Whilst reduced size, altered shape and diminished sexual dimorphism of the canine–premolar complex are diagnostic features of the hominin clade, little is known about the rate and timing of changes in canine size and shape in early hominins. The earliest Australopithecus, Australopithecus anamensis, had canine crowns similar in size to those of its descendant Australopithecus afarensis, but a single large root alveolus has suggested that this species may have had larger and more dimorphic canines than previously recognised. Here we present three new associated dentitions attributed to A. anamensis, recently recovered from the type site of Kanapoi, Kenya, that provide evidence of canine evolution in early Australopithecus. These fossils include the largest mandibular canine root in the hominin fossil record. We demonstrate that, although canine crown height did not differ between these species,
A. anamensis had larger and more dimorphic roots, more like those of extant great apes and Ardipithecus ramidus, than those of A. afarensis. The canine and premolar occlusal shapes of A. anamensis also resemble those of Ar. ramidus, and are intermediary between extant great apes and A. afarensis. A. afarensis achieved Homo-like maxillary crown basal proportions without a reduction in crown height. Thus, canine crown size and dimorphism remained stable during the early evolution of Australopithecus, but mandibular root dimensions changed only later within the A. anamensis–afarensis lineage, coincident with morphological changes in the canine–premolar complex. These observations suggest that selection on canine tooth crown height, shape and root dimensions was not coupled in early hominin evolution, and was not part of an integrated adaptive package.
One of the earliest derived features of the hominin clade is canine tooth size reduction, with a decrease in
sexual dimorphism in canine crown height, and the loss of maxillary canine tooth ‘honing’ against
the lower third premolar that occurs in most primate species. Canine tooth crown reduction was originally thought
to have first appeared in Australopithecus,1 but now is known to have characterised even earlier
taxa – Sahelanthropus,2 Orrorin,3 Ardipithecus
kadabba4,5,6 and Ardipithecus ramidus.7,8,9,10 However,
the morphology of the Australopithecus canine–premolar complex is derived morphologically
relative to these earlier hominins. Furthermore, canine tooth form appears to have changed throughout the
early evolution of Australopithecus.9,11,12 The pattern and timing of canine evolution is
significant for understanding early hominin evolution because alterations in canine tooth size and dimorphism
constitute evidence of social and/or dietary adaptations.13,14
The earliest member of the Australopithecus–human clade is Australopithecus anamensis
(4.17 Ma – 3.9 Ma).9,15,16,17,18 A. anamensis appears to represent the initial
part of a lineage culminating in the better-known Australopithecus afarensis (3.77 Ma –
3.0 Ma).9,11,12 Compared to A. afarensis, A. anamensis had larger canine basal crown dimensions relative to postcanine tooth size,
more ape-like canine and premolar shapes, and altered topography of the maxilla and mandible in
the regions of the canine juga.11,16,17,18 The canine tooth crowns known for
A. anamensis appear no more variable in their dimensions than those of either
A. afarensis,17,18 or Ar. ramidus,7 which would seem
to suggest that absolute canine crown height and breadth remained stable with minimal
dimorphism throughout the origin and evolution of early Australopithecus.
However, a single large A. anamensis mandibular canine alveolus (KNM-KP 29287),
and to some extent a large canine root with heavily worn crown from Fejej, Ethiopia (FJ-4-SB-1a),20 has
led to the suggestion that there may have been more canine sexual dimorphism early in this lineage than is represented
in the fossil record of preserved canine tooth crowns.17,18 This suggestion would be surprising, given that
data from Ardipithecus, Orrorin and Sahelanthropus indicate that reduction in canine tooth crown height and breadth,
as well as a decrease in dimorphism, are basal hominin traits.
The intermediate temporal position of A. anamensis, between the earlier
Ardipithecus, Orrorin and Sahelanthropus, and A. afarensis,
makes this species of great interest in documenting the rate and timing of changes
in the canine–premolar complex. Whilst a gradual, integrated change in the
complex5,9 might suggest a single vector of selective change to integrate
canine function with that of the incisors, a mosaic pattern of change11
implies a pattern of sequential selective pressures and possible unappreciated functional diversity over time.
