Dental development and life history in living African
and Asian apes
Jay Kelleya,1,2 and Gary T. Schwartzb
a
Department of Oral Biology, College of Dentistry, University of Illinois at Chicago, Chicago, IL 60612; and bInstitute of Human Origins and School of Human
Evolution and Social Change, Arizona State University, Tempe, AZ 85287
Life-history inference is an important aim of paleoprimatology, but
life histories cannot be discerned directly from the fossil record.
Among extant primates, the timing of many life-history attributes
is correlated with the age at emergence of the first permanent
molar (M1), which can therefore serve as a means to directly
compare the life histories of fossil and extant species. To date, M1
emergence ages exist for only a small fraction of extant primate
species and consist primarily of data from captive individuals,
which may show accelerated dental eruption compared with freeliving individuals. Data on M1 emergence ages in wild great apes
exist for only a single chimpanzee individual, with data for gorillas
and orangutans being anecdotal. This paucity of information limits
our ability to make life-history inferences using the M1 emergence
ages of extinct ape and hominin species. Here we report reliable
ages at M1 emergence for the orangutan, Pongo pygmaeus (4.6 y),
and the gorilla, Gorilla gorilla (3.8 y), obtained from the dental
histology of wild-shot individuals in museum collections. These
ages and the one reported age at M1 emergence in a free-living
chimpanzee of approximately 4.0 y are highly concordant with the
comparative life histories of these great apes. They are also consistent with the average age at M1 emergence in relation to the timing of life-history events in modern humans, thus confirming the
utility of M1 emergence ages for life-history inference and providing a basis for making reliable life-history inferences for extinct
apes and hominins.
dental histology
| great apes | tooth eruption | tooth growth
T
o fully understand the evolution of primate life histories, it is
important to be able to evaluate the life histories of fossil
species within the wider context of those of extant primates. The
correlations between molar eruption schedules and various lifehistory variables provide a means to do this (1). The age at first
molar (M1) emergence in particular provides a reliable proxy
from which to infer the overall pace of life history in fossil species (1). This requires accurate knowledge about the relationships between M1 emergence and various life-history attributes
in living species. However, there are currently deficiencies in the
data on extant primate M1 emergence ages that limit the accuracy and reliability of life-history inference for fossil species.
First, data on age at M1 emergence exist for only 26 primate
species (2–4), and there are major impediments to expanding this
database by the usual methods, most of which require long-term
field or laboratory studies wherein animals are sedated at regular
intervals for radiography and/or to monitor tooth emergence.
Given the difficulty of collecting dental emergence data from
living animals using these techniques, the number of species in the
sample is not likely to increase greatly. Apes in particular are
poorly represented, and no reliable data exist for any species other
than the common chimpanzee, Pan troglodytes (5, 6). Second, the
extant primate database has been constructed mostly using captive
animals (2). There is now evidence suggesting that captive animals
exhibit accelerated development compared with free-living individuals, including earlier emergence of teeth (4, 7–10).
These deficiencies have led some to question the reliability and
usefulness of M1 emergence ages for inferring the life histories of
www.pnas.org/cgi/doi/10.1073/pnas.0906206107
fossil species, including fossil hominins (11). Thus, it is critical
to obtain M1 emergence data on additional species, especially
African and Asian apes, and to obtain these data from noncaptive
animals. Here we report reliable ages at M1 emergence for the
orangutan (Pongo pygmaeus pygmaeus) and the gorilla (Gorilla
gorilla gorilla), obtained from wild-shot individuals in museum
osteology collections and calculated entirely from the incremental
growth lines preserved in the enamel and dentine of the teeth.
Teeth preserve both short- and long-period growth lines that are
visible in histological thin sections. These include, respectively,
daily cross striations and Retzius lines in the enamel and the corresponding von Ebner and Andresen lines in dentine (12–15).
Although there is intra- and interspecific variation in Retzius/
Andresen line periodicity (i.e., the number of daily short-period
lines between adjacent long-period lines), within any individual the
periodicity is constant (12, 14, 16). Counts of these lines, therefore,
reveal the time taken to form a tooth, including the tooth crown
and however much root had formed at the time of death (17) (Figs.
1 and 2; see Materials and Methods). The correlations between lifehistory variables and age at M1 emergence are particularly fortuitous because, in all higher primates, the M1 begins to form at or
just before birth. Therefore, in an individual that died when the M1
was emerging, the total formation time of the M1 yields both the
age at death as well as the age at M1 emergence.
Ground sections were prepared from the M1s (one maxillary,
one mandibular) of two Pongo individuals and one Gorilla (mandibular), all of whom died while the M1 was erupting (Fig. 1; see SI
Materials and Methods). All could be demonstrated to be just past
the stage of initial gingival penetrance—the clinical definition of
tooth emergence—despite the lack of gingival remnants in
osteological specimens. In one of the Pongo teeth, the cusps were
slightly higher than the level typical of gingival emergence, based
on comparisons versus other primate individuals for which there is
a radiographic record of eruption and direct observations of gingival emergence (18). In the second Pongo M1, and in that of the
gorilla, eruption had also proceeded slightly beyond initial gingival
emergence, indicated by the presence of food protein stain only at
the tips of some cusps and delimited by a distinct gingival line (19).
