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Approaches to dietary reconstruction in the western Cape: Are you what you have eaten?
Journal of’ Archaeological
Science 199 1, 18,33I-342
Approaches to Dietary Reconstruction in the Western Cape: Are You What You Have Eaten? John Parkington” (Received I October 1989, revisedmanuscript accepted I.5 September 1990) Debate has arisenover the extent of marine food consumption and by implication coastalresidenceamong Holocenehunter-gatherersin the Cape Province of South Africa. Whereasbioarchaeologicalmethodshave beenassumedto imply short-term probably seasonal coastalvisits, stablecarbon isotoperesultsare heldto demonstrate heavy marine food intake and almostpermanentcoastaldwelling. Here a metabolic critique of the isotopeinterpretation is offered, in which the suggestion is made that
marine food signals are exaggerated in skeletons because of protein induced high turnover rates.In a secondcritique, the palaeoenvironmentalimplicationsof isotope readingsare shownto demonstratea higher inland C, componentthan hashitherto beenrecognized.Someenrichmentof skeletonsmay-wellresultfrom terrestrialrather than marinefood consumption. DIETARY RECONSTRUCTION, ISOTOPES, BIOARCHAEOLOGY, WESTERN CAPE, COASTAL PREHISTORY. Keywords:
Introduction
Some debate has emerged from the contrast between the results of bioarchaeological and archaeometric analyses of prehistoric settlement strategies in the western Cape (Sealy, 1986; Sealy & van der Merwe, 1985, 1986, 1988; Parkington, 1986, 1988). Briefly, in this paper it is argued that a consideration of potential variability in bone turnover rates, related to protein intake, and of the isotopic value of precolonial terrestrial food resources, allows us to see the isotope results as less conflicting with traditional models. More specifically it is argued that enriched 613C values for skeletons from coastal sites need not reflect a restrictedly strandloping existence and that for some time periods at least the differences between coastal and inland samples are negligible. Some clarification of the temporal and spatial aspects of the issue is required. The original suggestion of seasonal mobility (Parkington, 1972) and its subsequent elaboration (Parkington, 1976, 1977) referred broadly to the Holocene time period but specifically to the area between the mouths of the Berg and Olifants rivers and between the coast and the Cape Fold Belt (Figure 1). The time frame was narrowed by two realizations. Bioarchaeological analyses of animal and plant remains from the stratified sequence at Elands Bay Cave illustrated (Parkington, 1980) that the timing of visits, and also perhaps the duration of visits, had changed significantly between Terminal Pleistocene and Late Holocene. Dating of both this and other sequences revealed the episodic nature of site use “SpatialArchaeologyResearch Unit, Department of Archaeology,Universityof CapeTown,Rondebosch 7700,SouthAfrica. 331 0305-4403/91/03033
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!-I nd
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Figure 1. (D!l): Non-heath shrublands, nutrient rich soils C, grasses frequently dominant; (H): non-heath shrublands, nutrient poor soils, C, and C, grasses codominant; (0): heath shrublands, nutrient poor soils, C, grasses dominant; ( n ): archaeological sites (seasonal mobility hypothesis); (@): no. of skeletons from western Cape localities.
and a widespread hiatus in coastal settlement in the mid-Holocene millennia (Parkington et al., 1988). The issue now is whether for particular time slices there is evidence for the integration of coastal and inland sites into settlement systems. Unfortunately human skeletal remains from the western Cape, particularly the inland region, are rare. Because of this, Sealy (1986), and Sealy & van der Merwe (1985, 1986) have sampled skeletal remains from a wider area than that covered by any of the seasonal mobility models. It is also possible that the term “inland” has been used variably to indicate regions more or less distant from the coast. The seasonal mobility hypothesis should not be taken to apply to regions more than 100 km inland. A Bioarchaeological
Approach
Regional reconstructions of subsistence in the western Cape (Figure 1) are based on observations which have been published previously (Parkington & Hall, 1987; Parkington
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et al., 1988). Here, a small number of patterns are selected which illustrate
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the regional and comparative approach and which highlight the relationship between group mobility and diet. The thesis put forward is that coastal settlement, though variable in detail throughout the millennia of the Holocene, was always integrated with inland settlement. A primary issue is the consumption patterns of plant foods and shellfish, which have often been seen as complementary resources available at inland and coastal locations respectively (Deacon, 1972; Parkington, 1972). Caves and rock shelters in the western Cape that lie further than about 8 km from the coast are littered with plant remains, particularly in levels dating to the past two millennia (Liengme, 1987). Most notable among these, apart from obvious grass bedding and firewood debris, are the casings, leaves and bases of underground corms of the iris family (Parkington & Poggenpoel, 1971,1987; Kaplan, 1987). Although the genera represented vary from site to site these plants usually appear to have been the most frequently brought to the sites as food. Early travellers noted that indigenous hunter-gatherers collected corms daily in large numbers; this emphasis also seems to make sense ecologically. In the absence of highly concentrated foods (such as the kernels and nuts of the bushveld and thornveld regions to the north) corms, bulbs and tubers probably reflect the best collecting opportunities ofthe fynbos landscape. Although gathering and preparation times are considerable they are probably less than that for abundant but tiny grass seeds. With only a modest moisture content corms offer substantially better kilojoule returns than any ofthe leaves, stems and inflorescences available. The logic of this is confirmed by the comments of early travellers. At sites within 8 km of the coast the situation is very different. Here shellfish are the tnost common food remains; plant remains, whilst often well-preserved in rock shelters. do not include substantial quantities of corm parts (Liengme, 1987). This inverse correlation between underground plant food remains and shellfish has been remarked on previously, and is best regarded as an active substitution by prehistoric people than a function of preservation. It is hard to imagine coastal populations seeking carbohydrate staples in the green parts of plants whilst ignoring the more productive corms. The most widely promoted reason for the substitution is the seasonal fluctuation in corm availability which reflects patterns of dormancy and growth in concordance with the winter rainfall regime of the western Cape (Parkington, 1976). By moving to the coast during the winter growing season people could collect shellfish and some greens whilst the geophytic plants themselves used up the carbohydrate store of their corms. If some integration of this sort did not take place we would have to envisage coastal groups almost never making use of a widespread and relatively productive resource that provided the bulk of energy requirements further inland. The challenge remains to suggest the nature of the integration for specific Holocene time frames. The millennium between about 1700 and about 3000 years ago is an intriguing one. In this time period only one kind of coastal site has emerged from our programme of field surveys and 14C dating. It seems that all near coastal settlement at this time was concentrated next to flat platforms of intertidal rock from which extensive mussel colonies were exploited. The shells accumulated in massive megamiddens, some tens of thousands of cubic metres in volume on top ofor behind the dunecordon immediately adjacent to these platforms. Both archaeological excavations of modest size and the more extensive disturbances by farmers have shown us that the content of these sites is as standardized as their context. There are very few animal bones, stone or bone tools, no features such as stone emplacements or hearth pits and the shellfish component is usually about 85% black mussel (Choromvtilus meridionalis). There are extensive scatters ofcharcoal strung across the stratigraphy of these sites but no other evidence of plant material. It is difficult to imagine groups of people pursuing such a restricted life-style with no release for a thousand years or more: we must consider likely alternative explanations. It is
