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Palaeontology of the human dentition ten structural stages in the evolution of the cheek teeth.

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American Journal of Physical
............... 404
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OF THE Dryopithecus PATTERN.
OF M9, Loss OF THE Dryopithecus PATTERN,
. . . . . . . . . . 426
The human upper and lower jaws in normal occulsion are part of a
mechanism, the wonder and beauty of which are nowhere more fully
appreciated than by the members of this Congress. Were these complex
relations of upper and lower cusps and surfaces created perfect, a t one
stroke, have they always been as we now find them, or have they attained their present status after a long and gradual series of modifications from more simple beginnings? And how far, in the present imperfect state of our knowledge, can we begin to discern an orderly succession of stages in the evolution of the dental apparatus?
'Read before the First International Orthodontic Congress, New York City,
August 16-20, 1926.
AM. J. PRYS. ANTHROP., 1926, Val. IX,
Since the evolution of the orthodontic relations of the human upper
and lower teeth is thus clearly a phase of the general study of the evolution of man, it may be fortunate for me that I can say what I have t o
say now, before a new constitutional amendment m&es the whole
topic legally taboo and our next Congress in this country subject to
closure by agents of the Federal Government. At least in New York at
the present time it is still reasonably safe to lecture in public on the
evolution of man, but all over this broad land fiery zealots, with the
energy of the Hebrew prophets, are calling upon their followers to throw
down the idols of the evolutionists. Thundering into loud speakers from
a thousand pulpits, they brand the scientific doctrine of the evolution of
man as a flimsy hypothesis. Professing their desire t o purge and purify
true science, they denounce as “pseudo-scientists” all university professors and laboratory workers who dare to teach anything contrary to
the traditional belief that Adam was literally made out of the dust of the
ground and Eve from one of Adam’s ribs.
But it is vain for them t o kick against the pricks. They may make it
illegal to teach evolution in state-supported universities, colleges and
schools, but they cannot stop the endless discoveries of facts that have
made the gradual evolution of the body and mind of man one of the
most reasonable and fully documented inferences in the whole field of the
biological sciences.
The radio orators naturally feel no embarrassing doubts as to their
own competence to dispose of the subject of the evolution of man after
they have read a few popular works on the subject and have got hold of a
few citations of so-called authorities who have expressed opinions
supposed to be damaging t o this or that point of the evolutionist’s argument. The scientist, however, is necessarily more modest. He realizes that thousands of important details of the story of human evolution
must ever remain in doubt, and that at the present time science has
reached no brief clear-cut picture as to the general mechanism of evolution, that is, the precise manner in which Natural Selection, the Environment, and other factors finally overcome the conservative forces of
heredity and induce a definite shift from one structural level t o the next.
The scientist, moreover, recognizes that there is no royal road to a firsthand knowledge of the broader aspects of the origin and evolution of
man. He perceives that the evolution of man is only a special case of
the evolution of animals in general, and so he must first labor unceasingly to disentangle and classify the myriad twigs and branches of the great
tree of vertebrate life. Fortunately a small army of investigators,
namely, students of systematic zoology, comparative anatomy and
palaeontology have long been fruitfully engaged in this field. Thousands
of studies in the anatomy and classification of recent and extinct vertebrates, all quite unknown to the pulpit and public press, have supplied
the outlines of a tolerably detailed picture of the main branches and
twigs of vertebrate life and of the principal stages in the ascent from fish
to man. From a wide synthesis of such studies it has been learned that
the evolution of the vertebrates may be sketched under the following
heads : first, the successive changes in the locomotor apparatus; secondly,
the evolution of the respiratory, vascular and alimentary systems, the
last including the masticatory apparatus; third, the evolution of the
reproductive organs and methods; fourth, the evolution of the coordinating organs of the peripheral and central nervous systems.
Scores of millions of years ago in Devonian times we meet the relatives and forerunners of the fishes with limbs and lungs, the hardy
pioneers that first struggled up out of the water on to the land. Next, in
the time of the Coal Measures, we find both the varied amphibians not
yet wholly freed from the water and the earliest common ancestors of
the reptiles and of all higher types. Coming on to the Permian and
Triassic ages, each many millions of years in duration, we encounter
several grades of organization among the mammal-like reptiles and some
of these almost deserve to be callFd pro-mammals, so progressive are
they in locomotor and masticatory apparatus. Proceeding to the
Jurassic ages we come upon true mammals, with the beginnings of the
tritubercular type of molar teeth ; and quite recently Cretaceous
Placental Mammals have been discovered in Mongolia, which as we
shall see, supply a long-needed link in the evolution of the molars.
Passing to the Lower Eocene we find early representatives of that great
order the Primates, which was destined eventually to give rise to man,
and which even at that far-distant epoch was already distinguished by
its superior development of eye and hand and brain. In the Lower
Oligocene of Egypt we meet the first ancestors of the branch which was
later to subdivide into the anthropoid apes on the one hand and the
races of man on the other, and in the Miocene we encounter traces of
various true anthropoids, among which are some that, according tomy
interpretation of the evidence, stand in or quite near to the common
stem of man and the modern anthropoids. In the Lower Pliocene our
anatomical records, apart from one or two debatable finds, are a blank,
but in the Pleistocene we have in gradually ascending levels, first the
Pithecanihrops, a true man although in a low stage of development,
secondly, the Dawn Man or Eoanihropus of England, thirdly, the Heidelberg man, fourthly, the Neanderthal man, and finally the Cro-Magnon,
true Homo sapiens, and his successors of the late Palaeolithic, Neolithic,
Copper, Bronze and Iron Ages.
At each stage of this long line of ascent we must analyse the known
osteological and dental characters and endeavor to discover what
forms lie in or near the line of human descent and which ones are leading
off in various directions to the specialized side lines.
The ten structural stages in the evolution of the human cheek teeth
described above are all represented by known forms, extending in time
through ascending geological horizons from the Permo-Carboniferousto
the Recent. Not one of them is in any sense hypothetical. The drawings depicting the verifiable facts of observation have been made with
the most painstaking care by a skilled artist, Mrs. Helen Ziska, under
the author’s close and constant supervision.
