CRANIAL EQUILIBRIUhf INDEX MVIUZAFI’ER SULEYMAN QENYtfREK Department of Anthropobgy, Harvard University INTRODUCTION I n the preparation of this work, the writer utilized the primate collections of the Museum of Comparative Zoology of Harvard University and of the American Museum of Natural History. I am extremely grateful to my teachers, Prof. E. A. Hooton and Prof. G . Allen, for limitless advice and kind permission t o study the specimens in their laboratories. I am also grateful to the staff of the American Museum of Natural History of New York for kind permission to work in their laboratories. Also, I am extremely indebted to Miss. E. T. West, of Boston University, who prepared the illustrations for this paper and the coming publications. CRANIAL EQUILIBRIUM INDEX Every anthropologist knows that in the evolution of the Hominidae from a common primate ancestor, the cranial I n t h e literature, the writer waa able t o find only the following references which would euggest the cranial equilibrium index, but which are different from it: Broca (Bulletin de la societh d’anthropologie de Parie, 1862) gives the relstivo poeition of basion on the alveolo-condylean plane. Martiii in hie Lehrbuch der Anthropologie mentions a n index which gives the relative position of baeion on the inner Skull length. Aurel V. Toriik (Qrundztige einer systematiechen kraniometrie, 1890) mentione three indices which give the relative position of basion and opisthion on a line parallel t o Frankfort plane. These indices give the relative position of baaion and opisthion which (LLB respectively in front and behind the cranial fulcrum, while the cranial equilibrium index deals with the cranial fulcrum. 23 A I E R I C A N J O U R N A L 0. PHYSICAL ANTHROPOLOOY, VOL. X X I V , NO. JUI.Y--8LPTIXBEO, 1938 1 24 XVZAFFER S ~ L E Y M A S ~ E S Y ~ R E K fulcrum at the occipital condyles, in connection with the upright posture, has moved forward with the result of a better cranial equilibrium. Since, so far this observation has not been the subject of satisfactory metric expression, the writer has undertaken the task. To reduce this rather important morphological observation to measurements, the writer devised the following technique : The cranium is a lever of the first order, the fulcrum being at the occipital condyles. However, if the middle points of the lower surfaces of the occipital condyles are taken we have a better approximation to the true fulcrum. I f these points a r e connected by a Line and the middle point of this line is marked we get the fulcrum on the sagittal plane of the skull. Figure 1 For convenience we may call this new point the intercondylar point (point B, fig. 2). The distance of this point from the prosthion (AB) was measured and then the distance from prosthion to the most posterior point of the skull (AC), projected on a plane passing through the prosthion and the lower-most points of the two condyles, was measured, as is shown in figure 1. Then in the calculation of the index this (AB) distance was expressed as a percentage of (AC) distance. The method of finding the intercondylar point and the distance from prosthion to the intercondylar point and to the most posterior point of the skull was as follows: It is observed that generally the two occipital condyles, when their anterior- and posterior-most points are connected, make an CRANIAL EQUILIBRIUM INDEX 25 isoceles trapezium in norma basilaris. This trapezium is represented in figure 2 by DEFK. If the middle points of EK and D F are connected by a line this line divides the sagittal condylar height (LM) into two equal parts or in other words it bisects this height at the intercondylar point (point B in fig. 2). Conversely, if a line parallel to DE and FK is drawn through the intercondylar point it bisects EK and DF, and this makes the measurement rather easy. To find the distance of intercondylar point from the prosthion, at first the most anterior and most posterior points of the two occipital condyles were marked with a pencil. The occipital condyles can be differentiated very easily by their shine, smoothness and the contrast between them and the surrounding parts. Always the points at which the convexity ended, sometimes sharply and sometimes slightly, were marked for we must get only the functional base on which the cranium is balanced. If the condyles were too low and their anterior- and posterior-most points could not be made out definitely such specimens were eliminated from the series. After markinp: these points the inside of the fixed arm of a sliding caliper was brought in line with the posterior-most points of the two condyles and the movable