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Proc Natl Acad Sci U S A. 2000 December 5; 97(25): 13506–13511. Published online 2000 November 28. | PMCID: PMC17605 |
Copyright © 2000, The National Academy of Sciences Anthropology Diet and the evolution of the earliest human ancestors Mark F. Teaford*† and Peter S. Ungar‡ *Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205; and ‡Department of Anthropology, University of Arkansas, Old Man 330, Fayetteville, AR 72701 Received August 4, 2000. Over the past decade, discussions of the evolution of the earliest
human ancestors have focused on the locomotion of the
australopithecines. Recent discoveries in a broad range of disciplines
have raised important questions about the influence of ecological
factors in early human evolution. Here we trace the cranial and dental
traits of the early australopithecines through time, to show that
between 4.4 million and 2.3 million years ago, the dietary capabilities
of the earliest hominids changed dramatically, leaving them well suited
for life in a variety of habitats and able to cope with significant
changes in resource availability associated with long-term and
short-term climatic fluctuations. Since the discovery of
Australopithecus afarensis, many researchers have emphasized
the importance of bipedality in scenarios of human origins ( 1, 2).
Surprisingly, less attention has been focused on the role played by
diet in the ecology and evolution of the early hominids (as usually
received). Recent work in a broad range of disciplines, such as
paleoenvironmental studies ( 3, 4), behavioral ecology ( 5), primatology
( 6), and isotope analyses ( 7), has rekindled interests in early hominid
diets. Moreover, important new fossils from the early Pliocene raise
major questions about the role of dietary changes in the origins and
early evolution of the Hominidae ( 8– 10). In short, we need to focus
not just on how the earliest hominids moved between food patches, but
also on what they ate when they got there. This paper presents a review of the fossil evidence for the diets
of the Pliocene hominids Ardipithecus ramidus,
Australopithecus anamensis, Australopithecus
afarensis, and Australopithecus africanus. These
hominids offer evidence for the first half of human evolution, from our
split with prehistoric apes to the earliest members of our own genus,
Homo. The taxa considered are viewed as a roughly linear
sequence from Ardipithecus to A. africanus,
spanning the time from 4.4 million to 2.5 million years ago. As such,
they give us a unique opportunity to examine changes in dietary
adaptations of our ancestors over nearly 2 million years. We also trace
what has been inferred concerning the diets of the Miocene hominoids to
put changes in Pliocene hominid diets into a broader temporal
perspective. From such a perspective, it becomes clear that the dietary
capabilities of the early hominids changed dramatically in the time
period between 4.4 million and 2.3 million years ago. Most of the
evidence has come from five sources: analyses of tooth size, tooth
shape, enamel structure, dental microwear, and jaw biomechanics. Taken
together, they suggest a dietary shift in the early australopithecines,
to increased dietary flexibility in the face of climatic variability.
Moreover, changes in diet-related adaptations from A.
anamensis to A. afarensis to A. africanus
suggest that hard, abrasive foods became increasingly important through
the Pliocene, perhaps as critical items in the diet. In 1970, Jolly ( 11) noted that australopithecines had
relatively small incisors compared with molars and speculated that this
might be associated with terrestrial seed eating, as seen in
Theropithecus today. Although this idea has been the subject
of some controversy ( 12), Jolly's efforts have stimulated considerable
research on the origins of hominid adaptations and on relative incisor
size in a wide variety of living and fossil primates. Hylander ( 13),
for example, examined the relationship of incisor row length (relative
to body size) in a range of living anthropoids and found that those
species with larger incisors tend to consume larger, tougher fruits,
whereas those with smaller front teeth tend to feed on smaller foods,
or those that require less extensive incisal preparation, such as
leaves or berries. Since the work of Jolly and Hylander, numerous
workers have looked to incisor size in early hominids and other fossil
primates for clues concerning diet. What can incisor size tell us of the diets of Miocene apes?