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conceivable that the imperfection of the archaeological record is responsible-people could have snacked a wider range of foods along the shore in a way not productive of the concentrations we call sites; much organic debris could have decomposed in the open circumstances of the sites. But it is simpler to assume that the megamiddens reflect only a segment of people’s subsistence activity; to understand them we must integrate them with neighbouring contemporary interior sites. Across the coastal plains large numbers of extensive stone tool scatters occur, unfortunately not directly dated, but some of them only 5-10 km inland of the megamiddens. The stone tool assemblages of these sites, surface scatters in deflated hollows, are similar in composition to dated assemblages older than 2000 years (Manhire, 1987; Robey, 1987). Rather than conclude that coastal people hardly made a stone or bone tool for a thousand years we believe the two kinds of site to be parts of a single settlement system. As the rocky intertidal platforms are most accessible at low tides we suggest that the megamiddens are shell gathering stations scheduled for spring tide days and thus reflect episodic and restricted visits by people focussed at least partly on the terrestrial resources of the coastal plains. Such episodes could have been of 3 or 4 days duration and may have been seasonally restricted or not. Judging by the locations it was indeed the dense mussel colonies that attracted people, an observation that is consistent with the preserved archaeological record. Interestingly the situation after about 1700 years ago is markedly different; in such sites potsherds and domestic animal bones occur. Coastal sites between 300 and 1700 years old in the western Cape may be in or out of rock shelters, are usually small, perhaps 10-l 00 m3 in volume, have substantial quantities of both marine and terrestrial animal bones as well as artefacts and include bedding patches, hearths, post holes and pits. The variety of location, foodwaste composition and artefactual assemblage makes these sites better candidates as temporary domestic camps and certainly implies a radically different coastal settlement response from that of the previous millennium. Combined with the far smaller volume of actual midden, the later phase might reflect longer residential visits to the coast by fewer people. The correspondence between the appearance of domestic animal bones and pottery in sites and the distinct shift in settlement details at about 1700 years ago has prompted us to see the appearance of pastoralists as the key factor in promoting the change (Parkington et al., 1986). Does the ceramic settlement pattern imply year round residence at the coast? Calculations of the volume of preserved midden, their kilojoule yield and band size have been taken to reflect stays of only a week or two per year (Buchanan, 1986). Even if only l/10 of the debris produced at the time has survived, the case for moves to inland sites is strong. Purely coastal settlement would imply narrow but elongated home ranges, a conspicuous avoidance of corms, an unhealthily high protein diet (Noli & Avery, 1988) and an ethnographically unusual scenario. Convincing evidence comes from the analysis of the age at death of animals found in the excavated deposits. Animals such as dassies and seals have known and restricted birth peaks. The increasing size of animal bones with ageing or the changing dental pattern in mandibles or maxillae can be seen as monitors of site occupation. Any tendency for animal deaths to be concentrated in particular segments of the year is as likely to reflect seasonal visits by people as it is seasonal behaviour by other predators or the prey animal. The absence of deaths in other segments of the year strongly suggests that site occupation was neither random nor year-round. It is in effect evidence of absence rather than absence of evidence. Observations of this kind from several animals allows cross-checking. Hyrax ages at death in the uppermost ceramic levels of Elands Bay Cave are concentrated in the winter and spring months June to October, whereas those from lower down the sequence and those from some inland sites show a summer November to March
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pattern with many new born animals. Seal mandible measurements from these same upper levels at EBC demonstrate a predominance of individuals killed between June and October, whereas the lower sample shows animals killed throughout the year. Is it a coincidence that June to October is the period of flowering of iridaceae and thus corresponds to the minimum size, palatability and availability of corms? September or October usually marks the earliest red tides of the summer, upwelled cold water episodes that almost annually make mussels toxic at Elands Bay for 4 months or more (Grindley & Nel, 1970). I do not believe that these kinds of observation prove the integration of coastal and inland phases of occupation or prove that such integration was seasonal. But a strong circumstantial case for such models is implied. To argue for year round coastal settlement we would have to promote a view that degrades the archaeological record, deviates from ethnographic and historic scenarios and descriptions, devalues ecological modelling and possibly conflicts with physiological and nutritional standards. That the coast was a Holocene settlement option taken up to different extents and in different fashions by people who always included terrestrial environments in their annual rounds is suggested. For part of the Holocene. between 8000 and 4000 years ago, the option was rarely exercised, as our archaeological investigations at Elands Bay Cave and Tortoise cave have made clear (Robey, 1987). An Archaeometric Approach It has been said that it would be better to reconstruct ancient diets by analysing what people did eat rather than what they did not (van der Merwe, pers. comm.). The recognition that different kinds of plant foods have different stable carbon isotope signatures and that these are passed on to herbivores and carnivores with predictable fractionations makes such an analysis possible (van der Merwe, 1982; Sealy 8c van der Merwe, 1986). The isotopes thus record what people did eat, although the complexity of this record is just emerging. In the fynbos vegetation of the terrestrial western Cape environment the dominant photosynthetic pathway produces isotope ratios which are very depleted in 13C. Plant foods. when measured, give average readings of - 26.5%0. Animal consumers fractionate these ratios in the steps up the food chain and in the course of metabolic processes, so that their isotope ratios though less negative than the plants they eat, are nevertheless related and predictable (average -23.6%0, range from about -22% to -25.8%). The marine plants of the near shore southern Atlantic have varied isotope ratios but the marine system in general is based on plants with relatively enriched values. Thus, marine animals such as shellfish, fish, birds and seals eaten by prehistoric people have considerably more enriched isotope values (average - 156%0,range from about - 12.00%0 to - 19.0%) than do their terrestrial counterparts. Those people who consume both plants and animals as well as the products of marine food webs should show values that reflect their choices. Sealy (1986) and Sealy & van der Merwe (1985, 1986) have documented the carbon isotope ratios of most of the foods we expect prehistoric people to have eaten. They have also measured the carbon isotope values of collagen from the bones of prehistoric skeletons from both coastal and inland locations and have concluded the following: “in direct contrast to nearly all models of prehistoric coastal subsistence, a significant proportion of the coastal population consumed an entirely marine diet and so must have spent all its time on the shore” (Sealy & van der Merwe, 1985: 140). I‘ . . . inland people apparently consumed negligible amounts of marine foods, whereas coastal people ate little else. These results contradict subsistence strategy models which postulate seasonal movements between coast and mountains. The isotopic data also demonstrate that intensive coastal exploitation ocurred throughout the Holocene and was
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not a late response to the arrival of Khoi herders or European sailors and settlers” (Sealy & van der Merwe, 1986: 143). Note that the fractionation between the food 13C and the consumers collagen 613C is reported as an enrichment of about 5%. The coastal skeletal sample gives average values of between - 13.0% and - 15.O%Odepending on the choice of subsamples (by sex or time period, for example) which implies a mean food value of about - 19.0%. Sealy & van der Merwe view this as inconsistent with any model of movement, seasonal or otherwise, between coast and interior. Effectively they suggest that coastal settlement and marine food consumption have been the core features of prehistoric subsistence in the western Cape near-coastal region throughout the Holocene. This appears to be a direct contradiction of the hypothesis put forward from the bioarchaeological evidence and obviously promotes some re-evaluation. But are the sets of observations strictly comparable? The resolution of carbon isotope values from prehistoric skeletons is quite unlike that of traditional archaeological hypotheses. Whereas these latter have been described as very good on place, modest on time and poor on person the former are very good on person, modest on place and poor on time. Isotope values on individuals resolve person perfectly and, unlike the site based nature of most archaeological subsistence reconstructions, lead directly to person based statements about diet. But the resolution of time and place is more ambiguous. In terms of place, isotope values can