship of squamosal suture to asterion on external skull surfaces versus endocasts of pongids: Implications for Hadar early hominid AL 162-28

The relationship between the squamosal suture and asterion was quantified in 15 hemispheres of eight chimpanzee endocasts that were aligned in the conventional lateral view (i.e., with frontal pole [FP]–occipital pole [OP] horizontal). Using a three-dimensional digitizer, x, y, and z coordinates were collected for the highest and lowest points of the squamosal suture, and the most rostral point of the suture approximate to the coronal suture. Our results were compared to a similar study of the squamosal suture on the external surfaces of chimpanzee skulls that were oriented in the Frankfurt horizontal (Holloway and Shapiro, 1992). The relationship between the squamosal suture and asterion differs markedly between the outsides of skulls and endocasts. Whereas the squamosal suture is very rarely below asterion on the external skull, we found that most of the squamosal suture is located inferior to asterion on endocasts. We also found that the squamosal suture courses approximately 2.0 mm lower on the right side than the left. (An asymmetry of the same magnitude was reported for the external skull but, curiously, in the opposite direction.) It may be that a lowered right squamosal endosuture on chimpanzee endocasts is associated with earlier closure on that side. The discrepancy in results for the external skull versus endocast is partially attributable to orienting chimpanzee skulls in the Frankfurt horizontal, which usually results in the endocasts being tilted so that FP is above OP, i.e., FP-OP is not parallel with the Frankfurt horizontal. Falk's (1985) orientation of the early hominid endocast from Hadar (AL 162-28) is consistent with data determined from endocasts of chimpanzees. © 1994 Wiley-Liss, Inc.     

Human parieto-occipital visual cortex: lack of retinotopy and foveal magnification.

We studied visual representation in the parietal cortex by recording whole-scalp neuromagnetic responses to luminance stimuli of varying eccentricities. The stimuli were semicircles (5.5 degrees in radius) presented at horizontal eccentricities from 0 degree to 16 degrees, separately in the right and left hemifields. All stimuli evoked responses in the contralateral occipital and medial parietal areas. The waveforms and distributions of the occipital responses varied with stimulus side (left, right) and eccentricity, whereas the parietal responses were remarkably similar to all stimuli. The equivalent sources of the parietal signals clustered within 1 cm3 in the medial parieto-occipital sulcus and did not differ significantly between the stimuli. The strength of the parietal activation remained practically constant with increasing stimulus eccentricity, suggesting that the visual areas in the parieto-occipital sulcus lack the enhanced foveal representation typical of most other visual areas. This result strengthens our previous suggestion that the medial parieto-occipital sulcus is the human homologue of the monkey V6 complex, characterized by, for example, lack of retinotopy and the absence of relative foveal magnification.     

LB1’s virtual endocast, microcephaly, and hominin brain evolution

Earlier observations of the virtual endocast of LB1, the type specimen for Homo floresiensis, are reviewed, extended, and interpreted. Seven derived features of LB1's cerebral cortex are detailed: a caudally-positioned occipital lobe, lack of a rostrally-located lunate sulcus, a caudally-expanded temporal lobe, advanced morphology of the lateral prefrontal cortex, shape of the rostral prefrontal cortex, enlarged gyri in the frontopolar region, and an expanded orbitofrontal cortex. These features indicate that LB1's brain was globally reorganized despite its ape-sized cranial capacity (417 cm³). Neurological reorganization may thus form the basis for the cognitive abilities attributed to H. floresiensis. Because of its tiny cranial capacity, some workers think that LB1 represents a Homo sapiens individual that was afflicted with microcephaly, or some other pathology, rather than a new species of hominin. We respond to concerns about our earlier study of microcephalics compared with normal individuals, and reaffirm that LB1 did not suffer from this pathology. The intense controversy about LB1 reflects an older continuing dispute about the relative evolutionary importance of brain size versus neurological reorganization. LB1 may help resolve this debate and illuminate constraints that governed hominin brain evolution.     

Differential roles for parietal and occipital cortices in visual working memory

Visual working memory (VWM) is known as a highly capacity-limited cognitive system that can hold 3-4 items. Recent studies have demonstrated that activity in the intraparietal sulcus (IPS) and occipital cortices correlates with the number of representations held in VWM. However, differences among those regions are poorly understood, particularly when task-irrelevant items are to be ignored. The present fMRI-based study investigated whether memory load-sensitive regions such as the IPS and occipital cortices respond differently to task-relevant information. Using a change detection task in which participants are required to remember pre-specified targets, here we show that while the IPS exhibited comparable responses to both targets and distractors, the dorsal occipital cortex manifested significantly weaker responses to an array containing distractors than to an array containing only targets, despite that the number of objects presented was the same for the two arrays. These results suggest that parietal and occipital cortices engage differently in distractor processing and that the dorsal occipital, rather than parietal, activity appears to reflect output of stimulus filtering and selection based on behavioral relevance.     

Brief communication: predatory bird damage to the Taung type-skull of Australopithecus africanus Dart 1925

In this issue of the Journal, McGraw et al. ([2006] Am. J. Phys. Anthropol. 000:00-00) present new data on the taphonomic signature of bone assemblages accumulated by crowned hawk eagles (Stephanoaetus coronatus), including characteristic talon damage to the inferior orbits of primates preyed upon by these birds. Reexamination of the Taung juvenile hominin specimen (the type specimen of Australopithecus africanus Dart 1925) reveals previously undescribed damage to the orbital floors that is nearly identical to that seen in the crania of monkeys preyed upon by crowned hawk eagles (as reported by McGraw et al., this issue). This new evidence, along with previously described aspects of the nonhominin bone assemblage from Taung and damage to the neurocranium of the hominin specimen itself, strongly supports the hypothesis that a bird of prey was an accumulating agent at Taung, and that the Taung child itself was the victim of a bird of prey.     

Examining affinities of the Taung child by developmental simulation

As a well-preserved juvenile and the type specimen of Australopithecus africanus, the Taung child figures prominently in taxonomic, ontogenetic, and phylogenetic analyses of fossil hominins. Despite general agreement about allocation of Sterkfontein and Makapansgat fossils to this species, limited morphological comparisons have been possible between these adult specimens and the juvenile Taung. Here, we used developmental simulation to estimate the adult form of the Taung child, and directly compare its morphology to that of other fossil hominins. Specimens were represented by 50 three-dimensional landmarks superimposed by generalized Procrustes analysis. The simulation process applied developmental trajectories from extant hominine species to the Taung fossil in order to generate its adult form. Despite differences found in the developmental patterns of these modern species, simulations tested on extant juveniles-transforming them into "adults" using trajectories from other species-revealed that these differences have negligible impact on adult morphology. This indicates that morphology already present by occlusion of the first permanent molar is the primary determinant of adult form, thereby supporting use of extant trajectories to estimate the morphology of an extinct species. The simulated Taung adult was then compared to other adult fossils. As these comparisons required assumptions about the pattern and magnitude of developmental change, additional analyses were performed to evaluate these two parameters separately. Results of all analyses overwhelmingly rejected the possibility that the Taung child was a juvenile robust australopith, but were consistent with the hypothesis that the Taung and Sterkfontein fossils are conspecific. Between Sts 5 and Sts 71, the latter is more likely to resemble the adult form of the Taung child.