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POSTILLA

PEABODY MUSEUM
YALE UNIVERSITY

NUMBER 143. 10 MARCH 1970

SHARPNESS OF TEETH IN MAN
AND OTHER PRIMATES

R. G. EVERY

POSTILLA

Published by the Peabody Museum of Natural History, Yale University

Postilla includes results of original research on systematic, evolution-
ary, morphological, and ecological biology, including paleontology.
Syntheses and other theoretical papers based on research are also
welcomed. Postilla is intended primarily for papers by the staff of
the Peabody Museum or on research using material in this Museum.

Editors: Jeanne E. Remington and Nancy A. Ahlstrom

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should be directed to:

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Lists of the publications of the Museum are available from the above
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and available back numbers of the discontinued journal, Bulletin of the
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in exchange for relevant publications of other scientific institutions
anywhere in the world.

SHARPNESS OF TEETH IN MAN AND
OTHER PRIMATES

ka Gu EVERY

Centre for the Study of Conflict, 25 Clifton Terrace, Sumner,
Christchurch 8, New Zealand, and Dept. of Zoology, Univer-
sity of Canterbury, Christchurch 1, N.Z.

(Received June 1968)

ABSTRACT

Analyses of wear characteristics on the teeth of a young adult
male baboon, a male native human from New Guinea and a
male Caucasian from New Zealand are made, and their insepa-
rable relationship to behavior emphasized. These characteristics
provide evidence of the evolution of precise anatomical and in-
nate behavioral tooth-grinding mechanisms specific to the pro-
duction of sharp teeth. Although the teeth on eruption have some
degree of morphological sharpness, the tooth-grinding behavior
perfects this sharpness and subsequently maintains it throughout
the functional life of the teeth. The male baboon, used here as
broadly representative of man’s non-hominid relatives and an-
cestors, has a specialized, sickle-like, vertically oriented upper
canine, sharpened specifically as a slashing weapon. The lower
anterior premolars are the honing tools which grind against the
upper canines in a motion opposite to that of the masticatory
stroke. These premolars are noticeably specialized for this tooth-
to-tooth grinding action by their enlarged buccal crown-faces,
thickened enamel gingival extensions, and by paired roots placed
perpendicular to the “whetstone” faces.

In contrast, man’s short-canine condition has evolved to pro-
vide a specialized, horizontally sharp shearing device. The con-
tinuous rows of even, constantly sharpened teeth, vertically
oriented and firmly anchored in jaws which provide greater force
at the biting teeth, give man the capacity for powerful, lethal,

POSTILLA 143: 30 p. 10 MARCH 1970.

2 POSTILLA

“segmentive’ biting. Thus man’s dentition is seen not as “gen-
eralized”, and certainly not as “regressed” or “weakened”, but
as highly specialized. The significance of the short-canine con-
dition (currently regarded as a diagnostic feature of hominids)
is not that man has become biologically defenseless, but that the
hominid dental mechanism has harnessed attritional wear to
provide a more stable and durably functional weapon.

It is concluded that tooth-sharpening and related phenomena
are evidence of innate behavior related to a specialized, viable,
biological weapon in Homo sapiens, and because this weapon —
the teeth — is the primary one and has been overlooked it
emphasizes a corollary: intraspecies use of the teeth is strictly
controlled by genetical determinants, whereas such control of
the secondary weapon — the hand — is slight.

TEETH SHARPNESS IN MAN AND PRIMATES 3

INTRODUCTION

The prime evolutionary advantage of mammalian and especially
primate teeth — their sharpness — has previously received little
discussion in the literature. The special morphology of these
teeth, and the fact that they are composed of enamel and dentine
(the hardest and most durable of biological substances) gives
them the capacity to penetrate and divide exogenous materials.
Because the initial morphological sharpness of unworn teeth is so
advantageous, mechanisms to perfect and maintain this sharpness
through progressive wear are further advantageous and have
evolved as important characteristics of mammals ( with very few
specialized exceptions, e.g., the toothless anteater and baleen
whale).

The fossil record suggests that in the evolution of the hominid
dental system leading up to Homo sapiens there was a period
of rapid transition in such mechanisms. Apparently with a few
million years, at the most, there was a switch from the pongid
condition of vertically sharp upper canines to the condition seen
in recent hominids of horizontally sharp teeth. As continuous
rows of short, even teeth assumed the role of weapons, the
weapon-like action of the dentition changed from slash to bite.
Moreover, the process of precise attrition-sharpening, (thegosis,
see below) was extended to include all upper and lower incisors
and canines. This was a considerable addition to the premolar-
plus-molar, horizontal shearing blades which emerging hominids
already possessed. Incision, furthermore, was profoundly im-
proved by the significant addition of a unique feature — antero-
posterior (retrusive) shearing; it was no longer a simple vertical
(orthal) action in the canine-incisor region, such as that seen
in the non-hominid anthropoids today.

In previous reports that introduced my _tooth-sharpening
hypothesis (Every, 1960, 1965), figures were not presented. In
the present paper, figures are included and the major arguments
of the hypothesis are discussed; special attention is given to con-
siderations of dynamic spatial relationships among dental struc-
tures. Furthermore, it is emphasized that these anatomical
considerations can be understood only in the light of corre-
sponding ethological considerations, of which they are an integral
part. Tooth-sharpening and related phenomena give a new

4 POSTILLA

dimension to the study of mammals (particularly). Out of the
immense amount of detail already emerging, this paper con-
centrates on that related to Homo sapiens, his hominid ancestors
and anthropoid relatives.

