Apith diet cf.Afr.apes, sedges...

Contributions of Biogeochemistry to Understanding Hominin Dietary Ecology
Julia Lee-Thorp & Matt Sponheimer 2006 Yb.phys.Anthrop.49:131­148

Dietary ecology is one key to understanding the biology, lifeways, and
evolutionary pathways of many animals. Determining the diets of long-extinct
hominins, however, is a considerable challenge. Although archaeological
evidence forms a pillar of our understanding of diet and subsistence in the
more recent past, for early hominins, the most direct evidence is to be
found in the fossils themselves. Here we review the suite of emerging
biochemical paleodietary tools based on stable isotope and trace element
archives within fossil calcified tissues. We critically assess their
contribution to advancing our understanding of australopith, early Homo, and
Neanderthal diets within the broader context of non-biogeochemical
techniques for dietary reconstruction, such as morphology and dental
microwear analysis. The most significant outcomes to date are the
demonstration of high trophic-level diets among Neanderthals and Late
Pleistocene modern humans in Glacial Europe, and the persistent inclusion of
C4 grass-related foods in the diets of Plio­Pleistocene hominins in South
Africa. Such studies clearly show the promise of biogeochemical techniques
for testing hypotheses about the diets of early hominins. Nevertheless, we
argue that more contextual data from modern ecosystem and experimental
studies are needed if we are to fully realize their potential.
....
The range of paleodietary methods applied to the South African hominins
provides a good case study for comparisons, and allows elimination of at
least some possibilities. Some firm results have emerged. For one, the d13C
data clearly show that overall both australopith taxa and early Homo
consumed significant proportions of C4 or C4-derived foods. These results
can only be accounted for by consumption of C4 grass, C4 sedges, or animals
which ate these plants, but we cannot tell what these possibilities are from
these data alone. The low d18O is consistent with consumptions of rhizomes
or other roots, as well as animal foods. The microwear data discounts
gelada-like graminivory, since the australopiths¹ pitted molars (Grine 1986;
Grine and Kay 1988) are unlike those of modern geladas whose molar micro-
wear is dominated by scratches (Teaford 1993). On the other hand, two recent
molar microwear studies of savanna Papio baboon populations noted a higher
frequency of pitting than was found in Theropithecus (Daegling and Grine
1999). These baboons consume moderate amounts of savanna grasses on a
seasonal basis. The trace element data from australopith tooth enamel showed
that Australopithecus, and to a lesser extent Paranthropus, had higher Sr/Ca
ratios than contemporaneous carnivores, browsers, and papionins. The unusual
combination of high Sr/Ca and low Ba/Ca in Australopithecus has only been
found in modern fauna that heavily utilize the underground portions of
grasses, such as warthogs (Phacochoerus africanus) and African mole rats
(Cryptomys hottentotus) (Sponheimer et al.2005b). These elemental data are
still preliminary, and certainly cannot be used to state firmly that early
hominins consumed grass rhizomes. Nevertheless, they are entirely consistent
with the possibility and suggest avenues for future research.
Comparing the results from the various techniques may also give us the
opportunity to question some of the assumptions on which we base
interpretations of the results. For instance, it has been suggested that
hominid dental anatomy was not well suited for the processing of animal
foods (Lucas and Peters 2000; Teaford et al.2002; Ungar, 2004), while the
chemical evidence points towards some consumption of animal foods. It has
perhaps not been appreciated that these anatomical observations pertain only
to a limited class of animal foods (ie. flesh or meat-eating), while a great
many animal foods require little if any oral processing. Termites,
grasshoppers, ants, grubs, eggs, and a variety of other insects may be eaten
whole. Soft tissues can also be consumed without oral processing if they can
be reduced to a suitable size through extra-oral means. Moreover, in some
cases apparent disjunctions between dental morphology and actual trophic
behavior can result from the dentition being adapted for other, more
mechanically challenging foods in an animal¹s diet. For example, capuchin
monkeys (Cebus apella) have large, bunodont dentition with thick enamel
adapted for consuming fruits and hard nuts. Nonetheless, close to 25% of
capuchin diets can come from animal foods (Rosenberger and Kinzey 1976;
Fleagle 1999). Similarly, Grine et al.(2006) showed that A.afarensis
microwear closely resembled that of gorillas while their dental and enamel
morphology suggested other affinities. These observations are consistent
with Ungar¹s (2004) argument that among hominoids, differences in dental
morphology primarily reflect their multifarious fallback foods, rather than
their preferred foods during times of plenty.
As for the australopiths, stable isotopes suggest that they broadened the
ancestral ape resource base to include C4 foods which, coupled with
bipedalism, allowed them to pioneer increasingly open and seasonal
environments. Yet, there are equifinality problems that are common in stable
isotope and trace element studies. That is, many different diets can lead
to the same stable isotope (or trace element) composition (Peters and Vogel
2005). Although some progress has been made using further indicators,
including d18O and trace elements, there is little reason to believe that
this problem can be circumvented entirely by relying on chemical means. In
the end, stable isotopes are one tool among many, all of which provide a
slightly different window into the diets of our ancestors. Stable isotopes
will prove most informative when pursued as part of a larger, integrated
paleodietary investigation.
All of these tools also require a great deal of active development to
improve our understanding of how they work in ecosystems today. For
instance, we still have much to learn about of the stable isotope
compositions of modern plants and mammals, and how physiology affects
diet-tissue spacing. We must also continue to test comfortable assumptions.
As a good example, earlier notions of a simple stepwise trophic system from
trace elements that distinguishes, herbivores, omnivores, and carnivores has
been gradually refined after a series of modern ecosystem studies in
different environments (Sillen 1988; Burton et al.1999; Sponheimer and
Lee-Thorp, Kruger National Park Project, unpubl.data). Rather than a simple
trophic level indicator, Sr/Ca and Ba/Ca ratios may ultimately provide just
as much information about plant foods. Hopefully, such actualistic and
experimental work will serve to further refine the entire suite of
paleodietary tools.




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