Total formation time (TFT) for each M1 was calculated from
the mesial cusp region (plus one section through the distal cusps
in one of the Pongo specimens) using two different methods, one
using the incremental lines in both the crown enamel and root
dentine, and the other using only the axial dentine extending
from the tip of the dentine horn underlying the tooth crown to
the last-formed dentine adjacent to the pulp chamber (see
Materials and Methods). TFT for one of the Pongo M1s was
Author contributions: J.K. designed research; G.T.S. performed research; J.K. and G.T.S.
analyzed data; and J.K. and G.T.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1
Present address: Institute of Human Origins, Arizona State University, Tempe, AZ 85287
2
To whom correspondence should be addressed. E-mail: jkelley@uic.edu.
This article contains supporting information online at www.pnas.org/cgi/content/full/
0906206107/DCSupplemental.
PNAS | January 19, 2010 | vol. 107 | no. 3 | 1035–1040
ANTHROPOLOGY
Edited by Alan Walker, Penn State University, University Park, PA, and approved December 02, 2009 (received for review June 4, 2009)
Fig. 1. Polarized light micrograph of a section through the mesial cusps (protoconid on left) of the Gorilla gorilla M1 (specimen ZSM 1913/1163). (A) Entire section
and (B) magnified view of the protoconid cusp showing the neonatal line (red arrows) in the enamel. (C) Magnified view of the lateral enamel illustrates a series of
successive striae of Retzius (white arrows) that appear as black lines running from the outer enamel surface (left) toward the dentine (lower right).
calculated using both the enamel-plus-dentine and the dentineonly methods to check for consistency of results.
Results
Summary results from each of the histological sections are shown
in Table1 (see Tables S1–S3 for full details). For the two Pongo
individuals, total M1 formation times, and therefore ages at
death, were nearly identical at 4.78 and 4.66 y, respectively. Different age determinations from the first individual were highly
consistent regardless of the method used (4.76 vs. 4.79 y) or
whether the section was from the mesial or distal cusps (4.79 vs.
4.80 y). As the M1s of both Pongo individuals had undergone
gingival emergence, age at M1 emergence in both would have
been somewhat earlier than the age at death. Based on the
position of the mesial cusps in relation the deciduous premolars
and the alveolar margin of the mandible, eruption had proceeded
somewhat further in the first individual. Therefore, slightly more
time had probably elapsed between M1 emergence and death in
this individual than in the second. We estimate this elapsed time
at approximately 2 months in the first individual and 1 month in
the second, or 0.17 and 0.08 y, respectively (see SI Materials and
Methods). This results in an estimated age at M1 gingival emergence of 4.6 y in both individuals. However, we note that, although
we did not observe prenatally formed enamel (delimited by an
accentuated neonatal growth line [Fig. 1B]) in either Pongo individual, if prenatal enamel was in fact present in these individuals,
the ages at death would be reduced by approximately 1 month and
ages at gingival emergence would be closer to 4.5 y (see Table S1).
Age at death in the gorilla individual was calculated to be 3.91 y.
As in the two orangutans, M1 emergence slightly preceded death
in this individual, and by approximately the same amount of time
judging by the extent of protein stains on the cusps. Therefore, age
at M1 emergence is estimated to have been at 3.8 y, or nearly 1
1036 | www.pnas.org/cgi/doi/10.1073/pnas.0906206107
year earlier than in the two orangutans. However, because, in
primates, the mandibular M1 nearly always emerges somewhat
earlier than the maxillary M1 (ref. 2; see SI Materials and Methods), and because one of our two orangutan specimens is a maxillary tooth, the difference between Pongo and Gorilla might not
be quite as great as suggested by these results.
Discussion
The median age at M1 emergence in captive chimpanzees is 3.18 y
for the maxillary M1 (range, 2.26–4.38 y) and 3.15 y for the
mandibular M1 (range, 2.14–3.99 y) (6). Based on these data, plus
questionable estimates of 3.5 y for both Gorilla (20) and two
captive Pongo individuals (2), mean ages at M1 emergence for all
of the great apes have commonly been reported to be in the range
of 3.0 to 3.5 y. Until now, the only reliable estimate for age at M1
emergence in noncaptive apes was for a single individual of Pan
troglodytes verus, at approximately 4.1 y for the maxillary M1 (4).
The likely age of emergence for the mandibular M1 in this individual was approximately 3.8 to 3.9 y, resulting in a combined age
of M1 emergence of approximately 4.0 y (see SI Materials and
Methods). The M1 emergence ages for noncaptive animals
reported here of 4.6 y in Pongo and 3.8 y in Gorilla (mandibular
M1 only) offer support for the likelihood of a later average age at
M1 emergence in free-living chimpanzees than in captive animals.
Although the M1 emergence age for Pongo is based on only two
individuals, the uniformity of the two values suggests that the
average age at M1 emergence in the orangutan is likely to be
substantially greater than in chimpanzees or gorillas, although
data from more individuals will be needed to confirm this. The age
at M1 emergence of 3.8 y in the Gorilla mandibular M1 is the same
as our estimate of 3.8 to 3.9 y for the mandibular M1 in the
noncaptive chimpanzee (4), but, again, more data will be needed
Kelley and Schwartz
A
B
29.93µm
29.42µm
30.52µm
30µm
to determine if the means for these two great apes are in fact
coincident (as discussed further later).