FUNCTIONAL ANATOMICAL CONSIDERATIONS
NON-HOMINID ANTHROPOIDEA

The baboon (Papio) shows an extreme specialization for vertical
sharpness of the canine and is therefore chosen to illustrate
functional anatomical adaptations in cercopithecoids (Figs. 1-3).

Important features of the baboon upper canine are as follows
(ig. 1):

1. Facet striated by wear from attrition’ (the discrete wear
from the forceful grinding of the surface of one tooth against that
of its opposite, i.e., tooth-to-tooth contact wear). See also Figure
4 and discussion.

2. The absence of wear from abrasion (the diffuse wear from
friction of exogenous material). Compare with Figure 4.

3. The thin enamel coating on the palatal surface (indistinctly
shown on the mesial side of the facet, and on the less extensively
worn gingival third of the crown).

4. The relatively thick enamel coating on the labial surface
(indistinctly shown by the strip of enamel which forms the distal
edge — the cutting edge — of the crown).

1 The basic material of this paper was first prepared as the legend and dis-
cussion of five illustrations (here figured 1-5) and submitted for publica-
tion in 1964 along with the article “The teeth as weapons; their influence
on behaviour” (Every, 1965). At that time I used “attrition” and “abra-
sion”, terms I had differentiated and given specific meaning (Every, 1960).
Hitherto in the literature these terms were used synonymously. Since then,
however, I have used a new term, thegosis (from the Greek, thego, to
whet, sharpen). My argument for the use of thegosis as more appropriate
to an evolutionary adaptive phenomenon, and taking precedence over
bruxism (a term poisoned by a current definition of mal-adaption, pathol-
ogy and myth) is developed in another paper, in preparation. Since num-
bers of scientists are already familiar with my term thegosis I shall, there-
fore, use it in the remainder of this paper.

TEETH SHARPNESS IN MAN AND PRIMATES 5

5. The concavity of the vertical cutting (distal) edge; it tends to
a sickle-like formation. The distal edge is also concave when
viewed directly from behind. This is because the palatal surface
itself is distinctly concave, and this extends the length of the
crown. The reflected light in this unretouched photograph, there-
fore, comes from the apical part of the facet only.

6. The continuity of the concavity of the thegosis-facet. This
extends vertically from the gingival border of the facet to the
apex of the crown. There is no rounding-over (convexity) at the
apex. The tooth is thus ground to a sharp spike.

7. The continuity of the thegosis-facet in the mesiodistal (hori-
zontal) aspect. There is no rounding-over at the distal edge of
the crown, the blade of which is thus ground to a fine edge.

An examination of the origin of the attrition-facet of the upper
canine reveals specializations characteristic of non-hominid
catarrhines; these facets are adapted for producing and maintain-
ing sharpness of the weapon. The grinder (the cutting tool, the
hone or whetstone) is the lower anterior premolar, P3 (Fig. 3). Its
morphological and structural characteristics make it possible to
identify this tooth as a specialized sharpening tool. Moreover,
this is its major function. It is not in any way specialized for
shearing or sectioning, as is widely believed. The buccal surface
of the crown of P; is an elongated area of thick enamel which is
continuous from apex to gingival extension and which forms a
hard grinding surface relatively unyielding to wear. In all other
teeth of Papio (except the lower canine at its tubercle — see
below) there is a gradual thinning of enamel towards the gingival
edge. Thin enamel extends over the whole of the palatal surface
of the upper canine, a feature which allows it to yield easily to
grinding against Ps, and the underlying dentine, when exposed,
yields even more readily.

Specialization of this sharpening tool is further evidenced by
its roots: it has two; the distolingual is heavier than the mes-
iobuccal, and the line in which they are placed is at right angles
to the crown’s grinding surface. Such precise arrangements are
clearly to withstand the force of the laterally directed sharpening
action.

Lower anterior premolar root formations, concomitants of the

6 POSTILLA

ric. 1, Palatal surface of upper right canine tooth: young adult male baboon.
This view is inverted so that the direction of its cutting (distal) edge faces
in the same direction (left) as in Figs. 2 and 3.

TEETH SHARPNESS IN MAN AND PRIMATES 7,

STRIATIONS IN DENTINE
CUT (GROUND) BY THE
BUCCAL ENAMEL OF
LOWER ANTERIOR
PREMOLAR

EDGE OF
THICK LABIAL
ENAMEL:

THE SHARP
CUTTING EDGE

ENAMEL

DENTINE

EDGE OF
THIN PALATAL

ENAMEL

Fic. 1. (cont.)

vertically bladed upper canines, reappear with significant fre-
quency in Homo sapiens (Tomes, 1923), strongly to suggest an
origin from long canined ancestors. Earlier hominids, the
Australopithecus, show this even more distinctly, as they do the

8 POSTILLA

Fic. 2. Terminal phase of premolar grinding the canine in young adult
male baboon.

TEETH SHARPNESS IN MAN AND PRIMATES 9

ABRASION ROUNDED
INCISAL EDGE
(RIGHT CENTRAL INCISOR)

CANINE

DENTINE

ENAMEL

ANTERIOR
PREMOLAR

Fic. 2. (cont.)

feature of a distogingival tubercle on the lower canine. The report
of these (overt) features (Robinson, 1956) does not, however,
relate their concomitance (see below).

Contrary to current understanding, the motion of the mandible
in this tooth-to-tooth wearing action is not as it is in mastication.
The masticatory stroke, in the terminal phase of the masticatory
cycle, is an approximation of the teeth, in a medial, i.e., buccal to

lingual (ectal) movement; it terminates in central position. In
contrast, the sharpening movement starts with the teeth in central

10 POSTILLA

Fic. 3. Beginning phase of canine grinding canine in young male adult
baboon.