Unfortunately, not as much as one would like. Ideally, to consider
relative incisor sizes among taxa, we need estimates of species body
weights based on attributes independent of the dentition. Such
estimates are unavailable for most taxa. Furthermore, Miocene apes as a
whole evidently had small incisors compared with extant hominoids, in
much the same way that platyrrhines as a whole have relatively smaller
incisors than do catarrhines, regardless of diet ( 14). Such
phylogenetic effects make it difficult to find an extant comparative
baseline series with which to compare these basal taxa of uncertain
phyletic affinities. On the other hand, incisor size might give us some clues to diet and
tooth use for the early australopithecines, and we have good,
consistent weight estimates from independent studies ( 15, 16) for many
of these taxa. If we look at a regression of maxillary central incisor
breadth on body size for species representing a variety of catarrhine
genera, we see a separation of cercopithecines (with relatively larger
incisors) above the line and colobines below (Fig.
). Furthermore, more frugivorous
chimpanzees and orangutans fall above the line, whereas gibbons and
gorillas fall close to the line, with relatively smaller incisors.
Indeed, values for the living frugivorous great apes fall above the
95% confidence limits of expected incisor size for modern catarrhines.
The human values fall below the 95% confidence limits, indicating that
we have very small incisors relative to body size.
Relative incisor sizes for the three “gracile” australopithecines
are remarkably similar, and they fall very close to the regression
line, much like the gorilla. These results are similar to those
reported by Kay ( 21) and Ungar and Grine ( 17) and suggest that these
hominids used their incisors in ingestion to a similar degree, although
they all probably used these teeth less than either the chimpanzee or
orangutan. These data can also give us some idea of whether a taxon
often eats foods that require incisal preparation. For instance, lar
gibbons have much smaller incisors than orangutans, and they depend on
smaller fruits requiring little incisal preparation ( 17, 22, 23). From
this perspective, the australopithecines probably put less emphasis on
foods that require substantial incisor use, such as those with thick
husks and those with flesh adherent to large, hard seeds. Body weight
estimates and incisor size data for Ardipithecus ramidus and
Australopithecus garhi should provide even more insights. One of the hallmarks of the australopithecines has always been
their large, relatively flat molars ( 24– 29). There are certainly
differences in the amount of occlusal relief between gracile and robust
australopithecines ( 30) (see below). However, by comparison with other
primates, the australopithecines' molars are still flat and huge. Even
in the earliest hominids, this can be seen in a simple plot of
mandibular postcanine tooth area (MD × BL, the product of maximal
mesiodistal and buccolingual diameters), where most taxa have teeth
larger than those of the modern orangutan (Fig.
).
The only exception is Ardipithecus, which is more
chimp-sized in the P 4–M 1
region, but intermediate between chimpanzees and orangutans in the
M 2–M 3 region. Again,
interpretations of such differences are hampered by the lack of body
size estimates for Ardipithecus, but if a body size estimate
of 51 kg is used for A. anamensis (the average of the two
different estimates based on the tibia) ( 18), McHenry's “megadontia
quotient” for this taxon is essentially identical to that for
A. afarensis (Fig. ). In
other words, its molars are large for a hominoid, but smaller than
those of A. africanus or the “robust”
australopithecines.
As one might expect, the Miocene hominoids show a tremendous
range of mandibular molar sizes (Fig. ). Many have postcanine tooth
areas larger than that of Ardipithecus, and some (such as
Ouranopithecus) even have larger postcanine tooth areas than
that of A. anamensis, but as all body size estimates for
them have been computed from dental remains, a megadontia quotient
cannot be computed. The main message from a simple look at postcanine
tooth size is that the earliest hominids make a nice progression
leading into subsequent hominids, but they do not have larger
postcanine teeth than all of the middle to late Miocene hominoids. This might just mean that there are a variety of body sizes sampled in
these taxa. However, as shown by the work of Lucas and colleagues ( 39),
variations in tooth size are a means of adapting to changes in the
external characteristics of foods, such as their size, shape, and
abrasiveness. Clearly, some of these food characteristics were changing
during the evolution of the earliest hominids, as postcanine teeth
became relatively larger and larger. However, evidence from the middle
to late Miocene shows that tooth size, by itself, cannot pinpoint the
initial change to a hominid diet, at least not with the samples at
hand. One other way of looking at postcanine tooth size is to look at
the ratio of the areas of M1 and M3 (Fig.