With such a general sketch before us we may take up the special
subject of the present paper, namely, the pakeontology, or development
in past ages, of the human dentition. And first let us pass over all the
earlier stages in the evolution of the teeth, when in the ancestors of the
air-breathing fishes the tough skin around the mouth, bristling with
small shagreen denticles, through a change of function became useful in
holding on to small living prey. We must begin rather with that prlnitive reptilian stage represented by the small pelycosaurian or therornorph reptiles of the Permo-Carboniferous (Fig. 1, Ia), in which the
upper teeth were simple and laniariform with compressed enamelcovered crown, each tooth with a single root embedded in a socket.
In such a reptile the teeth of the premaxillary and maxillary bone of the
upper jaw overhang and alternate with the similar teeth of the lower jaw.
This was the beginning of relations that still hold to some extent in the
human jaws. It need hardly be said that, in such a simple mechanism,
when the lower jaws are drawn upward by the action of the temporal
muscle-mass the food is both pierced and sheared by the pointed, sharpedged teeth.
The next stage of evolution is illustrated in the Pro-mammal or
Cynodont reptiles of the Triassic of South Africa. For in them (Fig. 1,
Ic) the dentition has become differentiated into incisors, canines, premolars and molars, and the available evidence indicates that the succession of the teeth was reduced to two sets, corresponding to the deciduous and permanent series of mammals. In many genera of the
Cynodonts both the upper and the lower cheek teeth were all compressed
and shearing, with one or more accessory cusps on the margins of the
shearing blade, but in some other genera of Cynodonts constituting the
family Diademodontidz (Fig. 1, II), while the original tips of the upper
still overhung the tips of the lower cheek teeth, the upper molar teeth in
the middle of the tooth row were extended transversely on the palatal
side, while the corresponding lower molar crowns had acquired a subcircular outline with a transverse ridge across the summit of the crown.
I n such a dentition the upper and the lower teeth are markedly unlike,
the uppers being transversely oval with a tuberculate transverse ridge,
the lowers much smaller and circular but also with a transverse ridge.
These Diademodont Cynodonts, if not themselves the direct ancestors
of the mammals, were a t least nearly related to such ancestors; they
already exhibited the beginning of the power of developing a cuspidate
molar crown, which became so marked a characteristic of the mammals
themselves, and they were also extraordinarily mammal-like in many
significant morphological characters of the skull. It is therefore significant that in all the Cynodonts the original tip of the molar crowns
lies on the outer or buccal side in both the upper and the lower teeth; and
that the upper and lower cheek teeth fall in inter-locking series such that
each lower articulates with two uppers; also that the basal swellings on
the lingual side of both upper and lower molars constitute the main overlapping parts of the crowns.
The exact stages between the pro-mammal with compressed shearing
teeth and the pretritubercular molars of Cretaceous mammals are still
in doubt, notwithstanding the famous theory of Cope and Osbom as t o
the origin of the upper and lower triangles by rotation or migration of the
accessory cusps outward in the upper and inward in the lower teeth.
Unfortunately our present evidence does not permit us to confirm this
theory, but on the contrary it indicates that the tritubercular molars of
later mammals did not originate in this manner. A fundamental postulate of the Cope-Osborn theory was that the original tip of the ancestral
reptilian tooth gave rise in the mammals, in the upper molars t o themain
Fig. 1
FIG.1. Ten Structural Stages in the Evolution of the Human Dentition, from
Ascending Geological Horizons.
I. Substage a. Permo-Carboniferous. Mycteroseurus, primitive Theromorph
Reptile. After Williston.
Substage b. Permian. Scylacosaurus, primitive Mammal-like Reptile. After
Substage c. Triassic. Cynognathus, advanced Mammal-like Reptile. After
11. Triassic. Diademodon, advanced Mammal-like Reptile. Mainly after
Seeley. Occlusion diagram by author.
111. Jurasic. Pantotherian (primitive pro-Placental). Kindness of Dr. G. G.
Sim son. Occlusion diagram by Simpson.
I&. Cretaceous. Pre-Trituberculate, Deltatheridium. From the original
specimen. Occlusion diagram by author.
V.. Lower Eocene. Primitive Placental, Didelphodus. From the original
specimen. Occlusion diagram by author.
FIG.2. Ten Structural Stages in the Evolution of the Human Dentition (con’t)’
VI. Middle Eocene. Primitive Primate, Pronycticebus. m e r Grandidier.
Ocdusion‘diagram by author.
VII. Upper Eocene. Advanced Tarsioid Primate, Microclwrus. After Stehlin.
Occlusion diagram by author.
VIII. Miocene. Primitive Anthropoid Primate, Dryopithecus. Upper molars
mainly after Pilgrim; lower molars from type of Dryopithecus cautleyi. Occlusion
dia am by author and Milo Hellinan.
1%. Pleistocene. Primitve Man, Mousterian. From stereoscopic photographs
by Professor J. H. McGregor and from the published photographs by Weinert and
by Virchow (m3). Occlusion diagram by author.
X. Recent. Modern Man, White. From the original specimen. Occlusion
diagram by author.
internal cusp, which was therefore called the protocone, and in the
lower molars to the main external cusp, called the protoconid.
All the available evidence, both from the ancestral Cynodonts and
from the mammals of the Cretaceous and Tertiary ages, now indicates
that at least in the lines leading to the later mammals the supposed
reversal of the relationship of the upper 2nd lower original tips did not
take place, but that on the contrary the original tips remained on the
buccal side in both the upper and the lower teeth, while the inner basal
part of the crown became extended toward the inner side t o give rise to
the crushing portions of the crown.
Another postulate of the Cope-Osborn theory was that in the supposed
stage of “reversed triangles” the upper and lower molar crowns were
exactly alike in form; but it now seems clear that at a very early stage the
upper and lower molars became quite unlike, the uppers being larger,
roughly triangular in form, with large interdental embrasures into
which the small sharply triangular lowers fitted. At this stage (Fig. 1,
111) , represented by the Jurassic Pantotheria originally described by
Professor Marsh and now most ably restudied by Dr, G. G. Simpson,
the original tip of the ancestral reptilian tooth in the upper molar was
apparently represented by the main high cusp in the middle of the premolars and molar crowns, a cusp which was destined t o give rise later to
the mesiobuccal and distobuccal cusps (para-and metacones). From the
anteroexternal (mesiobuccal) comer projected a rounded buttress, the
parastyle, surmounted by a raised rim or cingulum, while from the
posteroexternal (distobuccal) comer projected the metastyle. The internal angle of the triangular base of the crown was formed by a rounded
protuberance terminating in a low cusp, the mesiopalatal cusp, or socalled protocone. The front side of the triangular basal section is
relatively short and more transverse in position, the posterointernal
(distolingual) side is longer and more oblique. Both these sides oppose
the principal moving blades of the lower molars.