TEETH SHARPNESS IN MAN AND PRIMATES 11

ANTERIOR PREMOLAR FACET ON
DISTAL SURFACE
OF LATERAL

INCISOR

ANTERIOR PREMOLAR CANINE

CANINE

THICK
ENAMEL

THIN ENAMEL

TUBERCEB:
THICK ENAMEL

GINGIVAL EXTENSION OF CROWN: THICK ENAMEL
Fic. 3. (cont.)

occlusion. The mandible is extended laterally, i.e., from lingual
to buccal (ental) movement, and is concurrently depressed. This
is effected by action of the external pterygoid muscle on one side
alone, in combination with the depressor muscles (chiefly the
digastric) of the mandible (Every, 1965). The strokes are rapid,
and the sound produced by such grinding in many animals has
frequently been heard but has been recorded merely as tooth-
chattering (e.g., van Hooff, 1962); it has, moreover, been inter-

12 POSTILLA

preted solely as an action deriving from tension and perhaps as a
signal — nothing more.

The audibility of tooth-grinding (chattering) and the lack of
any such sound in masticatory tooth-to-tooth contact is consistent
with the proposition that man’s teeth seldom, if ever, meet in
masticatory or incisive action; they meet when swallowing, but
even then too lightly to produce significant wear (Jankelson,
Hoffman, and Hendron, 1953; Yurkstas and Emerson, 1954;
Anderson, 1955). Attrition (thegosis) does not occur during
chewing and swallowing because masticatory and incisive strokes
are terminated, presumably by proprioceptive reflex, just short
of contact between opposing teeth. The division of exogenous
material is achieved as the teeth approximate; should the teeth
make contact no further advantage would be achieved, and the
production of uncontrolled wear would be a serious disadvantage.

The presence of the tubercle situated distogingivally on the
baboon lower canine tooth (Fig. 3) provides a further example
of morphological, structural, and behavioral specialization which
has evolved as a result of the advantages of the sharpening proc-
ess. The previously unexplained function of the tubercle can
now be understood as a specialization to protect the apex of the
upper canine in the terminal phase of the grinding stroke. Fig.
2 depicts this position. At the critical phase, where the apex of the
canine is poised precariously on the gingival extension of the
premolar crown, the hazard of maintaining the necessary grinding
pressure without rounding over, and thus blunting the apex, is
eliminated by the presence of the tubercle. This tubercle contacts
the upper canine tooth higher (gingivally) on its crown, an
arrangement which (with further mandibular action) allows the
apex to disconnect from the grinder without change of direction
and thus without damage. The tubercle supports the lateral pres-
sure at the termination of the grinding stroke, and as a result,
possesses a thicker coat of enamel than the remainder of the lower
canine crown in this area.

Further sharpening of baboon canine apices is effected by
grinding the lower canine apex against the apex of the upper.
The beginning of this action (shown in Fig. 3) appears to require
a shift from lateral (extrusive) action of the mandible to re-
trusive action but with the mandible still held in a lateral position.
It is possible, however, that no change of action is necessary and

TEETH SHARPNESS IN MAN AND PRIMATES 13

that the two can occur independently. Retrusion is limited by the
postglenoid tubercle of the temporomandibular joint. There is,
nonetheless, sufficient freedom in the baboon’s temporomandi-
bular joint (only slight movement is necessary) to allow some
forceful contact with the distal surface of the upper canine, and
thus effect a mutual sharpening of each apex’.

The apex of the lower canine is further sharpened by yet an-
other action of the mandible which grinds it against the distal
surface (shown in Fig. 3) of the upper lateral incisor. This action,
which affects the mesiolingual surface of the lower canine, is
concurrent with the beginning of the major weapon-sharpening
action on the opposite (contralateral) side of the mouth.

In the baboon, as in many non-hominid Anthropoidea, the
enamel of the upper canine’s anterior surface is grooved to form
two vertical columns. The corresponding (anterior) and the op-
posing (distal) surfaces of the lower canine are not grooved;
moreover, both the ground (distal and lingual) surfaces have
only a thin coat of enamel. This arrangement favors sharpening of
the lower canine apex, yet maintains the continuity of buccal and
mesial enamel on the upper canine. This is vital to its piercing
and cutting efficiency. Although the apices of both canines are
sharpened, it is the upper canine, with its acute blade sharpness,
(entirely absent in the lower canine), which is the dominant
weapon. The action of the upper canine in the baboon (reflected
in the strong nuchal musculature; Every, 1965) is in slashing
in a downward, backward, and inward direction. Even when
biting, this distally sharpened blade, shaped as a sickle (see No.
5 above) and oriented posteriorly, remains exposed; it is the
crown’s mesial surface which is covered by the overlapping lower
canine. The upper canine serves as an efficient weapon rather than
as a grasping organ’.

2 The postglenoid tubercle is, significantly, absent in many mammals, par-
ticularly the rodents and lagomorphs. Its absence in the pig is part of this
animal’s specialized temporomandibular joint which permits retrolateral
mandibular action — an adaptation for sharpening the lower canine tusk.

3 This is in marked contrast to the typical carnivore condition where canines,
aided by a large diastema, double in function as weapons and grasping
tools, and although their rounded distal surfaces may show vertical ridges,
these are slight and do not impair the vital — for a carnivore — grasping
advantage of the relatively blunt, hook-shaped walls.

POSTILLA

14

Fic. 4. Upper left molars of a male human native of New Guinea.

