Current events

Dean Falk Reassessment of the Taung early hominid

Dcpartmcnt of Anthropology, State from a neurological perspective

Uniunsify of New York, Albany, NY 1.?2?.?, U.S.A.

Charles Hildebolt & Michael W. Vannier

Mallinckmdt In.&& of Radiology, Washington University School of Medicine, St. Louis, MO 63110, U.S.A.

Journal of Human Evolution (1989) 18,485-492

Introduction

Although it has been almost 65 years since its discovery, the Taung fossil that provides the holotype for gracile austrolopithecines (Austrolopithecus africanus) is the focus of current debate about whether or not its dental development was apelike or humanlike (Bromage & Dean, 1985; Conroy & Vannier, 1987, 1988; Mann, 1988; Smith, 1986; Smith & Garn, 1987; Beynon & Dean, 1988; Wolpoff et al., 1988). Since 1979 (Radinsky, 1979), questions have also reemerged concerning whether or not the natural endocranial cast (endocast) that was found in association with the Taung skull appears apelike or humanlike in its external morphology, which reproduces details of the cerebral cortex (see Falk, 1987 for review). This latter controversy partially stems from the relatively primitive techniques that were used for reading and comparing endocasts, including stereoplotting (Falk, 19836), measuring indices from photographs (Holloway, 1984), and a confounding of measurements taken from endocasts by different methods (Falk, 1985). In this report, the Taung endocast is analyzed with three-dimensional computer technology that only recently became available for paleoneurological research (Vannier et al., 1985; Falk et al.,

1986), and sulcal patterns derived from this analysis are compared with those of a newborn human and chimpanzee.

Endocasts prepared from skulls of humans (Symington, 1916; Connolly, 1950) and chimpanzees (Connolly, 1950; Clark et al., 1936) fail to replicate the sulcal patterns of the brains from corresponding skulls, a fact that appears to be true regardless of the individuals’ ages (see Falk, 1980 and Holloway, 1975, for details). On the other hand, for reasons that are not fully understood, South African natural endocasts including Taung reproduce clear details of sulcal pattern (Falk, 1980). Thus, the sulci reproduced on endocasts of fossil hominids are traditionally analysed by comparing them with sulcal patterns on human and ape brains rather than endocasts.

0047-2484/89/050485 + 08 $03.0010 @ 1989 Academic Press Limited

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Materials and methods

The Taung natural australopithecine endocast reproduces most of the right hemisphere of

the cerebral cortex. A white plaster copy of this endocast was compared to the original

specimen (by DF) and found to be an excellent replica. This copy reveals numerous sulci

that are reproduced in the frontal and temporal lobes, and a few sulci that appear near the

rostra1 portion of the occipital lobe. Three-dimensional surface coordinates were measured

for points spaced less than 2 mm apart along each sulcus, using an electromagnetic

digitizer (McDonnel Douglas 3Space). This unit allowed an operator to record the

geometry of the endocast surface using an electromagnetic stylus and to encode the

information in a machine readable form.

These data were used as input to a personal computer (Apple Macintosh TM) which

was interfaced to the digitizer using an RS-232C protocol. Sulcal lengths were determined

by “stringing” the three-dimensional points together and calculating the resultant

arclengths. The X, Y, Z coordinates for each point were entered into a Computer Aided

Design (CAD) system (McDonnell Douglas Unigraphics and D-100M) and viewed in the

form of three-dimensional images (Figure 1). The images can be retrieved with ease for

future manipulations. Sulcal lengths were computed from the data and analyzed (see

Figure 1).

The volume of the Taung endocast is approximately 404 cm3 (Holloway, 1983)) which is

much nearer to the average volume of383 cm3 ofa chimpanzee brain (Tobias, 1971) than it

is to 1350-1400 cm3 for the average human brain. Although the Taung endocast has been

compared with brains from 17 adult humans (Falk, 1980), it is difficult to determine the

extent to which differences between adult human and Taung sulcal patterns are due to the

differences in brain size and shape, i.e., to allometric scaling factors (Jerison, 1982).