). Lucas et al. ( 39) showed
that this ratio was inversely related to the percentage of leaves,
flowers, and shoots in the diet; that is, anthropoids with a high ratio
of M1 to M3 area consumed more fruit than did those with a low M1 to M3
ratio. When this is computed for the earliest hominids, plus a sample
of Miocene apes, a clear separation is evident, with the early
hominids, including Ardipithecus, showing higher ratios than
the Miocene apes. So, does this indicate more fruit in the diet of the
earliest hominids? To begin to answer this question, we must look at
analyses of tooth shape.
Variations in tooth shape are a means of adapting to changes in
the internal characteristics of foods, such as their strength,
toughness, and deformability ( 39– 43). Clearly, foods are complicated
structures; thus it is impossible to describe all of the internal
characteristics that might have confronted the earliest hominids'
teeth. However, another approach is to describe the capabilities of
those teeth. For example, tough foods, those that are difficult to fracture,
are generally sheared between the leading edges of sharp crests. In
contrast, hard brittle foods, those that are easy to fracture but
difficult to penetrate, are crushed between planar surfaces. As such,
reciprocally concave, highly crested teeth have the capability of
efficiently processing tough items such as insect exoskeletons and
leaves, whereas rounder and flatter cusped teeth are best suited for a
more frugivorous diet. Kay ( 21) has devised a “shearing quotient”
as a measure of the relative shear potential of molar teeth. Basically,
more folivorous species have the highest shearing quotients, followed
by those that prefer brittle, soft fruits; finally, hard-object feeders
have the lowest shearing quotients ( 21, 44). Shearing crest studies have been conducted on early Miocene
African apes and middle to late Miocene European apes. These studies
suggest a considerable range of diets in these forms. For example,
Rangwapithecus and Oreopithecus have relatively
long shearing crests, suggesting folivory; Ouranopithecus
has extremely short “crests,” suggesting a hard-object
specialization; whereas most other Miocene taxa studied, such as
Proconsul and Dryopithecus, have the intermediate
length crests of a frugivore ( 14, 45). As for the early hominids, A. africanus had more
occlusal relief than did Paranthropus robustus, suggesting a
dietary difference between these species ( 30). Additional preliminary
shearing quotient studies support this idea while reaffirming that the
australopithecines, as a group, had relatively flat, blunt molar teeth
and lacked the long shearing crests seen in some extant hominoids ( 28).
By itself, this indicates that the earliest hominids would have had
difficulty breaking down tough, pliant foods, such as soft seed coats
and the veins and stems of leaves—although they probably were capable
of processing buds, flowers, and shoots. Interestingly, as suggested by Lucas and Peters ( 46), another tough
pliant food they would have had difficulty processing is meat. In other
words, the early hominids were not dentally preadapted to eat
meat—they simply did not have the sharp, reciprocally concave shearing
blades necessary to retain and cut such foods. In contrast, given their
flat, blunt teeth, they were admirably equipped to process hard brittle
objects. What about soft fruits? It really depends on the toughness of
those fruits. If they were tough, then they would also need to be
precisely retained and sliced between the teeth. Again, early hominids
would be very inefficient at it. If they were not tough, then the
hominids could certainly process soft fruits. In sum, Miocene apes show a range of adaptations, including folivory,
soft-fruit eating, and hard-object feeding. This range exceeds that of
living hominoids and especially the early hominids. Although studies of
shearing crest length have been conducted on only some of the early
hominids, all evidence indicates that the australopithecines had
relatively flat molar teeth compared with many living and fossil apes.