The lower molar crowns of this stage consist of two radically distinct
parts, an elevated triad of cusps, the trigonid, and a small low posterior
extension, the incipient heel or talonid. An important difference from
the upper molars is that the homologue of the original tip is not central
but external in position. The cusps of the trigonid of the lower molars
in the Osbomian system of nomenclature are called protoconid (main
external), paraconid (anterointernal), metaconid (posterointernal), and
for convenience these names are retained, but with the express reservation that according to present evidence the cusps so designated are not
severally homologous with the similarly named cusps of the upper
If we reject, as seems necessary, the Cope-Osborn theory of the origin
of the upper and lower triangles by the folding in opposite directions of
simple triconodont teeth, and if we accept the evidence cited by various
authors and tending to show that in both upper and lower teeth the
original cusps remained on the outer or buccal side, then in the lower
molars the first appearance of the paraconid and metaconid as distinct
cusps must have been associated with the transverse, or obliquely transverse, widening of the crowns; and both these cusps arose in sibu, the one
at the anterointernal extension of the base of the crown and the other as
a swelling on the inner or lingual slope of the original tip or protoconid.
Meanwhile the so-called protocone or internal cusp of the upper molar is
simply a cusp or swelling on the inner or palatal extension of the crown.
It was not the original tip but it probably arose as far back as in the
Diademodont forerunners of the mammals. In the occulusion diagrams
(Fig. 1, 11, 111,IV) we can see the intimate functional relations of these
parts of the upper and lower molars.
The protocones of the upper molars became differentiated, we may
infer, at the same time with the metaconids of the lower molars. Even
in later mammals the metaconid blade shears in front of the anteroexternal blade of the protocone V, while the tip of the metaconid of the
lower molar grinds against the anterior side of the protocone of the
upper. In primitive insectivorous and carnivorous mammals the protocones are displaced forward, while the blade on the posterior slope of
the protocone shears past the blade on the antercexternal face of the
metaconid. This situation is already developed in the Jurassic Pantotheria (Fig. 1, 111).
The protocone of the upper molars very early came into functional
relations with another element of the lower molars, namely the talonid
or heel. In the predecessors of the mammals the upper teeth were set in
a crowded series and there were probably few or no interdental embrasures. Moreover the pressure of the lower jaw seems to have been chiefly
vertical and directly transverse, not obliquely transverse. The Triconodont mammals of the Mesozoic ages remained apparently in this stage
of evolution. But the Pantotherian mammals of the Jurassic (Fig. 1,
111) had already advanced to the stage where pressure of the lower jaw
in an oblique anteroexternal direction accompanied a forward displacement of the internal extension or protocone of the upper molars, with a
concomitant upgrowth of the paraconid and development of the paraconid blade, and finally with the opening up of the interdental embrasures.
Meanwhile the protoconid of the lower molars, from being directly
above the median forking between the anterior and posterior roots, had
shifted forward along with the protocone of the uppers, so that it finally
came to lie more above the anterior root, leaving room onitsposterior
slope for the backward growth of a new and highly important structure,
the talonid. This from the first impinged on the anterior slope of the
protocone of the next succeeding upper molar, but by further growth the
first cusp of the talonid, namely the entoconid, worked itself around
behind the tip of the protocone.
By this time each upper and lower molar consisted of two functionally
analogous parts: first, a shearing V and secondly, an overlapping heel.
The shearing V of the upper tooth was formed by the anterior and posterior slopes of the protocone, more or less connected respectively with
similar blades on the anterior and posterior comers of the crown (paraand metastyle). At this stage the large original tip of the upper molars
(the future paracone) held aloof from articulating with any part of the
lower molars, simply fitting into the interdental embrasures on the outer
sides of the lower molars and pressing the food against the nearby
blades on its own base and on the sides of the lower teeth. The shearing
V of the lower molar had its tip, the protoconid, on the outer side and its
sides (the paraconid and metaconid) directed inward.
Here then we have the true functional reversal of upper andlower
triangles which apparently misled Osborn and Cope into homologizing
the upper and the lower reversed V’s; but as already intimated, the reversed upper and lower v’s both have the primitive apex of the tooth on
the outer side of the crown, and in the upper the primitive tip stands
not at the internal angle but in the middle of the V, while in the reversed
lower V the original tip of the crown is located at the fork of the V.
The overlapping or crushing portions of the upper and lower teeth in
this stage consisted in the lower molar of the small talonid, with its sole
cusp the entoconid, and in the upper molar of the protocone. the tip of
which already fitted into the basin of the talonid as it does in nearly all
later mammals. Nevertheless I was formerly in error in supposing that
the protocone of the upper and its functional mate the talonid of the
lower were of equal age. The protocone apparently arose as early as the
Diademodont premammalian stage and accompanied a transverse
widening of the crowns; the talonid arose later in an early Pantotherian
stage and its appearance was made possible by an anteriorward displacement of the metaconid, associated with an anterior displacement of the
protocone of the upper molar.
Thus by Lower Jurassic times the upper and lower cheek teeth of
mammals, as illustrated in the Pantotheria, had already attained rather
complex interrelationships, and such a dentition as that of Amphitheriam, which was not dissimilar to that shown in Fig. 1, 111,had in it the
morphological potentiality of the highly diverse arrangements and
patterns of the molars that are found in existing placental andmarsupial
Until quite recently there was a considerable structural gap and an
enonnous chronological gap in the record of dental evolution between
the Jurassic Pantotheria and the swarming placental mammals of the
Eocene and later ages. Owing to this gap in our records it has been in
doubt, for example, whether the internal extension of the protocones of
the Pantotheria was truly homologous with the so-called protocones of
the molars of later mammals, whether the protocones of later mammals
arose puri passu with the development of the talonids, and so forth.