From 2003 to 2007, several new fossils attributed to A. anamensis were
recovered from the type site of Kanapoi, Kenya by a team led by one of us (F.K.M.).
The new Kanapoi hominin fossils include three partial, associated dentitions,
each including a canine tooth (Figure 1; Table 1). All are from the lower fluvial
sequence at the site, and are dated to between 4.195 Ma and 4.108 Ma.21
There are two mandibular dentitions: KNM-KP 47951 is a mandibular canine with associated premolars
and KNM-KP 47953 is a mandibular dentition preserving the right canine and premolars, along with the
second and third molar. KNM-KP 47952 is a maxillary dentition with two maxillary canines and an incisor.
These fossils provide important new evidence of canine evolution in early Australopithecus.
Here we present these fossils and consider their implications for understanding canine tooth evolution in early hominins.
The newly discovered A. anamensis teeth were compared with those of extant great apes
(Gorilla gorilla n = 25, Pongo pygmaeus n = 7,
Pan troglodytes n = 15, Pan paniscus n = 17 and Homo sapiens n = 25)
(Table 2), as well as A. afarensis (n = 11), previously described A. anamensis (n = 9) and Ar. ramidus
(n = 6) (Table 3).
Linear data from fossils were taken on original Kenyan A. anamensis specimens by one of us (C.V.W.).
A. afarensis data were kindly provided by William Kimbel and checked against measurements from casts
taken by CVW to ensure consistency amongst data sets. Data for the Fejej fossils were taken by C.V.W.
from casts kindly provided by John Fleagle and checked against the originals by C.V.W. Ar. ramidus data
were taken from Suwa et al.7 with supplementary data on crown heights kindly provided by Gen Suwa,
Tim White and Berhane Asfaw (2009, personal communication, December 1), with the stipulation that the
unpublished numbers are not for reproduction. Data for the Asa Issie A. anamensis specimens were
taken from White et al.9 and checked against the originals by C.V.W.
Maxillary canine basal dimensions are measured as ‘mesiodistal’ and ‘buccolingual’.
Morphologically, the maximum diameter of the canine embraces, or nearly embraces, the base of the mesial
and distal crests of the tooth in most non-human primates. Because the human tooth is mesiodistally
compressed relative to its buccolingual diameter, the maximum diameter of the tooth is not homologous
to its mesiodistal dimensions, as in other species. However, mandibular canine basal dimensions
are presented using ‘maximum’ and ‘minimum’ (being the greatest
dimension perpendicular to the maximum) diameters, because, in hominins, the relative position
of the tubercles are not located at the mesial and distal margins of the tooth. The same definitions
apply to the mandibular canine measurements for non-human primates. Similarly, because the P3
is normally oriented obliquely relative to the tooth row, basal dimensions of the P3 are also
measured as maximum and minimum in all human and non-human primates.
We measured all available crown heights for A. anamensis and A. afarensis (Table 4).
For many primates, wear is a normal and necessary part of canine function, and in some species
the apex of the tooth is worn before the tooth is finished erupting. Canine crown height data
from Plavcan23 for 89 extant primates demonstrate that ‘moderately worn’
(teeth showing some blunting of the apex) and unworn canines do not significantly differ in crown
height.24 Given that the criteria used for excluding worn canines for this study were more
stringent than for the Plavcan23 data set, apical wear had no significant impact on our results.
Here, we did not correct for wear, but have noted those teeth that clearly show apical blunting.
Whilst including worn specimens slightly depresses the mean canine height for the hominin sample,
and increases the variance, the overall change in variation and the range of crown height is small
by comparison to interspecific differences in canine size. Even adding several millimeters to
canine dimensions for worn teeth will not affect the results of this study. Given that the canine crowns of
A. anamensis and A. afarensis were measured by us in the same way, the interspecific
differences in canine size are robust, and our conclusions would not be altered by attempting to estimate
the unworn size of the canine teeth.
