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Further evidence on relative dental maturation and somatic developmental rate in hominoids.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY87:29-38 (1992)
Further Evidence on Relative Dental Maturation and Somatic
Developmental Rate in Hominoids
SCOTT
W. SIMPSON.
C. OWEN LOVEJOY.
.~
_ . _
~
-,
AND RICHARD s. MEINDL
Biological Anthropology Program, Diuision of Biomedical Sciences, Kent
State Uniuersity, Kent, Ohio 44242
~~~~~
KEY WORDS
mental pattern
~
~
~~~
~
Dental maturation, Hominoid, Human, Develop-
ABSTRACT
New data on hominoid dental development are presented.
Individual bivariate pairings of all mandibular teeth were made for African
apes and humans. Data were analyzed with a full linear regression model. No
statistically significant differences were found among apes, although a consistent pattern of earlier incisal development was observed in Pan relative to
Gorilla. This is concordant with an earlier fusion of the premaxil1ary:maxillary suture in Pan. Only one tooth pair differed significantly by sex among
apes. Two biologically distinct human samples (Libben and Hamann-Todd),
although assessed differently (extraction and radiography) yielded virtually
identical results. Humans differ from apes only by earlier relative calcification
of their anterior teeth. This can be viewed as a consequence of reduced facial
prognathism and a shift in hominid canine function.
That relative dental development (especially of the cheek teeth) directly reflects
prolongation of somatic maturation in hominids has become a long-standing, widely held
view (Mann, 1975; Gould, 1977; Smith, 1986,
1989; Dean and Wood, 1981; Dean, 1987,
1989).Although the timing of human dental
development is relatively well known (Nolla,
1960; Moorrees et al., 1963; Fanning, 1961;
Anderson et al., 19761,our knowledge of ape
dental development is restricted to a few
early reports (Schultz, 1935; Nissen and Riesen, 1964; Willoughby, 1978) which were
based on very limited samples. A more extensive study of pongid development was conducted by Dean and Wood (1981) for the
expressed purpose of establishing an archetypical developmental schedule for pongids.
A primary consequence of the methods employed to achieve this goal was a requisite
suppression of variation within the samples
used. Therefore, these same data cannot be
used to accurately judge the taxonomic or
developmental affinity of additional specimens. Thus, the use of these standards to
identify the developmental pattern of individual fossils is inappropriate (contra Smith,
1986).
@ 1992 WILEY-LISS.INC.
In an effort to resolve these issues, we
recently presented a comparison of the relative rates of maturation of the permanent
teeth in modern humans and the African
apes (Simpson et al., 1990).A primary goal of
our original study was to assess the feasibility of using relative dental maturation as an
indicator of achieved somatic development
in fossils. Our results clearly demonstrated
an absence of any reliable relationship between dental and somatic developmental
rates in these higher primates. While it is
obvious that the absolute timing of dental
development differs significantly between
modern humans and the African apes, all
substantial pattern differences were clearly
related t o functional differences in their dentognathic complexes, and not to the duration
of somatic growth. In fact, virtually all of
the developmental relationships previously
viewed as hallmarks of “delayed development in hominids, such as a greater latency
in the relative emergence of hominid permanent molars [although Schultz (1935) recognized the delayed M3 as a catarrhine characReceived May 18,1990; accepted April 30,1991
30
S.W. SIMPSON ET AL.
teristicl, were found not to distinguish
humans and apes in any reliable way. In
addition, the relative development of the
first, second, and third permanent molars
were found to be essentially indistinguishable in all taxa studied (Simpson et al., 1990:
Fig. 4 and Table 1)(Tables 7 and 8 and Fig.
2). Because these results contradicted previously held views on hominoid dental development (Mann, 1975; Smith, 1986; Conroy
and Vannier, 1987,1988; Beynon and Dean,
1988; Dean, 1989), we have undertaken a
replication of our original study and here
report the results of additional observations.
We have employed novel procedures in order
to determine whether methodology played a
significant role in our original findings.
In our original study the following procedures were employed:
1.The human sample was taken entirely
from the Libben population (Lovejoy et al.,
1977), a n American aboriginal skeletal sample.
2. Relative development of the human
sample was determined by direct observation following extraction.
3. The human sample was composed of
both maxillary and mandibular dentitions.
4. Only pongid maxillary dentitions were
used, and these observations were made
from radiographs.
5 . In order to assure consistency across
taxa, all estimates of completed dental
growth were made by only one of the authors
(C.O.L.).
For the current study we have altered our
methods in the following ways:
1. Two human samples are used: the Hamann-Todd and Libben collections.
2. The Hamann-Todd human sample has
been radiographed and observed in a manner identical to that applied to the ape sample.
