The following points are made by Stanley H. Ambrose (Science 2006
314:930):

1) Seasonal variations in temperature, rainfall, and food
availability drive many animals to hibernate or migrate. Animals
that are tethered to their home ranges and remain active in all
seasons may need flexible adaptive strategies for survival,
especially in arid African savannas, where seasonal and annual
rainfall can vary widely. About 2.4 to 1.4 million years ago, our
earliest stone tool-making ancestors, Homo habilis and H.
erectus, shared African savannas with their close relatives,
commonly referred to as “robust” australopithecines or
Paranthropus species (1). How variable were their environments?
How much did their diets overlap in different seasons? And how
did these two bipedal hominins manage to coexist for 1 million
years? New work (2) documents the seasonal variation in diet and
climate of four robust australopithecines from Swartkrans Cave in
South Africa. The authors use laser ablation of tooth enamel — a
method that causes minimal damage to the precious fossils —
followed by advanced methods of isotope analysis. They are
literally blazing a new trail to answers to fundamental questions
about early hominin paleoecology and evolution.

2) With their huge molar teeth and massive jaw muscles, robust
australopithecines are considered dietary specialists that fed
mainly on small, hard, tough, fibrous plant foods. Their
extinction between 1.0 and 1.4 million years ago is often
attributed to their low-nutrient, high-fiber diets. However,
systematic assessments of the cranial and dental anatomy (1) and
dental microwear (3) suggest that their diets were less
specialized than previously thought and more similar to those of
their ancestors and hominin competitors.

3) Dietary niche separation between closely related species is
usually greatest when resources are scarce. For example,
chimpanzees and lowland gorillas that live in the same area eat
similar amounts of fruit for most of the year, but during the
leanest season, gorillas rely entirely on herbaceous vegetation
(4). The powerful teeth and jaws of Paranthropus may have been
essential for survival only when they resorted to tough
“fallback” foods to mitigate competition with Homo.

4) How can stable-isotope variations in teeth provide insight
into seasonality in diet and climate? The answer lies in the
different 13C/12C ratios of different types of plants (5).
Tropical grasses (and a few herbaceous broadleaf plants) fix
atmospheric CO2 using the C4 photosynthetic pathway; these plants
have high 13C/12C ratios. Conversely, most broadleaf plants,
including trees, shrubs, and herbs, use the C3 pathway and have
low 13C/12C ratios. The isotope ratio of the diet controls that
of the consumer, such that grazing (grass-eating) and browsing
(broadleaf-eating) herbivores — and the carnivores that prey on
them — preserve the isotopic difference at the base of the food
web. The carbon-isotope ratios of mixed feeders reflect the
proportions of C3 and C4 plants in their diets.

References (abridged):

1. B. Wood, D. Strait, J. Hum. Evol. 46, 119 (2004).

2. M. Sponheimer et al., Science 314, 980 (2006).

3. R. S. Scott et al., Nature 436, 693 (2005).

4. C. B. Stanford, J. B. Nkurunungi, Int. J. Primatol. 24, 901
(2003).

5. T. E. Dawson, Annu. Rev. Ecol. Syst. 33, 507 (2002).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

The fragile bones of infants rarely survive long enough to make
it into the hominin fossil record. But if they do, they provide
precious evidence about the growth and development of the
individual and its species. This helps researchers not only to
understand how such processes have changed during hominin
evolution, but also to interpret the…

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