sometimes be generally helpful in the sense that highly enriched values may point to a high marine food intake although they can be no more specific than that. In some circumstances, as when enriched terrestrial food systems (those dominated by C, grasses) are juxtaposed to marine systems even this broad resolution of place is lost. Moreover, isotopes cannot distinguish between fynbos plant foods gathered metres from the shoreline and those gathered hundreds of kilometres inland. Resolution of time in the isotope values deserves more thought. Because turnover rate in bone collagen is low in adults (Avioli, 1977) collagen values reflect the periods of rapid growth in individuals, essentially the growth pulses of early childhood and adolescence. They represent an integration of dietary intake over several years, and, in the case of adult skeletons, are representations of earlier, average diets. Perhaps more significantly, isotope values cannot, at least in currently used models, resolve the temporal patterns of dietary mixes that are inherent in seasonal models of prehistoric mobility. A 30% contribution of marine foods, for example, could be obtained either by never moving away from the coast, by spending one prolonged period there as a seasonal visit or by making regular but short trips to the coast at low spring tides throughout the year. These differences in resolving power are important because they affect the ways in which bioarchaeological and isotopic interpretations can be assessed relative to one another. Metabolic Critique There are two routes open to us if we wish to use isotopic observations to test bioarchaeological hypotheses of prehistoric subsistence. The first is to set aside, temporarily, the question of the actual diet implied by the readings and, instead, to compare the values obtained from skeletons distributed around a hypothesized seasonal round. Thus, we might argue, if the hypothesized movements about the landscape really did take place, and if people died and were buried randomly throughout those moves, skeletons should be homogeneous in the segment of the landscape implied. Conscious of the fact that hypothesized movements vary through the Holocene, and that most of the isotope readings do not come from the same area as the bioarchaeological remains but from a wider geographic region, we can examine the stable carbon isotope pattern in this way. Unfortunately the samples particularly for inland locations, are very small and widely dispersed (Figure 2). A group of four inland skeletons all less than 1000 years old, gives a
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mean 6°C value of - 16.67%, not very close to the hypothesized terrestrial value of - 2 1.5% and not far from the average value of - 16.04%0 for five roughly contemporary coastal skeletons. The possibility of people having moved seasonally between the Cape Fold Belt and the coast is obviously not contradicted by these observations. There are no s13C values for inland skeletons between 1000 and 1700 years old and only four for skeletons from the inland region older than 1700 years (Sealy &van der Merwe, 1985). At least two of these latter group come from east of the Olifants River Valley. No assessment of models of transhumance for this period is possible with such small samples. The second is to understand the dietary significance of the isotope readings in quantitative terms. Just as we recognize the fauna1 and plant remains as a complex and imperfect reflection of diet, so must we examine the relationship between collagen isotopic ratios. food intake and residence patterns. The initial assumption was that all foods contributed proportionally to collagen readings so that the isotopes are a simple mean of food values. However, it is now widely recognized that the human metabolic system preferentially uses amino acids in food to make its proteins so long as the supply of protein is adequate (Krueger & Sullivan, 1984; Klepinger & Mintel, 1986; Schwarz, 1986). This means that not only all of the essential amino acids but also most of the inessential ones in human collagen come from protein in the food, and predominantly meat in the diet. Carbohydrate components ofdiet are thus under-represented in collagen values. This is of enormous significance to all archaeologists working in coastal localities because marine collectable resources are usually shellfish, whereas terrestrial collectable resources are usually plant foods. The role of marine plants in prehistoric diets is unknown, but most archaeologists would assume that any kind of integration of marine and terrestrial foods is a mix of high and low protein components respectively. Coastal diets, whatever the problems of precise food percentage calculations, were protein rich. An abundance of dietary amino acids results in systemic acidosis and thus stimulates rapid bone turnover through PTH secretion. Although ethnographic evidence for this is still minimal, the high level of osteoporosis noted by Mazess (1970) from Arctic populations living on high protein diets, is consistent with the notion of PTH-driven high
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rates of resorption and formation, with the former predominating in old age. Other kinds of observations (Tschudy et al., 1959; Nelson, 1975) also support the view that “in adults, doubling the dietary protein from 40 to 80 gm/day with no change in caloric intake greatly increases the total protein synthesized and the turnover rates of protein metabolism” (Nelson, 1975: 73). In terms of dietary signals entering bone collagen, we note that whatever foods are eaten at times of rapid turnover are exaggerated in the overall collagen signal by comparison with those consumed when bone is less active. Visits to the coast may thus stimulate episodes of rapid collagen turnover particularly because the PTH effects would be complemented and exaggerated by higher levels of both vitamin D and A than in inland situations. Such periods would amount to temporary phases of “active” osteoporosis (Hough, 1988) caused by an acid-base imbalance that would be reversed by a shift back to carbohydrate staples. Shifts of a few days to a few weeks toward high protein, relatively high vitamin A and D foods would increase bone resorption, stimulate osteoid secretion and thus generate an expanded opportunity for the isotopic signal of such foods to enter and dominate the bone collagen reading, particularly in growing individuals. Ironically this recognition of the impact of dietary decisions on protein metabolism brings us to the position of needing to know the diet, at least in general terms, before we can interpret the isotope results. Fortunately, the archaeological record reflects meat intake fairly well, particularly the mix of marine and terrestrial protein at coastal and inland sites. During the millennium 2000-3000 years ago coastal sites reflect a >95% marine protein pattern, whereas after 1700 years ago the figure may drop to 80% or even 60% in some sites. Inland sites can be taken as 100% terrestrial. All identifiable factors point to an exaggeration of the marine signal in populations moving systematically between coastal and inland sites. As yet we have no way of quantifying the exaggeration but we could start by assuming it is proportional to protein intake. Coastal diets based on gathered shellfish were probably at least three times as rich in protein as inland diets based on gathered plant foods. If this is correct then the marine signal replaces pre-existing isotope ratios at three times the rate at which the inland signal does, leading in the long-term to an exaggerated impression of marine food intake and, by implication, coastal residence. Shellfish, fish and other seafoods may well have made up a substantial proportion of protein intake but it could have been obtained during relatively transient coastal visits. The assumption implicit in the isotope reconstruction of diet to date, and now challenged by metabolic research, is that no exaggeration, no increased turnover is likely. Note that this is not inconsistent with the notion that the isotope value is a long-term average but emphasizes the uneven rate of contribution to that average with changing diet. Sealy & van der Merwe (1988) have noted differences in male and female skeletal readings and may be correct in looking to gender differences in access to foods. Their figures show male and female means in the period 2000-3000 years ago of - 12.5 1%O and - 13.9%. (N= 11, N=8) respectively, whereas the equivalent values in the period 3002000 years ago are-14.75% and - 14.88% (Sealy & van der Merwe, 1988). Given the ethnographic evidence for prolonged lactation among San women (Howell, 1979) and the enormous effects of lactation and pregnancy on calcium demands, another dietary possibility emerges. If, in women, the physiological response to a seasonal shift to high protein diets were less pronounced, leading to less signal exaggeration than among men, their readings would be less enriched than men eating essentially the same food. This would happen if women turned over faster in the interior and thus experienced both coastal and inland signal exaggeration by comparison with men. As the whole population moved toward a less marine diet this difference between men and women would naturally diminish without the need to invoke differential access to food.