3. All observations have been restricted to
the mandible.
4.All estimates of completed dental development have been made by one of the authors (S.W.S.).
MATERIALS AND METHODS
We examined the mandibles of 49 preadult
chimpanzees (Pun troglodytes), 50 lowland
gorillas (Gorilla gorilla), and 38 modern
Homo supiens from the Hamann-Todd collection, which is housed at the Cleveland
Museum of Natural History, Cleveland,
Ohio. We also examined 81juvenile humans
from the Libben population, which is curated
by the Department of Anthropology, Kent
State University, Kent, Ohio. All HamannTodd specimens were radiographed a t least
twice with low voltagellong duration exposures (Hewlett-Packard Faxitron) using
Kodak Ultraspeed Dental Film (DF 50). Individual exposures were made of the postcanine and anterior regions for each specimen.
In addition, a large format exposure (Kodak
X-Omat TL) of each mandible was also made.
The mandibular teeth of the Libben sample
were extracted and directly observed. Each
tooth in this study was scored on a continuous scale of 0.00 to 2.00 where 0.00 equals
absence of any crown calcification, 1.00
equals crown complete with no root formation, and 2.00 represents apical closure ofthe
root (Simpson et al., 1990) (Fig. 1).
In order to evaluate relative growth, bivariate comparisons of all possible tooth
combinations were conducted using a full
linear covariance model as previously described (see Simpson et al., 1990)l. In each
such comparison, the regression slope is assumed to reflect the relative growth rate,
while the y-intercept is assumed to represent
the onset of calcification of the later appearing tooth in each bivariate pair. In each
scatterplot the y-axis therefore represents
the earlier calcifying tooth. The following
group comparisons were made: PanlGorilla,
male apesffemale apes, L i b b e a a m a n n Todd humans, and apes combinedhuman
combined.
RESULTS: COMPARISONS
African apes
Several authors have noted the strong
similarity in both absolute and relative dental developmental among apes (Schultz,
1935; Dean and Wood, 1981), and our data
‘Each sample comparison was analyzed statistically with a full
linear regression model. For example, in order to compare chimpanzee and gorilla samples for relative state of development of
the followingmodel would
the first molar (Ml) and the canine (0,
be used:
M1 = Po + pl(C) + p,(Taxon) + p3(Interaction)
where “taxon” is the dichotomous variable depending on the
comparison (0, chimpanzee 1, gorilla, 0, male, 1, female; 0,
Hamann-Todd, 1 = Libben; 0, human combined, 1, ape combined) and “interaction”is the product of “canine”and “taxon.”p3
is thus the difference in the slopes (relative developmental rate)
between the two groups and pz is the difference in y-intercepts
(relative initiation of calcification).
31
RELATIVE DENTAL DEVELOPMENT
c 'A
.25
a
.50
.75
1.oo
1.25
F3
Gs
W
c 'h
Gs
C%
cs
cc
W
R 'A
R 'h
1.50
R%
1.75
RC
2.00
Fig. 1. Schematic of dental maturation from Dean
and Wood (1981). 0.00, no crown apparent; 1.00, crown
complete with no root development; 2.00, apical closure.
are fully consistent with these earlier obser- logical phenomenon. That is, we applied a
vations. None of our bivariate PanlGorilla linear statistical model t o our data, but tooth
comparisons (Tables 1 and 2) yielded a sig- development is not constant (Fanning, 1961;
nificant difference (a= 0.01) in either slope Moorrees et al., 1963; Swindler et al., 1982).
or y-intercept. Indeed, the similarity in pat- In the M1:C comparison, a linear model protern is striking. Despite obvious morpholog- duced slight y-intercept and slope differical and size differences between Pan and ences between the taxa. However, when a
Gorilla, the African apes clearly exhibit a third-order polynomial was fit to these data,
the taxa exhibited virtually identical funcsingular pattern of dental maturation.
Although there are no significant differ- tions (Fig. 3). Therefore, the differences were
ences in any of the ape bivariate paired not a result of different relative growth but of
comparisons, it is worthwhile to examine sample composition. Specifically, they were
those in which differences in y-intercept ap- a consequence of comparing samples with
proach statistical significance (0.10 > P > slightly different age compositions. Neither
0.01) (Table 2). These pairs are Il:C, I2:C, sample compositionnor nonlinearity emerged
Il:P4, I2:P4, Ml:C, Il:M2, I2:M2, and Il:M3. as a dominant factor in any of the remaining
In all of these pairs, chimpanzees display bivariate comparisons.
accelerated or earlier incisor development
Sexual dimorphism
compared to gorillas.