But these doubts have at last been resolved, in my own mind at least,
through the fortunate discovery by the Third Asiatic Expedition of the
American Museum of Natural History, under the leadership of Roy
Chapman Andrews, of the long-sought Cretaceous mammal skulls.
These priceless specimens, which have recently been described in a joint
paper by myself and Dr. G. G. Simpson of Yale University, still retain
certain primitive features inherited from the Jurassic Pantotheria: thus
their main high central cusps are still flanked on the outer sides by prominent parastyle and metastyle projections, while the basal section of the
crown is obliquely and asymetrically triangular with the internal apex
displaced anteriorly.
On the whole the Cretaceous mammals are already nearer in dental
structure to the later placental mammals than to the Jurassic Pantotheria. In one of them, which we have named Deliatheridium petrituberculare, as indeed also in the others, the upper molars (Fig. 1, IV,
Fig. 2) show a further progression beyond the primitive Pantotherian
condition in the direction of those of typical later mammals. In the
first place, the total number of postcanine teeth has been much reduced,
from about eleven or twelve to six or seven, the reduction apparently
having occurred through the loss of the posterior molars, so that seven
larger teeth now occupy the whole tooth row instead of twelve small
ones. Secondly, the interdental embrasures of the upper teeth have
widened t o accommodate the enlarged talonid of the lower teeth.
Thirdly, the talonids of the lower molars have now captured and definitely engage with the entire protocone
of the upper molars, the latter being reduced to the condition of a pestle that
works in the basin of the talonid. Fourthly, the main central cusp of the upper
molar has assumed a more dominant
position, since the blades on its anteroexternal and posteroexternal slopes now
shear past the blades on the correspondFIG.3. Palate of Primitive ing slopes of the protoconids. Fifthly,
Placental Mammal (Deltathrthe main central cusp, the original “repidium frretritubercwhre) from
tilian cone” is now in process of being
t h e Cretaceous of Mongolia.
x 2/1
divided into two cusps, still connate at
the base, the para- and metacones (mesiobuccal, distobuccal),
Another of these Mongolian Cretaceous genera described by us
(Zalambdalestes, Fig. 4 ) , although more in the line of ascent to the
Zalambdodont insectivores than to the higher Insectivora, shows a
further advance in the direction of later mammals in that the trigonid is
now completely displaced to the forward part of the crown, while the
talonid is widened transversely and is now even wider than the trigonid.
In correlation with this transverse widening of the talonid the main
FIG.^. Right Lower Cheek
Teeth of Primitive
Eeclzei) from the Cretaceous of Mongolia. X 3/1.
FIG.5. Occlusal diagram of ZaZamb
dalestes lechei X 5/1.
external cusp of the upper molar, already subdivided into the para- and
metacones, is retreating toward the outer border of the crown (Fig. 5).
These Mongolian Cretaceous genera of placental mammals (Fig. 6 ,
A, B, C) combine the characters of the insectivores and carnivores of the
Eocene and later ages t o such a degree that we regard them as standing
in or quite close to the common ancestry of these two orders. And it has
long since been inferred from numerous comparative studies that this
FIG.6. Skulls of Cretaceous mammals from Mongolia.
A. Deltetheridium pretrituberculare. X 1.
B. Zalambdalestes lechei, young. X 1.
C. Zabmbdalestes lechei, extremely old X 1.
hitherto undiscovered primitive insectivore-carnivore group also constituted the common stock of all the later placental orders, including the
Primates .
It would be too much to expect that the first discovered Cretaceous
placental mammals should be the very ones that would tend to bridge
the gap between the Jurassic Pantotheria and the Eocene and later
placental mammals and yet such is the fortunate fact. It had already
been inferred by several authors from comparative studies of Eocene and
later mammals that the main high cusp of the upper molars, standing in
line with and behind the similar main cusp of the premolars and likewise
supported equally by the two main roots on the outer side of the crown,
was serially homologous with that cusp; in other words, that the paracone and not the so-called protocone represented the summit of the
original reptilian crown. But in the Cope-Osborn theory of reversed
triangles this fact was ignored, although it was tacitly admitted that in
the lower molars the main high cusp of the molars, the protoconid, was
homologous with the main high cusp of the premolars in front of it.
This inherent inconsistency of the Cope-Osborn theory was exposed by
me in 1916, and in 1922 I showed that the occlusal relations of the upper
and lower cheek teeth made it far more probable that in the upper
molars the homologue of the original reptilian cone was the paracone or
FIG.7. Relations of Main Tips of Molars and Premolars in a Primitive Cretaceous
Placental (Deltatheridium pretrituberculare).
A. Oblique rear view of left lower cheek teeth, showing t h e protoconids of the
molars in line with the tips of the premolars.
B. Oblique rear view of right upper cheek teeth, showing the high paracone in
line with the tips of t h e premolars and the “protocone” as a low process from t h e
base of t h e paracone.
the combined para- plus metacone. The newly discovered Mongolian
Cretaceous placental mammals afford strong support to this view,
namely that in the upper teeth the original reptilian tip lies on the
paracone and that in the lower molars it is the protoconid. These
relations may be seen at a glance in Fig. 7 ; hereafter the burden of prooE
must be upon anyone who would maintain that in the upper jaw the
original reptilian tip shifts as we pass backward from the main high cusp
to the internal basal extension, the so-called protocone.
From the Cretaceous Mongolian placentals to the highly vaned
placentals found in the Basal and Lower Eocene of North America is
but a short step. Already in the Lower Eocene and perhaps even earlier
the Order of Primates had become separated from other mammals.
The existing Tree Shrews (Tupaiidae) of Borneo and adjacent regions
seem to be living and but little modified survivors of the basal Primate
stock, so that after many studies on their anatomy and osteology certain
authors classify them as the first division of the Primates rather than as a
highly progressive family of Insectivores. While they have become
more or less specialized in their front teeth, their cheek teeth have retained many features seen in the Cretaceous placentals and in other
primitive placental mammals of Eocene times.