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Palaeoenvironmental Critique In large part the confidence with which isotopic researchers had characterized coastal diets derives from the recognition of the marine food web as enriched and the terrestrial foodweb as depleted. Here, the problem of distinguishing between enriched isotope readings which are the result of marine food consumption and those that are the result of an enriched terrestrial food component is raised. A number of botanical, geological, bioarchaeological and archaeometric observations are significant. Mot1 (1987; pers. comm.) has pointed to the apparently residual distributions of Themeda triandra, a C, grass, along geological ecotones between granite and quartzite. or shale and quartzite, in various parts of the western Cape (Figure I). His suggestion has been that these are the relict traces of a formerly much more widespread grass-rich vegetation type located primarily on the nutrient rich granite- or shale-derived soils of the south-western Cape. Precolonial and subsequent colonial sheep herders would have preferentially used such pastures, leading through burning and overgrazing to the long-term replacement of palatable grasses by unpalatable renosterbos (Elytropappus rhinocerotis) such as characterizes much of this landscape today. Wheat farming has most recently dominated this nutrient-rich part of the western Cape, effectively removing the Themeda triandra grassland from the sampling range of modern archaeometrists. It is clear from the juxtaposition of geological substrates and isotope sampling patterns (Figure 1) that Mall’s reconstruction has considerable implications for the interpretation of isotope readings. The enormous majority of skeletons sampled come from coastlines backed by shale-or granite-derived soils. 6°C enrichment could come from the sea, from animals grazing on C, grasses or, most likely, from both. The absence of enrichment reported from modern steenbok or other terrestrial animal bones would be expected from an area now dominated by C, grasses (wheat) and shrubs. The isotope readings of bovidae reported by Sealy & van der Merwe (1986) Sealy ( 1986) and February (1986) represent then, in the absence of direct palaeobotanical remains the best statement on prior vegetation communities (Figure 2). Two grysbok, the nearest animal to a pure browser in the fynbos landscape, from the south-western Cape have given readings of - 2 1.63%0 and - 21.83%, almost exactly the expected C, mean. Six steenbok. animals known to be mixed feeders, from Bushmanland, described by Sealy & van der Merwe as a region “with 955100% C, grasses” (1986: 140), give a mean isotope reading of -- 18.34%0. This reflects a deviation of over 3%0toward the enriched end of the scale. Nine steenbok from the coastal plains, including five from the Elands Bay area, give a mean reading of - 20.13%0. Even in the modern environment there seems to be a perceptible shift in the sandveld coastal plains away from the mean C, value of - 2 1.5%0. Moreover, two modern sheep measured from the Kasteelberg area ofgranite intrusions give readings of - 17*42%0 and - 19.52%0. Although it might be that exotic food additives have caused this, the mean for five precolonial sheep from the same area give a mean reading of - 17.63%, quite substantially different from the C, mean. It is hard to avoid the conclusion that precolonial sheep had considerable access to C, grasses presumably somewhere on the coastal plain. The same must also be true for the sample of 16 precolonial steenbok from Elands Bay Cave (all dated to between 300 and 3000 years ago by 14C) which give a mean reading of - 17.63%0, more enriched than the Bushmanland sample. Either 6’jC readings on archaeological fauna1 remains are systematically enriched after burial, in which case the same may be true for human bone, or there was a stronger C, component in western Cape vegetation than has previously been considered. Interestingly, if the Elands Bay Cave steenbok bones were - 17.63%0, the flesh would have been about - 20X and any human consumer of steenbok would have been about - 15%~~Yet humans with readings of between - 14X and - 15%0have been described as having lived “almost permanently on the sea shore” (Sealy & van der Merwe, 1986: 143), and by
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implication as having consumed “an entirely marine diet” (Sealy & van der Merwe, 1985: 140). Further support for the suggestion that the terrestrial environment in the vicinity of Elands Bay Cave was more enriched than has been promoted, comes from the 613C values obtained by John Vogel as a part of the 14C dating process (1990, pers. comm.). In this context coastal sites are all within 100 m of the coast and lie along a 15 km stretch of coastline close to EBC. Coastal plain sites are 4-25 km from the sea in the sandveld, whereas inland sites are more than 25 km inland in the relatively mountainous terrain of the Cape Fold Belt (Figure 1). Quite obviously people were able to gather firewood which was far more enriched than the global average for C, environments of - 26.5%~ This was apparently not true prior to 10,000 ears ago, in which time coastal, coastal plain and inland sites have indistinguishable 6’ 3C records. Although the analyses of mammals and charcoals have come from different laboratories and are not strictly comparable, these observations show that resources in the Verlorenvlei region over the past 3500 years were more enriched than has been supposed (Sealy, 1986; Sealy & van der Merwe, 1988). The end-point from which we should measure marine food intake is thus not - 21.5%0 or even - 19%0. Humans whose collagen measures - 17%0may have eaten no more marine foods than the local eland or steenbok. Conclusion
To interpret carbon isotope readings in terms of prehistoric diets we need to have (a) an accurate 613C measurement, (b) a knowledge of the diet--collagen spacing, (c) comprehensive measurements on foods available in the past and (d) an understanding of the metabolic processes that turn food signals into measurable bone signals. In the absence of adequate inland, and for some areas coastal, samples from contemporary time frames, the testing of bioarchaeological models of settlement rest on proper interpretations of coastal readings. In current interpretations there are some assumptions that are opaque and implicit, but which may well be wrong. The assumption that the level of protein intake does not affect signal manufacture is not consistent with research results from bone metabolic studies. The assumption that the precolonial vegetation mosaic contained negligible C, components is not consistent with isotope readings on archaeological fauna! remains. Under these circumstances it is premature to claim that isotope readings “contradict subsistence strategy models” (Sealy & van der Merwe, 1986: 143) derived from bioarchaeological remains. Acknowledgements
I am grateful to Judy Sealy, Ed February, John Vogel and Nikolaas van der Merwe for access to unpublished material. I thank Royden Yates, Judy Sealy and Tony Manhire for comments, Professors Stephen Hough, Alistair Stephen and Wieland Gevers for much technical assistance and Andrew Sillen, as well as participants in the Dietary Workshop in Cape Town, for encouragement and editorial help. I thank Professor Eugene Moll for his botanical suggestions. References Avioli, L. V. (1977). Osteoporosis: pathogenesis and therapy. In (L. V. Avioli & S. M. Krane. Eds) Metabolic Bone Disease Vol. I. New York. Academic Press, pp. l-643. Buchanan, W. F. (1986). Sea shells ashore. Unpublished Ph.D. thesis. University of Cape Town. Deacon, H. J. (1972). A review of the Post-Pleistocene in South Africa. South African Archaeological Society Goodwin Series 1,2645. February, E. (1986). An investigation ofprehistoric environments in the southwestern Cape. Honours project. Archaeology Department, University of Cape Town.