Although limited, the cumulative data therefore suggest that
the average age at M1 emergence in noncaptive extant great apes
ranges from just younger than 4 y to just older than 4.5 y, or
approximately 1 year later than previously supposed.
The ages at M1 emergence reported here, particularly those
for Pongo, as well as the later age reported for noncaptive Pan
(4), are also consistent with the comparative life histories of
these species in relation to one another and in comparison with
that of modern humans. Ages at M1 emergence between 3.0 and
3.5 y for great apes represent only approximately 52% to 60% of
the average age at M1 emergence of 5.8 y in modern humans, the
latter based on an average of various non-European/nonwhite
American populations (21, 22) (see SI Materials and Methods). In
contrast, the ages for M1 emergence reported here for wild great
apes represent 65% to 80% of the modern human value. This
range is more concordant with the durations or ages of attainment for many key life-history attributes of great apes, which are
generally in the range of 55% to 80% of the modern human
values (Table 2) (23–27).
The relatively late age at M1 emergence for Pongo compared
with those of the other great ape species is particularly noteworthy. Recent findings on the life histories of wild orangutans
reveal that many life-history milestones occur much later than in
chimpanzees or gorillas, and that life stages are consequently of
longer duration (26, 28, 29) (Table 2). The late ages at M1
emergence in the two Pongo individuals described here are
consistent with this more prolonged life-history profile. The
estimated age at M1 emergence in the gorilla, at 3.8 y, is likewise
broadly compatible with the comparative life-history data for
great apes and humans (Table 2). Based on the even shorter lifehistory schedule in gorillas than in chimpanzees, we would predict that analyses of larger samples would show a somewhat
Table 1. Summary of crown formation time, root formation time, total tooth formation time,
and age at death for the two P. pygmaeus M1s and the G. gorilla M1
P. pygmaeus pygmaeus
Parameter
Individual 1
Individual 2
G. gorilla gorilla
Prenatal formation, d (y)
CFT , d (y)
RFT , d (y)
TFT (CFT + RFT), d (y)
TFT (dentine)‡, d (y)
Age at death, y
0*
1,079 (2.96)†
671 (1.84)†
1,749 (4.79)
1,737 (4.76)
4.78§
0*
1,152 (3.16)
547 (1.49)
1,699 (4.66)
NA
4.66
48 (0.13)
1,025 (2.81)
449 (1.23)
1,474 (4.04)
NA
3.91||
CFT, crown formation time; RFT, root formation time.
*Prenatal enamel was not observed, but some prenatal enamel formation is common in Pongo M1s (see text).
†
Average of crown and root formation times from mesial and distal M1 sections (see Table S1 for details).
‡
Method II determination for this individual only (see Materials and Methods and Table S1 for details).
§
Represents the average TFT derived from both the mesial and distal sections (both determined by method I; see
Materials and Methods) and from the axial dentine (method II).
||
Age at death = 4.04 − 0.13 y.
Kelley and Schwartz
PNAS | January 19, 2010 | vol. 107 | no. 3 | 1037
ANTHROPOLOGY
Fig. 2. Close-ups of a section through the mesial cusps of the Pongo pygmaeus M1 (UIC specimen). (A) Striae of Retzius in the lateral enamel (long white
arrows) outcropping at the outer enamel surface (right) and daily cross-striations (short white arrows). (B) Close-up of a field of dentine tubules that run
obliquely from the upper left to lower right; arrows indicate linear measurements across 10 successive daily von Ebner lines, which can be seen oriented
perpendicular to the long axis of the tubules. In this region of dentine, the daily lines are spaced, on average, 2.99 μm apart.
Table 2. Comparative life-history variables and age at M1 emergence in extant great apes and
humans
Variable
Age at first reproduction, y
Interbirth interval, y
Survivorship, y||
Age at M1 emergence, y
Gorilla
10.1* (51.3%)
4.3
?
3.8 (65.5%)
Pan
†
14.3 (72.6%)
5.8†
29.7 (54.9%)
4.0† (69.0%)
Pongo
‡
15.7 (79.7%)
6.9‡
43.0 (79.5%)
4.6‡ (79.3%)
Homo
19.7
3.4§
54.1
5.8
Numbers in parentheses indicate the percentage relative to values in humans. Life history sources: Gorilla (23,
24); Pan (25); Pongo (26); Homo (27); all survivorship data from ref 27. M1 sources: Pan (4); Homo (21, 22); Gorilla
and Pongo (present study).
*Value for mountain gorilla, G. g. beringei, which is likely to be earlier than in G. g. gorilla (see text for
explanation).
†
Values for P. t. verus only (Taï Forest, Ivory Coast).
‡
Values for P. p. pygmaeus only (Borneo), omitting a single value for age at first reproduction from Gunung
Palung (26), which involves an age estimate.
§
Interbirth interval is anomalously low in modern humans compared with other anthropoid species. This lifehistory variable is included for between-ape comparisons only and percentages of human values were not
calculated.
||
Expected age at death at age 15 y based on empirically derived survival curves (see Materials and Methods for
further discussion).
earlier mean age at M1 emergence in the former. Moreover, the
schedule of M1 eruption among Gorilla subspecies might be
expected to vary, with the faster-growing and more folivorous
mountain gorilla (G. g. beringei) predicted to possess an even
earlier eruption age than the more frugivorous western lowland
gorilla analyzed here (30, 31).