These teeth were well suited for breaking down hard, brittle foods,
including some fruits and nuts, and soft, weak foods, such as flowers
and buds; but again, they were not well suited for breaking down tough
pliant foods such as stems, soft seed pods, and meat. Another area of interest regarding dental functional anatomy
is the study of enamel thickness. There are certainly methodological
differences between studies ( 47– 52), but the consensus still seems to
be that the australopithecines had relatively thick enamel compared
with living primates, and that many of the Miocene apes also had thick
enamel ( 24, 28, 48– 49, 51, 53– 54). Interestingly, this perspective
may be changing as we get glimpses of more and more new taxa. For
instance, Conroy et al. ( 55) have noted that
Otavipithecus may have had thin enamel, and White et
al. ( 8) have made the same observation for
Ardipithecus. Granted, in neither case do we have a detailed
series of measurements over the tooth crown, but still, the figures
that have been quoted (less than 1 mm for Otavipithecus and
1.1–1.2 mm for Ardipithecus) are far less than those quoted
for the australopithecines. So what might be the functional significance of enamel thickness? The
most frequently cited correlations are between the consumption of hard
food items, or abrasive food items, and thick molar enamel ( 58– 59).
There are many potential complicating factors ( 51, 56, 59– 60); thus it
is perhaps not surprising that the correlation between enamel thickness
and diet is not a perfect one ( 57). Moreover, thick enamel by itself
does not necessarily provide protection against hard objects, which
commonly cause fracture of enamel ( 61). The best protection against
this is prism or crystallite decussation or interweaving. Maas ( 62,
63), Rensberger ( 64, 65), and others ( 42, 59) have shown that prism and
crystallite orientations can give clues to intricate details of dental
function, and that decussation can be an effective crack-stopping
mechanism in many animals. Only anecdotal references to this phenomenon
in Miocene apes and early hominids have been made thus far, largely
because more detailed work generally requires the sectioning and
etching of teeth. Still, after some discussion and debate ( 48– 49, 53),
a consensus now seems to be that they did have a significant degree of
prism decussation. Thus, the thick enamel of the early hominids may
have been a means of resisting breakage during the consumption of hard
objects and an adaptation that prolonged the life of the tooth, given
an abrasive diet. Numerous workers have recognized that microscopic wear on the
incisors and molars of primates reflects tooth use and diet. For
example, those primates that often use their front teeth in ingestion
have high densities of microwear striations on their incisors.
Furthermore, folivores have a high incidences of long narrow scratches
on their molars, whereas frugivores have more pits on those surfaces.
Among frugivores, hard-object feeders have even higher pit incidences
than soft-fruit eaters. These and other relationships between microwear
and feeding behaviors in living primates have been used to infer diet
in fossil forms. Miocene apes have a remarkable range of microwear
patterning, greatly exceeding that of living hominoids. For example,
relatively high scratch densities suggest that Micropithecus,
Rangwapithecus, and especially Oreopithecus ( 66)
included more leaves in their diets. In contrast, high pit percentages
suggest that Griphopithecus and Ouranopithecus
( 66) were hard-object specialists. Finally, intermediate microwear
patterns suggest that most other species studied, such as
Gigantopithecus, Dendropithecus, Proconsul,
Dryopithecus, and, perhaps, Sivapithecus ( 66– 68), had
diets dominated by soft fruits. These data give us a glimpse of the
extraordinary variation from which the last common ancestor of apes and
hominids evidently arose. Unfortunately, little is known about the microwear of early
australopithecines. No microwear research has yet been published for
either Ardipithecus ramidus or A. anamensis,
although there has been some done on A. afarensis and
A. africanus. The work done on A. afarensis has
been largely qualitative and focused on the anterior teeth, and it
suggests that these hominids were beginning to exploit savanna
resources ( 69). Furthermore, Ryan and Johanson ( 70) argued that
A. afarensis had a mosaic of gorilla-like fine wear striae
and baboon-like pits and microflakes, indicating the use of incisors to
strip gritty plant parts such as seeds, roots, and rhizomes. These
authors also suggested that there was a functional shift in the
P 3 complex from ape-like slicing and cutting to
hominid puncture-crushing. Work done on A. africanus has been more quantitative but has
focused on comparing this taxon to Paranthropus robustus
rather than to extant hominoids. Grine ( 71) found that A.