But to illustrate the passage from the Cretaceous t o the Eocene, in the
line of ascent toward man, I have chosen a form that is not a primate at
all but is of so generalized a type that according to Dr. W. D. Matthew
it can most conveniently be referred to the Order Insectivora. I refer to
the Lower Eocene genus Didelphodus (Fig. 1, V). I n this genus we see
very clearly several distinct advances upon the conditions characteristic
of the Mcngolian Cretaceous mammals. This dentition affords an ideally central type for the derivation of all specialized molar patterns of
later placental mammals, although it must be borne in mind that since
Didelphodus is a Lower Eocene rather than a Basal Eocene or even
Upper Cretaceous form, it was even in its own day a relic of still oldeiforms and that it lived beside more progressive contemporaries that had
already attained more advanced modifications of the dentition in several
Didelphodus then, representing a primitive placental stage of the
dentition, had advanced beyond the more primitive Deltatheridiurn of
the Cretaceous in the fallowing respects: first, it was no longer in a
pre-tritubercular stage, since the para- and inetacones of its upper
molars were now well separated, thus producing the typical tritubercular
upper molar, which Cope and Osborn rightly chose as the starting point
for the more complex niclar patterns of all higher placental mammals.
Secondly, the lower molars had attained the typical tuberculo-sectorial
condition with a nearly balanced development of the trigonid and
That this combination of tritubercular upper molars and tuberculosectorial lower molars was in very truth the starting point for the
diverse molar pattenis of later mammals would need no emphasis if
addressed to the few living pal2eontologists who have devoted their
lives to the study of the swarming families of Eocene and later fossil
and recent mammals. But unhappily the published evidence for this
statement is scattered in scores of technical papers and the main bulk of
the evidence can hardly ever be published at all by reason of its very
extensiveness. It is therefore still possible even for certain eminent
students of anthropology and human morphology to treat with equal
scepticism all theories of the early evolution of the molar teeth of mammals and to imagine that they can safely start de nmo and invent a
new theory each one for himself.
But to return to Didelphodus, it does, as I have said, present ideally
primitive upper and lower molar patterns. Every one, but one, of the
standard main cusps of the typical primitive placental molar is present
and normally developed. The proto-, para- and meta- cones, the protoconule and metaconule and the para- and meta- styles are all present and
functioning normally, as may be seen in the occlusion diagram (Fig. 1,
The one molar cusp which normal placental mammals now have and
which Didelphodus had not yet developed is the hypocone, a later derivative of the posterointernal region of the crown. In Didelphodus the
well developed trigonids still fit into the interdental embrasures, the
talonid had not yet outstripped the trigonid in size, and its hypoconid
had not yet pushed its way laterally beyond the line of the inner bases of
the protoconids.
Nature’s beautiful mechanisms sooner or later get thrown out of
balance and seek new adjustments through the overgrowth and crowding of cedain parts and the retreat and final disappearance of other
parts. This phenomenon is clearly illustrated in Fig. 2 , VI, representing
one of the numerous Primates of the Middle Eocene and quite representative in a general way of all its relatives. For in this form the primitive trigon of the upper molar has become modified by the addition of a
fourth main cusp, the hypocone, which in this case is derived from a
swelling on the posterior cingulum or basal ledge. The upgrowth of this
hypocone tends to reduce and finally to obliterate the interdental embrasure and to transform a triangular tritubercular upper molar into a
quadrangular quadritubercular one. This process has not yet affected
the third molar, is just beginning in the first, and is well advanced in the
second molar.
Meanwhile the talonid of the lower molars has increased rapidly in
size so that it now surpasses the trigonid; its hypoconid pushing laterally
has passed the line of the protoconids and invaded the space formerly
occupied by the main high central cusp; while the para- and metacones,
derivatives as we have seen of the primitive central cusp, have not only
parted widely from each other but have retreated laterally before the
advance of the hypoconid. Meanwhile the paraconid is apparently
being crowded out of existence, a t any rate, it retreats before the backwardly-growing talonid of the preceding tooth and before the inwardlygrowing hypocone of the upper molars. Thus, as first perceived by Cope
and Osborn, we have the upper molars transformed from the tri- to the
quadritubercular stage and the lowers from the tuberculo-sectorial into
the tubercular stage with four main cusps, the protoconid and hypoconid
on the outer side, the meta- and entoconids on the inner side.
A fifth small cusp, the hypoconulid, on the median posterior border,
had already appeared in the primitive placentals (Fig. 1, V). In the
earlier Primates it was poorly developed on the first and second lower
molars, but large and prominently develcped on m3; i t occluded on or
slightly behind the posterior basal cingulum of the upper molar especially
in m3, where it formed a powerful piercing organ since it was located
far back near the fulcrum and having a small area, its piercing power was
correspondingly high.
In this Middle Eocene p r k a t e (Pronycticebus, Fig. 2, VI) we observe
that while the first and secc id upper premolars are small, the third and
fourth are tending to be su )equal and bicuspidate. A further emphasis
of this difference together m th a crowding of the whole tooth row might
result in the entire elbinatit n of the first and second premolars, so that
the originally third premolar 1 .odd be in contact with the canine. This
condition is realized in certain progressive tarsioid primates of the
Upper Eocene (e.g., Microchczrus, Fig. 2, VII) and in all the Old
World monkeys, anthropoid apes and man.
Microcharw itself is distinctly too specialized in certain particulars to
lie in the direct line of human ascent. Nevertheless it belongs to a
division, the Tarsioidea, which in many respects is progressive. Its
skull is distinctly progressive toward the higher primate type and as investigations multiply it seems more and more probable that the' line
leading to the higher primates including man, branched off from the
tarsioid stem before the latter acquired its various aberrant specializations. Therefore it seems legitimate to consider the molar patterns of
Microcharus (Fig. 2, VII) in the present connection, since in many ways
they are intermediate between the primitive primate stage typified by
Pronycticebus and the common stem leading to the anthropoids and man.