TEETH SHARPNESS IN MAN AND PRIMATES 15

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HOMINIDS

Figure 4 shows some of the features of wear and sharpening
processes in the molars of Homo sapiens. The third molar, with
six to eight years’ less use than the second molar, shows a clear
picture of wear principally from the tooth-to-tooth contact wear

16 POSTILLA

of thegosis. This wear is the result of two discrete mandibular
movements which have produced two equally discrete sets of
facets (some of these are high-lighted in the photographs); the
facets form ridges and grooves which meet at distinct boundaries,
The transverse boundaries (vertical in the photograph) are
formed by a left lateral (extrusive) thegosis action. In this action
the mandible rotates about an axis at the left postglenoid tubercle
(see discussion of Fig. 5, below), i.e., about a point posterior to
the mandibular condyle on the same (ipsilateral) side as the
teeth in the figure. This movement is in exact opposition to that
of the masticatory stroke in its terminal phase, and therefore must
occur apart from mastication. The oblique boundaries are formed
by a lateral mandibular movement, i.e., the mandible rotates
about an axis at the postglenoid tubercle on the opposite (con-
tralateral) side. As this movement is beyond the terminal phase
of a masticatory stroke, the oblique facets are also fashioned
apart from mastication.

Wear from abrasion (in this instance, the friction of food) is
predominant in the second molar because it has had from six
to eight years’ more use than the third molar; there is, however,
a small thegosis-facet on the remnant of the distal surface of the
distopalatal cusp (hypocone). Mesial to this distal thegosis-facet,
with its discrete transverse ridge, are irregular, but still trans-
versely oriented, lines. These are the result of friction scouring
by exogenous material. In contrast to the thegosis-facet, the more
heavily abraded remainder of the occlusal surface of this second
molar has no precise occlusal conformity with an antagonist.

Three other key features are to be seen on this second molar:

a) The prominent sharp edge to the buccal cusps, from which
the chewing table slopes down to the less prominent palatal cusps
(the incline is more apparent in a mesial or distal view). This
buccal edge is (relative to the movement of the lower molars)
the leading (cutting) edge of the upper molar; it forms a crest
on the tooth’s vertical, relatively flat, buccal surface. In the lower
molars the corresponding leading edge and vertical, relatively
flat, surface is lingual.

b) The worn concavity of the palatal aspect of the buccal cusps.
This feature, which helps to maintain the sharpness of the lead-
ing edge, is formed as a result of specializations in the temporo-

TEETH SHARPNESS IN MAN AND PRIMATES 17

mandibular joint, in the mandibular symphysis, and in the
proportions of the basal skull, maxillae, and mandible.

c) The remnant of the oblique ridge connecting the distobuccal
cusp with the mesiopalatal cusp. Midway along the palatal section
of this ridge is a small area of exposed dentine. With progressive
wear from the friction of masticatory action other areas of dentine
are exposed and gradually increase in size, and because dentine
is a softer substance these areas become hollowed out. This
phenomenon is characteristic of most mammals and provides a
most important adaptive feature in allowing additional enamel
cutting edges to appear (i.e., around the boundaries of the
abraded dentine), thus maintaining — even enhancing — the
sharpness of the tooth throughout its life.

Figure 5 shows some of the features of wear and sharpening
processes on the incisors and canines of Homo sapiens. The
principle reason for inclusion of this figure is to demonstrate the
incorrectness of the hypothesis that states that excessive tooth
wear in civilized man is the result of a “heavy bite”. This hypo-
thesis also often includes the proposition that heavy wear may
have atavistic components, occurring more frequently in cul-
turally primitive races (the traditional example given is the
Australian aborigine ).

Heavy wear is most often accounted for (e.g., Zuckerman,
1958) by assuming that use (age) wears hominid teeth down
from their tips, and that they become flat and blunt as a result.
Fig. 5, however, clearly shows a picture of wear which is domi-
nated by thegosis, with the remaining loss of tooth substance
resulting from decay, abrasion, and erosion. There is also evi-
dence that thegosis occurs on distinctly separate occasions from
incision and mastication, i.e., in the absence of exogenous ma-
terial. Furthermore, it is possible to determine that the mandibular
stroke producing thegosis is both oblique to the incisive stroke,
and extrusive. Its action is in contrast to that of the terminal
phase of incision, which is directly retrusive. This is shown by:

1) The thegosis ridges on the right (left side of the picture)
canine, and (indistinctly) on the right central incisor. These
thegosis ridges form arcs which are concentric with the trans-
verse ridges on molars situated on the same side (ipsilateral),

POSTILLA

Fic. 5. Three remaining incisors, two canines, and one premolar from the
upper jaw of a Caucasian male New Zealander.

19

TEETH SHARPNESS IN MAN AND PRIMATES

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and with the oblique ridges on the opposite (contralateral) side

of the dental arch.

2) The gold-alloy restoration of the mesial incisal angle of the

left lateral incisor which has the appearance of being repeatedly

20 POSTILLA

beaten. The wear on all the anterior teeth shows that their oc-
clusal relations to the lower anterior teeth are now relatively
edge-to-edge; there is evidence of an overjet but no overbite. If
the current notion were true — that normal wear on anterior
teeth is a result of incision — this action would be expected to
produce a palatal rolling over of the malleable alloy, since the
terminal phase of incision is a retrusive mandibular stroke (par-
ticularly in this edge-to-edge specimen where there is no over-
bite). But the alloy is rolled to a labially projecting ledge. It has
been hammered over to produce a ledge, as in a rivet. This can
only have occurred under an action of forceful occlusion — an
action distinct from and incompatible with incision or mastica-
tion (Every, 1965).