africanus molars have lower incidences of pitting than seen for
Paranthropus. A. africanus scratches are also longer and
narrower and show more homogeneity in orientation. Grine argued that
compared with the “robust” forms, A. africanus ate
more soft fruits and leaves. Comparisons with work from Teaford ( 72)
places A. africanus between Cebus olivaceus on
one hand and Pan troglodytes on the other. Work on A.
africanus incisors has shown that this taxon has higher microwear
feature densities on all surfaces examined than does
Paranthropus ( 17). This suggests that A.
africanus processed a greater variety of foods with its front
teeth, including larger, more abrasive ones, than were encountered by
Paranthropus. Comparisons with an extant baseline series
examined by Ungar ( 73) puts Australopithecus between
Pongo pygmaeus and the seed predator/folivore
Presbytis thomasi in degree of anterior tooth use in
ingestion. In sum, then, the microwear suggests that, by the end of the Miocene,
hominoids had a wide range of diets. In contrast, A.
afarensis probably focused on soft fruit but also began to
incorporate into its diet abrasive, terrestrial resources that required
incisal stripping. A. africanus may still have focused on
soft fruit, particularly that which required a moderate amount of
incisal preparation. Clearly, considerably more work is needed on these
and other early hominids to put together a reasonable picture of diet
based on microwear evidence. Finally, there are other lines of evidence that we can examine to
look for evidence of diet. Mandibular fragments are among the most
common bony remains found at hominid fossil sites, and the architecture
of this bone has been adapted to withstand stresses and strains
associated with oral food processing. Thus its morphology probably
reflects some aspects of diet. Analyses of australopithecine mandibular
biomechanics have focused on corpus size and shape. Comparisons with extant hominoids have shown that A.
afarensis and A. africanus have relatively thick
mandibular corpora ( 74, 75). The same pattern was also found for
Paranthropus boisei and P. robustus. Fig.
shows mandibular robusticity index
values for extant great apes, some Miocene apes, and early
australopithecines. The early hominids show relatively thicker
mandibular corpora than extant great apes and Miocene catarrhines,
suggesting a morphological shift in the former.
Both functional and nonfunctional interpretations have been offered to
explain this phenomenon. For example, it may simply be that a thick
mandibular corpus is an effect of large cheek teeth or a reduced
canine. This is not a likely explanation, however, as
australopithecines still have relatively broad mandibles when
considered relative to molar size, and there appears to be no
relationship between mandibular robusticity and relative canine size
among the australopithecines ( 75). Despite some inherent difficulties, it seems more likely that the
unique shape of the australopithecine mandibular corpus relates to the
functional demands of mastication. Thickened mandibles can act to
resist extreme stresses associated with transverse bending (that is,
“wishboning”) and torsion. Because wishboning stresses decline
toward the back of the corpus, torsion is likely a more important
explanation. Corpus torsion can result from bite force and muscle
activity during mastication. Therefore, it may be that
australopithecine mandibular morphology reflects elevated stresses
associated with unusual mechanical demands. Daegling and Grine ( 75)
suggest that australopithecines may have eaten fibrous, coarse foods
that required repetitive loading. While this fails to explain why
colobines do not have thick corpora, it does suggest a fundamental
difference between australopithecines and living great apes that may
reflect a shift in diet in the early hominids. Studies of corpus shape in A. anamensis and
Ardipithecus ramidus will likely provide further clues
regarding differences in mandibular architecture between great apes and
later australopithecines. Corpus robusticity indices for A.