The M1:C comparison is, however, better
Previous studies (Swindler et al., 1982;
explained as a statistical, rather than a bio- Garn et al., 1958; Gleiser and Hunt, 1955;
32
S.W. SIMPSON ET AL.
TABLE 1. Slopes and y-intercepts of the Pan: Gorilla
comoarisons'
Tooth
type
Pan
y-intercept
Slooe
MI-I1
M1-I2
M1-C
M1-P3
MLP4
Ml-M2
M1-M3
11-12
0.607
0.619
0.882
0.890
0.845
0.982
1.893
0.026
0.372
0.299
0.285
0.309
0.982
0.387
0.251
0.249
0.296
1.135
-0.018
-0.031
0.007
0.525
0.091
0.201
0.946
0.119
1.056
0.938
0.937
0.960
1.309
0.763
0.850
0.826
0.126
1.010
1.253
0.894
0.885
0.893
0.977
1.154
0.909
0.891
0.883
0.723
0.704
0.709
0.705
0.762
0.904
0.850
0.776
0.939
0.704
0.815
11-c
I1-P3
Il-P4
I1-M2
I1-M3
12-c
I2-P3
I2-P4
I2-M2
I2-M3
c-P3
c-P4
C-M2
C-M3
P3-P4
P3-M2
P3-M3
P4-M2
P4-M3
M2-M3
Gorilla
y-intercept Slope
0.512
0.558
1.125
0.827
0.937
0.980
1.928
0.055
0.582
0.422
0.502
0.497
1.351
0.587
0.436
0.446
0.472
1.271
0.055
0.017
0.027
0.778
0.080
0.146
0.859
0.099
0.976
0.965
0.937
0.860
0.915
1.035
0.904
0.864
0.083
1.001
1.106
0.988
0.887
0.895
0.692
1.010
0.940
0.899
0.858
0.695
0.728
0.723
0.713
0.557
0.883
0.832
0.737
0.916
0.688
0.729
'The earlier developing tooth in each tooth pair is listed first. See
also Table 2.
Swindler and Gavan, 1962; Hurme and VanWagenen, 1961; Fanning, 1961; Anderson et
al., 1976; Schultz, 1935; Nolla, 1960) have
identified sexual differences in the absolute
but not relative timing of dental development and emergence within anthropoids.
Swindler and co-workers (1982) have examined sexual differences in developmental
pattern in nonhuman primates. Following
an analysis of dental ontogeny in three anthropoid species (Macaca mulatta, M. nemestrina, and Homo sapiens), they concluded
that "males and females follow the same
pattern of development; the only difference
is the chronological age at which the events
occur" (1982:51).
Because the apes demonstrated no interspecific differences, we combined both taxa
into a single group in order to identify any
differences attributable to sex (Tables 3 and
4).With the exception of the Ml:P3 comparison, no bivariate pair differed significantly.
That the Ml:P3 pair differed in both slope
and y-intercept is striking in light of the fact
that no other pair involving either the P3 or
TABLE 2. Ranking of bivariate tooth pairs based on
magnitude of difference between the y-intercept
for Pan us Gorilla'
Tooth
11-M3'
M1-C2
11-P4'
11-c2
12-c2
12-P4'
C-M3
Il-M2'
I2-P3
I2-M2'
I2-M3
I1-P3
MLP4
Ml-M2
c-P3
M1-I2
c-P4
Ml-M3
C-M2
11-12
P4-M3
P3-P4
P4-M2
P3-M2
Ml-P3
P3-M3
M1-I1
y-intercept
Slope
difference difference
0.369
0.243
0.217
0.210
0.200
0.197
0.193
0.188
0.185
0.180
0.136
0.123
0.092
0.088
0.073
0.061
0.048
0.035
0.020
0.029
0.027
-0.011
-0.020
-0.055
-0.063
-0.087
-0.095
-0.285
-0.394
0.002
-0.147
-0.144
0.008
-0.205
0.002
0.031
-0.025
-0.028
0.094
0.054
0.038
0.024
-0.100
0.014
-0.043
0.008
-0.008
-0.086
-0.022
-0.023
-0.018
0.272
-0.039
0.000
Probability
y-intercept Slope
0.012
0.081
0.056
0.078
0.060
0.031
0.166
0.086
0.146
0.047
0.221
0.427
0.253
0.276
0.569
. ...
0.670
0.700
0.128
0.853
0.374
0.750
0.911
0.684
0.584
0.724
0.503
0.541
0.181
0.081
0.986
0.346
0.231
0.914
0.163
0.985
0.772
0.738
0.837
0.507
0.514
0.664
0.809
~.