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Grindley, J. & Nel. E. (1970). Red water and mussel poisoning at Elands Bay, December 1966. Fisheries Bulletin South Africa 6, 3655. Hough. S. (1988). The Osteoporosis syndrome: pathogenesis, diagnosis and management. South Afkican Journal oj’Continuing Medical Education t&39-56. Howe]]. N. (1979). Demography qfthe Dobe !Kung. New York: Academic Press, Kaplan, J. (1987). Settlement and subsistence at Renbaan Cave. In (J. Parkington & M. Ha]], Eds) Papers in the Prehistory of the Western Cape, South Africa. Oxford: British Archaeological Reports (BAR) 332(ii). 350-372. Klepinger, L. L. & Mintel, R. W. (1986). Metabolic considerations in reconstructing past diet from stable carbon isotope ratios of bone collagen. In (S. A. Olin & M. S. Blackburn, Eds) Proceedings qf the 14th International Archaeometry Symposium. Washington D.C.: Smithsonian Institution Press, pp. 4348. Krueger. H. W. & Sullivan, C. H. (1984). Models for carbon isotope fractionation between diet and bone. In (J. R. Turland & P. E. Johnson, Eds) Stable isotopes in Nutrition. Washington D.C.: American Chemical Society Symposium series No. 258, pp. 205-220. Liengme, C. (1987). Botanical remains from archaeological sites in the Western Cape. In (J. Parkington & M. Ha]], Eds) Papers in the Prehistory ofthe Western Cape, South Africa. Oxford: British Archaeological Reports 332(i), 237-261. Manhire. A. H. (1987). Sandveld deflation hollows: as study of open site assemblages in the southwestern Cape. In (J. Parkington & M. Hall, Eds) Papers in the Prehistory ofthe Western Capt. South A./i-ica. Oxford: British Archaeological Reports (BAR) 332(ii), 32&349. Mazess. R. B. (1970). Bone mineral content in Wainwright Eskimos: preliminary report. .4rctic. .4n/hropolog~ VII(l), I 14-l 16. Mall. E. ( 1987). Review of some new concepts in “Fynbos” ecology. In (J. Parkington & M. Hall, Eds) Papers in the Prehistorj, qfthe Western Cape, South A.fiica. Oxford: British Archaeological Reports (BAR) 332( I), 120-131. Nelson, R. A. (1975). Implications of excessive protein. In (P. L. White & N. Solvy, Eds) Proceea’ings ll’th #‘estern Hemisphere Nutrition Congress. Acton, MA.: Publishing Sciences Group, pp. 71-76. Noli. H. D. & Avery. G. (1988). Protein poisoning and coastal subsistence. Journal o/’ A rchueological Science 15, 39540 I Parkington. J. (1987). On stable carbon isotopes and dietary reconstruction. Current .4nthropologv 28( I ). 9 I-94. Parkington, J. E. (1972). Seasonal mobility in the Late Stone Age. A-frican Studies 1,223-243. Parkington. J. E. ( 1976). Coastal Settlement between the mouths of the Berg and Olifants rivers. Cape Province. South African Archaeological Bulletin 31, 127-l 40. Parkington. J. E. ( 1977). Soagua: hunter fisher gatherers of the Olifants River Valley, W. Cape. South A,frican Arc~haeological Bulletin 33, 89-93. Parkington. J. E. ( 1980). The effects of environmental change on the scheduling of visits to the Elands Bay Cave, Cape Province, S.A. In (N. G. Hammond. I. Hodder & G. L. Isaac. Eds) Pattern ofthe Past. Cambridge: Cambridge University Press. Parkington. J. E. (1986). Comment on Judith C. Sealy & Nikolaas J. van der Merwe. Isotope assessment and the seasonal mobility hypothesis in the south-western Cape of South Africa. Current ,4nthropo/ogy 27(2), 145-146. Parkington. J. E. (1988). Holocene coastal settlement patterns in the western Cape. In (G. Baile) & J. Parkington. Eds) The Archaeology qff’rehistoric Coastlines. Cambridge: Cambridge IJniversity Press, pp. 32-41. Parkington. J. E. 81 Hall, M. ( 1987). Patterning in recent radiocarbon dates from Southern Africa as a reflection of prehistoric settlement and interaction. Journal of’ African Histon, 28, I-25. Parkington. J. E. & Poggenpoe]. C. (1971). Excavations at De Hangen 1968. South Afkican .4rchaeological Bulletin 26, 3-36. Parkington. J. E. & Poggenpoel, C. (1987). Diepkloof rock shelter. In (J. Parkington & M. Hall, Eds) Papers in the Prehistory ofthe Western Cape, South Ajkica. Oxford: British Archaeological Reports (BAR) 332(ii), 269-293.
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Parkington, J. E., Yates, R. J., Manhire, A. H. & Halkett, D. J. (1986). The social impact of pastorahsm in the southwestern Cape. Journal of Anthropological Archaeology 5,313-329. Parkington, J. E., Poggenpeol,C., Buchanan,W. F., Robey, T., Manhive, A. H. & Scaly,J. (1988) Holocenecoastalsettlementpatternsin the westernCape.In (G. Bailey& J. Parkington, Eds) The Archaeology of Prehistoric Coastlines. Cambridge.CambridgeUniversity Press,pp. 22-41. Robey, T. S.(1987).The stratigraphicand cultural sequence at Tortoise Cave,Verlorenvlei. In (J. Parkington & M. Hall, Eds)Papers in the Prehistory ofthe Western Cape, South Africa. Oxford: British ArchaeologicalReports(BAR) 332(ii), 294-325. Schwartz, H. P. (1986).Commenton SealyJ. C. &N. J. van der Merwe. Current Anthropology 27(2), 146-147. Sealy,J. (1986).Stable Carbon Isotopes and Prehistoric Diets in the South- Western Cape Province, South Africa. Oxford: British ArchaeologicalReports(BAR) 293. Sealy,J. C. & vander Merwe, N. J. (1985).Isotopeassessment of Holocenehumandietsin the southwesternCape,South Africa. Nature 315,138-140. Sealy,J. & van der Merwe, N. J. (1986).Isotopeassessment and the seasonal mobility hypothesis in the southwesternCapeof South Africa. Current Anthropology 27(2), 135-150. Sealy,J. & van der Merwe, N. J. (1988).Social,spatialand chronologicalpatterning in marine food useasdeterminedby 6’-‘Cmeasurements of Holocenehumanskeletonsfrom the southwesternCape,South Africa. World Archaeology 20,87-102. Tschudy,D. P., Bacchus,H., Weissman,S.,Watkin, D. M., Eubanks,M. &White, J. (1959). Studiesof the effect of dietary protein andcaloric levelson the kineticsof nitrogenmetabolism usingN- 15L-asparticacid. Journal of Clinical Investigation 38,892-901. van der Merwe, N. J. (1982).Carbon isotopes,photosynthesisand archaeology.American Scientist 70,596606.
Science 199 1, 18,33I-342
Approaches to Dietary Reconstruction in the Western Cape: Are You What You Have Eaten? John Parkington” (Received I October 1989, revisedmanuscript accepted I.5 September 1990) Debate has arisenover the extent of marine food consumption and by implication coastalresidenceamong Holocenehunter-gatherersin the Cape Province of South Africa. Whereasbioarchaeologicalmethodshave beenassumedto imply short-term probably seasonal coastalvisits, stablecarbon isotoperesultsare heldto demonstrate heavy marine food intake and almostpermanentcoastaldwelling. Here a metabolic critique of the isotopeinterpretation is offered, in which the suggestion is made that
marine food signals are exaggerated in skeletons because of protein induced high turnover rates.In a secondcritique, the palaeoenvironmentalimplicationsof isotope readingsare shownto demonstratea higher inland C, componentthan hashitherto beenrecognized.Someenrichmentof skeletonsmay-wellresultfrom terrestrialrather than marinefood consumption. DIETARY RECONSTRUCTION, ISOTOPES, BIOARCHAEOLOGY, WESTERN CAPE, COASTAL PREHISTORY. Keywords:
Introduction
Some debate has emerged from the contrast between the results of bioarchaeological and archaeometric analyses of prehistoric settlement strategies in the western Cape (Sealy, 1986; Sealy & van der Merwe, 1985, 1986, 1988; Parkington, 1986, 1988). Briefly, in this paper it is argued that a consideration of potential variability in bone turnover rates, related to protein intake, and of the isotopic value of precolonial terrestrial food resources, allows us to see the isotope results as less conflicting with traditional models. More specifically it is argued that enriched 613C values for skeletons from coastal sites need not reflect a restrictedly strandloping existence and that for some time periods at least the differences between coastal and inland samples are negligible. Some clarification of the temporal and spatial aspects of the issue is required. The original suggestion of seasonal mobility (Parkington, 1972) and its subsequent elaboration (Parkington, 1976, 1977) referred broadly to the Holocene time period but specifically to the area between the mouths of the Berg and Olifants rivers and between the coast and the Cape Fold Belt (Figure 1). The time frame was narrowed by two realizations. Bioarchaeological analyses of animal and plant remains from the stratified sequence at Elands Bay Cave illustrated (Parkington, 1980) that the timing of visits, and also perhaps the duration of visits, had changed significantly between Terminal Pleistocene and Late Holocene. Dating of both this and other sequences revealed the episodic nature of site use “SpatialArchaeologyResearch Unit, Department of Archaeology,Universityof CapeTown,Rondebosch 7700,SouthAfrica. 331 0305-4403/91/03033
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!-I nd
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Figure 1. (D!l): Non-heath shrublands, nutrient rich soils C, grasses frequently dominant; (H): non-heath shrublands, nutrient poor soils, C, and C, grasses codominant; (0): heath shrublands, nutrient poor soils, C, grasses dominant; ( n ): archaeological sites (seasonal mobility hypothesis); (@): no. of skeletons from western Cape localities.