It has recently been claimed that body mass is a better predictor of great ape life histories than “dental development,” and
that the latter is only weakly related to the timing of life-history
events (11), but there are several problems with this analysis.
First, the authors of this study (11) included data from the entire
dentition rather than just the first molar, whereas it is the latter
that is most strongly correlated with the timing of life-history
events (1). Second, they included data on tooth crown formation
as well as crown emergence, despite there being almost no difference in M1 crown formation times among Pongo, Gorilla, and
humans (11), resulting in poor correlations with the timing of
life-history events. Last, their data on the timing of molar
emergence include some unreliable, anecdotal information and
also combine data from both wild and captive animals (11). Our
results and the life-history data in Table 2 demonstrate that,
contrary to the conclusions of Robson and Wood (11) and when
the emergence of only the first molar is considered, dental
eruption is strongly related to the timing of life-history events in
humans and free-living great apes. It follows that age at M1
emergence should provide reliable inferences about the timing of
life-history events in fossil members of this clade, including
early hominins.
Finally, our results demonstrate the utility of obtaining ages at
M1 emergence entirely from the histology of first molars that
were in the process of erupting. Whereas previous studies of both
extant and extinct primate taxa have demonstrated the feasibility
of obtaining ages at death using dental histology (13, 17, 32–39),
and these have occasionally included the first molar, we propose
that analyses of dental histology be extended specifically to
individuals that died while the M1 was erupting in a variety of
extant primate species, with the express purpose of providing
more reliable ages at M1 emergence and therefore more accurate representation of the correlations between age at M1
emergence and the timing of life-history events for primates as a
whole. This is the most practical method for developing an
inventory of ages at M1 emergence from noncaptive individuals
of many primate species. As we have done here, this can be
accomplished by sampling wild-shot individuals from museum
1038 | www.pnas.org/cgi/doi/10.1073/pnas.0906206107
collections that died when their first molars were erupting. Using
this method, the relationships between various life-history variables and age at M1 emergence in extant primates can be portrayed more accurately than is currently possible with M1
emergence ages that have been obtained mostly from captive
animals. This will lead to more accurate interpretations of life
history in extinct primates, including early hominins, and a better
understanding of the evolution of life-history variation within the
primate order.
Materials and Methods
Specimen Preparation. Erupting M1s were extracted from the skulls of three
African and Asian ape osteological specimens and then prepared for histological analysis. Two of the M1s were from Borean orangutans (P. pygmaeus
pygmaeus): a maxillary M1 from a skull in the osteology collection of the
College of Dentistry, University of Illinois at Chicago, and a mandibular M1
(specimen 1981/233) from a skull in the collections of the Zoologische
Staatssammlung, Munich, Germany. The third specimen was a mandibular
M1 from the skull of a western lowland gorilla (G. gorilla gorilla, specimen
1913/1163), also from the Zoologicshe Staatssammlung, Munich, Germany.
Following extraction, the teeth were photographed and examined for any
wear or staining, either of which would indicate that the molar had pierced
the gingiva and/or was in functional occlusion. Each molar was molded,
cleaned, and embedded in epoxy to prevent shattering during sectioning.
Two sections were prepared from each tooth, one through the apices of the
mesial cusps and one through the distal cusps. All sections were initially
approximately 500 μm thick and were progressively lapped to a final
thickness of approximately 100 μm using a graded series of abrasive discs
and then polished with a 3-μm aluminum oxide powder. The sections were
then ultrasonicated, rinsed in both 90% and anhydrous alcohol, cleared in
CitraSolve, and mounted with DPX medium. The sections were examined
using polarized light microscopy and images were captured using a Spot
Insight 4-MB digital camera and image analysis system.
TFTs. To determine TFTs, we employed two different methods as a test of
methodological reliability. One method used both crown and root components (method I) and the other used only coronal/axial dentine (method II).
Method I. TFT equals crown formation time (cuspal enamel formation time
plus lateral enamel formation time) plus the time to form however much root
was present at the time of death. Cuspal enamel formation time equals the
cuspal enamel thickness measured along a prism divided by the average daily
secretion rate of ameloblasts, the enamel matrix forming cells. Secretion rate
was calculated as the grand mean from >10 measurements of daily crossstriation lengths in each of the inner, middle, and outer regions of the cuspal
enamel (secretion rate typically increases from inner to outer enamel; Fig. 2).
Lateral enamel formation time equals the total number of lateral enamel
Retzius lines multiplied by their periodicity, which is constant in any indi-
Kelley and Schwartz
Life-History Variables. For the comparative life-history data, we chose three
key variables relating to fecundity or lifetime reproductive success: age at first
reproduction, interbirth interval, and survivorship, the latter being an
expression of longevity (Table 2). We chose survivorship rather than maximum lifespan as an expression of longevity for several reasons. First, with
very small samples, maximum lifespan is not particularly reliable as a species
characteristic, and for all the included taxa save modern humans would be
based on a single individual. Second, because even the longest periods of
continuous observation of great apes in the wild have not been of sufficient
duration to record the birth dates of the oldest individuals that have died,
ages at death for these individuals are estimates. Finally, and specifically
concerning comparisons to modern humans, the ability in human societies
for others to partly assume the care of and provide for elderly individuals
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Kelley and Schwartz
makes it more likely that some individuals will live nearer to the actual
physiological maximum of the species. This combined with samples sizes that
are many orders of magnitude larger than those for great apes renders
comparisons of maximum lifespan between humans and great apes somewhat questionable. Nevertheless, maximum lifespan data are still useful as
supporting documentation of species longevity, especially for comparisons
between great ape species (discussed further later).