Canine data for Ar. ramidus were, as reported to us, ‘corrected’ for wear and damage.
Having not studied the original specimens, we cannot quantify whether the measurements are exactly comparable
to ours or not. Nevertheless, restricting crown height comparisons to only unworn teeth does not alter any of
our results and conclusions. Here we report only the results for the entire sample. Therefore, conclusions drawn
from comparisons between the Ardipithecus and Australopithecus canine crown heights appear to be robust.
Standard parametric and non-parametric statistical tests were used for most comparisons,
as noted where appropriate. To compare the range of variation in root length between hominins
and extant apes and Homo, a bootstrap analysis was carried out using a program written
in Matlab.25 For each fossil taxon comparison, 1000 random samples from each extant
taxon were selected with replacement, and without regard to sex, selecting the same number of
specimens as available for the fossil sample. The number of samples with a range equal to or
exceeding that of the fossil sample was tabulated.
To evaluate basal canine shape proportions, we used the SMATR26 software package
to test for differences in both slope and elevation of reduced major axis lines fit through
ln-transformed mesiodistal and buccolingual canine tooth dimensions amongst all extant ape species,
A. anamensis and A. afarensis, using 1000 iterations (following Wharton et al.27).
To confirm these results, we also performed a least-squares regression through ln-transformed canine mesiodistal
versus buccolingual dimensions of apes only. We calculated the analysis of variance of residuals for extant hominoids, A. anamensis and A. afarensis derived from this least-squares regression,
using Tukey’s honestly significantly different two-tailed tests for post-hoc contrasts
between groups at an alpha level of 0.05. In no case did the results differ from the SMATR
results, so only results from the latter analysis are reported here as they are statistically most appropriate.27
FIGURE 1: New associated dentitions from Kanapoi, Kenya.
All are comparable
morphologically to other published specimens attributed to Australopithecus
anamensis from Kanapoi, Asa Issie and Allia Bay. (a) KNM-KP 47952, labial and
occlusal views: left I11-2 and Cx . (b) KNM-KP 47951, occlusal and lingual views:
LCx , RP3-4 shown, canine reversed for comparison. LP3 and two tooth fragments
not figured. These teeth are missing much of their enamel in places, and the
canine crown is broken off. Despite the missing enamel, the large centrally
placed protoconid of the P3, the metaconid that existed mainly as a tubercle
along a large lingual ridge, a small anterior fovea, and oblique occlusal profile
are typical of other Kanapoi P3s. Similarly, the cusp and basin morphology of the
P4 match those described for A. anamensis from Kanapoi. The premolar roots
are widely splayed, but splaying is a variable characteristic of Australopithecus.
The root morphology is typical of that of other early Australopithecus premolars
with the double fused mesiobuccal root and independent lingual root. (c) KNMKP
47953, occlusal and lingual views: RCx-P4, M2-3. We attribute the molars to
second and third because of the broad, centrally placed interstitial facet on the
mesial face of the M2, the distally elongate shape of the third and the matching
contact facets. Tooth crowns are complete, but the roots are broken towards
their tips. The distal third molar root is just completing formation. Although this
specimen is amongst the larger A. anamensis fossils, even when it is included,
A. anamensis mandibular canine crowns are equivalent in height and variation
to those of A. afarensis. KNM-KP 47953 displays the blade-like morphology
of the mandibular canine crown characteristic of A. anamensis, and indeed is
among the most extreme specimens yet known in this regard. Similarly, its P3
has the most centrally placed paraconid and the most ovoid crown outline of
any discovered so far.
TABLE 1a: Dimensions (mm) of the newly discovered Kanapoi hominin maxillary dentition.
TABLE 1b: Dimensions (mm) of the newly discovered Kanapoi hominin mandibular dentitions.