arm was applied to prosthion and this distance recorded, viz., AM. Then the inside of the fixed arm of the caliper was brought in line with the anterior-most points of the two condyles and again the movable arm was applied to prosthion and this distance recorded, viz., AL. These two distances AL and AM were added up and then divided by two, with the resulting figure giving the distance of the intercondylar point from prosthion, viz., AB distance. I n measuring AL and AM it must be noticed that the bar of the caliper is parallel to the median sagittal axis of the cranium. Here, necessarily, consideration must be given to the fact that the measured intercondylar point, because of technical difficulties, does not coincide with the true intercondylar point. This is due to the fact that in norma lateralis the occipital condyles present a downward convexity and that the 26 MUZAPFER S ~ L E Y M A N ~ E N Y U R E K measurements had to be made from the ends of this convex surface. This difference is illustrated in figure 3. I n this drawing the two occipital condyles are represented as one and the true intercondylar point is represented by (b). Since the measurements were made from E and K, it is evident that the measured intercondylar point lies at (B), that is, a few millimeters above the true intercondylar point. However, the error introduced thus is negligible and further, since all the Primates were measured by the same technique, it does not affect the final conclusions. Hence, in figure 1, no attempt has been made to mark the exact position of B and it has been p u t at the place of (b) for convenience. I n measuring both the AB and AC distances it was highly desirable that these two lines coincided. But in the preceding paragraph reasons have been given to show that this could not be attained and consequently the AB line deviated by a very small angle from the AC line. To measure AC distance it was first necessary to have the lowermost points of the condyles and the prosthion fall i n the same plane. To secure this plane two methods had to be used in case of different Primates. I n case of large Primates the skull was laid on an osteometric board such as is used to measure the length of the long bones. I n doing this care was taken to see that the sagittal plane of the cranium formed a right angle with the vertical plate of the measuring board. After that, wooden plates of known height were placed under the condyles (both condyles supported by the same plate). Then the height of the prosthion, already marked with a pencil, from the surface of the horizontal plate of the measuring board, was measured by means of a vertically placed ruler. These processes were continued until the height of prosthion from the surface equaled the height of the materials placed under the condyles. Then it was seen once more that the sagittal plane of the skull was at a right angle to the vertical plate of the board and that the posterior-most point of the cranium touched this board. After this the distance of prosthion from the vertical plate was read on thin paper rulers on the horizontal plate of the measur- CRANIAL EQUILIBRIUM INDEX 27 ing board,2 I n case of some small Primates, like lemurs, a different method was used. I n forms where there was a considerable gap between the upper central incisors the bar of the caliper was passed between them touching the bone, and the fixed arm of the caliper was brought against the most Figures 2 and 3 posterior point of the skull. Then care was taken so that the lower-most points of the condyles were on the same plane as the top of the bar of the caliper and when this was assured the movable arm was applied to prosthion. I n case of small Primates this is an easier method. * I would suggest that a more accurate and standardized measurement would be obtained if zoologists measured the maxinium skull €ength by the technique employed here. 28 MUZAFFER SULEYMAN ~ E N Y U R E K CALCULATION O F THE CRANIAL EQUILIBRIUM I N D E X I n the calculation of this index, which for convenience we mag call the cranial equilibrium index, AB distance was expressed as a percentage of AC distance as is shown in the following formula : AB X .