3) The flat incisal facet on the right central incisor. Especially
when there is a normal incisor overbite and overjet relationship,
this characteristic flat formation can be caused only by a mandi-
bular movement which extensively crosses over the two rows
(upper and lower) of anterior teeth. That is, the row of lower
anterior teeth must cross over, and become diagonally related to,
the row of upper anterior teeth. This is actually achieved by
extensions of the same movement which grinds facets on the
molars. (See evidence outlined in discussion of Fig. 4.)

4) The angle that a typical upper incisal facet makes with the
vertical labial surface is about 76°. (Although this is not apparent
in the figure, the incisal facet is inclined palatally). The fallacy
that these teeth wear blunt is exposed by noting that the flat
facet is the bevel to the sharp, leading, labial edge. In the lower
teeth, the sharp, leading, cutting edge is the lingual boundary
of the incisal facet. In contrast to man, the baboon’s incisors (see
Figs. 2 and 3) are more likely to wear blunt; but this is an abra-
sive bluntness: the baboon cannot grind bevelled facets on its
incisal edge — any possible crossing over of occluding upper and
lower rows of anterior teeth is prevented by the baboon’s long
canines and jaw proportions. Furthermore, relative to the pongid
condition and, by inference, that of the hominid’s long-canine-
toothed ancestors, the lingual enamel which forms the sharp
cutting edge of lower incisors and canines is thickened. Although
the corresponding sharp edges of upper incisors and canines are
labial, the palatal enamel of these teeth is also thickened. This,
however, is to the advantage of the stage of wear when dentine

TEETH SHARPNESS IN MAN AND PRIMATES 21

becomes exposed and hollowed out by abrasive wear (see dis-
cussion of Fig. 4). When this occurs the thickened palatal enamel
provides an additional cutting edge, and is also subject to
thegosis. This feature, of course, correspondingly occurs, but in
the reverse relationship, in the lower teeth; the new cutting edge,
which appears when dentine is exposed, forms a crest on the
labial enamelt.

DIscussIon

When man fabricated his first stone chisels, he found an optimum
angle of bevel which was advantageous in cutting and durability.
He had probably learned much from his use of other animals’
teeth as chisels. In his use of either stone or teeth he would not
have directly applied the flat bevel in the cutting stroke. Yet this
inefficient action is imagined by some to occur in the functioning
of man’s own teeth; the flat facet on incisors of Homo sapiens is
called the “incisal edge”, whereas the real incisal edge is the
boundary of the facet (the edge of the bevel), not its flat surface.

Hundreds of millions of years before man fabricated a function-
ing blade — a blade which, when blunted by use, was resharpened
— natural selection had achieved numerous specializations for
the maintenance and function of sharp teeth. It is a common-
place that, failure to discern a specific function for an anatomical
feature does not imply that it is truly functionless. The “function-
less” lateral incisors of the lagomorphs, for example, actually
serve as specialized sharpeners; this is their dominant, if not
exclusive, function (Every, 1967). By a stroke of the mandible at
right angles to the incisive stroke an extremely fine “razor-edge”
to the lower central incisor is honed’. The precision of this action
in lagomorphs becomes even more apparent when it is seen that

4 This system of consecutive blades oriented relatively in a horizontal plane
is seen more specifically in the selenodont molars of herbivores. Selenodont
teeth only come into full function when dentine is exposed and hollowed
out and the precise shearing blades (not rough, grinding surfaces) are
brought into accurate alignment by thegosis — a genetically programmed
behavior as in most other mammals. A characteristic selenodont molar then
presents pairs of four consecutive, sickle-like blades, and each of the eight
blades is precisely oriented to face the direct line of the masticatory stroke.

5 The leading edge of the lower central incisor of lagomorphs and rodents
in particular, and of most mammals in general, is labial.

22 POSTILLA

the striations of the remainder of the extensive bevel are cut by the
twin gouges which crest the incisal edge of the labial surface of the
upper central incisor. This action involves a forceful protrusive
mandibular stroke. Thus the two sets of striations on each bevel
of the lower incisors are at right angles to each other and cannot
have been caused either by grasping or by cutting of exogenous
material. In marked contrast to this wear on the lower incisors is
the characteristic wear from abrasion on the upper incisors, which
likewise contributes to sharpening. (A detailed analysis of this
will be presentéd in a later paper. )

The significance of all this to man is that it clarifies not only
human dental morphology but also his innate tooth-sharpening
behavior. It is important to emphasize that, for an animal to kill,
two basic components must exist: a) the anatomical weapon, and
b) the physiological mechanisms to activate it. These two com-
ponents, though separate conceptually, are in functional terms
inseparable. Aggressive behavior is unlikely to evolve, ie., be-
come genetically programmed, when there is no _ biological
weapon. Learned improvements in aggressive behavior, more-
over, are still less likely to occur in such a case.

In this light the hypothesis that the earliest hominid was
biologically defenseless, and that improvements in the brain and
hand produced aggressive behavior and cultural weapons “in
compensation’, is unconvincing.

The important part played by teeth in the evolution of most
mammals is generally recognized, and particularly in the evolu-
tion of the hominids the teeth are considered to have played a
vital part. But, paradoxically, this part is considered to be a
negative one — the weapon is thought to have disappeared. In the
light of the phenomenon of thegosis, however, these erroneous
notions can now be discounted. The clarification of man’s evolu-
tion concerns not only the evolution of his biologically inheritable
features but also the evolution of his culture, which he does not
inherit but acquires.

These subsequently learned cultural improvements in killing,
however, today so occupy our attention that we tend to be ob-
livious of any unlearned components which may be programming
our behavior. Any suggestion that innate aggression is a com-
ponent of, human behavior we tend to regard as objectionable, as
“animal” and “inhuman”. And, at best, when its existence is recog-

TEETH SHARPNESS IN MAN AND PRIMATES 23

nized, we hopefully believe that we may effect its control by
avoidance, redirection, and sublimation.