KNM-KP 47951 has a strikingly large and robust mandibular canine root that is the largest known for any early hominin,
in length, cervical dimensions and volume (Figure 2). The KNM-KP 47951 canine root is substantially larger than the
alveolus for KNM-KP 29287, which was sufficiently large to suggest greater canine size and variation in
A. anamensis compared with all later hominins.17,18 KNM-KP 47951 demonstrates that
neither KNM-KP 29287 nor FJ-4-SB-1a have unusually large canines, nor would KNM-KP 29287 even have belonged
to a particularly large male individual. The large root of KNM-KP 47951 increases the observed range of
variation in length and occlusal dimensions in A. anamensis canine teeth, and so also in overall
size (Figure 2). It is far greater in length and size than any A. afarensis specimen. Long, large
mandibular canine roots also are seen in Ar. ramidus (9) and extant great apes, suggesting that this
is a primitive trait for the hominin clade (Figure 2).
Even though sample sizes of complete root lengths are small, A. anamensis has greater variation
than minimally dimorphic Homo (Figure 2; Table 5). A. afarensis and Ar. ramidus root
length variation, by contrast, is minimal, although too few specimens are preserved with which to assess
degrees of variation in either species. Mandibular root size in both A. anamensis and Ar. ramidus is similar,
and both are substantially greater than A. afarensis. There is no overlap in mandibular root volume between
A. anamensis and A. afarensis (Figure 2). In combination, the data suggest a decline in mandibular root
size from the primitive size in A. anamensis to a derived condition in A. afarensis.
The canine crown of the other new mandibular dentition, KNM-KP 47953, supports the observation that although
canine crowns were not absolutely taller or broader in
A. anamensis than in A. afarensis (11, 16, 17, 18), A. anamensis canines have larger basal
dimensions relative to the size of their postcanine teeth (Figure 3). This shift in relative size ratios apparently
continues a general trend seen when comparing Ar. ramidus to Australopithecus, and at least partly reflects
increasing postcanine tooth size.7,9,13
Together, KNM-KP 47951 and KNM-KP 47953, along with previously known specimens, suggest that mandibular canine crown height,
breadth and root size variation were not coupled in early Australopithecus. Specifically, the new specimens suggest
that whilst canine crowns appear to have reduced in height and dimorphism prior to the appearance of the genus
Australopithecus,5,7 root size and variation decreased within the A. anamensis–afarensis
lineage independently of crown dimensions. Thus, the apparently large alveolus of KNM-KP 29287 reflects the relatively large
roots in the earlier species, and not greater canine crown size as previously
Preserved A. anamensis and A. afarensis fossils do not differ in relative maxillary canine
basal crown and root size, but all four A. afarensis specimens are small, suggesting that they may be
female individuals, thereby obscuring comparisons. Maxillary canine crown shape does differ as part of an overall
shift in morphology of the C–P3 complex during the evolution of A. anamensis into
A. afarensis.5,7,9,11,19 KNM-KP 47952 demonstrates the previously documented
A. anamensis condition of having mesiodistally longer maxillary crowns and roots than does
A. afarensis (Figure 4a; Table 6) (see also Leakey et al.15 and Ward et al.18).
Notably, A. anamensis is nearly identical to Ar. ramidus in the absolute size and occlusal
proportions of the maxillary canines, but both differ from A. afarensis, which has a shape equivalent to
that of the more diminutive human canines (Figure 4a; Tables 2 and 3). Thus, A. anamensis
retained the primitive condition, and shape change occurred during the evolution of A. afarensis.