Various parts of the brain also scale allometrically (Passingham, 1973, 1975). These

Figure 1. Computer illustrations of sulci that were digit&d in three dimensions from a chimpanzee brain (above), brain ofa human newborn (below right), and Tang endocast (below

left); right lateral views. Abbreviations of sulci: a, anterior branch of ts; d, descending branch of

ts; fi, inferior frontal; fm, middle frontal; fo, fronto-orbital: fs, superior frontal; Ic, lateral calcarine

(rostra1 part); oci, inferior occipital (lateral part); pci, precentral inferior; tm, middle temporal;

ts, superior temporal. The inclusion of a and identification of Ii on the Tang endocast departs

from an earlier description by Falk (1980); ts ofTaung includes straight lines that were digitised for portions of the sulcus obscured by a bony chip (rostral) and vessels (caudal). See text for

discussion.

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complications may be partially overcome by comparing the Taung endocast with a small (albeit younger) brain from an infant human.

In this report, we compare the Taung endocast to an apparently normal human brain

that weighs 411 g (i.e., the equivalent of 432 cm3, see Jerison, 1973). The latter specimen was obtained from a human neonate that lived for approximately 2 weeks. Besides being of

nearly equal size to the Taung endocast, the brain from a human neonate may also be a more appropriate specimen for comparison than would be a brain from an adult human because, on the basis of recent dental evidence, it has been suggested that Taung’s estimated age at death be reduced from the traditional 5 to 6 years to a new estimate of 2.7-3.7 years (Bromage, 1985). For similar reasons, the Taung endocast is also compared to a brain from a juvenile chimpanzee. The weight of the chimpanzee brain is 292 g (307 cm3).

Initially, only sulci that appeared equivalent to those on the Taung endocast were digitised from the right hemispheres of both the human neonate brain and the chimpanzee brain (Figure 1). These sulci include: the superior frontal, middle frontal, inferior frontal, and parts of the precentral inferior in the frontal lobe. In addition, the fronto-orbital sulcus was not present in the human brain but was digitised in the chimpanzee brain, as it was on the Taung endocast. Temporal lobe sulci that were digitised in all three specimens include the superior temporal, its anterior and descending branches, the middle temporal, and the lateral portion of the inferior occipital sulci. The only other sulcus that was digitised in all three specimens was the rostra1 portion of the lateral calcarine in the occipital region. Each of the three specimens was digitised a second time to assess the repeatability of the 3Space digitiser for determining sulcal lengths. The instrument was found to be extremely reliable, with r for the two readings exceeding O-998 in all three specimens. The root mean squares of the difference scores between the two readings for each sulcus were O-073, 0.092 and 0.036 cm for the human brain, chimpanzee brain, and Taung endocast, respectively.

Because endocasts reflect only features of the brain that have become imprinted on the inside of the skull, they do not reproduce as much details as do actual brains. Nevertheless, as Figure 2 shows, a good deal of detail is reproduced on the frontal and temporal lobes of the Taung endocast. To have a baseline for analyzing that portion of the sulcal pattern which is reproduced on the Taung endocast, we next digitised the entire lateral surfaces of the right hemispheres of the human and the chimpanzee brains.

Results

A consistent relationship exists between the summed lengths of sulci and the volume of neocortex across species (Elias & Schwartz, 1969) that is expressed by the formula (Jerison, 1982) :

ES = 3.56VO.76 (1)

where ES = expected total length of sulci, and V = brain volume. The expected sum of sulcal lengths for all surfaces of one hemisphere of the human neonate (432 ems) is 179 cm and for the juvenile chimpanzee (307 ems) is 138 cm. Table 1 shows that the lateral surface of the right hemisphere of the human neonate has a total of 13 1 cm of sulci, while that of the chimpanzee has 98.4cm. Thus, the summed sulci on the lateral surface of the human neonate right hemisphere is 73% ofthe value expected for its entire surface (i.e., lateral and basal surfaces) while the comparable measure for the chimpanzee is 7 1%. These figures for

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Figure 2. Photographs of right lateral views of a juvenile chimpanzee brain (above), brain of a human newborn (below right), and the Taung endocast (below left). The sulci in Figure I were digitized in three dimensions from these specimens. See text.

Tahie 1 Summed lengths of sulci on lateral surface of right hemisphere

Human neonate Juvenile chimpanzee brain’ (cm) brain* (cm)

Frontal lobe 46.1 31.7 Temporal lobe 24.2 23.1 Occipital lobe 17.8 10.0 Parietal lobe 27.4 18.8 Dividing sulci 15.5 14.8 Total 131.0 98.4

1 Cranial capacity = 432 ems. 2 Cranial capacity = 307 cm3. Note: Dividing sulci include the Sylvian fissure and central and

parieto-occipital s&i.

the human and chimpanzee brain are remarkably similar, a finding that is consistent with

Radinsky’s (1975) observation that “human brains have the expected amount of neocortex for an anthropoid brain of their size”.