It seems that the phenomenal success of our (learned) cultural
achievements has caused us to fail to perceive the importance of
precultural improvements in learning as facilitating genetical
programming of behaviors. The increased capacity to learn is, of
course, a biological improvement. The cultural improvement
(particularly the overwhelming advantage of the accumulation
of experience through the capacity to speak) is merely an ex-
tension of the same advantage. It is as if learning were the leaven
to the dough: leaven does not constitute the food; it merely im-
proves the food and enhances its ultimate utilization.

Interpretations of these hitherto unsuspected phenomena re-
quire reappraisals of both the palaeontological and the recent
record of reptilian and mammalian life. In the case of man, the
reappraisal includes not only aspects of his morphological charac-
teristics, of his behavior and social organization (of those aspects
which are genetically determined), but also of his culture — the
feature which distinguishes him from other animals.

Furthermore, these interpretations are antithetical to many
current ideas of the selective forces which produced man. They
suggest, not merely a modification, but a reversal of ideas, par-
ticularly related to man’s aggression. Certain notions, such as
those aptly stated by Washburn (1960), become untenable:
“The skull of the man-ape has transferred to its hands the func-
tions of seizing and pulling, and this has been attended by a
reduction of its incisors. Small canines and incisors are biological
symbols of a changed way of life; their primitive functions are
replaced by hand and tool.”

Such misinterpretations of the relative grasping capacities of
the dentition of higher primates can be disproved in the light of
phenomena already well documented. For example: the assump-
tion that there had been a transfer, in the “man-ape’, of functions
of seizing and pulling (to hands from teeth), suggests that no
such transfer had occurred in the ape itself; yet I have seen no
report of anyone having observed an ape transporting its young
by its teeth, let alone supporting its own weight by grasping with
its teeth; nor any report of any ape seizing and grasping an ad-
versary, or a struggling prey, by its teeth. In contrast, the seizing
and pulling capacity of man is demonstrated by his capacity to

24 POSTILLA

seize and hold an adversary by his teeth, and by the circus per-
former’s act where the force from the combined weights of two
individuals, plus the centrifugal force from their swinging as a
pendulum, is supported by the strength of one dentition.

Such a capacity is made possible in man by the improved
leverage of his jaws as a result of the shortening of the snout. It
is a feature which has allowed the reduction in the size of the
“masticatory” muscles without loss of force available at the
teeth (Every, 1965).

The grasping capacity of the sub-human primate dentition is
impaired, furthermore, by the acute sharpness of the distal edges
of the upper canine teeth. These canines do not double in func-
tion as grasping tools as do the carnivore’s; they are specialized
as slashing weapons, and are more formidable than the carni-
vore’s, which are poorly adapted for slashing. The grasping
function of a carnivore’s canine teeth, however, is enhanced by
the bluntness of the walls of their crowns and by diastemata
posterior to, and thus exposing, their bluntness. Although the
slashing advantage is to the detriment of grasping, this is of little
consequence to a primate; the arboreal ancestors of all primates
show no evidence of their teeth having had dominant grasping
functions.

Washburn’s statement quoted above that the transference of
functions had been “attended by a reduction of [the man-ape’s]
incisors” suggests that there is a correlation between these factors,
i.e., large incisors are advantageous for seizing and pulling. Yet
the dentitions of characteristic carnivores, specialized for seizing
and pulling, have minute incisors, relatively a fraction of the
size of those of any anthropoid, including man.

Until the tooth-sharpening hypothesis first appeared (Every,
1960), there was, apparently, no suggestion that the hominid’s
short canine teeth gave direct evolutionary advantage per se
to their possessor. On the contrary, evolutionary theorists widely
and confidently held that the canines had “regressed” and had
become “weak and inefficient”. Moreover, they believed that, in
use, the canines “wore down from their tips” and soon become
“flat and blunt” — as did the hominid incisors, premolars, and
molars (e.g., Zuckerman, 1958; Leakey, 1960; Le Gros Clark,
1962). This was taken as evidence that biological progress in
early hominids had occurred in other features, particularly in the

TEETH SHARPNESS IN MAN AND PRIMATES 25

brain and in the use of the hand, and that these morphological
and cortical adaptations were “necessary” to offset the disadvan-
tages of short, small, and weak teeth. This theory suggests that
Ramapithecus, and certainly the earliest Australopithecus, must
have had sufficient intelligence to use tools, if not to make them.
It also suggests that this capacity must have been developed
sufficiently to compensate for the absence of dental weapons dur-
ing millions of years of what must on this theory have been ex-
tremely precarious existence. It is clear, however, that during
the Pleistocene there was no lack of sizable predators. Early
hominids were not fast runners; they had, as a result of biped-
alism, a reduced climbing capacity, a low procreative rate, a
reduced sense of smell and hearing, probably a lengthening
period and increasing intensity of infant dependence, a small
brain, no capacity to transmit accumulated experience by speech,
and a comparatively limited capacity to transmit any experience
by signals. In addition, evolutionary theorists widely held that the
teeth were inefficient, not only as weapons, but also as tools of
mastication, incision, and grasping. Despite all these disadvan-
tages and hazards, early hominids were not overwhelmed; they
managed to survive the long and critical epoch unprotected by
the intelligence which is concomitant with a capacity for true
speech, and supposedly unprotected by an effective biological
weapon. This supposed achievement was made the more re-
markable by the absence of one other significant and fundamental
advantage which a biological weapon gave and still gives today.
This advantage, moreover, is one which no artificial weapon pre-
sents or can ever present. It is that a biological weapon is built
in; it cannot be dropped, mislaid, or lost, nor can its possessor
be dispossessed, or taken by surprise “unarmed”; in an emergency
it is always immediately available.