Accompanying this shift in maxillary canine crown proportions, mesial crest length reduced as a function
of the mesial shoulder of the tooth shifting apically5,7,9,11 (Figure 4b). This change mirrors
the broadening of the mandibular canine, which also experienced morphological alterations through time,
becoming less blade-like. Additionally, the mandibular premolar transformed, with the protoconid shifting buccally,
affecting the fovea form, and the metaconid expanded in
Metric changes in basal shape from A. anamensis to
A. afarensis occurred in the maxillary canine and mandibular premolar, the honing pair, but not in the
mandibular canine or maxillary premolar (Figure 4c). This shape change reflects change in C–P3
function, increasing transverse contact area between maxillary and mandibular teeth, most logically due to increased
use of the canine in food acquisition or preparation. This shape change suggests that any associated change in function
occurred between A. anamensis and
A. afarensis, and not with the origin of Australopithecus. Also, it is now clear that shape changes in the
canine–premolar complex did not accompany selection for reduced canine crown height, which was already diminished
in earlier hominins (Ardipithecus, Orrorin and
TABLE 2a: Descriptive statistics for extant ape and human maxillary canine teeth used in this analysis.
TABLE 2b: Descriptive statistics for extant ape and human mandibular canine teeth used in this analysis.
TABLE 3: Descriptive statistics for fossil hominin canine teeth used in this analysis.
TABLE 4: Listing of canine crown heights for specimens used in this analysis, with an assessment of wear.
FIGURE 2: Crown height, root cervical area
(calculated as maximum x minimum diameters),
root length and an estimate of root volume (calculated as root cervical area
x length) for mandibular canines. Black symbols represent Australopithecus anamensis, circles represent previously
known specimens, stars represent KNM-KP 47951
and triangles represent KNM-KP 47953. Grey diamonds represent Australopithecus afarensis specimens
(bars through points indicate moderately worn to worn crowns).
Squares represent Ardipithecus ramidus specimens. Crown height is similar in both Australopithecus species,
but root length is smaller and less variable in A. afarensis.
TABLE 5: Results from a bootstrap analysis of mandibular root length variation tabulating how often a randomly drawn sample from an extant species shows a range
matching or exceeding that of each fossil hominin sample.
FIGURE 3: Mandibular canine crown basal area (computed as maximum x
minimum diameters) divided by basal area for each postcanine tooth (maximum
x minimum for P3, mesiodistal (MD) x buccolingual (BL) for P4–M3). Black
circles represent Australopithecus anamensis and grey diamonds represent
A. afarensis. Canines tend to be larger relative to the postcanine dentition in
A. anamensis than in A. afarensis; the differences are only statistically significant
for the canine compared to the premolars (p < 0.05, two-tailed Kruskal Wallis
test for both comparisons: C–P3, n = 10, p = 0.045: C–P4, n = 8, p = 0.025), but the
trend is the same for all teeth.
Canine crown reduction is one of the hallmarks of hominin evolution and so plays an
important role in identifying potential adaptive changes at the origins of the clade.
Multiple hypotheses have been put forward to explain canine reduction amongst hominins in general,
including the loss of canine teeth as weapons, dental crowding and selection altering the canines for
food ingestion and/or
29,30,31,32,33 Hypotheses concerning
the co-option of the canine for food processing or gathering either implicitly or explicitly
link canine reduction directly to selection for a change in dietary function. Furthermore,
whilst large canine roots have been noted in early hominins, the relationship between root and
crown reduction (and dimorphism) has not been evaluated.
These new A. anamensis fossils help demonstrate that whilst canine tooth size reduction
probably occurred basally in hominin evolution prior to the evolution of Australopithecus,
changes in canine shape, in both crowns and roots, occurred in a mosaic fashion throughout the
A. anamensis–afarensis lineage. Whatever selective pressure led to canine tooth crown
size reduction in human evolution did not occur at the same time as that leading to tooth crown shape change.
This finding suggests, in turn, that multiple independent factors altered the complex over time. These phenomena,
therefore, appear to have been a result of different pressures. These pressures, in turn, suggest at least the
possibility that the canines of australopithecines may have served a different function from those of either
their ancestors or their descendants.