-100 Cranial equilibrium index = --AC IXPLICATIO&?3 O F T H E I N D E X First implication is that the index gives the relative position of the cranial fulcrum on AC line. Also it tells us the relative position of foramen magnum on AC line because the foramen magnum is invariably bordered by the occipital condyles in its anterior part, and thus it accompanies the condyles in their evolutionary migration. The second implication is that the higher the index the less suited is that specimen for erect posture and that if the cranium is going to be kept erect on the vertebral column it would require stronger riuchal musculature and stronger occipital attachments than one with a lower index. However, since this conclusion is the result of a complicated process of correlation it is necessary to elucidate it. The formula expressing the cranial leverage in a n erect Primate is as follows : W-Q'R = B H * W ' + BC*F Here W is the weight of the skull anterior to the fulcrum, and including the mandible which has been omitted from the drawing, GB ' is the distance from fulcrum to the center of gravity of the prefulcral portion of the skull, W' is the weight of the postfulcral portion of the skull, BH' the distance from the fulcrum to the center of gravity of the postfulcral part, BC the distance from fulcrum to the most posterior point of the skull projected on AC, coinciding with the attachments of the nuchal musculature in most Primates, and (F) is the This equation i s an approxiniation to the leverage formula of IIomo sapiens. F o r comparison, i t was also applied t o the erect primates. B y the terin 'erect primate' is meant the relatively erect position of the hend and body in the actions of sitticig, elimhing, brachiation and bipedal loconiotion. I t should he mentioned here that in quadropedal locomotioii the whole skull is pulled clown by gravity. CRhNIAL EQUILIBRIUM INDEX 29 force applied by the nuchal musculature to keep the head balanced on the vertebral column. These points are shown in figure 4. From this formula it is clear that if the relations of these dimensions and weights are changed, depending on the nature of changes, F (muscle force) will have to change, that is, it will have to be either decreased or increased. The sagittal sections of a number of Primate crania were projected carefully on thick cardboard and two or three copies were made for each skull. On these cardboard copies a BX line perpendicular to AB was drawn. In one copy the Figure 4 cardboard figure was cut along BX line. Then the centers of gravity of the two parts of the skull were determined by hanging the cardboard figures from three corners. I n figure 4 the center of gravity of the prefulcral part is represented by G and that of postfulcral part by H. G’ and H’ represent the vertical projections of these points on AC line. The G’B and BH’ distances were recorded. I n the second copy of the same skull the point B was moved forward to B’ and B’X’ vertical was drawn and the cardboard was cut along this line. Then again the centers of gravity of the pre- and 30 MUZAFFER SULEYMAN EJENYUREK postfulcral parts were found. These new centers of gravity are represented by g and h, and their projections on AC by g‘ and h’. As the fulcrum was moved forward it was found that B’g‘ distance became shorter than G’B and R’h’ became longer than H’B. By the anterior movement of the fulcrum the weight of the prefulcral portion is decreased while that of the postfulcral portion is increased and also BC distance is increased. These findings, when applied to the leverage formula of the erect cranium, show that as the fulcrum moves forward, that is, as the cranial equilibrium index decreases, the cranium can be balanced on the vertebral column with a reduced (F). This would mean that as the fulcrum moves forward this cranium can be kept in balance with weaker nuchal musculaturc and weaker occipital attachments. In a number of cases the process was reversed, that is, the fulcrum was moved backward and it was found that the results also were reversed and to keep this cranium well balanced (F) had to be increased. Besides this, BH’ distance was expressed as a percentage of BG’ distance. The index thus obtained is 56.25 for a Homo sapiens and 23.4 for a chimpanzee. This finding corroborates the results obtained by changing the position of cranial fulcrum of a single skull. It is from the correlation of the cranial equilibrium index with these experiments that the second implication of the index is concluded, that is, the higher the index, the less suited is that cranium for erect posture and conversely, the lower the index, the easier it is to keep a well-balanced cranium. CRANIAL EQUILIBRIUM INDEX OF THE PRIMATES I n table 1, where the indices of the adult male members of the leading genera of the order are given, it is seen that, in general, the index decreases from suborder Lemuroidea to suborder Anthropoidea. The suborder Lemuroidea ranges from 93.53 to 80.4, one Tarsioid has an index of 79.94, which is lower than that of any lemur, and the Anthropoidea run from 31 CRANIAL EQUILIBRIUM INDEX 94.39 in Alouatta to 