and a widespread hiatus in coastal settlement in the mid-Holocene millennia (Parkington et al., 1988). The issue now is whether for particular time slices there is evidence for the integration of coastal and inland sites into settlement systems. Unfortunately human skeletal remains from the western Cape, particularly the inland region, are rare. Because of this, Sealy (1986), and Sealy & van der Merwe (1985, 1986) have sampled skeletal remains from a wider area than that covered by any of the seasonal mobility models. It is also possible that the term “inland” has been used variably to indicate regions more or less distant from the coast. The seasonal mobility hypothesis should not be taken to apply to regions more than 100 km inland. A Bioarchaeological
Approach
Regional reconstructions of subsistence in the western Cape (Figure 1) are based on observations which have been published previously (Parkington & Hall, 1987; Parkington
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et al., 1988). Here, a small number of patterns are selected which illustrate
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the regional and comparative approach and which highlight the relationship between group mobility and diet. The thesis put forward is that coastal settlement, though variable in detail throughout the millennia of the Holocene, was always integrated with inland settlement. A primary issue is the consumption patterns of plant foods and shellfish, which have often been seen as complementary resources available at inland and coastal locations respectively (Deacon, 1972; Parkington, 1972). Caves and rock shelters in the western Cape that lie further than about 8 km from the coast are littered with plant remains, particularly in levels dating to the past two millennia (Liengme, 1987). Most notable among these, apart from obvious grass bedding and firewood debris, are the casings, leaves and bases of underground corms of the iris family (Parkington & Poggenpoel, 1971,1987; Kaplan, 1987). Although the genera represented vary from site to site these plants usually appear to have been the most frequently brought to the sites as food. Early travellers noted that indigenous hunter-gatherers collected corms daily in large numbers; this emphasis also seems to make sense ecologically. In the absence of highly concentrated foods (such as the kernels and nuts of the bushveld and thornveld regions to the north) corms, bulbs and tubers probably reflect the best collecting opportunities ofthe fynbos landscape. Although gathering and preparation times are considerable they are probably less than that for abundant but tiny grass seeds. With only a modest moisture content corms offer substantially better kilojoule returns than any ofthe leaves, stems and inflorescences available. The logic of this is confirmed by the comments of early travellers. At sites within 8 km of the coast the situation is very different. Here shellfish are the tnost common food remains; plant remains, whilst often well-preserved in rock shelters. do not include substantial quantities of corm parts (Liengme, 1987). This inverse correlation between underground plant food remains and shellfish has been remarked on previously, and is best regarded as an active substitution by prehistoric people than a function of preservation. It is hard to imagine coastal populations seeking carbohydrate staples in the green parts of plants whilst ignoring the more productive corms. The most widely promoted reason for the substitution is the seasonal fluctuation in corm availability which reflects patterns of dormancy and growth in concordance with the winter rainfall regime of the western Cape (Parkington, 1976). By moving to the coast during the winter growing season people could collect shellfish and some greens whilst the geophytic plants themselves used up the carbohydrate store of their corms. If some integration of this sort did not take place we would have to envisage coastal groups almost never making use of a widespread and relatively productive resource that provided the bulk of energy requirements further inland. The challenge remains to suggest the nature of the integration for specific Holocene time frames. The millennium between about 1700 and about 3000 years ago is an intriguing one. In this time period only one kind of coastal site has emerged from our programme of field surveys and 14C dating. It seems that all near coastal settlement at this time was concentrated next to flat platforms of intertidal rock from which extensive mussel colonies were exploited. The shells accumulated in massive megamiddens, some tens of thousands of cubic metres in volume on top ofor behind the dunecordon immediately adjacent to these platforms. Both archaeological excavations of modest size and the more extensive disturbances by farmers have shown us that the content of these sites is as standardized as their context. There are very few animal bones, stone or bone tools, no features such as stone emplacements or hearth pits and the shellfish component is usually about 85% black mussel (Choromvtilus meridionalis). There are extensive scatters ofcharcoal strung across the stratigraphy of these sites but no other evidence of plant material. It is difficult to imagine groups of people pursuing such a restricted life-style with no release for a thousand years or more: we must consider likely alternative explanations. It is
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conceivable that the imperfection of the archaeological record is responsible-people could have snacked a wider range of foods along the shore in a way not productive of the concentrations we call sites; much organic debris could have decomposed in the open circumstances of the sites. But it is simpler to assume that the megamiddens reflect only a segment of people’s subsistence activity; to understand them we must integrate them with neighbouring contemporary interior sites. Across the coastal plains large numbers of extensive stone tool scatters occur, unfortunately not directly dated, but some of them only 5-10 km inland of the megamiddens. The stone tool assemblages of these sites, surface scatters in deflated hollows, are similar in composition to dated assemblages older than 2000 years (Manhire, 1987; Robey, 1987). Rather than conclude that coastal people hardly made a stone or bone tool for a thousand years we believe the two kinds of site to be parts of a single settlement system. As the rocky intertidal platforms are most accessible at low tides we suggest that the megamiddens are shell gathering stations scheduled for spring tide days and thus reflect episodic and restricted visits by people focussed at least partly on the terrestrial resources of the coastal plains. Such episodes could have been of 3 or 4 days duration and may have been seasonally restricted or not. Judging by the locations it was indeed the dense mussel colonies that attracted people, an observation that is consistent with the preserved archaeological record. Interestingly the situation after about 1700 years ago is markedly different; in such sites potsherds and domestic animal bones occur. Coastal sites between 300 and 1700 years old in the western Cape may be in or out of rock shelters, are usually small, perhaps 10-l 00 m3 in volume, have substantial quantities of both marine and terrestrial animal bones as well as artefacts and include bedding patches, hearths, post holes and pits. The variety of location, foodwaste composition and artefactual assemblage makes these sites better candidates as temporary domestic camps and certainly implies a radically different coastal settlement response from that of the previous millennium. Combined with the far smaller volume of actual midden, the later phase might reflect longer residential visits to the coast by fewer people. The correspondence between the appearance of domestic animal bones and pottery in sites and the distinct shift in settlement details at about 1700 years ago has prompted us to see the appearance of pastoralists as the key factor in promoting the change (Parkington et al., 1986). Does the ceramic settlement pattern imply year round residence at the coast? Calculations of the volume of preserved midden, their kilojoule yield and band size have been taken to reflect stays of only a week or two per year (Buchanan, 1986). Even if only l/10 of the debris produced at the time has survived, the case for moves to inland sites is strong. Purely coastal settlement would imply narrow but elongated home ranges, a conspicuous avoidance of corms, an unhealthily high protein diet (Noli & Avery, 1988) and an ethnographically unusual scenario. Convincing evidence comes from the analysis of the age at death of animals found in the excavated deposits. Animals such as dassies and seals have known and restricted birth peaks. The increasing size of animal bones with ageing or the changing dental pattern in mandibles or maxillae can be seen as monitors of site occupation. Any tendency for animal deaths to be concentrated in particular segments of the year is as likely to reflect seasonal visits by people as it is seasonal behaviour by other predators or the prey animal. The absence of deaths in other segments of the year strongly suggests that site occupation was neither random nor year-round. It is in effect evidence of absence rather than absence of evidence. Observations of this kind from several animals allows cross-checking. Hyrax ages at death in the uppermost ceramic levels of Elands Bay Cave are concentrated in the winter and spring months June to October, whereas those from lower down the sequence and those from some inland sites show a summer November to March