0.430
0.882
0.290
0.926
0.723
0.367
0.750
0.535
0.806
0.167
0.798
0.998
'None of the comparisons differ significantly ( u = 0.01).
'Tooth pairs discussed in text.
Refer to table 1 for absolute slope and y-intercept values.
the M1 also differed significantly. Some of
the variance in this particular case is undoubtedly due to our small sample size
(n < 30). Because this was the only comparison that yielded a considerable difference
between any of our groups, we concur with
previous authors (Swindler et al., 1982) that
anthropoids do not exhibit significant sexual
differences in relative development.
Small sample size (Hamann-Todd, n =
38) and indeterminate sex identification
(Libben) precluded a similar analysis of our
human material.
Methodology
Methodology was a potential source of
nonbiological variation in our original analysis. While we extracted and scored Libben
dentitions directly, ape dentitions were
scored from radiographs. In addition, Libben
maxillary and mandibular dentitions were
combined into a single dental score for each
individual, whereas ape dental scores were
based only on maxillary teeth. Despite the
33
RELATIVE DENTAL DEVELOPMENT
TABLE 3. Slopes and y-intercepts by sex for
mandibular deuelopment in apes'
Tooth
type
M1-I1
MI-I2
M1-C
Ml-P3
Ml-P4
Ml-M2
Ml-M3
11-12
11-c
I1-P3
I1-P4
I1-M2
Il-M3
12-c
I2-P3
I2-P4
I2-M2
I2-M3
c-P3
c-P4
C-M2
C-M3
P3-P4
P3-M2
P3-M3
P4-M2
P4-M3
M2-M3
Female
y-intercept
Slope
0.375
1.105
1.475
0.646
1.253
1.253
1.930
0.056
0.707
0.285
0.594
0.555
1.270
0.786
0.549
0.577
0.529
1.287
0.076
0.003
-0.026
0.707
0.021
0.119
0.934
0.090
1.016
0.971
1.053
0.566
0.498
1.101
0.565
0.577
0.081
0.997
0.970
1.044
0.760
0.772
0.672
0.803
0.825
0.763
0.783
0.596
0.730
0.758
0.764
0.637
0.942
0.878
0.713
0.939
0.695
0.792
TABLE 4. Male ape:female ape differences ordered by
absolute difference in the y-intercept'
Male
y-intercept
Slope
0.981
0.985
1.395
1.548
1.071
1.182
1.933
0.077
0.843
0.746
0.532
0.549
1.307
0.769
0.685
0.483
0.528
1.244
0.122
-0.002
-0.025
0.622
0.016
0.111
0.812
0.140
0.981
0.926
0.627
0.668
0.710
0.339
0.373
0.665
0.063
0.986
0.848
0.705
0.840
0.866
0.722
0.877
0.699
0.825
0.817
0.698
0.631
0.679
0.723
0.590
0.921
0.868
0.818
0.905
0.711
0.754
similarity of development in the maxillary
and mandibular teeth (Anderson et al., 1976;
Demirjian, 1980; Swindler, 1985), we tested
the possibility that these slight procedural
differences might have introduced bias into
our analysis. We therefore used the same
linear model as described above to test for
differences in the dental maturation pattern
of Libben (aboriginalNorth American; direct
observation) and Hamann-Todd (contemporary BlackNhite; radiographs). Small Sample sizes (combined n < 20) precluded any
useful comparisons involving the M3. All but
one of the 21 remaining bivariate comparisons failed t o yield any significantly different slopes or y-intercepts (Tables 5 and 6).
The single exception (P3:P4) is problematic,
although it should be noted that Anderson et
al. (1976) found that the P3 and M3 were the
most developmentally variable teeth in their
Caucasian sample.