64.18 in Saimiri sciureus, and to a minimum of 50.74 in Homo. However, it must be noted that this difference is only general and that all the three suborders overlap. The only gap in the series is between Homo sapiens TABLE 1 Cranial equilibrium inde5 of adult male Primates OEANIAL EQUILXBEKIM INDEX SUBOXDEB FAMILY GENUS - Lemuroidea Lemuridae Indriidae Lorisidae Lemur Lepilemur Microcebus Propithecus Perodictieus Nycticebua Loris Galago Euoticus Tarsius Tarsiidae Cercopithecidae Erythrwbus Colobus Pygathrix Papio M8CaCuS Cercopitheous Cercaeebus ____.Hylobatidae Symphahngus Hylobates Simiidae Pongo Anthropoidea Pan Gorilla Hapalidae Mico Hapale Mystax Oedipomidas -~ Alouatta Cebidae Pithecia Callicebus Ateles Aotus &bus - Saimiria Hominidae Homo sapiena Tarsioidea lpecimens Range 8 1 2 5 5 3 2 8 5 1 2 14 4 5 7 23 5 1 5.2 3 7 8 2 8 3 14 5 5 11 4 15 10 27 86.98-90.23 -- - I .......... 83.27-84.16 _____ 85.43-88.93 91.92-93.53 88.59-91.22 82.60-84.14 80.40-87.46 81.10-83.26 .......... 78.76-87.61 75.25-83.33 77.50-80.31 74.87-80.23 72.90-78.81 66.66-81.08 71.42-76.25 .......... 79.24-83.64 77.14-83.95 78.42-82.08 69.84-75.47 74.78-78.93 75.68-77.14 71.95-77.45 71.30-75.56 84.68-94.39 76.31-82.53 72.50-81.88 74.78-80.08 74.13-77.50 68.10-78.78 64.18-69.26 50.74-59.57 Average 88.67 88.44 83.71 86.86 92.71 90.18 83.37 83.02 82.24 --79.94 83.18 79.41 78.93 77.02 76.09 76.01 75.00 81.10 80.99 80.54 79.67 _ 72.44 _ 77.72 76.41 74.19 73.98 89.65 78.42 78.05 76.72 75.82 75.68 66.91 54.34 32 MUZAFFER SULEYMAN ~ E N Y U R E K and the rest of the Primates. Most advanced species of the infrahuman Anthropoidea in this respect are Cercopithecus talapoin, Saimiri sciureus and Saimiri oerstedi, whose respective averages are 67.01, 66.94 and 66.88 (table 2). The lowest individual value of the infrahuman Primates is 64.18, and it is that of a Saimiri sciureus; the highest individual value of Homo sapiens, 59.57, that is, there is a gap of approximately 5 index units, which is a considerable difference. The Eocene Lemuriformes undoubtedly had high indices and the foramen magnum looked backward.' These primitive features have been retained by the existing Lemuriformes. Also the early Eocene tarsioids, before and at the Necrolemur stage, had high indices and their foramen magnum also looked backward. However, by the end of the Eocene period the modern condition found in the living Tarsius, that is, an anteriorly placed and downward looking foramen magnum, was almost attained. We have no direct evidence about the position of the foramen magnum of the Anthropoidea in the early part of the Tertiary era, but from the fact that the living members of this suborder have a more anteriorly placed and downward looking foramen magnum we can infer that their Eocene ancestors, after they were separated from the tarsioids, were possibly more advanced than the Lemuriformes. Thus it is clear that the evolution in this index was from a high t o a low one. With the exception of Alouatta the Anthropoidea have values below 83 and from this and from the condition in the Tarsioidea, which undoubtedly have a community of origin with the Anthropoidea, it may be conjectured that the earliest Anthropoidea probably had an index not lower than 83 to 82. It has been thought (Clark) that in Alouatta the foramen magnum has secondarily moved backward and that the condition found in this genus, where the foramen magnum looks rather backward, is by no means a primitive feature. The highest individual value in the order is 94.39 and is found in the genus Alouatta, but I suspect that if there were more ' W. K. Gregory. On t h e structure and relations of Notharctus, an American eoceiie primate, 1920. CRAXIAL EQUILIBRIUM INDEX 33 specimens of Lemuroidea we would have still higher indices, as would be expected from the fact that Perodicticus and Nycticebus have the highest generic averages. It is also seen that the Hylobatidae and, with the exception of gorilla, the Simiidae have relatively higher indices than some of the lower monkeys. Among the anthropoid apes, gorilla has the lowest index, and though this is consequent, in a great measure, upon the enormous development of the occipital crests, nevertheless, as is shown by the female gorilla whose cranium is devoid of such ridges, it is somewhat nearer to Homo sapiens than the other anthropoid apes. Living Homo sapiens have the lowest indices, explaining the fact that modern man has a relatively round and smooth occiput, but, geologically speaking, this is undoubtedly a recent development for such archaic Hominidae as the Pithecanthropus erectus, Sinanthropus pekinensis, Homo neanderthalensis and Homo soloensis