Canine crown size and dimorphism were already reduced in all earlier hominins
(Ardipithecus, Sahelanthropus and Orrorin) prior to the appearance of
Australopithecus,7,8 suggesting that the ancestor of Australopithecus
probably had reduced crown size and dimorphism as well. However, substantial shape change did not
accompany this crown height reduction. This observation stands in contrast to the hypothesis that
shape changed in association with crown height reduction and incorporation of the tooth into an incisal
functional field.13 The short canine crowns imply that canines no longer played a role as weapons
for intrasexual or intraspecific aggression early in hominin evolution. It follows that changes in canine
shape almost certainly do not signal changes in social behaviour in later hominins.
Therefore, further alterations in canine shape within early Australopithecus
by default probably reflect changes in food processing. Shorter canine crowns also did
not accompany a shift towards thicker tooth enamel and enhanced mastication with the
origins of Australopithecus. Rather, canine crown reduction in earlier hominins
likely exapted the canines to serve a unique, derived function in Australopithecus,
probably in food acquisition and/or processing. The development of the mesial cristid,
which contacts the lateral maxillary incisor, and the elevation of the shoulders of the
maxillary canine accompanies the shift in canine occlusal shape in Australopithecus,
strongly suggesting a dietary function of the canines.13 However, the lack of
simultaneous canine size reduction suggests that this change did not reflect a gradual
integration of the canine into an integrated anterior incisal mechanism. Rather,
it suggests a dietary function unique to Australopithecus,
and not simply human-like. Unfortunately, little is known about anterior
tooth use and function in early hominins. Recent work suggesting similar
overall diets in A. anamensis and A. afarensis is based on molar morphology
and microwear.34 These data demonstrate that the diets of both species involved
heavy mastication of tough food items with similar material properties, but do not address
possible variation in incision or ingestion behaviours, nor do they provide evidence of canine use,
which may have differed. To date, evidence of canine use in early hominin canines, such as with microwear, has not been evaluated.
The addition of the new fossils of A. anamensis presented here reveals a dissociation of canine
root morphology that appears to have accompanied morphological shifts in the C–P3 complex,
but not canine crown height reduction. Canine tooth root size likely accounts, at least in part, for the
inflated anterolateral margins of the mandible seen in A. anamensis as compared with A. afarensis,
in which the canines are set directly anterior to the postcanine tooth rows in A. anamensis,
but more medially in A. afarensis. Relatively large canine roots may also contribute to the
inflated canine jugal area and rounded lateral nasal aperture seen in A. anamensis and possibly
the earliest A. afarensis (Garusi 1).17 These changes in facial and mandibular form,
which may in turn affect masticatory biomechanics, may be spatially linked to the reduction in canine
root size and dimorphism. Unfortunately, little is known about the functional significance of variation
in canine root size or morphology. At the least, these results suggest that crown and root size are not tightly integrated functionally.
The new Kanapoi fossils underscore the complex, mosaic nature of evolution in the hominin canine honing
complex during early hominin evolution, and highlight new questions about hominin dentognathic adaptations.
Identifying the order and timing of these morphological and proportional changes provides a basis for developing
accurate hypotheses to explain the selective factors acting on crown height, crown shape and root dimensions in
Australopithecus that will play a key role in understanding the adaptive transition from
A. anamensis to A. afarensis.
FIGURE 4: (a) Scatter plot of ln-transformed maxillary
canine mesiodistal length
compared to buccolingual breadth in extant and fossil samples. White squares
represent extant great apes (Pan troglodytes, Pan paniscus, Gorilla gorilla
and Pongo pygmaeus); white triangles represent Homo sapiens; black circles
represent Australopithecus anamensis; grey diamonds represent A. afarensis
and grey squares represent Ardipithecus ramidus. A. afarensis canines are
similar in buccolingual size to A. anamensis, but are clearly mesiodistally shorter
than those of A. anamensis. A. afarensis canines are proportionally identical to
humans but humans are smaller overall, with almost no overlap in size with A.
afarensis. (b) Morphological differences in canines and third premolars (5, 9,
11, 17, 18). The maxillary canines of A. anamensis have a lower mesial shoulder
and are more symmetrical than those of A. afarensis (A. afarensis maxillary
canine reversed for comparison). In the mandibular teeth, A. anamensis has a
lower mesial crown shoulder and longer mesial crest, a narrower, more bladelike
mandibular crown with pronounced distal tubercle and a more unicuspid
P3 with centrally placed paraconid compared with A. afarensis. (c) Crown basal
proportions measured as mesiodistal ÷ buccolingual diameters for maxillary
canine and P3, maximum ÷ minimum breadths for mandibular canine and P3.