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pattern with many new born animals. Seal mandible measurements from these same upper levels at EBC demonstrate a predominance of individuals killed between June and October, whereas the lower sample shows animals killed throughout the year. Is it a coincidence that June to October is the period of flowering of iridaceae and thus corresponds to the minimum size, palatability and availability of corms? September or October usually marks the earliest red tides of the summer, upwelled cold water episodes that almost annually make mussels toxic at Elands Bay for 4 months or more (Grindley & Nel, 1970). I do not believe that these kinds of observation prove the integration of coastal and inland phases of occupation or prove that such integration was seasonal. But a strong circumstantial case for such models is implied. To argue for year round coastal settlement we would have to promote a view that degrades the archaeological record, deviates from ethnographic and historic scenarios and descriptions, devalues ecological modelling and possibly conflicts with physiological and nutritional standards. That the coast was a Holocene settlement option taken up to different extents and in different fashions by people who always included terrestrial environments in their annual rounds is suggested. For part of the Holocene. between 8000 and 4000 years ago, the option was rarely exercised, as our archaeological investigations at Elands Bay Cave and Tortoise cave have made clear (Robey, 1987). An Archaeometric Approach It has been said that it would be better to reconstruct ancient diets by analysing what people did eat rather than what they did not (van der Merwe, pers. comm.). The recognition that different kinds of plant foods have different stable carbon isotope signatures and that these are passed on to herbivores and carnivores with predictable fractionations makes such an analysis possible (van der Merwe, 1982; Sealy 8c van der Merwe, 1986). The isotopes thus record what people did eat, although the complexity of this record is just emerging. In the fynbos vegetation of the terrestrial western Cape environment the dominant photosynthetic pathway produces isotope ratios which are very depleted in 13C. Plant foods. when measured, give average readings of - 26.5%0. Animal consumers fractionate these ratios in the steps up the food chain and in the course of metabolic processes, so that their isotope ratios though less negative than the plants they eat, are nevertheless related and predictable (average -23.6%0, range from about -22% to -25.8%). The marine plants of the near shore southern Atlantic have varied isotope ratios but the marine system in general is based on plants with relatively enriched values. Thus, marine animals such as shellfish, fish, birds and seals eaten by prehistoric people have considerably more enriched isotope values (average - 156%0,range from about - 12.00%0 to - 19.0%) than do their terrestrial counterparts. Those people who consume both plants and animals as well as the products of marine food webs should show values that reflect their choices. Sealy (1986) and Sealy & van der Merwe (1985, 1986) have documented the carbon isotope ratios of most of the foods we expect prehistoric people to have eaten. They have also measured the carbon isotope values of collagen from the bones of prehistoric skeletons from both coastal and inland locations and have concluded the following: “in direct contrast to nearly all models of prehistoric coastal subsistence, a significant proportion of the coastal population consumed an entirely marine diet and so must have spent all its time on the shore” (Sealy & van der Merwe, 1985: 140). I‘ . . . inland people apparently consumed negligible amounts of marine foods, whereas coastal people ate little else. These results contradict subsistence strategy models which postulate seasonal movements between coast and mountains. The isotopic data also demonstrate that intensive coastal exploitation ocurred throughout the Holocene and was
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not a late response to the arrival of Khoi herders or European sailors and settlers” (Sealy & van der Merwe, 1986: 143). Note that the fractionation between the food 13C and the consumers collagen 613C is reported as an enrichment of about 5%. The coastal skeletal sample gives average values of between - 13.0% and - 15.O%Odepending on the choice of subsamples (by sex or time period, for example) which implies a mean food value of about - 19.0%. Sealy & van der Merwe view this as inconsistent with any model of movement, seasonal or otherwise, between coast and interior. Effectively they suggest that coastal settlement and marine food consumption have been the core features of prehistoric subsistence in the western Cape near-coastal region throughout the Holocene. This appears to be a direct contradiction of the hypothesis put forward from the bioarchaeological evidence and obviously promotes some re-evaluation. But are the sets of observations strictly comparable? The resolution of carbon isotope values from prehistoric skeletons is quite unlike that of traditional archaeological hypotheses. Whereas these latter have been described as very good on place, modest on time and poor on person the former are very good on person, modest on place and poor on time. Isotope values on individuals resolve person perfectly and, unlike the site based nature of most archaeological subsistence reconstructions, lead directly to person based statements about diet. But the resolution of time and place is more ambiguous. In terms of place, isotope values can sometimes be generally helpful in the sense that highly enriched values may point to a high marine food intake although they can be no more specific than that. In some circumstances, as when enriched terrestrial food systems (those dominated by C, grasses) are juxtaposed to marine systems even this broad resolution of place is lost. Moreover, isotopes cannot distinguish between fynbos plant foods gathered metres from the shoreline and those gathered hundreds of kilometres inland. Resolution of time in the isotope values deserves more thought. Because turnover rate in bone collagen is low in adults (Avioli, 1977) collagen values reflect the periods of rapid growth in individuals, essentially the growth pulses of early childhood and adolescence. They represent an integration of dietary intake over several years, and, in the case of adult skeletons, are representations of earlier, average diets. Perhaps more significantly, isotope values cannot, at least in currently used models, resolve the temporal patterns of dietary mixes that are inherent in seasonal models of prehistoric mobility. A 30% contribution of marine foods, for example, could be obtained either by never moving away from the coast, by spending one prolonged period there as a seasonal visit or by making regular but short trips to the coast at low spring tides throughout the year. These differences in resolving power are important because they affect the ways in which bioarchaeological and isotopic interpretations can be assessed relative to one another. Metabolic Critique There are two routes open to us if we wish to use isotopic observations to test bioarchaeological hypotheses of prehistoric subsistence. The first is to set aside, temporarily, the question of the actual diet implied by the readings and, instead, to compare the values obtained from skeletons distributed around a hypothesized seasonal round. Thus, we might argue, if the hypothesized movements about the landscape really did take place, and if people died and were buried randomly throughout those moves, skeletons should be homogeneous in the segment of the landscape implied. Conscious of the fact that hypothesized movements vary through the Holocene, and that most of the isotope readings do not come from the same area as the bioarchaeological remains but from a wider geographic region, we can examine the stable carbon isotope pattern in this way. Unfortunately the samples particularly for inland locations, are very small and widely dispersed (Figure 2). A group of four inland skeletons all less than 1000 years old, gives a
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Figure 2. Stablecarbon isotope meanvalues.