appear t o me to have high indices. This, together with the fact that the great anthropoid apes, which are man’s nearest relatives, have high indices would indicate that the ancestral Hominidae also had high indices. Eanthropus dawsoni would seem to have a relatively low index and from this it may be conjectured that the modern condition of Homo sapiens was attained at least in early Pleistocene times. Thus, I infer that while there is a gap between the living Homo sapiens and the rest of the living Primates in this respect, this hiatus is fully filled in by the fossil Hominidae. Since the anterior portion of the head is heavier than the posterior, the most advantageous and ideal condition for the Homo sapiens would be an index lower than 50, but so far as the thirty-four specimens studied are concerned this desirable condition has not yet been attained. From an examination of tables 1 and 2, it appears that certain monkeys such as Cercopithecus talapoin, Saimiri, Oedipomidas and Mystax are more advanced in this respect than the great anthropoid apes. These monkeys are lowly pronograde Primates and this feature is certainly not of much use when they pursue a quadrupedal locomotion, but when they are climbing they can keep their heads erect more easily 34 MUZAFFER S ~ L E Y M A NS E N Y ~ R E K TABLE 2 Cranial equilibrium indsz of adult males and adult female8 - ADULT MA130 Lemur fulvus Propithecua verreauxi Perodicticus potto Qalago demidovii Euoticus elegantulua Tnrsius spectrum Colobus polykomoe Colobus badiue Pygathrix aurata Papio doguerr Macacue nimistrinus Cercopithecus mitis Cercopithecus nictitam Cercopithecus aethiops Cercopithecus mona Cercopithecus cephua Cercopithecus talapoin Cercocebue torquatus Cercocebus agilis Hylobates lar P a n satyrus C)orillagorilla Hapale santaremensis Mico argentata Mystax ruflmanus Oedipomidaa geoff royi Alouatta seniculus Alouatta beelzebul Alouatta palliata Pithecia monachus Callicebus remulue Ateles vanegatus Ateles geoffroyi Ateles paniecus Aotus zonalis Aotua trivirgatus Cebus flatuellus Cebua capucinus Saimiri sciureue Saimiri oeretedi Homo eapiens ADULT R M A U J I - -- Bpeeimenr Range ireraga Spscimans Range 4 87.71-89.83 85.43-86.8 7 91.92-93.53 80.40-81.62 81.10-83.26 88.90 86.35 92.71 81.01 82.24 79.94 81.61 77.70 79.33 76.08 77.41 79.05 77.95 76.58 74.26 74.62 67.01 75.77 73.83 81.48 80.30 71.94 76.41 76.79 74.04 73.98 92.03 89.98 87.79 70.42 77.10 77.58 76.62 76.12 76.66 75.83 77.13 74.28 66.94 66.88 54.34 2 3 3 2 82.97-87.95 85.59-86.53 92.18-93.65 80.00-81.59 4 5 3 5 1 7 6 2 3 1 5 5 3 2 4 2 3 2 3 2 6 2 7 7 3 4 4 5 2 4 1 1 1 2 1 6 6 6 4 27 - .......... 79.91-83.33 76.19-79.36 78.35-80.31 74.87-77.01 .......... 77.27-81.08 75.25-80.80 75.24-78.57 73.78-74.74 72.39-77.04 66.6667.36 75.00-76.25 71.42-76.25 79.24-83.64 78.53-82.08 69.84-74.53 75.68-77.14 74.78-78.93 71.95-77.45 71.30-75.56 89.61-94.39 89.25-90.97 84.68-92.98 76.31-82.53 72.50-78.75 .......... .......... .......... 75.83-77.50 .......... 75.25-78.78 68.10-77.55 64.1848.79 65.48-69.26 50.74-59.57 -- - 1 1 5 3 3 2 1 5 3 5 2 1 1 1 1 3 5 4 3 2 4 1 4 2 5 1 4 1 LTerage . .......... .......... 78.71-82.03 75.75-78.65 73.53-77.65 70.27-73.20 .......... 75.28-77.89 75.96-77.88 71.51-72.57 72.15-73.62 .......... .......... .......... .......... 75.70-79.89 74.46-81.73 71.32-78.89 74,5677.87 77.14-77.96 70.95-76.51 .......... 87.50-89.21 86.13-88.78 81.30-85.57 .......... 75.16-79.08 85.46 86.08 92.88 80.79 84.10 75.64 80.87 76.74 75.57 71.73 72.54 76.79 76.69 72.07 72.88 72.04 67.09 76.15 72.47 77.95 78.74 75.63 75.86 77.55 73.89 73.88 88.40 87.45 84.01 76.42 i 77.02 .......... 4 74.77-75.90 i, 2 2 1 5 2 3 1 6 76.98-81.81 76.27-76.72 I 79.39 .......... ! 60.34-66.66 iI - 50.55-55.05 1 70.17-73.09 70.22-75.00 .......... I 75.34 77.96 76.49 74.17 72.39 73.61 64.45 62.58 52.73 CRANIAL EQUILIBRIUM INDEX 35 than the great anthropoid apes. It is a zoological fact that if the size of a species is increased, as compared with the size of its near relatives the face grows more than the neurocranium, and conversely, if it is reduced, the face is decreased more than the brain case that is the result is a more infantile form. All these low monkeys with a low index are small and infantile and, as will be shown in the discussion of age differences, since the young Primates usually have lower indices than the adults of the same species it becomes clear that in this respect also they have been infantilized. This low index in these monkeys has thus been attained independently of the forward migration of the condyles in Homo sapiens for I infer from the fossil Hominidae and the living great apes that the ancestral