First, then, Microcharus shows a marked advance beyond Pronyciicebus in the development of the hypocones, which have now very
nearly obliterated the interdental embrasures. Concomitantly the paraconids of the second and first molars are nearly or quite gone, but the
paraconid of the first molar is still well developed in harmony with the
feeble development of the posterointernal cusp of the fourth upper
premolar. Secondly, the talonids now considerably surpass the trigonids
in size, the hypoconids being predominant over the protoconids. Accordingly the para- and metacones of the upper molars are larger and
even further separated than they were in Pronycticebus and lie still
nearer to the outer border of the crown. This outer border from its
great prominence in the earlier stages is thus greatly reduced, the external cingulum being the last trace of its former extensive development.
A specialization of Microcharus is the great development of the metaconule, which occludes behind the enlarged hypoconid and occupiesthe
field left vacant by the retreat of the metacone. Another specialization
is the doubling of the hypoconulids in the lower molars, which occlude
in the space in front of and between the metaconule and the hypocone.
Dryopithecus PATTERN.
The upper teeth (Fig. 2, VIII) of the primitive anthropoid Dryopithecus from the Miocene of India and Europe exhibits a marked advance
beyond earlier Primates and in the human direction in many particulars.
In the first place the two premolars remaining out of the original four,
namely p3, p4,are both fully bicuspid and have become closely similar in
form. In the upper molars all the cusps have lost their crested shearing
form and have become obtusely conical. They have lost all trace of the
original outer border of the crown and they have lost the protocondes,
retaining however in a’ somewhat modified form the anterior cingular
ridge connecting with the anterior crest of the protocone. The metacones have become somewhat smaller than the paracones and do not
project so far laterally. The metaconule, no longer distinct, has apparently been fused with the base of the metacone, the well developed
hypocone has reduced but not wholly obliterated the interdental embrasure. The third molar has acquired an irregular outline, usually
broader on the inner than on the outer border.
Meanwhile the lower cheek teeth have undergone profound modifications. The anterior lower premolar (correspondingto the third of the
primitive dentition) has remained compressed and is only incipiently or
potentially bicuspid. The posterior premolar, on the contrary, is definitely bicuspid with a tendency to become molariform by the development of the talonid. The most conspicuous advance in the molars lies
onule a
FIG.8. Names of the cusps of the upper and lower molars of Dryopithecus, according
t o the Osbornian System.
in the marked changes in the proportions; the width of the tooth in
proportion to its length has materially increased in comparison with the
earlier Primates, especially in m2. The ridges on the cusps, that formerly formed the shearing blades, are vestigial or absent, the cusps
themselves having become very large‘and conical and being arranged in
two main pairs, the first pair (protoconid-metaconid) transverse, the
second pair (hypoconid-entoconid) obliquely transverse, with the entoconid a little behind the hypoconid. The hypoconulid varies somewhat
in the different species of Dryopithecus but especially on m2 is a prominent conical cusp lying obliquely behind the hypoconid. With the
complete loss of the paraconid, the whole trigonid basin has been reduced to a narrow fissure, the “fovea anterior;” while a forward displacement of the protoconid, together with the widening of the whole
tooth and the flattening down of all the cusps, has effected a great in-
crease in size of the talonid basin, which now forms the great central
fossa of the tooth. With the reduction in height of the trigonid and the
levelling of the general surface of the crown there has been an almost
complete loss of the old tuberculo-sectorial pattern and functions. The
shearing-grinding effect of bluntly conical cusps grinding past each
other has taken the place of the shearing effects of delicate sharp-edged
blades of the earlier trigonid, while the crushing-shearing-grinding
function of the talonid has become dominant. With the loss of the
sharp blades on the trigonid and talonid the conical cusps are no longer
integrated into the primitive tuberculo-sectorial pattern, but a new
pattern called the Dryopithecus pattern has grown up in place of it.
This pattern (Fig. 2, VIII) consists of the collocation of the five main
cusps in the positions already noted and with their bases defined by
certain deep sulci, which during development appear along zones of
contact between the expanding centers of growth of the five main
cusps. An essential feature of the Dryopithecus pattern is that there is
such a zone of contact between the posteroexternal slope of the metaconid and the anterointernal slope of the hypoconid.
The occlusion diagram of Dryopithecus brings out even more closely
the functional significance of the characters already noted. AS compared with the occlusion diagrams of earlier forms it reveals the marked
increase in width of the lower as compared to the upper teeth, the complete loss of the prbitive shearing blades of the trigonid, the enhanced
functional importance of the talonid and of the hypoconulid, the lateral
growth of the hypoconid between the paracone and metacone. As a
whole there is decidedly less alternation between the upper and lower
teeth than there was in the primitive stages; that is, the reduction of the
trigonid and the increase of the talonid causes each lower tooth to articulate more with the correspondingly numbered upper tooth than with
the preceding upper tooth.
But beneath these changes the old fundamental relations of the main
cusps of the upper and lower molars are still very apparent. Thus the
remnant of the trigonid still lies between two upper teeth, thehypoconid
articulates between the para- and the metacone (although further
laterad), the protocone still falls between the metaconid and the entoconid and the latter articulates on the anterior slope of the hypocone.
The oldest human dentition known in which complete upper and
lower sets belong t o one individual is that of the fossilized skull of a
young man of the Neanderthal race (Homo neanderthalensis) discovered
a t Le Moustier in France and described originally by Klaatsch as
Homo rnousferiensis hauseri (Fig. 2, IX). From stereoscopic photographs taken by my colleague Professor J. H. McGregor and from an
excellent cast of the specimen presented to the American Museum by
Dr. J. Leon Williams, I am enabled to give diagnostic drawings of the
upper and lower cheek teeth and to construct a diagram of the upper
and lower teeth in occlusion. The European anthropologists who have
described this dentition stressed its resemblances to and difierences from
other specimens referred to the Neanderthal race and to Homo sapiens
and have said nothing about the whole series of fundamental characters
which it has inherited from earlier primates and which the present series
of drawings brings into sharp relief.
As to the upper cheek teeth, most of the description of the upper
teeth of the anthropoid Dryopifhecus, given above, would apply word for
word to this human specimen. Thus on each side of the palate there are
not only the same number of cheek teeth, but also the same total number of principal cusps on each corresponding tooth, as follows:
3 4
3 4
Dryopithecus. . . . . . . . . . . . . . . . . . . . 2
LeMoustier.. . . . . . . . . . . . . . . . . . .
