Robust australopithecine diet ablated

Sponheimer and colleagues (2006, link) zapped some Swartkrans teeth with lasers to measure their 13C content. I wrote quite a bit here last year about australopithecine diets, including a long review of isotopic evidence for australopithecine diets.

With respect to dietary differences between A. africanus and A. robustus (the two species with any substantial isotopic sampling), there are four essential observations:

  1. The apparent C4</sup> dietary content of the two species is basically the same, and fairly high. </li>
  2. High C4 foods are not so easy to come by, they include some grasses and sedges and the animals who eat them.
  3. The Sr/Ca ratios of the two species are fairly different.
  4. The postcanine teeth of A. robustus seem to be adapted to crushing and grinding, moreso than A. africanus.
  5. </ol>

    One hypothesis for the difference in Sr/Ca ratios is exploitation of underground tubers (warthogs and mole rats have elevated Sr/Ca similar to A. africanus). A mix of C4 foods has been proposed to solve the grass-eating problem, including seeds, rhizomes, insects, lizards, and herbivore meat. But these don't really solve the postcanine tooth conundrum, and while they may both be true; neither is really testable.

    OK, so does the new laser ablation study solve any problems? First, let's read a bit about what exactly it is, and why it might be useful. Ann Gibbons has written a ScienceNOW article:

    [A] team of American and British researchers studied the teeth of four individuals of Paranthropus robustus (also known as Australopithecus robustus) from the Swartkrans Cave in South Africa. The team scanned the teeth with a sensitive laser, which did not destroy the teeth but etched them lightly enough to free carbon gases long trapped in the enamel. Because different plants absorb atmospheric carbon dioxide differently, the researchers were able to see what types of vegetation the hominids ate based on the ratio of carbon isotopes in their teeth.

    An accompanying perspective by Stanley Ambrose explains:

    In contrast to conventional methods, the laser ablation technique used by Sponheimer et al. barely penetrates the enamel surface of an area of less than 0.5 mm2 and is thus nearly nondestructive (2). Laser ablation also avoids the problem of time averaging in large drilled grooves. Moreover, perikymata can be counted, providing a good estimate of the minimum time interval sampled and of the duration of tooth formation.
    The Paranthropus teeth studied by Sponheimer et al. show interesting patterns of seasonal variation in diet and climate. All have the isotopic composition of mixed feeders, and two show at least ca. 40% variation in the proportions of C3- and C4-based resources over 1 year. One individual had a predominantly C3-based diet and foraged in a cooler, more humid environment; it may have formed its tooth in a very wet year. The others ate more C4-based foods in a warmer, drier environment. Their average carbon-isotope ratios are similar to those of adaptively versatile savanna baboons (2). Analyses of seasonal variation in teeth of modern and fossil baboons and of other hominin species are necessary to evaluate dietary specialization in Paranthropus and niche overlap with other hominin species.

    Back to me. There are two possibilities. First, the differences between 13C values for different samples might be sampling the actual dietary variability of single A. robustus individuals over the course of their tooth development (in this paper, sampled over a course of a couple hundred days).

    Or second, they may just be sampling noise.

    The paper presents comparative data to suggest that this is actual variability in diet and not isotopic noise. They sampled some steenbok teeth from Swartkrans with the same technique. Steenbok are consistent C3 browsers; their diet doesn't vary much in its 13C proportion over time. And the samples from the steenbok teeth didn't show very much variation across different sampling zones from the same tooth. Hence, it looks like the samples from different perikymata actually may give a consistent picture of dietary 13C composition over time.

    Compared to the steenbok, the A. robustus samples show great heterogeneity in 13C content. This heterogeneity is manifested when looking at multiple samples from the same tooth, and it is also manifested when looking at different individuals. So far, that would seem to indicate dietary heterogeneity -- the A. robustus individuals ate a different mix of foods over time, and different individuals ate different foods.

    On the basis of the magnitude of difference (particularly within the single specimen SKX 5939), Sponheimer et al. propose that some individuals must have gone from a diet predominantly composed of C3 foods to one predominantly C4 within the span of two years (estimated 644 days).

    Here's how their paper concludes:

    A dental microwear study of the earlier (3.0 to 3.7 Ma) hominin Australopithecus afarensis found no evidence that its diet changed over time or in different habitats (20). In contrast, stable carbon isotope (3, 4) and dental microwear texture analyses (1) of the slightly younger (3.0 to 2.4 Ma) hominin A. africanus demonstrated that its diet was far more variable. This suggests the possibility that a major increase in hominin dietary breadth was broadly coincident with the onset of increasing African continental aridity and seasonality after 3 Ma (21, 22) and only shortly antedated the first probable members of the genera Homo and Paranthropus (23-25) and the earliest stone tools (26). The undoubted toolmaker Homo is thought to have been a dietary generalist that consumed novel foods such as large ungulate meat and tubers that are abundant in savanna environments (27-30). Paranthropus, in contrast, with its extremely large and flat cheek teeth, thick enamel, robust mandible, and heavily buttressed facial architecture, is often portrayed as a dietary specialist (27-29). Further, it has been argued that this specialization contributed to its extinction when confronted with increasingly dry and seasonal environments later in the Pleistocene, whereas Homo's generalist adaptation was crucial for its success (28, 29). Our results suggest that Paranthropus had an extremely flexible diet, which may indicate that its derived masticatory morphology signals an increase, rather than a decrease, in its potential foods. Thus, other biological, social, or cultural differences may be needed to explain the different fates of Homo and Paranthropus (31).

    We have lots of other reasons to believe that robust australopithecines were not dietary specialists, as pointed out by Wood and Strait (2004). Robust australopithecines had broad geographic ranges, were able to disperse over long distances, and persisted despite substantial climatic and environmental changes. The evidence for dietary differences across the lifespan is certainly consistent with this.

    It does, however, make for an interesting conundrum: if australopithecines were selected on the basis of their ability to find different foods over the course of years, that suggests a strong role for social learning of more food types and broader geographic ranges. But if this was the path taken by robust australopithecines, what was the path taken by Homo?

    References:

    Ambrose SH. 2006. A tool for all seasons. Science 314:930-931. DOI link

    Gibbons A. 2006. Not just nuts and berries for these hominids. ScienceNOW 9 Nov. Full text

    Sponheimer M, Passey BH, de Ruiter DJ, Guatelli-Steinberg D, Cerling TE, Lee-Thorp JA. 2006. Isotopic evidence for dietary variability in the early hominin Paranthropus robustus. Science 314:980-982. DOI link

    Wood B, Strait D. 2004. Patterns of resource use in early Homo and Paranthropus. J Hum Evol 46:119-162. DOI link