CONCLUSIONS

The evidence for the hypothesis of man’s biological killing capac-
ity can be studied in three aspects:

1. The anatomical weapon, i.e., the biological instrument of

killing.

26 POSTILLA

2. The permanent (inflexible) genetical determinants of killing
behavior.

3. The transient (flexible) learned adjustments to the genetically
programmed behavior.

The simple but fundamental hypotheses of tooth-sharpness and
tooth-sharpening processes (thegosis) throw further light on the
evolutionary events leading to man. The evidence suggests that
in hominid evolution there was no period of defenselessness; the
increased number of attrition-sharpened teeth, introducing the
new and specialized feature of anterior shearing blades with a
capacity for “segmentive biting”® (Every, 1965), was defensively,
predatorily, and aggressively advantageous. It was especially
advantageous when coupled with an increasing capacity to know
and to signal when and where not to use the primary biological
weapon.

The origin of the short canine by paedomorphic novelty is
generally accepted (e.g., Koestler, 1966). Also accepted is the
significance to evolutionary processes of paedomorphic novelty,
which is not so much the initial event itself but that selection, here
(operating on a more plastic, less committed stage) allows a
sudden advance in a new direction. It is this sudden (now
gerontomorphic) advance which, in the hominid short-canine
condition, evolutionists have overlooked.

Once this process advanced to the stage where the relatively
sudden appearance of a chin altered the shape of the oral cavity
and the face, these prior advantages permitted the evolution of
the further, and overwhelming, advantage of a capacity to speak
(Every, 1965).

But along with this overwhelming advance in the capacity to
learn came an inevitable disruption of the biologically balanced
(unlearned) controls: the physiological reactions evoking and
attenuating agonistic behavior. This disruption clearly resulted
from two principal causes:

1. The introduction of the exogenous (artificial) weapon im-
mediately gave lethality to the secondary, and fundamentally
non-lethal, weapon — the hand. Being non-lethal (except by

6 I use this term (Every, 1965) to describe the separation and removal (in
one action) of a large chunk of material.

TEETH SHARPNESS IN MAN AND PRIMATES 27

accident) the hand is subject to relatively scant intraspecies
(ritualized ) control of its use, whereas the lethal primary weapon
— the teeth — has its intraspecies use strictly controlled. That this
control of agonistic behavior should be built in, i.e., automatic,
stereotyped, species-specific (universal) and unlearned, is
clearly advantageous. For, in agonistic behavior, especially when
the weapon is sexually dimorphic (Every, 1965) in the adult,
there is no margin of time allowance for learning: uncontrolled,
random, trial-and-error actions of a built-in lethal instrument
would rapidly lead to chaos. And if the species were to survive
there would need to be strong selective pressure against learning
the controls. This feature accounts for the restricted use of the
primary weapon today, i.e., it is restricted in frequency of oc-
currence, force of bite, and selection of site; seldom are teeth
used, seldom is a bite segmentive, and it is almost unknown for it
to be sited at the fatally vulnerable neck.

2. The introduction of the exogenous weapon immediately ex-
tended the distance from the attacker in which the adversary
could be injured. With sophistication of the weapon and pro-
gression of the distance this concomitantly and progressively
dilutes the efficacy of the biological perceptions (seeing, hearing,
touching, smelling, tasting) through which the controls are in-
strumented. Today, a kill can be made in circumstances devoid
of direct biological perception. There is, as a result, almost com-
plete disruption of the biologically balanced controls. The evoker,
inevitably and tragically, is favored.

Genetical determinants of an animal’s behavior can be con-
sidered permanent in the sense that they evolve in relation to
permanent, or even relatively permanent, environmental features
such as seasons, tides, bisexual reproduction, prey-food, and so
on. Even less consistent but reasonably cyclic features such as
droughts and famine can, in this sense, qualify as “permanent”.
When this permanency, i.e., inflexibility, is applied in the con-
sideration of innate controls of killing behavior, whether it be
predation, defense from predation, or intraspecies decimation in
caged (e.g., overpopulation) circumstances, it is clear that
especially for the higher animal, the capacity to learn is crucial.
Because of the ever present non-permanent (i.e., transient) en-
vironmental circumstance, learning allows adjustment to the

28 POSTILLA

timing and the movement of the innate sequence, and thus facili-
tates its consummation. In this view the capacity to learn is an
inherent concomitant of the unlearned determinant of behavior
— not a replacement.

Direct and comparative morphological studies of man’s denti-
tion, together with dental’, psychological®, psychosomatic’,
vertebrate paleontological’®, and paleoanthropological™ studies
of the conditions which evoke his sharpening behavior, suggest
that man sharpens his teeth as weapons; he does not directly
sharpen his teeth as tools of mastication or grasping. His teeth
are adequately maintained as sharp tools in the normal frequency
of their preparation as weapons.

The tooth-sharpening phenomenon uniquely presents a discrete
entity of innate behavior in man which appears to be unclouded
by argument that it could be learned. It, moreover, elucidates un-
learned determinants of man’s agonistic behavior, and strongly
suggests that a partial measure of the (pre-cultural) biologically

7 Experimental studies in loss (other than by caries and accident) of tooth
substances; force and direction of mandibular action; i.e., using and de-
vising techniques of conservative restoration, periodontia, therapeutic
splinting and orthodontic appliance, full and partial prosthesis (allowing
unique control of nocturnal action). (Every, 1939, 1949, 1960, 1965;
Craddock and Johnston, 1961; Reed, 1968).