Table 2 presents the summed lengths of sulci for the Taung endocast, and the

corresponding sulci (only) in the human and chimpanzee brain. The summed lengths of sulci are presented as ratios of the values for the human neonate brain (% H) . Expected ratios of

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Table 2

Human

(cm)

Lengths of corresponding sulci in right hemisphere

Chimpanzee Taung

cm (%H)I (%H-%Ez, %H-%G’) cm (%H) (%H-%E, %H-%G)

Frontal lobe 18.4 13.9 (0.76) (-0.01, -0.03)

Temporal lobe + anterior branch superior temporal sulcus 18.2 16.3 (0.90) (+0.13, +o.ll)

Lateral calcarine (rostal end) 1.8 1.6 (0.89) (+0.12, +o.lo)

Total 38.4 31.8 (0.83) (+oa6, +0.04)

%E: (0.77) %G: (0.79)

14.5 (0.79) (-0.16, -0.17)

17.8 (0.98) (+0.03, +0.02)

1.8 (100.0) (+0.05, +0.04)

34.1 (0.89) (-0.06, -0.07) (0.95) (0.96)

1 %H (actual ratio) is the chimpanzee or Taung sulcal length divided by human s&al length. 2 %E is the expected ratio of sulci relative to the human brain based on equation (1). 3 %G is the expected geometric ratio based on solid geometry.

summed sulci when brain size (allometry) is taken into account are also provided in Table 2. These were determined from equation (1) i.e., by computing the appropriate ratios with the values of ES determined for human neonate, chimpanzee, and Taung (358, 276 and 341 cm respectively). Thus, the expected ratio (%E) for chimpanzee is 276/358 = 0.77; that for Taung is 3411358 = 0.95.

Because equation (1) presumably was determined from data for adult mammals (Jerison, 1982; Elias & Schwartz, 1969), one might question whether or not it should be used for juveniles such as Taung. At birth, human brains manifest a complete set of sulci and subsequent growth of the brain is “attributed to changes in cortical cell-packing density and the growth of cortical-subcortical fibre systems” (Chi et al., 1977). Postnatal growth of the brain may therefore be viewed in geometric terms, i.e., as an increase in volume of an object that maintains its shape. If the brains of the neonate, chimpanzee, and Taung are treated as spheres, their volumes yield radii of 4.69, 4.18 and 4.59 cm respectively. Because surface features (sulci) are functions of r*, one can estimate the expected geometric ratio (%G) of sulci for chimpanzee relative to human neonate to be 4.18*/4.69* = O-79 and the ratio for Taung to be 4*59*/4*69* = 0.96. These ratios (%G) are remarkably close to those determined empirically from adult mammalian brains (% E, see Table 2), lending support to the assumption that juvenile brains are allometrically smaller versions of adult brains.

In Table 2, actual ratios (%H) are compared to expected ratios (%E) and geometric ratios (%G) for sulci in the chimpanzee brain and Taung endocast relative to those in the human brain. For the chimpanzee, sulci in the frontal lobe (0.76 of the value for the human) are close to the predicted values (O-77 and 0.79), whereas the temporal lobe has a ratio that is O- 13 and 0.11 greater than predicted. For the total sum of sulci, the chimpanzee brain has approximately 0.05 greater ratio relative to the human brain than predicted.

The picture is quite different when one turns to the Taung endocast in Table 2. The ratio of temporal lobe sulci relative to those in the human brain is very close to expectation (0.98 US. 0.95 and 0.96). However, the percentage of summed sulci in the frontal lobe relative to the human neonate is 0.16 to O-1 7 below expectation (O-79 us. 0.95 and O-96). The low sum of sulci in the frontal lobe of Taung causes the ratio for all of the sulci listed in Table 2 to be

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O-06 and 0.07 below the expected and geometric ratios of0.95 and 0.96. In short, when brain size is accounted for, the chimpanzee brain has longer sulci in all areas relative to the human brain than does the endocast of the Taung early hominid.