Therefore, population differences are not a
major source of variation in human dental
development. In addition, we conclude that
radiography and direct observation yield essentially identical results. New cases (i.e.,
fossils) may therefore be compared with
Tooth
type
y-intercept
differences
Slope
differences
MlLP3
M1-I1
Il-P3
I2-P3
0.902*
0.606
0.461
0.136
0.136
0.050
0.046
0.037
0.022
0.003
0.001
-0.001
-0.005
-0.006
-0.006
-0.008
-0.016
-0.035
-0.043
-0.045
-0.062
-0.071
-0.085
-0.085
-0.094
-0.120
-0.122
-0.182
-0.762*
-0.426
-0.339
-0.126
-0.122
-0.034
-0.099
0.050
-0.011
-0.018
-0.041
0.034
-0.021
0.094
-0.079
11-c
P4-M2
c-P3
Il-M3
11-12
Ml-M3
C-M2
I2-M2
P3-P4
I1-M2
c-P4
P3-M2
12-c
P4-M3
I2-M3
M2-M3
IlLP4
MI-M2
M1-C
C-M3
I2-P4
MI-I2
P3-M3
M1-P4
~~
~
~
-0.010
0.074
0.016
0.102
-0.038
0.080
0.088
0.212
-0.047
0.062
0.102
0.105
0.192
Probability
y-intercept Slope
0.001
0.049
0.053
0.526
0.466
0.594
0.840
0.786
0.713
0.853
0.995
0.993
0.970
0.977
0.979
0.960
0.918
0.751
0.687
0.552
0.766
0.572
0.646
0.531
0.585
0.676
0.348
0.190
0.003
0.057
0.082
0.441
0.550
0.601
0.536
0.805
0.772
0.617
0.767
0.789
0.834
0.549
0.593
0.929
0.631
0.902
0.478
0.655
0.607
0.388
0.372
0.755
0.620
0.627
0.496
0.121
these data with confidence regardless of the
method of observation.
Ape:human Comparison
Having confirmed the overall developmental uniformity of both ape and human
samples, respectively, we conducted a bivariate comparison of human (combined Libben
and Hamann-Todd) and ape (combined
chimpanzee and gorilla) mandibular dentitions (Fig. 2: Tables 7 and 8). Differences in
slope were, except for P3:P4, insignificant
for all comparisons. Only systematic differences in y-intercepts could be detected. We
conclude that hominoid dentitions differ
only with respect to relative and absolute
timing but not in relative growth rate.
For further comparisons, we will define
three moieties within the dentition: incisors,
canines, and postcanine teeth.
Comparisons within moieties(incisa1 or
postcanine)
Comparisons of teeth within moieties [e.g.,
P3, P4, M1, M2, M3 (Fig. 2) or 11, I21 show
34
S.W. SIMPSON ET A L
TABLE 5. Slopes and y-intercepts of the Libben
population and the Humann-Todd human sum le for
mandibular relative dental development
P
Tooth
type
M1-I1
M1-I2
M1-C
Ml-P3
Ml-P4
Ml-M2
11-12
11-c
Il-P3
Il-P4
Il-M2
12-c
I2-P3
I2-P4
I2-M2
c-P4
c-P3
C-M2
C-M3
P3-P4
P3-M2
P3-M3
P4-M2
P4-M3
M2-M3
Hamann-Todd
y-intercept
Slope
0.094
0.126
0.132
0.682
0.851
0.961
0.021
0.040
0.514
0.750
0.748
0.020
0.304
0.569
0.599
0.541
0.445
0.656
1.713
0.144
0.377
1.451
0.261
1.304
1.179
1.049
1.034
1.293
0.970
0.882
0.851
1.013
1.220
0.942
0.767
0.855
1.216
1.156
0.979
1.020
0.756
0.773
0.703
0.182
0.963
0.823
0.355
0.842
0.477
0.598
Libben
y-intercept Slope
0.092
0.211
0.173
0.667
0.922
0.967
0.209
0.279
0.668
0.847
0740
0.928
0.873
1.089
0.869
0.781
0.698
0.864
0.980
0.814
0.094
1.117
0.775
0.820
0.723
0.615
0.732
0.644
0.743
0.676
0.718
1.167
0.984
1.030
0.992
0.648
0.816
0.838
0.714
0.487
0.655
1.145
0.476
0.383
0.659
-0.033
0.693
0.760
o.8nF;
..0884
.
TABLE 6. Rank ordering of differences in magnitude
of y-intercepts for Hamann-Todd us Libben'
Tooth
type
y-intercept
difference
Slope
difference
I2-P3
P3-P4
I2-P4
11-c
I2-M2
11-12
c-P4
Il-P3
Il-P4
M1-I2
12-c
M1-P4
c-P3
M1-C
Ml-M2
P3-M2
C-M2
MI-I1
Il-M2
Ml-P3
P4-M2
0.344
0.331*
0.247
0.239
0.238
0.188
0.173
0.154
0.097
0.085
0.074
0.071
0.042
0.041
0.007
0.006
-0.001
-0.003
-0.008
-0.015
-0.294
-0.381
-0.287*
-0.159
-0.240
-0.297
-0.149
-0.141
-0.128
0.038
-0.161
-0.099
-0.101
-0.041
-0.204
-0.153
-0.105
-0.059
-0.120
0.029
-0.101
0.142
Probability
y-intercept Slope
0.124
0.000
0.120
0.027
0.172
0.034
0.172
0.585
0.691
0.285
0.518
0.630
0.750
0.683
0.954
0.959
0.993
0.975
0.972
0.942
0.032
0.123
0.000
0.380
0.068
0.149
0.082
0.202
0.693
0.899
0.031
0.442
0.548
0.720
0.838
0.316
0.329
0.582
0.153
0.914
0.654
0.230
'All M3 comparisonsare omitted because of smallsample sizes ( N <
20). Only the P3:P4 comparison yields a significant (a = 0.01)
difference.