Moreover, each one of the corresponding cusps and ridges has the
same general character in the Mousterian youth as it has in Dryopithecus.
The lower cheek teeth of Le Moustier likewise show the same number
of teeth and the same total number of principal cusps as in Dryopithecus,
as follows:
Dryopithecus.. . . . . . . . . . . . . . . . . . .
LeMoustier.. . . . . . . . . . . . . . . . . . .
Except in the third lower molar, in which the fundamental pattern
has been obscured by secondary grooves and wrinkles, as it sometimes is
in the Chimpanzee and still more in the Orang, the Dryopithecus pattern
is still evident in the lower molars of Le Moustier, which retains the
essential contact between the adjacent bases of the metaconid and
Moreover the occlusion diagrams reveal the extraordinary degree of
correspondence between Dryopilhecus and Le Moustier in all fundamental characters, so that it may be stated as a fact that in the general
pattern and in the occlusal relations of its cheek teeth, the primitive
anthropoid Dryopithecus is far nearer to the primitive human stage
represented by Le Moustier than it is to the early primate Pronycticebus.
For example, the tendency toward an end-to-end rather than a broadly
alternating relation of the upper and lower molars, already established in
Dryopithecus, is retained in Le Moustier, which is in this respect much
nearer to Dryopithecus than the latter is t o Pronycticebus.
Finally, many of the differences between Dryopdhecus and Le Moustier
represent further advances in the same direction in which the former
differs from primitive primates. In Le Moustier, for example, pa has
progressed considerably toward the molar pattern since its posterior
moiety and its incipient hypoconid and entoconid are now better
developed than they were in Dryopithecus; p3 from being at most incipiently bicuspid has become completely so, its main axis now being
transverse instead of anteroposterior. But that this difference between
Le Moustier and Dryopithecus may fairly be classed as merely a progressive character of the former and not a divergence tending to remove
Dryopithecus from the line of human ascent, is indicated by the wide
degree of variability in the form of p3 among recent chimpanzees and
gorillas, in some of which ps retains largely the compressed Dryopithecus
form, while in others the crown of p3 has been rotated so that the
direction of its main axis is obliquely labiolingual, its lingual border
bearing a strong cusp, so that the tooth as a whole approaches the
bicuspid type.
In the lower molars of Le Moustier the broadening of the crown has
progressed much further than it had in Dryopithecus; m3 has completely
lost its tuberculo-sectorial character and has gone a step further on the
road toward degeneration, delayed eruption and eventual disappearance. The hypoconulid, but moderately developed in Dryopifhecus,
is larger in Le Moustier, especially in ml. Turning again to the occlusion
diagram we note in Le Moustier the further tendency of the hypoconulid to crowd into the territory formerly occupied by the hypoconid,
with an associated tendency in ml for the hypoconid to expand laterally.
But in addition to all these characters in which Le Moustier has
either retained the pure Dryopithecus status or progressed further in the
same direction in which Dryopiithecus had advanced beyond the earlier
primates, we witness many new features in Le Moustier which were in
general associated with a profound change in the form of the dental arch
as a whole. For whereas in primitive anthropoids the jaw is relatively
narrow transversely, the opposite halves of the lower jaw and the
opposite rows of cheek teeth being more or less parallel, in man the
mandible has been greatly widened and shortened, the front part of the
alveolar border crowded backward, so that the lower incisors and canines are no longer procumbent, but in the more advanced races even
pass the vertical. Meanwhile the canine tooth has diminished in
length and its tip has finally withdrawn beneath the closed up row of
upper teeth, while the forepart of the dental arch has shrunk transversely so that the premolar series incline more toward the midline, all
these changes producing the characteristic paraboloid upper and lower
arches which had already been attained in the Pleistocene species of
man represented by Le Moustier.
It is true that apart from the doubtful exception of the Talgai palate we
do not yet possess the structurally intermediate stages between the
Dryopithecus condition with narrow dental arch and enlarged canines
and the condition seen in Le Moustier and other early human races in
which the canine is definitely human. But in view of the extraordinary
resemblances in the cheek teeth already noted, taken in connection with
the cumulative evidence from comparative anatomy as t o the derivation
of the human family from a primitive anthropoid stock, there can be
little reasonable doubt that there once were such intermediate stages, in
which the lower canine was smaller and more erect than in the primitive
anthropoid and the front part of the jaw already in course of being
crowded backward. Indeed the mandible of the Piltdown race may,
when its front part is better known, afford just such an intermediate
The lower molars of the Mousterian youth also exhibit another major
advance toward the modernized human stage, since its second molar
already shows signs of substituting the cruciform for the Dryopithecus
pattern. That is, with the crowding forward of the entoconid the transverse sulcus in front of the entoconid has been brought nearly in line
with the transverse sulcus between the protoconid and the hypoconid.
Meanwhile the pairing of the hypo- and entoconid in a transverse row
prepares the way for the completion of the cruciform pattern in the
present stage of evolution.
OF M;,
LOSS OF THE Dryopiflzecus PATTERN, M'
The cheek teeth of modern man, as the members of this Congress
well know, vary widely both in size and form, but the dentition of the
female Bedouin illustrated in Fig. 2, X. may be taken perhaps as fairly
representative for the white race.
That this dentition is in some respects considerably simplified and
reduced, as compared with the more vigorously developed dentitions of
infra-human primates, there can be no reasonable doubt. Professor
Cope long ago noted that in certain races, especially the Esquimaux
but also in allied Mongolian races, there was a tendency for the second
upper molar to exhibit only the three “primary cusps,” the fourth or
hypocone being reduced or absent. , Cope spoke of this phenomenon as
a “reversion” to a “lemurine” “tritubercular” stage of evolution but the
use of the words reversion and lemurine only brings in an unnecessary
and confusing element of mystery. For all stages in the reduction of the
hypocone may be observed in any large collection of human skulls.
Moreover, the occlusal relations of the three main cusps are substantially the same as in the quadritubercular second molars of Le Moustier
and Dryopithecus, to both of which the resemblance is far closer than to
the tritubercular second upper molar of primitive Eocene primates.
The third upper molar in this fully adult individual is well developed
but was not yet in contact with the third lower molar, this delay in
assuming its functional responsibilities presaging the final degeneration
of these teeth attained in some human jaws.