8 Communication by facial expression, particularly (innate) signals and their
auditory concomitants. Communication by olfaction. Relationship of
(innate) unlearned and (cultural) learned determinants of behavior.
Behavioral and morphological changes permitting the evolution of the
capacity to speak. Pain thresholds in fight and flight. Use of tooth-sharpen-
ing as a marker of stress. (Every, 1960, 1965; Hughes, 1969).

9 Oral symptoms of repression. Presentation of the Syndrome of extreme
mandibular movements. Experimental therapy. (Every, 1946, 1960; Crad-
dock and Johnston, 1961).

10 The reptilian-mammalian transition. Mesozoic mammalian dentition, par-
ticularly in respect to the origin of the talonid and protocone, the contact
point, the transition of cusp-interdigitation to cusp-apposition and
opposing-convexities of shearing blades, and the significance to sharpness
of differentially hard tooth-substances. Post-Mesozoic radiation. (Every,
“Reinterpreting the dentitions of Amphitherium and Peramus,” paper read
at the Symposium of Vertebrate Palaeontology and Comparative Anatomy,
London, 1967).

11 The transition from vertically sharp slashing blades to horizontally sharp
shearing blades. Evolutionary advantages of the hominid short-canine
condition. Relationship of changes in the dentition to brain enlargement.
(Every, 1965, and paper cited in Footnote 10).

TEETH SHARPNESS IN MAN AND PRIMATES 29

balanced controls is restorable. But this only when man’s
(flexible) culture is made to harmonize with his (inflexible)
biology.

ACKNOWLEDGMENTS

Dr. Elwyn L. Simons (Peabody Museum of National History,
Yale University) has given much assistance in the preparation
of this manuscript, for which I am deeply grateful. Others also
deserve my sincere gratitude; they gave facilities, access to col-
lections and clinical material, discussion, service, encouragement
and support. Too numerous to mention individually, they are
spread out among universities, museums, hospitals, miscellaneous
institutions, private practices, and private individuals, in New
Zealand, Australia, Germany, England and the U.S.A. To Pro-
fessor Dr. Walter G. Kiihne, Geologisch Palaontologisches In-
stitut, Freie Universitat, Berlin; Dr. Fraser McDonald, Medical
Superintendent and Director Research Unit, Kingseat Psychiatric
Hospital, Papakura, New Zealand; Dr. Ronald Singer, Dept. of
Anatomy, University of Chicago, and Dr. Albert A. Dahlberg,
Dept. of Anthropology and Zoller Memorial Dental Clinic, Uni-
versity of Chicago, I give special thanks for the additional assis-
tance of short-term posts in their departments. The whole basis
of this work was formulated in a psychiatrically oriented private
dental practice in Christchurch, N. Z. In the final stages, crucial
financial support, most of it anonymous, was given by private
well-wishers. For this I am extremely grateful. Part of the prepar-
ation for publication was later supported by grants from the
Canterbury Medical Research Foundation, N.Z.; The Golden
Kiwi Lottery Committee for the Promotion of Medical Research,
N.Z.; and The Explorers Club, New York, U.S.A.

LITERATURE CITED

Anderson, D. J. 1955. Physiology of mastication. Dent. Pract. 5: 389-394.
Craddock, F. W., and J. W. Johnston. 1961. A new explanation of facial
pain. N.Z. Dent. J. 57: 153-161.
Every, R. G. 1939. Contact points. N.Z. Dent. J. 35: 287-291.
1946. The problem of the thumb-sucker. N.Z. Dent. J. 42:
178-183.

30 POSTILLA

1949. The elimination of destructive forces in replacing teeth
with partial dentures. N.Z. Dent. J. 45: 207-214.
1960. The significance of extreme mandibular movements.
Lancet 2: 37-89.
__ 1965. The teeth as weapons; their influence on behaviour.
Lancet I: 685-688.

Hughes, R. N. 1969. Social facilitation of locomotion and exploration in
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Jankelson, B., G. M. Hoffman, and G. A. Hendron. 1958. Physiology of
the stomatognathic system. J. Am. Dent. Assn. 46: 375-386.

Koestler, Arthur. 1966. Biological and mental] evolution — an exercise in
analogy, p. 95-107. In Knowledge among men. Simon and Schuster,
Washington, D.C.

Leakey, L. S. B. 1960. Finding the world’s earliest man. Nat. Geog. 118:
240-435.

Le Gros Clark, W. E. 1962. The antecedents of man, 2nd ed., University
Press, Edinburgh. 388 p.

Reed, J. W. 1968. A new design for a partial lower denture. N.Z. Dent. J.
64: 23-26.

Robinson, J. T. 1956. The dentition of the Australopithecinae. Transvaal
Museum (Pretoria) Memoir 9. 178 p.

Tomes, C. S. 1923. A manual of dental anatomy, human and comparative,
8th ed. J. and A. Churchill, London. 547 p.

Washburn, S. L. 1960. Tools and human evolution. Sci. Am. 203: 3-15.

Yurkstas, A. A., and W. H. Emerson. 1954. Study of tooth contact during
mastication with artificial dentures. J. Pros. Dent. 4: 169-174.

van Hooff, J. A. R. A. M. 1962. Facial expression in higher primates, p.
97-125. In Evolutionary aspects of animal communication. Symp. Zool.
Soc. Lond. 8.

Zuckerman, S. 1958. Correlation of change in the evolution of higher
primates, p. 300-352. In Huxley, Julian, A. C. Hardy, and E. B. Ford
[eds.] Evolution as a process. George Allen and Unwin, London.

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