In contrast to maximum lifespan, survivorship curves incorporate longevity
data from the entire sample, which allows for statistically meaningful
comparisons between taxa. Among other useful metrics from survivorship
curves, in addition to projected age at death for individuals that survive until
age 15 y (used in Table 2), is the age at which the probability of survival
becomes very low. For example, the ages at which the probability of survival
equals 0.1 are approximately 35 y in Pan and 54 y in Pongo (28, 40).
Unfortunately, the only comparison that can be made for gorillas at this
time is the estimated age of the longest living individual in the wild. Among
mountain gorillas, the longest-lived individual, a female, was estimated to
have been somewhat older than 42 y at death. In contrast, the oldest surviving chimpanzee, again a female, was estimated to be 55 y and still alive
when this age was reported, whereas the oldest chimpanzee male was
estimated to be 46 y at death (40). Among orangutans, the oldest wild
female was estimated to be 52 y and still alive, whereas the oldest male was
estimated to be 58 y at death (27). Although the longevity records for
chimpanzees and orangutans are fairly similar, it has been noted that those
for orangutans are based on much smaller samples and suggested that
samples equivalent to those of chimpanzees (and with similarly long periods
of observation) might be expected to reveal even older orangutan individuals (27). It was also noted that this would be compatible with the consistently higher probabilities of survival at all ages in orangutan survival
tables compared with those of chimpanzees (27).
ACKNOWLEDGMENTS. We thank Dr. Richard Kraft, Director, and the
Zoologische Staatssammlung, Munich, Germany, for allowing us to remove
and section the molars from specimens housed at their institution. We also
thank Kierstin Catlett, Frank Cuozzo, William Hughes, and Don Reid for
assistance with specimen preparation and/or analysis, and Chris Dean, Terry
Ritzman, Holly Smith, Tanya Smith, and David Watts for helpful conversations or for providing unpublished data. G.S. was supported by the Institute
of Human Origins at Arizona State University.
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van Schaik CP (Oxford University Press, Oxford), pp 171–188.
27. Kaplan H, Hill K, Lancaster JA, Hurtado M (2000) A theory of human life history
evolution. Evol Anthropol 9:156–185.
28. Wich SA, et al. (2004) Life history of wild Sumatran orangutans (Pongo abelii). J Hum
Evol 47:385–398.
29. Shumaker RW, Wich SA, Perkins L (2008) Reproductive life history traits of female
orangutans (Pongo spp.). Interdiscip Top Gerontol 36:147–161.
30. Breuer T, Hockemba MB, Olejniczak C, Parnell RJ, Stokes EJ (2008) Physical
maturation, life-history classes and age estimates of free-ranging western gorillas –
insights from Mbeli Bai, Republic of Congo. Am J Primatol 71:106–119.
31. McFarlin SC, et al. (2009) Recovery and preservation of a mountain gorilla skeletal
resource in Rwanda. Am J Phys Anthropol S48:187–188.
PNAS | January 19, 2010 | vol. 107 | no. 3 | 1039
ANTHROPOLOGY
vidual (Figs. 1 and 2). Root formation time was calculated in different ways
depending upon the preservation of growth lines in the root dentine (see
Fig. S1): (i) by measuring the root cone thickness along an odontoblast path
(or dentine tubule) at the tooth cervix (the point of root initiation) and
dividing that distance by the average daily secretion rate of odontoblasts
(the innermost dentin of the root cone adjacent to the pulp chamber represents the time of death); (ii) by multiplying the number of Andresen lines
in the cervical root cone by their periodicity (in d), which is the same as the
periodicity of Retzius lines in the enamel; (iii) by measuring the maximum
length of root formed (measured along the granular layer of Tomes from
the cervical margin to the last formed dentine at the tip of the root cone)
and dividing that distance by the average root extension rate (RER), a
measure of the rate of root elongation in μm/d, which was determined by
averaging extension rates taken from many different regions along the root
surface from the cervical margin to the tip of the developing root cone (see
Fig. S1 legend for details on calculating regional RERs); or (iv) the same
procedure as in i, but with the root segmented into three portions defined
by prominent accentuated Andresen lines and then summing the formation
times of the three segments.
Method II. Axial dentine thickness—measured in the coronal dentine along a
single dentine tubule from the tip of the dentine horn to the pulp chamber—
is divided by the average daily odontoblast secretion rate. This yields an
estimate of TFT as it records the time span between the first-formed dentine
at the dentine horn (coincident with the initiation of tooth formation) and
the last-formed dentine (at the time of death) at the margin of the pulp
chamber (see Fig. S1).
32. Beynon AD, Dean MC, Reid DJ (1991) Histological study on the chronology of the
developing dentition in gorilla and orangutan. Am J Phys Anthropol 86:189–203.