*P5 0.01.
'Small sample size for the Ml:M3, Il:M3, and I 2 M 3 comparisons
preclude statistical analysis.
canine is significantly accelerated within the
dentition relative t o the apes.
only limited distributional differences.
There are two exceptions to this generalization, each involving the P3 (P3:P4 and
P3:M2). These differences may be due to
sampling error or to the sectorial role of the
P3 in apes. As described earlier, however,
the only significant difference which
emerged in our Hamann-TodaLibben comparison also involved the P3 (P3:P4). With
the exception of these isolated cases, all
other intramoiety comparisons failed to
demonstrate any significant differences in
hominoid dental developmental pattern.
The most striking differences in the dental
development of apes and humans are bivariate comparisons of teeth in different moieties (e.g., incisors:postcanine, incisors:canine, and canine:postcanine).
Canine
As shown in Table 8, all bivariate pairings
which included the canine differed significantly. In that table, positive y-intercept differences indicate that the later developing
tooth in each pair is accelerated, or earlier, in
its relative development. In humans, the
Incisors
All pairings involving the incisor moiety
yield very significant (P< 0.01) y-intercept
differences. While incisorlcanine differences
are best explained by changes in the timing
of the canine, incisorlpostcanine differences
clearly result from modification of incisor
growth parameters. Like the canine, human
incisors are also accelerated relative to cheek
teeth.
DISCUSSION
Chimpanzees and gorillas demonstrate almost identical dental developmental patterns, although chimpanzees show a slightly
earlier onset of incisor calcification. Chimpanzees also exhibit less alveolar prognathism relative to gorillas (Moore and Lavalle, 1974). We previously hypothesized
that an acceleration of development of the
human premaxilla was the developmental
mechanism by which human incisors underwent reduction. The present parallel case,
within the African apes, is therefore fully
consistent with this hypothesis and therefore justifies its review here.
35
RELATIVE DENTAL DEVELOPMENT
A
20
15
15
0
-
i
0
a't,
B
20
0
0
10
c
10
2
Y-
L L
i
05
05
00
00
05
10
15
20
00
00
0.5
10
15
20
--
0.0
0.0
C
05
Fourth Premolar
Fourth Premolar
0.0
05
1.0
1.5
2.0
Second Molar
00
05
1.0
Third Molar
15
20
D
Fig. 2. Four ape:human bivariate comparisons ofthe
postcanine moiety. Least-squares regression and the
99% confidence interval of the regression are included
for each. Data from Table 7. (A) P4 and P3. Both the
slopes and y-intercepts are significantly (a= 0.01) different. Apes show coincident onset of premolar calcification; humans show a delay in onset of the P 4 relative to
the P3. (B)M1 with P4. Humans and apes show virtually
identical development of the P4 and M1. (C) M1 with M2.
Apes and human show identical relative development of
these molars. (D)M2 with M3. Ape and human distributions overlap completely. Note that humans do not demonstrate a n y delay of either the second or third molar. In
all scatterplots, the ape regressions are represented by
solid lines and humans by stippled lines. Circles = apes;
Squares = humans.
In his survey of the primate premaxilla,
Ashley-Montagu (1935) noted a strong relationship between its size and age a t fusion,
prognathism, and the relative timing of incisor eruption. These developmental relationships were also recognized by Schultz (1948)
and Wallace (1978).
The alveolar portion of the premaxilla is
fused perinatally to the maxillary corpus in
chimpanzees, whereas, in gorillas the premaxillary suture is patent well into adolescence and adulthood (Ashley-Montagu,1935;
Krogman, 1930; Schultz, 1948). In humans,
the premaxillae fuse with the maxillae, being overgrown by the latter by the third fetal
month (Ashley-Montagu, 1935). These fusion patterns strongly correlate with the de-
gree of alveolar projection and pattern of
incisor development in hominoids.
Fenart and Deblock (1972) described the
ontogenetic trajectory of the developing
teeth in apes and humans. They noted that
gorilla permanent incisors have a growth
path oblique to the occlusal plane, and are
continuously displaced anteriorly throughout maturation. Human incisors, as a result
of facial orthognathism and early premaxillary involution, have a more vertical developmental path. Chimpanzees display an
intermediate route which reflects their median attenuation of premaxillary growth. In
essence, the earlier the cessation of premaxillary growth, the more accelerated is the
development of the incisors.