Hominidae had a rather high index which is a relatively more primitive condition than the low indices of these monkeys. SEX DIFFERENCES From an examination of table 2 it will be seen that in females also, the index, in general, decreases from suborder Lemuroidea to suborder Anthropoidea. The suborder Lemuroidea range from 93.65 to 80. One Tarsius has an index of 75.64. The Anthropoidea run from 89.21 in Alouatta to 60.34 in a Saimiri sciureus and to a minimum of 50.55 in Homo. The gap between the most advanced of the infrahuman Primates and the least advanced of Homo is about 5 index units. From this table it is seen that females of most Primates have lower averages than the males of the same species, that is, in this respect the females are more advanced than the males. This is due to the fact that female Primates are relatively more infantile than the males, that is, they have a less snonty face and this is responsible for the decrease in the index. This would mean that the female Primates need less muscular power and weaker nuchal attachments than the males, explaining the fact that usually the female Primates have rounder and smoother occiputs than the males of the same kind. 36 MUZAFFER SULEYMAN ~ E N Y U R E K However, there appear to be a few exceptions to this rule. Most of these exceptions can be ruled out for the time being by the fact that these species are represented by insufficient specimens. But in the case of the gorilla this condition is due to an entirely different factor. I n the gorilla the females on the average are about 4 index units higher than the males. This is because of the fact that the male gorilla has developed large occipital tori which makes an addition to the posterior part of the skull, and thus decreases the index. This is further confirmed by the fact that a sub-adult male gorilla (table 3) has an index higher than any female. This exception in the case of the gorilla shows that when the front of the head is extremely heavy it requires strong occipital attachments. This observation also explains the low index of the baboons which have large snouts. However, in the genus Alouatta there has been a secondary backward displacement of the foramen magnum and hence the development of large occipital tori does not decrease the index. AGE DIFFERENCES Age criteria. Any individual in which the basal suture was completely synostosed and the number of permanent teeth peculiar to that species was completed was called adult. However, the age was not determined by the complete number only, but also by the complete eruption of the individual permanent teeth which, in turn, was decided upon by a comparison of dental morphology in each species. Individuals in which the basal suture was open and which had not completed their permanent dentition, that is, if some of the deciduous teeth were not replaced or some of the permanent teeth had only slightly erupted were called sub-adults. Those that had completed their permanent dentition while only a trace of the basal suture was left open were considered young adults and were included in tables 1and 2. Individuals in which the basal suture was open and only milk teeth were present were called young. Range ADULT Y A L E 3peci. mens - - Average __ Range ADULT FEMALE Average 3peci. mens 78.35-80.31 72.90-78.81 75.00-76.25 74.87-77.01 .......... 78.53-82.08 79.24-83.64 3 3 5 5 6 1 2 3 1 3 7 5 4 1 6 1 1 27 Cerccopithecusaethiops Cercopithecusmitie Cercopithecusnictitans Gorilla gorilla Gorilla beringei Pan satyrus Hylobates lar Sympbalangus syndaetylus Oedipomidaa geoffroyi Mystax rufimanus Alouatta palliata Alouatta seniculus Alouatta caraya Cebus flatnellus Atelea geoffroyi Ateles paniscus Homo sapiens 81.01 81.61 75.25 79.33 75.05 75.71 50.74-59.57 .......... .......... 75.25-78.78 .......... 71.30-75.56 71.95-77.45 84.68-92.98 89.61-94.39 .......... 69.84-74.53 77.27-81.08 75.25-80.80 75.24-78.57 76.62 76.12 54.34 77.12 73.98 74.04 87.79 92.03 88.18 81.10 75.47 80.30 81.48 71.94 79.05 77.95 76.58 76.08 .. .......... .... .......... 80.40-81.62 79.91-83.33 3 7 1 2 4 3 80.00-81.59 78.71-82.03 80.79 80.87 1 1 5 78.74 77.95 70.95-76.51 81.30-85.57 87.50-89.21 .......... 73.88 73.89 84.01 88.40 4 2 6 5 74.77-75.90 76.98-81.81 50.55-55.05 70.17-73.09 75.34 79.39 52.73 72.39 .. .......... .... 1 4 5 4 .. .......... .... 3 74.46-81.73 75.70-79.89 .......... .... 5 .. 75.63 76.79 76.69 71.32-78.89 75.28-77.89 75.96-77.88 72.07 76.15 76.45 71.73 4 3 71.51-72.57 74.16-78.75 70.27-73.20 2 2 5 .......... 1 a . Average 3peci. men8 - .......... .......... .......... 68.07-71.05 72.52 69.56 70.95 67.39 72.07 71.71 1 f 0 9 P d 0 6 d 1 0 1 T 0 d 80.55 80.35 74.45 81.25 I .......... 