33. Dean CM, Beynon AD, Thackeray JF, Macho GA (1993) Histological reconstruction of
dental development and age at death of a juvenile Paranthropus robustus specimen,
SK 63, from Swartkrans, South Africa. Am J Phys Anthropol 91:401–419.
34. Beynon AD, Dean MC, Leakey MG, Reid DJ, Walker A (1998) Comparative dental
development and microstructure of Proconsul teeth from Rusinga Island, Kenya. J
Hum Evol 35:163–209.
35. Dirks W (1998) Histological reconstruction of dental development and age of death in
a juvenile gibbon (Hylobates lar). J Hum Evol 35:411–425.
1040 | www.pnas.org/cgi/doi/10.1073/pnas.0906206107
36. Dirks W, Reid DJ, Jolly CJ, Phillips-Conroy JE, Brett FL (2002) Out of the mouths of
baboons: Stress, life history, and dental development in the Awash National Park
hybrid zone, Ethiopia. Am J Phys Anthropol 118:239–252.
37. Dirks W (2003) Effect of diet on dental development in four species of catarrhine
primates. Am J Primatol 61:29–40.
38. Smith TM, Toussaint M, Reid DJ, Olejniczak AJ, Hublin JJ (2007) Rapid dental development
in a Middle Paleolithic Belgian Neanderthal. Proc Natl Acad Sci USA 104:20220–20225.
39. Smith TM, et al. (in press) Dental development of the Taï Forest chimpanzees
revisited. J Hum Evol.
40. Hill K, et al. (2001) Mortality rates among wild chimpanzees. J Hum Evol 40:
437–450.
Kelley and Schwartz
Supporting Information
Kelley and Schwartz 10.1073/pnas.0906206107
SI Materials and Methods
Calculating Age at M1 Emergence. All three great ape individuals in
our sample died shortly after M1 gingival emergence based on the
criteria described in the main text. We estimated the amount of
time between M1 gingival emergence and the time of death based
on the rate of mandibular M1 eruption in baboons and modern
humans (1, 2). In the latter, the mean time interval between alveolar eruption and complete eruption (see ref. 2 for definitions
of these eruption stages) is 0.85 y, or 10.2 months (alveolar
eruption: mean, 5.25 y; complete eruption: mean, 6.10 y). These
data do not permit an estimate of when during this interval gingival emergence occurred, although it was inferred that it occurs
closer to complete eruption than to the penultimate stage (i.e.,
late eruption) (2). In baboons, the duration of the interval between alveolar and complete eruption as defined by Dean (2) is
approximately 5 months, with gingival emergence occurring 3
months after alveolar eruption (ref. 1 and J.K., unpublished observations). Thus, the amount of time from gingival emergence to
complete eruption is 2 months. If it is assumed that gingival
emergence in humans also occurs somewhat later than the midpoint of the 10.2-month interval between the alveolar and complete phases of eruption (2), the amount of time between gingival
emergence and complete eruption can be estimated at 4 to 4.5
months. According to the overall chronologies of dental eruption
(3), apes should be intermediate between baboons and humans
for this time interval, which we have therefore estimated at 3
months. As none of the individuals in our study had reached the
stage of complete eruption at the time of death, the amount of
time between gingival emergence and death would have been less
than 3 months. Based on the limited amount of staining at the
cusp tips, we therefore estimated these individuals to have been,
at most, between 1 and 2 months past gingival emergence at
death. It is our opinion that the actual times were somewhat
shorter than this and that gingival emergence was a bit closer to
the calculated ages at death, but we used these maximum estimates to be conservative about the age at gingival emergence.
This is a different procedure than has been used to determine
age at M1 emergence in most species, including captive chimpanzees (3–5). In these, age at tooth emergence is determined
using some form of statistical interpolation (generally probit
analysis), using longitudinal records of dental eruption in which
the tooth is scored as either present or absent (i.e., emergent or
nonemergent through the gingiva) (5). When the observation
periods are random with respect to the time of tooth emergence
in longitudinal studies, or the equivalent for individuals in crosssectional samples, such procedures are necessary to ensure unbiased estimates of the age at emergence (6). Such was not the
case with our great ape sample. The specimens were chosen
specifically to correspond to the same precise eruption stage of
the M1, that of being immediately postemergent. In this case, the
only necessary treatment of the data is having a means to accurately assess the relatively brief amount of time between the
age at M1 emergence and the age at death, as described earlier.
In principle, we could have included in our sample individuals
in which the erupting M1 was still preemergent, as well as
additional postemergent individuals, and carried out a statistical
interpolation procedure similar to that used for other species. We
did not do this for two reasons. First, there is no signal on or in
preemergent teeth equivalent to the protein stains on emergent
teeth to assess the amount of time that would have passed until
Kelley and Schwartz www.pnas.org/cgi/content/short/0906206107
emergence. As the individuals in our sample were all very near the
same stage of immediate postemergence, statistical treatment
incorporating preemergent individuals at various stages of preemergence would likely lead to an underestimate of the mean age
at M1 emergence. Second, and as noted earlier, the absolute
amount of time that had elapsed between the ages at M1 gingival
emergence and death in our sample individuals is demonstrably
quite small relative to the ages at death. Thus, whatever error is
present in our estimates of this time interval is unlikely to be
material and is probably no greater than that resulting from
statistical interpolation.