Black circles represent A. anamensis and grey diamonds represent A. afarensis.
Interspecific differences are seen only in the maxillary canine (p = 0.001, twotailed
Kruskal Wallis test, n = 19) and mandibular premolar (p < 0.001, twotailed
Kruskal Wallis test, n = 26), that is, in the teeth that hone, but not in the
mandibular canine (p = 0.529, two-tailed Kruskal Wallis test, n = 16) or maxillary
premolar (p = 0.44, two-tailed Kruskal Wallis test, n = 13), illustrating that
observed shape changes are associated with a shift in occlusal relationships in
TABLE 6a: Results for tests in elevation for reduced major axis regressions of all apes and hominins comparing maxillary canine mesiodistal (dependent) versus
maxillary canine buccolingual (independent) dimensions: Post-hoc multiple comparisons for pair-wise differences in elevation between groups.
TABLE 6b: Results for tests in slope for reduced major axis regressions of all apes and hominins comparing maxillary canine mesiodistal (dependent) versus maxillary
canine buccolingual (independent) dimensions: Post-hoc tests for shift along the common slope.
We thank the curators and staff of the National Museums of Kenya, National Museum of Ethiopia, Turkana Basin Institute,
Cleveland Museum of Natural History, National Museum of Natural History and Royal Museum of Central Africa for access to
collections in their care and assistance. We would also like to thank Gen Suwa, Tim White, Berhane Asfaw and William Kimbel
for data and access to original fossils; John Fleagle and William Kimbel for casts; and William Kimbel, Milford Wolpoff,
Alan Walker, Bernard Wood, Luke Delazene, Yohannes Haile-Selassie, Scott Simpson, John Fleagle and anonymous
reviewers for helpful comments and discussion. This project was supported by the Leakey Foundation, PAST (South Africa),
Turkana Basin Institute and the Wenner Gren Foundation for Anthropological Research.
We declare that we have no financial or personal relationships which may have inappropriately influenced us in writing this article.
F.K.M. led the team that recovered the fossils; J.M.P. conducted the statistical analyses;
C.V.W. described the fossils; and all authors contributed to writing the manuscript.
1. Dart RA. Australopithecus africanus: The man-ape of South Africa. Nature. 1925;115:195.
2. Brunet M, Guy F, Pilbeam D, et al. A new hominid from the Upper Miocene of Chad, Central Africa. Nature. 2002;418:145–151.
3. Senut B, Pickford M, Gommery D, Mein P, Cheboi K, Coppens Y. First hominid from the Miocene (Lukeino Formation, Kenya).
C R Acad Sc Paris. 2001;332:137–144.
4. Haile-Selassie Y. Late Miocene hominids from the Middle Awash, Ethiopia. Nature. 2001;412:187–191.
5. Haile-Selassie Y, Suwa G, White TD. Late Miocene teeth from middle Awash, Ethiopia, and early hominid dental evolution.
6. Haile-Selassie Y, WoldeGabriel G, editors. Ardipithecus kedabba: Late Miocene evidence from the Middle Awash,
Ethiopia. Berkeley: University of California Press; 2009.
7. Suwa G, Kono R, Simpson S, Asfaw B, Lovejoy C, White T. Paleobiological implications of the Ardipithecus ramidus dentition.