36
S.W. SIMPSON ET A L
TABLE 7. Slopes and y-intercepts of ape and
human relative dental deuelopment,
mandibular dentition'
Tooth
type
M1-I1
M1-I2
M1-C
MLP3
Ml-P4
Ml-MZ
Ml-M3
11-12
11-c
I1-P3
Il-P4
I1-M2
I1-M3
12-c
12-P3
I2-P4
I2-M2
I2-M3
c-P3
c-P4
C-M2
C-M3
P3-P4
P3-M2
P3-M3
P4-M2
P4-M3
M2-M3 -
Ape
y-intercept
Slope
0.581
0.655
1.008
0.876
0.891
0.935
1.923
0.035
0.471
0.367
0.389
0.401
1.268
0.482
0.362
0.358
0.385
1.247
0.029
0.000
0.019
0.670
0.082
0.174
0.878
0.107
0.992
0.959
0.912
0.896
1.079
0.887
0.887
0.856
0.087
1.010
1.190
0.941
0.897
0.902
0.707
1.091
0.914
0.893
0.878
0.660
0.709
0.711
0.636
0.613
0.895
0.827
0.776
01926
0.720
0.762
Human
y-intercept
Slope
0.100
0.150
0.153
0.663
0.912
0.978
1.974
0.092
0.141
0.650
0.835
0.740
1.930
0.044
0.598
0.786
0.766
1.963
0.479
0.684
0.654
1.552
0.421
0.389
1.267
0.050
1.130
1.059
0.942
0.950
1.141
0.901
0.798
0.722
-0.082
0.961
1.115
0.825
0.782
0.879
0.000
1.174
0.840
0.811
0.810
-0.044
0.741
0.641
0.667
0.294
0.727
0.746
0.465
0.952
0.590
0.687
TABLE 8. Rank ordering of differences in magnitude
of y-intercepts for apes us humans'
Tooth
type
M1-C
M1-I2
M1-I1
12-c
11-c
Ml-P3
P4-M2
M1-M3
Ml-P4
Ml-MZ
11-12
M2-M3
P4-M3
P3-M2
I2-P3
Il-P3
P3-P4
I1-M2
P3-M3
I2-M2
I2-P4
c-P3
I1-P4
C-M2
I1-M3
c-P4
- ~.
I2-M3
C-M3
Significances
Slope
y-intercept
differences differences y-intercept Slope
0.855**
0.505**
0.481**
0.438**
0.330**
0.213
0.057
0.052
-0.021
-0.043
-0.057
-0.100
-0.138
-0.215*
-0.236
-0.283
-0.339**
-0.339*
-0.389*
-0.401**
-0.428**
-0.450**
-0.508**
-0.635**
-0.662
-0.684**
-0I..
71fi**
_"
-0.882**
-0.062
0.054
0.030
-0.083
0.075
-0.014
-0.026
-0.169*
0.089
0.133
0.049
0.075
0.128
0.081
0.074
0.116
0.168*
0.023
0.311
0.068
0.082
-0.032
0.117
-0.031
0.707
0.070
0.704*
0.319
0.000
0.000
0.000
0.000
0.000
0.060
0.327
0.018
0.744
0.584
0.242
0.200
0.179
0.006
0.022
0.021
0.000
0.002
0.002
0.000
0.000
0.000
0.000
0.000
0.034
0.000
0.000
0.000
0.589
0.416
0.684
0.342
0.441
0.914
0.594
0.001
0.237
0.095
0.230
0.442
0.321
0.166
0.505
0.382
0.001
0.846
0.074
0.476
0.342
0.651
0.291
0.548
0.225
0.299
n no&
0.039
'The ape sample includes both Pun and Gorilla dentitions. The
human sample combinesdata from the Hamann-Toddcollection and
the Libben population.
'The extreme values of the y-intercepts are made up entirely of
comparisons involving the incisors and canines, and each of these
indicates a n acceleration of the human anterior dental field relative
to that of the apes (a = 0.01).