Age at M1 Emergence in Modern Humans and Captive Chimpanzees.
Average age at M1 emergence in modern humans (Table 2 in the
main text) was calculated from two sources (7, 8). The first includes
data only for mandibular M1s, whereas the second includes data
for both mandibular and maxillary M1s. The calculated average
from the two compilations is the average of the maxillary mean
and the mandibular mean. Studies of questionable reliability were
eliminated, as were the redundancies in the two compilations. The
remaining studies included populations from Africa (Gambia,
Cote d’Ivoire, Kenya, Zambia, Zulu, Bantu, Togo, South Africa),
Iraq, and Russia, as well as African-American and Native American populations (Pima, Ontario, Inuit). Average age at M1
emergence in captive chimpanzees was taken from a single sample
(5) and is also the average of the maxillary and mandibular means.
Age at M1 Emergence in Free-Living Chimpanzees. In primates,
including humans, the mandibular M1 almost always emerges
earlier than the maxillary M1. This is evident in the data on
average ages of tooth emergence reported for several modern
human populations (8) and for nonhuman primates (3). It has
also been observed in a study being conducted by one of the
present authors on root growth during the eruption process in
extant great apes, which involves examination of osteological
specimens in which the M1 is undergoing eruption (9). In virtually all of the individual specimens, the mandibular M1 precedes the maxillary M1 in the eruption process. The amount of
time by which eruption of the mandibular M1 precedes that of
the maxillary M1 varies among species (3). In chimpanzees, the
delay in the emergence of the maxillary M1 is very slight (4, 5), a
finding that has generally been confirmed in the study on root
development during molar eruption (9).
The estimate of age at M1 emergence in free-living chimpanzees in Table 2 in the main text is based on a single individual
(10). In this individual, which died at 3.8 y, the maxillary molar
was just past alveolar emergence and was estimated (10) to be
approximately 3 months from gingival emergence based on the
rate of M1 eruption in baboons (1). The mandible of this individual was also recovered but the M1 had fallen out and was
lost (11). Based on the studies cited earlier regarding the minimal offset in the timing of emergence of maxillary and mandibular M1s in chimpanzees, we infer that the mandibular M1
had probably not yet undergone gingival emergence, although we
allow that it was possibly at the point of emergence (11). We
therefore estimated that the mandibular M1 would have achieved gingival emergence at approximately 3.8 to 3.9 y. We used
this range and an estimate of 4.1 y for gingival emergence of the
maxillary M1 (10) to arrive at an average age at M1 emergence
of 4.0 y for this individual.
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1. Kelley J, Smith TM (2003) Age at first molar emergence in early Miocene Afropithecus
turkanensis and life-history evolution in the Hominoidea. J Hum Evol 44:307–329.
2. Dean MC (2007) A radiographic and histological study of modern human lower first
permanent molar root growth during the supraosseous eruptive phase. J Hum Evol
53:635–646.
3. Smith BH, Crummet TL, Brandt KL (1994) Age of eruption of primate teeth: a compendium
for aging individuals and comparing life histories. Yearb Phys Anthropol 37:177–231.
4. Nissen HW, Riesen AH (1964) The eruption of the permanent dentition of
chimpanzees. Am J Phys Anthropol 22:285–294.
5. Kuykendall KL, Mahoney CJ, Conroy GC (1992) Probit and survival analysis of tooth
emergence ages in a mixed-longitudinal sample of chimpanzees (Pan troglodytes).
Am J Phys Anthropol 89:379–399.
6. Smith BH (2001) Dental development and the evolution of life history in Hominidae.
Am J Phys Anthropol 86:157–174.
7. Smith RJ, Gannon PJ, Smith BH (1995) Ontogeny of australopithecines and early
Homo: evidence from cranial capacity and dental eruption. J Hum Evol 29:155–168.
8. Liversidge H (2003) Patterns of Growth and Development in the Genus Homo, eds
Thompson JL, Krovitz GE, Nelson AJ (Cambridge University Press, Cambridge), pp 73–113.
9. Kelley J, Dean MC, Ross S, Interdisciplinary Dental Morphology, eds Koppe T, Meyer G,
Alt KW (Karger, Basel), pp 128–133.
10. Zihlman A, Bolter D, Boesch C (2004) Wild chimpanzee dentition and its implications
for assessing life history in immature hominin fossils. Proc Natl Acad Sci USA 101:
10541–10543.
11. Smith TM, et al. (in press) Dental development of the Taï Forest chimpanzees
revisited. J Hum Evol.
12. Winkler LA, Schwartz JH, Swindler DR (1991) Aspects of dental development in the
orangutan prior to the eruption of the permanent dentition. Am J Phys Anthropol 86:
255–272.
O1
D1
O2
2000 m
D2
Day of death
O3
D3
Fig. S1. Illustration of different methods for calculating root formation time. (Left) Photomontage of the mesial section of P. pygmaeus RM1 (specimen ZSM
1981/233) illustrating coronal dentine thickness in the axial plane (green arrow), which was used for determining age at death (method II; see Materials and
Methods and Table S1). The day of death is indicated as the last formed dentine at the developing root apex and is indicated. (Center) Close-up of the
mesiobuccal root; blue line indicates the path of the dentine tubule associated wth the initiation of root formation used for measuring root cone thickness.