Furthermore, as Table 2 shows, Taung is more apelike than humanlike in the length of sulci in its frontal lobe even when the differences in brain volume of the three specimens are not taken into account. That is, O-76 and 0.79 are closer to each other (a difference of only 0.03) than they are to the 1 .OO%H for the human (differences of 0.24 and 0.2 1). The same is true for total sulcal lengths. The chimpanzee with an actual ratio of0.83 is more Taunglike (ratio = 0.89) with a difference of only 0.06 than either chimpanzee or Taung is humanlike (with differences of 0.17 and 0.11, respectively). This observation holds for allometric expectations based on both mammalian and geometric data.

Discussion and conclusions

Compared to other regions of the brain, the ratios of sulci in the frontal lobe relative to the human brain are low for both the chimpanzee and the Taung endocast. According to a recent comparative study (Zilles et al., 1988), human and pongid brains resemble each other in most of their rostrocaudal patterns of convolutedness. However, human brains are more convoluted (have higher “gyrification indices”) than great apes in the rostra1 portions of their frontal lobes. This finding is concordant with our observations regarding summed sulcal lengths for this region of the frontal lobe. It appears that the surface morphology of the frontal lobes of hominids underwent changes subsequent to the divergence from pongids.

The present study also confirms earlier reports (Falk, 1980) that the configuration of sulci reproduced on the Taung endocast is apelike rather than humanlike. The pattern of sulci in the frontal lobe is extremely simple and apelike, cf. Connolly’s numerous illustrations of chimpanzee brains (Connolly, 1950). In particular, Taung reproduces a fronto-orbital sulcus, as does the chimpanzee brain. In fact, this sulcus is found in brains of all the great apes, but is lacking in human brains (Connolly, 1950; Falk, 1983a). The temporal lobe of the Taung endocast is also clearly more apelike than humanlike. In both the chimpanzee and Taung, the middle temporal sulcus is long. It is broken up in the human brain. Although our sample includes only one brain of a human neonate and only one brain of a chimpanzee, it is important to note that the distinctions outlined above are representative of the species under discussion (Connolly, 1950; Falk, 1980).

Contrary to the literature (Holloway, 1975), the shape of the Taung endocast is clearly more apelike than humanlike in those areas where one can make any distinction at all. In particular, the cerebelli of the chimpanzee brain and Taung endocast are located farther back relative to the temporal lobe than is the case for the human brain (Figure 2). Thus, the occipital lobe greatly overhangs the cerebellum in the human brain, but not in the Taung endocast or chimpanzee brain. Because of this (and the position of the sigmoid sinus on the Taung endocast), the supposedly humanlike position of the foramen magnum needs to be reexamined for the Taung specimen (Dart, 1925).

The true identity of the dimple that one of us (DF) identified as a possible medial end of the lunate sulcus (Holloway, 1984; Falk, 1980; Holloway, 1988) is of no consequence to these observations. Even if this dimple proved to be just an artifact (but see Falk, 1989), the entire sulcal pattern that is reproduced on the Taung endocast is more apelike than

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humanlike, as demonstrated here with computer technology that facilitates extremely

accurate comparisons in three dimensions. Finally, our findings should be viewed within the framework of other recent revelations

about the Taung fossil. Its dental development may be apelike and not humanlike as previously believed (Bromage & Dean, 1985; Conroy & Vannier, 1987, 1988; Mann, 1988; Smith, 1986; Smith & Garn, 1987; Beynon & Dean, 1988; Wolpoff et al., 1988). Growth of the face of Taung may also be characterised by some apelike features (Bromage, 1985). We now add apelike brain shape, possibly apelike position of the foramen magnum, and an apelike sulcal pattern of the cerebral cortex to this list.

It is also interesting that the Taung specimen shares certain features with robust australopithecines that are not seen in other gracile australopithecines including an enlarged occipital/marginal sinus not seen in four other gracile australopithecine specimens but seen in 7/7 robust australopithecines (Tobias & Falk, 1988; Falk, 1986)) and

an intrapalatal extension of the maxillary sinus (Conroy & Vannier, 1987). As Tobias first suggested (Tobias, 1973), it is time to reassess the status of the Taung specimen relative to robust and other gracile australopithecines.

Acknowledgements

We thank P. V. Tobias for providing the copy of the Taung endocast; B. Ghetti for allowing us to digitise the brain of the human neonate; and M. Levy, J. Neely, and C. Helmkamp for assitance and comments. We are especially grateful to H. Jerison for providing constructive suggestions about the analysis. This research is supported by Public Health Service grant 7 ROl NS24904.

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