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Isotopic Evidence for the Diet of an Early Hominid, Australopithecus africanus

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Science  15 Jan 1999:
Vol. 283, Issue 5400, pp. 368-370
DOI: 10.1126/science.283.5400.368

Abstract

Current consensus holds that the 3-million-year-old hominidAustralopithecus africanus subsisted on fruits and leaves, much as the modern chimpanzee does. Stable carbon isotope analysis ofA. africanus from Makapansgat Limeworks, South Africa, demonstrates that this early hominid ate not only fruits and leaves but also large quantities of carbon-13–enriched foods such as grasses and sedges or animals that ate these plants, or both. The results suggest that early hominids regularly exploited relatively open environments such as woodlands or grasslands for food. They may also suggest that hominids consumed high-quality animal foods before the development of stone tools and the origin of the genus Homo.

Little is known about the diets of hominids that predate the genus Homo, because they did not leave archaeological traces such as “kitchen middens” and stone tools. Consequently, researchers have made dietary inferences based on craniodental morphology (1–4), gross dental wear (1, 2, 5), and dental microwear (6, 7). Some researchers have stressed the importance of animal foods in the diets of these hominids (1, 8, 9); others have suggested that they were primarily adapted for the consumption of plant foods such as grass seeds and roots (3, 4). The current consensus, however, is that these early hominids ate fleshy fruits and leaves (6, 7, 10, 11). This agrees with evidence that they occupied relatively heavily wooded habitats, not open savannas (12–16). In contrast, there is evidence suggesting that the later hominids (∼2.5 million years ago) Homo and Paranthropusinhabited more open environments (13, 16) and were omnivorous (17–20). Here we provide direct isotopic evidence of the diet of an early hominid, the 3-million-year-old Australopithecus africanus from Makapansgat Limeworks in Northern Province, South Africa (13, 16, 21).

Previous studies have shown that the ratio of 13C to12C in tooth enamel can be used to provide dietary information about extinct fauna (19, 22). The foundation of this approach is our knowledge of photosynthesis in plants. Trees, bushes, shrubs, and forbs (C3 plants) discriminate more markedly against the heavy 13C isotope during fixation of CO2 than do tropical grasses and sedges (C4 plants). As a result, C3 plants have δ13C values of –22 per mil (‰) to –30‰, with an average of about –26.5‰ (23), whereas C4plants have δ13C values of –10 to –14‰, with an average of about –12.5‰ (24, 25). Animals incorporate their food's carbon into their tooth enamel with some additional fractionation (26). Hence, the relative proportions of C3 and C4 vegetation in an animal's diet can be determined by analyzing its tooth enamel with stable isotope mass spectrometry. Animals that eat C3vegetation (including fruits, leaves, and the roots of trees, bushes, and forbs) have δ13C values between about –10 and –16‰; animals that eat C4 tropical grasses (including blades, seeds, and roots) have δ13C values between 2 and –2‰; and mixed feeders that eat both fall somewhere in between these two extremes. Carnivores have tooth enamel δ13C values similar to those of their prey (26).

We sampled molar tooth enamel from 4 of the 14 A. africanusindividuals (MLD 12, MLD 28, MLD 30, and MLD 41) that have been unearthed at Makapansgat. We also analyzed the enamel of associated 3-million-year-old fauna (65 individuals from 19 mammalian taxa) in order to place A. africanus within a broader ecological context (Table 1) (27). Makapansgat's C4 consumers (grazing taxa) include the equid Hipparion lybicum; the suid Notochoerus capensis; and the bovids Parmularius braini,Parmularius sp. nov., and Redunca darti. The C3 consumers (browsers and browser/frugivores) include the giraffid Giraffa jumae; the rhinocerotid Diceros bicornis; the chalicothereAncylotherium hennigi; and the bovids Tragelaphussp. aff. angasi, T. pricei, Oreotraguscf. oreotragus, and Cephalophus sp. The bovidsMakapania broomi and Simatherium cf.kohllarseni are the only mixed feeders that we analyzed.Aepyceros sp., Gazella gracilior, and G. vanhoepeni were all C3 consumers (28), despite the fact that their extant kin are generally mixed feeders. This demonstrates the danger of assuming that fossil taxa had the same diets as their closest extant relatives.

Table 1

Mean δ13C values, ranges, standard deviations, and number of individuals we sampled for each taxon from Makapansgat Member 3. Modern cercopithecids are also included for comparison. Permanent molars were sampled, except for larger species such as Diceros bicornis, from which mostly juvenile dentition was available (23).