8. White T, Asfaw B, Beyene Y, et al. Ardipithecus ramidus and the paleobiology of early hominids. Science.
9. White T, WoldeGabriel G, Asfaw B, et al. Assa Issie, Aramis and the origin of Australopithecus. Nature.
10. White TD, Suwa G, Asfaw B. Australopithecus ramidis, a new species of early hominid from Aramis,
Ethiopia. Nature. 1994;371:306–312. http://dx.doi.org/10.1038/371306a0,
11. Kimbel W, Lockwood C, Ward CV, Leakey M, Rak Y, Johanson D. Was Australopithecus anamensis ancestral to A. afarensis?
A case of anagenesis in the hominin fossil record. J Hum Evol. 2006;51:134–152.
12. White TD. Earliest hominids. In: Hartwig WC, editor. The primate fossil record. Cambridge:
Cambridge University Press, 2002; p. 407–417.
13. Greenfield L. Origins of the human canine; a new solution to an old enigma. Yearb Phys Anthropol.
14. Plavcan JM. Inferring social behavior from sexual dimorphism in the fossil record. J Hum Evol. 2000;39:327–344.
15. Leakey MG, Feibel CS, MacDougall I, Ward CV, Walker A. New specimens and confirmation of an early age for
Australopithecus anamensis. Nature. 1998;363:62–66.
16. Leakey MG, Feibel CS, McDougall I, Walker A. New four-million-year-old hominid species from Kanapoi and Alia Bay, Kenya.
17. Ward CV, Leakey MG, Walker A. Morphology of Australopithecus anamensis from Kanapoi and Allia Bay,
Kenya. J Hum Evol. 2001;41:255–368.
18. Ward CV, Walker A, Leakey MG. The new hominid species Australopithecus anamensis. Evol Anthropol. 1999;7:197–205.
19. Haile-Selassie Y, Saylor B, Deino A, Alene M, Latimer B. New hominid fossils from Woranso-Mille (Central Afar, Ethiopia)
and taxonomy of early Australopithecus. Am J Phys Anthropol. 2010;141:406–417.
20. Fleagle JG, Rasmussen DT, Yirga S, Bown TM, Grine FE. New hominid fossils from Fejej, southern Ethiopia. J Hum Evol.
21. McDougall I, Brown F. Geochronology of the pre-KBS Tuff sequence, Omo Group, Turkana Basin. J Geol Soc.
22. Wolpoff M. Paleoanthropology. New York: Knopf; 1980.
23. Plavcan J. Sexual dimorphism in the dentition of extant anthropoid primates. Durham: Duke University; 1990.
24. Plavcan J, Ward CV, Paulus F. Estimating tooth crown height in early Australopithecus.
J Hum Evol. 2009;57:2–10.
25. Matlab. Version 7. Natick, MA: Mathworks; 2010.
26. SMATR (Standardized Major Axis Tests and Routines). Version 2. Sydney: Falster D, Wharton D, Wright I; 2006.
27. Wharton D, Wright I, Falster D, Westoby M. Bivariate line-fitting methods for allometry. Biol Rev. 2006;81:259–291.
28. Bailit H, Friedlaender J. Tooth size reduction: A hominid trend. Am Anthropol. 1966;68:665–72.
29. Brace C. Structural reduction in evolution. Am Nat. 1963;97:39–49.
30. Calcagno J, Gibson K. Human dental reduction: Natural selection or probable mutation effect?
Am J Phys Anthropol. 1988;77:505–517.
31. Wolpoff M. The effect of mutations under conditions of reduced selection. Soc Biol. 1969;16:11–23.
32.Jungers W. On canine reduction in early hominids. Curr Anthropol. 1978(19):155–156.
33. Szalay FS. Hunting-scavenging protohominds: A model for hominid origins. Man. 1975;10:420–429.
34. Ungar P, Scott R, Grine F, Teaford M. Molar microwear textures and the diets of Australopithecus anamensis and
Australopithecus afarensis. Philos Trans R Soc London. 2010;365:3345–3354.