The relationship between prognathism
and premaxillary development is readily demonstrable in extinct hominid species (Simpson et al., 1990).Australopithecus afarensis
and A. africanus both show marked facial
prognathism. Concomitant with this morphology is the patency of the premaxillary/
maxillary suture as seen in the following
specimens: AL 333-105, AL 333-86, Taung,
and MLD-6 (Kimbel et al., 1982; LeGros
Clark, 1978;Wallace, 1978;Clarke, 1977).In
these specimens, there is a clear association
between incisor size and degree of alveolar
prognathism, and there is consequently no
basis whatever to equate what is clearly a
dentofacial adaptation to an alteration of
overall somatic growth rate. Conversely, the
later robust australopithecines and modern
humans are more orthognathic and show a
common pattern of premaxillary involution.
In these species, the premaxilla is much
reduced in size and overgrown by the maxilla
(Ashley-Montagu, 1935; Grine, 1989). Al-
though there exists a commonality in
premaxillary/maxillary relationship and a
similarity in both degree of facial prognathism and relative incisor development in
both humans and robust australopithecines,
this trait complex merely belies a common
developmental response in historically independent lineages. Because a precocial union
of the premaxillae and the maxillae is the
proximate cause of decreased prognathism
in humans (Ashley-Montagu, 1935), the relative differences in facial projection between
chimpanzees and gorillas (see above), and
probably in robust australopithecines, it is
not unexpected that the associated dentary
structures are similarly accelerated. This
accounts for the widely identified similarity
in anterior dental developmental patterning
between robust australopithecines and modern humans (Broom and Robinson, 1951;
Garn and Koski, 1957; Wallace, 1977; Mann,
1975; Dean, 1985; Smith, 1986).
Although hominids and pongids differ
*P5 0.01.
**P5 0,001.
RELATIVE DENTAL DEVELOPMENT
37
In earlier hominids there has been a reduction in canine crown size but not root size
(Robinson, 1956; White et al., 1981). Therefore, the alveolar requirements necessary to
1.5
accomodate this large rooted tooth have been
little altered. We have proposed (Simpson et
al., 1990) that the canine crown underwent
0
reduction
and was incorporated into the in10
cisal developmental field. This involution
r
.LL
may have been a consequence of a change in
its role in inter- and intrasexual relationo Pan
05
ships (Johanson and White, 1979; Lovejoy,
o Gorilla
1981). Hominid males neither achieve nor
retain reproductive access through competitive displays involving the canine tooth. Un0.0
like apes, the human canine erupts before
0.0
0.5
1 .0
15
20
somatic and sexual maturity. This change in
Canine
role of the canine receives strong support
from both its accelerated relative developments (Simpson et al., 1990) and incisiform
B.
structure (LeGros Clark, 1959) in hominids.
Whatever mechanisms of differential canine and incisor size and development are
eventually identified, our results show that
African apes and humans otherwise exhibit
the same fundamental pattern of postcanine
dental development. They are thus consistent with those of previous authors who have
demonstrated a singular postcanine dental
developmental pattern not only among hominoids but throughout catarrhine primates
0.5 1
o Pan
(Gavan, 1953; Swindler and Gavan, 1962;
I
OGoriIla
1
Dean and Wood, 1981; Schultz, 1935; Hurme
and Vanwagenen, 1961). This synonymy of
I
0.0
pattern has frequently been overlooked in
0.0
0.5
1 .o
1.5
2.0
analyses of hominid remains. Swindler
(1985)has noted a similarity in dental develCanine
opmental timing in a variety of primates to
Fig. 3. Comparison of linear and third order regresbe a consequence of similar body size, and
sion models of Pan and Gorilla canine with first molar
Hurme and Vanwagenen (1961) identified a
distribution. Differences are clearly a consequence of
strong relationship between longevity and
sample composition and not of relative growth.
the timinig of dental emergence in primates.
They proposed that dental development represents a stable proportion of total life span,
and maximum life span of a species can be
with respect to their incisal developmental estimated from a knowledge of the absolute
pattern, the most striking distinction is the timing of its dental developmental events.
earlier relative development of the human These analyses suggest that humans do not
canine. Although the hominid canine crown exhibit specialized molar [or somatic (Gavan
has been greatly reduced, this does not sin- and Swindler, 196611 development (as has
gularly account for its acceleration, because been long held), but merely a proportional
differences in crown size are not necessarily extension of the dental developmental peassociated with changes in relative develop- riod.
ment (compare male and female canines in
apes). Furthermore, the canine develops
ACKNOWLEDGMENTS
within the maxilla, thus, premaxillary reWe would like to thank Dr. Bruce Latimer
duction need not lead to any change in cafor allowing us t o examine materials in his
nine size.
A.
2.0
i
0
Y
'
1
I
I
1
38
S.W. SIMPSON ET AL
care and Mr. Bobby Spell for help in radiographing the specimens. This research was
partially funded by NSF Grant BNS
8718856 awarded to Dr. Steven C . Ward.
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