When divided by the mean daily dentine secretion rate (see Fig. 2B), root cone thickness yields an estimate of total root formation time (method I; see
Materials and Methods, Table 1 in the main text, and Table S1); Red Line indicates total root length measured from the cervical margin to the developing root
apex (image cropped apically). When divided by the average RER, it yields total root formation time. RERs were calculated as follows: The formation time of a
portion of root length was calculated by measuring the distance an odontoblast traveled from the root surface to a given Andresen line, and then dividing this
distance by the local daily dentine secretion rate. That portion of root length was determined by following the Andresen line back to the root surface. Dividing
the length of that root portion by its associated formation time yields a local RER. The average RER for the entire root was calculated from a series of local RERs
from different regions along the root, which were then averaged (see Table S2). (Right) Same as in center image but with the root segmented to calculate
formation times for each root segment: distances along an odontoblast (dotted white lines, O) were measured in μm and traced to where they intersect
prominent Andresen lines (white arrow), which when traced back to the root surface defines root segment lengths (D). Dividing the linear distance, e.g., O1, by
the average daily dentine secretion rate associated with that segment, yields the amount of time (in d) taken to form that particular segment of root length,
D1. Total root formation time therefore equals the summed time taken to form segments D1 to D3 (see Table S3).
Kelley and Schwartz www.pnas.org/cgi/content/short/0906206107
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Table S1. P. pygmaeus pygmaeus, UIC specimen, LM1
Parameter
Method I
Crown formation
Cuspal enamel formation time
Prenatal enamel formation time*
Enamel thickness, μm
Formation time, d (y)
Lateral enamel formation time
Retzius line count
Retzius line periodicity, d
Formation time, d (y)
Crown formation time, d (y)†
Root formation
Root cone thickness at cervix, μm
Dentine daily secretion rate, μm/d
Andresen line count
Andresen line periodicity, d
Root formation time, d (y)
M1 total (crown + root) formation time, or age at death, d (y)
Method II
Dentine only
Coronal/axial dentine thickness, μm
Axial dentine daily secretion rate, μm/d
M1 total (dentine) formation time, or age at death, d (y)
Metacone/hypocone (observer 1)
Paracone/protocone (observer 2)
NA
1,270
327 (0.90)
NA
1,250
323 (0.88)
67
11
737 (2.02)
1,064 (2.92)
70
11
770 (2.11)
1,093 (2.99)
1,512
2.22
–
–
681 (1.87)
1,745 (4.79)
–
–
60
11
660 (1.81)
1,753 (4.80)
3,857
2.22
1,737 (4.76)
*As no neonatal line was present, crown initiation was taken to occur at day 0. In Pongo, M1s can initiate prenatally (12) with prenatal enamel formation time
in the range of 29–34 days (n = 2; T.M. Smith, personal communication).
†
Sum of cuspal enamel formation time plus lateral enamel formation time.
NA, not applicable.
Table S2. P. pygmaeus pygmaeus, ZSM 1981/233, RM1
Parameter (method I)
Crown formation
Cuspal enamel formation time
Prenatal enamel formation time
Enamel thickness, μm
Formation time, d (y)
Lateral enamel formation time
Retzius line count
Retzius line periodicity, d
Formation time, d (y)
Additional Retzius lines*
Additional formation time, d (y)†
Crown formation time, d (y)
Root formation
Length of root present, μm
Average extension rate, μm/d
Root formation time, d (y)
M1 total (crown + root) formation time, or age at death, d (y)
Protoconid/metaconid (section 1)
Protoconid/metaconid (section 2)
NA
1,256
261 (0.72)
–
–
–
91
9
819 (2.24)
–
–
1,152 (3.16)
–
–
–
8
72 (0.20)
–
–
–
–
1,699 (4.66)
5,273
9.64
547 (1.49)
*Additional striae were measured from a second mesial cusp section (section 2) that captured 8 additional cervical striae formed subsequent to the last of the
91 Retzius lines counted from section 1. This reveals that section 2 preserved a cervical extension of the enamel crown that did not lie in the plane of section 1.
†
The additional 8 striae multiplied by the periodicity (equal to 9) yields an extra 72 d, which were added to the 819 d of crown formation calculated from
section 1.
Kelley and Schwartz www.pnas.org/cgi/content/short/0906206107
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Table S3. G. gorilla gorilla, ZSM 1913/1163, LM1
Parameter (method I)
Crown formation
Cuspal enamel formation time
Prenatal enamel formation time, d (y)
Enamel thickness, μm
Formation time, d (y)
Lateral enamel formation time
Retzius line count
Retzius line periodicity, d
Formation time, d
Crown formation time, d (y)
Root formation
Root segment 1, d
Root segment 2, d
Root segment 3, d
Andresen line periodicity, d
Root formation time, d (y)
M1 total (crown + root) formation time, d (y)
Age at death, d (y)*
Protoconid
48 (0.13)
919
242 (0.66)
107
7
749
1,025 (2.81)
275
125
49
7
449 (1.23)
1,474 (4.04)
1,474 − 48 = 1,426
(4.04 − 0.13 = 3.91)
*Equals M1 total formation time less the time taken to form the prenatal enamel.
Kelley and Schwartz www.pnas.org/cgi/content/short/0906206107
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