73.94 .......... 72.50 .......... 82.18 .......... 82.77 .......... 84.53 .......... 71.66 67.07 .......... 69.60 .......... 74.51 .......... .... .......... .......... .......... 71.51-77.40 .......... 73.45 .......... 71.48 6 .......... 80.02 .. .......... .... P 0 .......... d ........... d d 6 79.43 79.17 70.31 68.83 ? d ~~ 63.15-66.66 I . . i9.20 i4.90 i7.10 52.38 i1.26 50.94 i4.58 . Average - __ i6.60 .. .. .......... .... .. .. .......... .... .. .. .......... .... .. .. .......... .... .. .. .......... .... .. .. .......... .... .. .. .......... .... .. .. .......... .... .. .. .... .. .. .... 1 1 1 1 .......... .......... .......... .......... .......... 0 .......... 0 ? ? 2 Range YOUNQ .. .......... .... .. .......... .... .. .......... .... .. .......... .... .. .......... .... ? .......... 62.66 .. .......... .... .. .......... d .......... 71.35 .. .......... .... - Sex - 1 1 1 .. 1 .. .... .. .. .. d .......... ? .. .......... .. P .......... 1 .. .......... .... .. Range .. .......... d .......... Jex - SUBADULT .. .. 1 1 1 1 1 1 1 1 1 1 1 1 2 ..1 1 1 1 1 1 1 2 .. .......... .... 1 .. 73.53-77.65 75.57 3 .......... .... 1 5 2 2 82.60-84.14 83.37 .. .......... .... .. .. .......... .... .. .......... .... 1 Ipeci. mens -. Loris tardigratus Loris gracilis Galago demidovii Colobus polykomos Colobus kirki Pygathrix aurata Maeseus rhesus Cbrcocebus torquatue Cercocebus albigena Papia doguera SPECIES TABLE 3 A g e differences in cranial equilibrium indez 38 MUZAFFER SULEYMAN ~ E X Y U R E E An examination of table 3 would indicate that the young and sub-adults of most of the Primate species have lower indices than the adults, that is, a low index is a neo-genetic feature that appeared in the young of the infra-human Primates. I n the ontogeny of the infra-human Primates the index increases from young to sub-adult and then to adult. Homo sapiens has evolved by retaining the infantile character of the infra-human Primates i n this respect, as also in other features, but in human evolution the index has further been decreased, for the lowest index observed in a young infra-human Anthropoidea is 60.94, whereas the highest index found i n an adult male Homo sapiens is 59.57. A 9-month-old human infant has an index of 56.6, but whether this means that man has become fixed in this respect or that this specimen is exceptional is a question that cannot be answered without the examination of larger series of specimens. Whatever we can deduce from this meager series of young Primates is sufficient to caution us not to compare the position of the foramen magnum of a young Primate with that of the adult of another species, as has been done by Dart of South Af r i ~ a . ~ PROBLEM OF TARSIUS Clark,6 in his illuminating book on the evolution of Primates, expresses suspicion that the pithecoid appearance of the tarsioid cranium may be a secondary development consequent upon the distortion caused by the enlarged orbits. Therefore, since it involves the position of foramen magnum, it is not superfluous to consider this problem in some detail. I n the Advanced Insectivores and Lemuriformes a line, perpendicular to the alveolar process, dropped from the most anterior point of the orbits intersects the upper dental border between the last pre-molar and first molar. In Tarsius this line passes between the first and second pre-molars, while in Nycticebus it passes between the second and third pre-molars. Raymond A. Dart. Australopithecus afrieanus : The Man-Ape of South Africa. Nature, 1925. Sir Arthur Keith. New discoveries reiating t o the antiquity of man. a W. Le Gros Clark. Early Forerunners of Man, pp. 56-60. CRANIAL EQUILIBRIUM INDEX 39 So we have evidence that in Tarsius and to a slightly less extent in Nycticebus the orbits, besides their enlargement, have moved forward and, consequently, have shortened the face. In Old World hthropoidea this line passes, except in gibbons, on or behind the gap between the last pre-molar and first molar. I n New World monkeys this line is either on the gap between the last pre-molar and first molar or before it. I n Anthropoidea where this line is rather anterior it is undoubtedly due to the bending of the face on the basi-cranial axis. The skull of Tarsius is characterized by a short face and large orbits, and also by the partial closure of the back wall of the orbits, by a rounded neurocranium and by an anteriorly situated and downward facing foramen magnum. Professor Clark is inclined to think that these are all secondary features consequent upon the anteroposterior pressure caused by the enlarged orbits. In Lemurs the orbits look lateralward, but in