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Analysis of variance shows that the δ13C values forA. africanus are significantly different from the values for grazers, browsers, and mixed feeders from Makapansgat (P < 0.01) (Table 1). The only taxon from which they are not significantly different is the carnivore Hyaena makapani. Three of the four hominid specimens (MLD 12, MLD 28, and MLD 30) fall outside the range of C3 (fruit, herb, or leaf) feeders at Makapansgat. MLD 30 is so enriched in 13C (–5.6‰) that it falls closer to the mean of the grazers than of the browsers. The δ13C values for these specimens are inconsistent with a diet of fruits and leaves (C3 plants). One specimen (MLD 41), however, does fall within the range of C3 eaters. These data show that (i) A. africanus had a highly variable diet (its range of δ13C values is greater than 18 of the 19 other taxa analyzed), and (ii) the majority of the Makapansgat hominids habitually obtained dietary carbon from C4 plants such as grasses and sedges or from animals that ate C4foods, or both.

Modern South African cercopithecids that range into open woodland and grassland (Cercopithecus aethiops and Papio cynocephalus ursinus) can become similarly enriched in13C, though they usually have nearly 100% C3diets (Table 1). Ecological studies of Papio have shown that grasses can be a dominant component of its diet, particularly in areas with little tree cover and during the dry season (9,29). Field studies suggest that the only modern primates that consistently consume as much or more C4 foods than A. africanus are Theropithecus gelada(9, 30), Erythrocebus patas(31), and P. hamadryas (32), all of which inhabit areas with few trees. In contrast, the majority of the Makapansgat hominids consumed somewhere between 25 and 50% C4 foods despite a relatively high density of trees in the ancient Makapansgat Valley (12, 13,16).

This raises the following question: Why did the Makapansgat hominids exploit C4 resources to such an extent, despite an abundance of C3 resources as well as evidence that they may have been well adapted for foraging in trees (33,34)? C3 feeders are abundant at Makapansgat (16), so it is possible that A. africanusminimized competition by foraging for C4 grasses and sedges, particularly during the dry season when a large portion of available nutrition is found in their roots (which cannot be readily accessed by most animals). An equally plausible explanation is thatA. africanus had a preference for high-quality animal foods, which included C4 plant-eating insects such asTrinervitermes trinervoides or the young of grazing mammals like R. darti, or both. There is limited evidence to help us decide whether one or both of these alternatives is correct. Researchers comparing the dental microwear of A. africanusand modern primates did not conclude that these hominids ate grasses (6, 7), but these studies included no grass-eating modern primates. A comparison of the dental microwear ofA. africanus and modern Papio populations from open environments, whose diet is 25 to 50% grasses (9), would be ideal, but no such study has been undertaken. Recent research demonstrates, however, that the percentage of pitting versus scratching on the molars of modern T. gelada (∼10%) (35) is different from the percentage previously reported for A. africanus (∼30%) (7). Theropithecus gelada consumes grass blades, seeds, and roots nearly exclusively (9, 30), making it a less than ideal analog for a hominid eating 25 to 50% grass foods. Nonetheless, the large difference in microwear features between Theropithecus andA. africanus suggests that we must seriously consider the possibility that these hominids were 13C-enriched because they consumed animal foods.

Carbon isotope analysis of 1.8- to 1.0-million-year-oldParanthropus robustus from Swartkrans in South Africa demonstrates that it too is enriched in 13C as compared to pure C3 consumers (19). The mean δ13C value for A. africanus (–8.2‰) is very similar to the mean for P. robustus (–8.5‰), although the range for the Makapansgat hominids is greater (5.7‰, as compared to 3.2‰). This is surprising because P. robustuslived in an area with more abundant C4 grasses (13, 16) and, concomitantly, animals that ate C4 grasses. This similarity in δ13C values, combined with the fact that the craniodental specialization ofP. robustus may be in an incipient state in A. africanus (2, 36), might mean that these species ate similar food items but that the further specializedP. robustus ate them more efficiently. Thus, P. robustus may have included a higher proportion of tough, fibrous foods in its diet (6, 7) and may have orally processed foods (such as Sclerocarya nuts) thatA. africanus could only access with hammerstones (like chimpanzees use today) (37). Presently there is no evidence that A. africanus used tools, although it is not unreasonable to assume that they could have used tools in the same manner as chimpanzees do today. Despite this, it is suggestive that the last appearance date of Australopithecus and the first appearance date of Paranthropus are close in time (∼2.5 million years ago) (13, 16, 38). It is possible that P. robustus was better able to process foods that A. africanus also favored, contributing to the latter's eventual extinction.

It is believed that the encephalization of early Homo was made possible by the consumption of energy- and nutrient-rich animal foods to “pay” for its metabolically expensive brain (39, 40). Our results raise the possibility, however, that dietary quality improved (through the consumption of animal foods) before the development of Homo (41) and stone tools (42) about 2.5 million years ago. Moreover, trace element (Sr/Ca) (43) and stable carbon isotope analyses (44) do not seem to indicate that earlyHomo from Swartkrans consumed more animal foods than didA. africanus. Therefore, the primary dietary difference between A. africanus and Homo may not have been the quality of their food but their manner of procuring it. One key difference may have been that stone tools allowed Homo to disarticulate bones and exploit bone marrow from large carcasses (obtained through hunting or scavenging) that A. africanuscould not (17, 18, 45).

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