anamensis below M 1 average 53.5 (M. Leakey,
personal communication). These values fall at the upper range for
extant hominoids ( Pan = 39.2–57.8;
Gorilla = 43.5–59.7; Pongo =
35.7–52.0) and at the lower end of the range for later fossil hominids
( A. afarensis = 48.4–68.9, A.
africanus = 54.8–79.0) (Fig. ) (data from Daegling and
Grine and Lockwood et al.) ( 75, 85). In sum, the architecture of the mandibular corpus suggests that the
“gracile” australopithecines differed from living apes in their
abilities to dissipate masticatory stresses. Taken with other lines of
evidence, this certainly suggests a difference in diet between living
apes and A. anamensis, and between A. anamensis
and later hominids, with A. anamensis intermediate between
the African ape and later australopithecine conditions. The australopithecines exhibited a complex of morphological
features related to diet that are unique compared with living hominoids
or Miocene apes. These early hominids all had small- to moderate-sized
incisors; large, flat molars with little shear potential; a ratio of
first to third molar area that was low compared with those of extant
apes, but generally higher than those of Miocene apes; thick tooth
enamel; and thick mandibular corpora. This suite of traits is
distinctive of australopithecines and suggests a dietary shift at or
near the stem of hominid evolution. Their thick-enameled, flattened
molars would have had great difficulty propagating cracks through tough
foods, suggesting that the australopithecines were not well suited for
eating tough fruits, leaves, or meat. The dental microwear data agree
with this conclusion, as the australopithecine patterns documented to
date are most similar to those of modern-day seed predators and soft
fruit eaters. Furthermore, given their comparatively small incisors,
these hominids probably did not specialize in large, husked fruits or
those requiring extensive incisal preparation. Instead, the
australopithecines would have easily been able to break down hard,
brittle foods. Their large flat molars would have served well for
crushing, and their thick enamel would have withstood abrasion and
fracture. Their mandibular corpora would probably have conferred an
advantage for resisting failure, given high occlusal loads. In essence,
for much of their history, the australopithecines had an adaptive
package that allowed them ready access to hard objects, plus soft foods
that were not particularly tough. The early hominids could also have
eaten both abrasive and nonabrasive foods. This ability to eat both
hard and soft foods, plus abrasive and nonabrasive foods, would have
left the early hominids particularly well suited for life in a variety
of habitats, ranging from gallery forest to open savanna. Does this mean we can talk of a characteristic
“australopithecine” dietary pattern? Perhaps to some extent, but
although the australopithecines shared many features in common, they
also differed from one another, suggesting a change in diet through
time. Such morphological changes occurred as a mosaic, much as that
seen for locomotor anatomy. Much of the evidence for Ardipithecus ramidus is not yet
available, but despite its thin molar enamel and absolutely smaller
teeth than those of later hominids, it shows molar size proportions
that may hint at dietary changes to come. A. anamensis shows
the first indications of thicker molar enamel in a hominid, and its
molar teeth were equivalent in size to those of A.
afarensis. Still, its mandibular corpus is intermediate in
robusticity between those of living great apes and later
australopithecines. This combination of features suggests that A.
anamensis might have been the first hominid to be able to
effectively withstand the functional demands of hard and perhaps
abrasive objects in its diet, whether or not such items were frequently
eaten or were only an important occasional food source. A.
afarensis was similar to A. anamensis in relative tooth
sizes and probably enamel thickness, yet it did show a large increase
in mandibular robusticity. This increase may be due to changes in peak
force magnitude or degree of repetitive loading in mastication. Either
way, hard and perhaps abrasive foods may have become even more
important components of the diet of A. afarensis. A.