Nycticebus they are not only larger but also look more forward. The neurocranium of Nycticebus is also relatively rounder than that of Lemur. Thus, it is clear that some of the changes that characterize the tarsioid cranium have been attained by Nycticebus, though to a lesser degree. Let us connect the anterior and posterior borders of the two external auditory meatuses with two lines and obtain a horizontal band as is shown in figure 5. Now, if we observe the relation of occipital condyles to this band we observe that in Tupaiidae and all the Lemuroidea the condyles are way behind this band, while in Tarsius and Anthropoidea, excepting a few cases, the anterior part of the condyles fall invariably into this band. In man they reach to a more anterior position in this band than in the other Primates. This condition is undoubtedly an advanced and specialized feature peculiar to higher Primates. However, in Alouatta, some gibbons and a few other Anthropoidea the condyles are behind this band. I n Nycticebus not only the index is very high but also the condyles are way behind this band, probably even more posterior than in the Lemurs. From this we may infer that if Clark’s suspicion that the enlargement of orbits was responsi- 40 MUZAFFER SULEYMAN ~ E N Y U R E K ble for the anterior position of the foramen magnum in Tarsius was right we should expect a more anteriorly placed foramen magnum and thus a lower index in Nycticebus. Let us suppose that the most primitive Primate neurocranium was oval, or, for the sake of discussion say, rectangular, and that the foramen magnum was at the very back of the skull; then, if the orbits were enlarged as in Tarsius, and if, on account of enlargement, the breadth of the neurocranium was widened at the expense of the length, the distance between the condyles and the band between the two auditory meatuses would be gradually decreased, but always the foramen magnum would be a t the very back of the skull. Figure 5 It would just be pulled forward with the posterior wall of the cranium and would always be at the back. If we suppose that this process was continued for a long time, then the occipital condyles would fall in this band, but in this case the auditory meatuses would be at the very back of the skull rather than both the condyles and auditory meatuses being nearer the center of the neurocranium as in Tarsius. The pull of the enlarged orbits would be exerted along the zygomatic arches and along the occipital crests and it is plausible that this would tilt the foramen magnum and make it look even more backward than it did before. I n view of these theoretical objections, and the evidence of Nycticebus, Clark’s suspicion is unjustified. We are justified in concluding that the foramen CRANIAL EQUILIBRIUM INDEX 41 magnum of Tarsius has moved forward irrespective of the enlargement of the orbits. However, it appears from the fossil record that the relatively low index of the Tarsius has been acquired independently of the low index in the Anthropoidea; for even at the Necrolemur stage, where the index was still high, the Tarsioids were already specialized and thus could not give rise to the living Anthropoidea. The Anthropoidea and Tarsioidea could only have separated before the Necrolemur stage when the index was relatively high. Thus it seems that this relatively anteriorly placed foramen magnum of Tarsius was attained independently, and also that it was not a secondary development consequent upon the distortion caused by the enlarged orbits as Clark is inclined to believe. I believe it was brought about by latent potentialities in this direction, which were developed by the common ancestors of the Tarsioidea and Anthropoidea, and thus, in the course of time, this advanced feature was independently acquired in the two suborders. SUMMARY AND CONCLUSIONS The evolution in the order Primates, except in Alouatta, has been from a high to a low index, or, in other words, the cranial fulcrum has moved forward. Homo sapiens has the most anteriorly placed cranial fulcrum of all the Primates. This provides a mechanical advantage in the erect posture. But it is a relatively recent feature, for the ancestral Hominidae undoubtedly had a much more posteriorly placed cranial fulcrum. The female Primates usually have a more anteriorly located cranial fulcrum than the males of the same species. The anteriorly placed cranial fulcrum appeared as a neogenetic feature in the young of the infra-human Primates. Homo sapiens in his evolution has retained this infantile feature of the infra-human Primates ;but in his phylogeny the cranial fulc.rnm has moved more forward than it has in any of these.