Along with the diminution in size of m2,m3, as compared with the
corresponding teeth of older types, we observe in this Bedouin the dominance of m*and of its fellow in the lower jaw, which is one of the outstanding features of modernized human dentitions.
In the lower molars we now find another conspicuous reduction,namely the loss of the hypoconulid as a distinct cusp,-in which the
modernized dentition has sacrificed some of the features slowly won
through long geological ages and in which there is a marked tendency
toward simplification. The reduction of the hypoconulid has been traced
again and again by numerous observers, in various races and individuals.
In general this loss is more pronounced in m2than in ml, which although
progressive in dmensions is more conservative in pattern than mz.
Usually with the loss of the hypoconid of mzthe substitution of the plusshaped or cruciform arrangement of the principal sulci has been completed.
As a result of the changes in dimension noted above we find that with
regard to anteroposterior length rns is usually shorter than ml it is also
longer than ms.This is the direct opposite of the relations obtaining in
Dryo@zecus, where the molars typically increase in length from ml to
m3, but many intergrading conditions between these extremes have
been observed in fossil and recent human jaws.
Another marked characteristic of such modernized dentitions as
that shown in Fig. 2, X is for the two upper bicuspids to become like
each other; a similar tendency is seen in the two lower bicuspids. The
result of all these modifications is an apparent simplicity and a degree of
resemblance between adjacent teeth which has led some authors t o farreaching theories of the early evolution of the teeth, based upon the
development or upon the teratological variations in modernized human
Who would have suspected, from an examination of the human dentition in its present simplified condition, that it had passed through the
long and complex series of changes which we have followed above?
Unfortunately the detailed labors of palaeontologists and of students
of the major classification and evolutiQn of the mammals are so little
known to most of their own colleagues in other branches of the biological
sciences that few can realize from first-hand knowledge that the study
of the evolution of human molar teeth is no longer in the vague stage of
fog and uncertainty. The cumulative and ever-widening evidence for
Darwin’s view that man is an off-shoot from the base of the anthropoid
stem securely links the study of the eyolution of human cheek teeth with
the history of dental evolution in the infrahuman primates. And here we
have clear documentary evidence, resting on no hypothetical stages,
that enables us to follow the patterns of the human dentition backward
to the Dryopithecus stage, thence t o the primitive Primates of the
Eocene, the dentition of which in turn closely approaches those of
many other Eocene phyla that sprang from the central tuberculosectorial Placentals of Basal Eocene times. The discovery of definitely
pre-tritubercular Placental mammals in the Cretaceous of Mongolia
carries the history backward to a point near or a t the origin of the
“stem Placental” mammals, long expected but never hitherto known
from fossil specimens. Back of this the long gap to the very beginnings
of the future tritubercular stock in the Lower Jurassic still remains to be
explored in detail, but even from present evidence little doubt can remain that the relation of reversed triangles between the upper and lower
molars was not arrived a t by the steps inferred by Cope and Osborn but
took the general course inferred in outline by Wortman and several
subsequent authors. The known Jurassic Pantotherian mammals
again are advanced far beyond the most advanced mammal-like reptiles
of the Triassic, but these in many respects are nearer in structure to the
mammals than they are to the stem reptiles of the Permo-Carboniferous. Among the latter Seymouria again divides the difference between the most primitive amphibians and the typical reptiles, while
there is the strongest morphological and palaeontological evidence for
connecting the amphibians with some undiscovered group of air-breathing fishes related both to the Fringe-finned Ganoids and the Dipnoans.
Reading the story the other way, the ten structural stages in the evolution of the human cheek teeth, as described in the present paper, may
be summarized as follows:
Substage a. Permo-Carboniferous. Mycberusaurus, primitive
Theromorph Reptile.
Substage b. Perrnian. Scyhusaurus, primitive Mammal-like
Substage c. Triassic. Cynognathus, advanced Mammal-like
Triassic. D i a d e m d o n , advanced Mammal-like Reptile.
Jurassic. Pantotherian (primitive pro-Placental).
Cretaceous. Pre-Trituberculate, Deltatheridiunz.
Lower Eocene. Primitive Placental, Didelphtodus.
Middle Eocene. Primitive Primate, Prunycticebus.
Upper Eocene. Advanced Tarsioid Primate, Microchmus.
Miocene. Primitive Anthropoid Primate, Dryopithecus.
Pleistocene. Primitive Man, Mousterian.
Recent. Modern Man, White.
The older palaeontological literature dealing with the subject under consideration
is cited and discussed in “The Ori ‘n and Evolution of the Human Dentition,” by
William K. Gregory, Baltimore, 8 2 2 . Consequently only a few of the more important older papers are cited below.
1901-02. Wortman J. L. Studies on the Eocene Mammalia in the Marsh Collection,
Peabody Museum. A m . J. Sci., XI-XIV. 11 Pls.
Studies on the Eocene Mammalia in the Marsh Collection,
Peabody Museum. A m . J . Sci., XV, 419-436.
[The Premolar-Analogy Theory]
1906. Gidley, J. W. Evidence bearing on Tooth Cusp Development. Proc. Wash.
Ac. sci., VIII, 91-110, Pls. IV, v.
1907. Osborn, Henry Fairfield. Evolution of Mammalian Molar Teeth to and from
the Triangular Type. The Macmillan Company. N. Y.
1910. Gregory, William K. The Orders of Mammals. Bull. Am. Mus. Nut. Hist.,
XXVII, 1-524.
, Studies on the Evolution of the Primates. Part I. The CopeOsborn “Theory of Trituberculy” and the Ancestral Molar Patterns of the
11. Phylogeny of Recent and Extinct Anthropoids with Special
Reference t o the Origin of Man. Bull. A m . Mus. Nut. Hist., XXXV, Art. XIX,
1925. Simpson, G. G. Mesozoic Mammalia 111: Preliminary Comparison of
Jurassic Mammals. Am. J . Sci., X 559-569.
1926. Gre ory, William K. and Milo Hellman. The Dentition of Dryo~thecus
and the &gin of Man. Anthrop. Papers Am. Mus. Nat. Hist., XXVIII, Pt. I.
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