africanus shows yet another increase in postcanine tooth size,
which by itself would suggest an increase in the sizes and abrasiveness
of foods. However, its molar microwear does not show the degree of
pitting one might expect from a classic hard-object feeder. Thus, even
A. africanus has evidently not begun to specialize in hard
objects, but rather has emphasized dietary breadth. In contrast,
subsequent “robust” australopithecines do show hard-object
microwear and craniodental specializations, suggesting a substantial
departure in feeding adaptive strategies early in the Pleistocene. In sum, diet was probably an important factor in the origin and early
evolution of our family. The earliest australopithecines show a unique
suite of diet-related features unlike those of Miocene apes or living
hominoids. Such features suggest that the earliest hominids may have
begun to experiment with harder, more brittle foods at the expense of
softer, tougher ones early on. This does not mean that all of the
australopithecines were specialized hard-object feeders. It merely
means that, through time, they acquired the ability to feed on hard
objects. Many modern primates need to consume critical “fall-back
foods” at certain times of the year ( 6), and it may well be that the
earliest australopithecines resorted to the consumption of hard objects
only in such situations, whereas the robust australopithecines relied
on them far more regularly. Another important aspect of early hominid trophic adaptations is
evident from data presented here—the dietary shift from apes to early
hominids did not involve an increase in the consumption of tough foods,
and so the australopithecines were not preadapted for eating meat. This
conclusion runs counter to ( i) recent isotope work
suggesting that the australopithecines did in fact consume significant
amounts of meat ( 7) and ( ii) nutritional work suggesting
that meat may have provided critical nutrients for both young and old
hominids ( 77– 79). There would seem to be three different ways to
reconcile these perspectives. First, the present study has reviewed
only craniodental features related to diet. If the australopithecines
used other means for ingesting and processing meat (e.g., tools), they
might have been able to process meat more efficiently than the
craniodental evidence suggests ( 80, 81). Second, the heavy C3 signature
found in A. africanus ( 7) may reflect the consumption of
underground storage organs of C3 plants rather than meat ( 82). Third,
the functional analyses of the teeth assume that all meat has the same
degree of toughness. This may not be the case. Studies of the physical
properties of food have thus far focused on plant remains, with only
brief mention of the toughness of materials like skin ( 40, 46).
Variations in toughness between animal tissues might well be due to
variations in the arrangement and density of collagen matrix.
Furthermore, the physical effects of decomposition might render meat
less tough and more readily processed by hominids. If this is so, it
could be further evidence in support of scavenging as part of the early
hominid way of life. Investigators have tried to relate patterns of hominid evolution
to patterns of climatic change for some time ( 3, 4). The focus of much
of the recent work has been on the origin of the genus Homo.
Can the dietary shifts in the earliest hominids also be tied to such
changes? Whereas there is some evidence of large-scale climatic changes
around the Mediterranean ( 83) and unusual faunal turnover in parts of
western Asia ( 84), there are no large-scale changes evident in
sub-Saharan Africa until after the earliest hominids have arrived on
the scene (i.e., not until 1.5–2.5 million years ago). There is the
slow and inexorable cooling and drying of the Miocene, but perhaps the
crucial result of this was an increase in microhabitat variability.
Certainly, there are limits to our paleoecological evidence from this
period, but as Potts ( 4) has noted, “in general, the oldest hominids
were associated with a diverse range of habitats.” These included
lake and river margins, woodland, bushland, and savanna. Potts ( 4) has
emphasized that locomotor versatility was a crucial adaptation of the
earliest hominids in the face of such varied environmental conditions.
We feel that this perspective needs to be extended to the dietary
adaptations of the earliest hominids as well. In such a land of
variable opportunities, the generalized craniodental toolkit of the
earliest hominids may have had a distinct advantage, as it allowed our
forbears the flexibility to cope with short-term and long-term climatic
variations and the resultant changes in resource availability. We are grateful to the Governments of Ethiopia, Kenya, and
Tanzania and especially to the National Museums of Ethiopia, Kenya, and
Tanzania for permission to study early hominid specimens in their care.
This work was supported by National Science Foundation Grants SBR
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