mean 6°C value of - 16.67%, not very close to the hypothesized terrestrial value of - 2 1.5% and not far from the average value of - 16.04%0 for five roughly contemporary coastal skeletons. The possibility of people having moved seasonally between the Cape Fold Belt and the coast is obviously not contradicted by these observations. There are no s13C values for inland skeletons between 1000 and 1700 years old and only four for skeletons from the inland region older than 1700 years (Sealy &van der Merwe, 1985). At least two of these latter group come from east of the Olifants River Valley. No assessment of models of transhumance for this period is possible with such small samples. The second is to understand the dietary significance of the isotope readings in quantitative terms. Just as we recognize the fauna1 and plant remains as a complex and imperfect reflection of diet, so must we examine the relationship between collagen isotopic ratios. food intake and residence patterns. The initial assumption was that all foods contributed proportionally to collagen readings so that the isotopes are a simple mean of food values. However, it is now widely recognized that the human metabolic system preferentially uses amino acids in food to make its proteins so long as the supply of protein is adequate (Krueger & Sullivan, 1984; Klepinger & Mintel, 1986; Schwarz, 1986). This means that not only all of the essential amino acids but also most of the inessential ones in human collagen come from protein in the food, and predominantly meat in the diet. Carbohydrate components ofdiet are thus under-represented in collagen values. This is of enormous significance to all archaeologists working in coastal localities because marine collectable resources are usually shellfish, whereas terrestrial collectable resources are usually plant foods. The role of marine plants in prehistoric diets is unknown, but most archaeologists would assume that any kind of integration of marine and terrestrial foods is a mix of high and low protein components respectively. Coastal diets, whatever the problems of precise food percentage calculations, were protein rich. An abundance of dietary amino acids results in systemic acidosis and thus stimulates rapid bone turnover through PTH secretion. Although ethnographic evidence for this is still minimal, the high level of osteoporosis noted by Mazess (1970) from Arctic populations living on high protein diets, is consistent with the notion of PTH-driven high
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rates of resorption and formation, with the former predominating in old age. Other kinds of observations (Tschudy et al., 1959; Nelson, 1975) also support the view that “in adults, doubling the dietary protein from 40 to 80 gm/day with no change in caloric intake greatly increases the total protein synthesized and the turnover rates of protein metabolism” (Nelson, 1975: 73). In terms of dietary signals entering bone collagen, we note that whatever foods are eaten at times of rapid turnover are exaggerated in the overall collagen signal by comparison with those consumed when bone is less active. Visits to the coast may thus stimulate episodes of rapid collagen turnover particularly because the PTH effects would be complemented and exaggerated by higher levels of both vitamin D and A than in inland situations. Such periods would amount to temporary phases of “active” osteoporosis (Hough, 1988) caused by an acid-base imbalance that would be reversed by a shift back to carbohydrate staples. Shifts of a few days to a few weeks toward high protein, relatively high vitamin A and D foods would increase bone resorption, stimulate osteoid secretion and thus generate an expanded opportunity for the isotopic signal of such foods to enter and dominate the bone collagen reading, particularly in growing individuals. Ironically this recognition of the impact of dietary decisions on protein metabolism brings us to the position of needing to know the diet, at least in general terms, before we can interpret the isotope results. Fortunately, the archaeological record reflects meat intake fairly well, particularly the mix of marine and terrestrial protein at coastal and inland sites. During the millennium 2000-3000 years ago coastal sites reflect a >95% marine protein pattern, whereas after 1700 years ago the figure may drop to 80% or even 60% in some sites. Inland sites can be taken as 100% terrestrial. All identifiable factors point to an exaggeration of the marine signal in populations moving systematically between coastal and inland sites. As yet we have no way of quantifying the exaggeration but we could start by assuming it is proportional to protein intake. Coastal diets based on gathered shellfish were probably at least three times as rich in protein as inland diets based on gathered plant foods. If this is correct then the marine signal replaces pre-existing isotope ratios at three times the rate at which the inland signal does, leading in the long-term to an exaggerated impression of marine food intake and, by implication, coastal residence. Shellfish, fish and other seafoods may well have made up a substantial proportion of protein intake but it could have been obtained during relatively transient coastal visits. The assumption implicit in the isotope reconstruction of diet to date, and now challenged by metabolic research, is that no exaggeration, no increased turnover is likely. Note that this is not inconsistent with the notion that the isotope value is a long-term average but emphasizes the uneven rate of contribution to that average with changing diet. Sealy & van der Merwe (1988) have noted differences in male and female skeletal readings and may be correct in looking to gender differences in access to foods. Their figures show male and female means in the period 2000-3000 years ago of - 12.5 1%O and - 13.9%. (N= 11, N=8) respectively, whereas the equivalent values in the period 3002000 years ago are-14.75% and - 14.88% (Sealy & van der Merwe, 1988). Given the ethnographic evidence for prolonged lactation among San women (Howell, 1979) and the enormous effects of lactation and pregnancy on calcium demands, another dietary possibility emerges. If, in women, the physiological response to a seasonal shift to high protein diets were less pronounced, leading to less signal exaggeration than among men, their readings would be less enriched than men eating essentially the same food. This would happen if women turned over faster in the interior and thus experienced both coastal and inland signal exaggeration by comparison with men. As the whole population moved toward a less marine diet this difference between men and women would naturally diminish without the need to invoke differential access to food.
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Palaeoenvironmental Critique In large part the confidence with which isotopic researchers had characterized coastal diets derives from the recognition of the marine food web as enriched and the terrestrial foodweb as depleted. Here, the problem of distinguishing between enriched isotope readings which are the result of marine food consumption and those that are the result of an enriched terrestrial food component is raised. A number of botanical, geological, bioarchaeological and archaeometric observations are significant. Mot1 (1987; pers. comm.) has pointed to the apparently residual distributions of Themeda triandra, a C, grass, along geological ecotones between granite and quartzite. or shale and quartzite, in various parts of the western Cape (Figure I). His suggestion has been that these are the relict traces of a formerly much more widespread grass-rich vegetation type located primarily on the nutrient rich granite- or shale-derived soils of the south-western Cape. Precolonial and subsequent colonial sheep herders would have preferentially used such pastures, leading through burning and overgrazing to the long-term replacement of palatable grasses by unpalatable renosterbos (Elytropappus rhinocerotis) such as characterizes much of this landscape today. Wheat farming has most recently dominated this nutrient-rich part of the western Cape, effectively removing the Themeda triandra grassland from the sampling range of modern archaeometrists. It is clear from the juxtaposition of geological substrates and isotope sampling patterns (Figure 1) that Mall’s reconstruction has considerable implications for the interpretation of isotope readings. The enormous majority of skeletons sampled come from coastlines backed by shale-or granite-derived soils. 6°C enrichment could come from the sea, from animals grazing on C, grasses or, most likely, from both. The absence of enrichment reported from modern steenbok or other terrestrial animal bones would be expected from an area now dominated by C, grasses (wheat) and shrubs. The isotope readings of bovidae reported by Sealy & van der Merwe (1986) Sealy ( 1986) and February (1986) represent then, in the absence of direct palaeobotanical remains the best statement on prior vegetation communities (Figure 2). Two grysbok, the nearest animal to a pure browser in the fynbos landscape, from the south-western Cape have given readings of - 2 1.63%0 and - 21.83%, almost exactly the expected C, mean. Six steenbok. animals known to be mixed feeders, from Bushmanland, described by Sealy & van der Merwe as a region “with 955100% C, grasses” (1986: 140), give a mean isotope reading of -- 18.34%0. This reflects a deviation of over 3%0toward the enriched end of the scale. Nine steenbok from the coastal plains, including five from the Elands Bay area, give a mean reading of - 20.13%0. Even in the modern environment there seems to be a perceptible shift in the sandveld coastal plains away from the mean C, value of - 2 1.5%0. Moreover, two modern sheep measured from the Kasteelberg area ofgranite intrusions give readings of - 17*42%0 and - 19.52%0. Although it might be that exotic food additives have caused this, the mean for five precolonial sheep from the same area give a mean reading of - 17.63%, quite substantially different from the C, mean. It is hard to avoid the conclusion that precolonial sheep had considerable access to C, grasses presumably somewhere on the coastal plain. The same must also be true for the sample of 16 precolonial steenbok from Elands Bay Cave (all dated to between 300 and 3000 years ago by 14C) which give a mean reading of - 17.63%0, more enriched than the Bushmanland sample. Either 6’jC readings on archaeological fauna1 remains are systematically enriched after burial, in which case the same may be true for human bone, or there was a stronger C, component in western Cape vegetation than has previously been considered. Interestingly, if the Elands Bay Cave steenbok bones were - 17.63%0, the flesh would have been about - 20X and any human consumer of steenbok would have been about - 15%~~Yet humans with readings of between - 14X and - 15%0have been described as having lived “almost permanently on the sea shore” (Sealy & van der Merwe, 1986: 143), and by
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implication as having consumed “an entirely marine diet” (Sealy & van der Merwe, 1985: 140). Further support for the suggestion that the terrestrial environment in the vicinity of Elands Bay Cave was more enriched than has been promoted, comes from the 613C values obtained by John Vogel as a part of the 14C dating process (1990, pers. comm.). In this context coastal sites are all within 100 m of the coast and lie along a 15 km stretch of coastline close to EBC. Coastal plain sites are 4-25 km from the sea in the sandveld, whereas inland sites are more than 25 km inland in the relatively mountainous terrain of the Cape Fold Belt (Figure 1). Quite obviously people were able to gather firewood which was far more enriched than the global average for C, environments of - 26.5%~ This was apparently not true prior to 10,000 ears ago, in which time coastal, coastal plain and inland sites have indistinguishable 6’ 3C records. Although the analyses of mammals and charcoals have come from different laboratories and are not strictly comparable, these observations show that resources in the Verlorenvlei region over the past 3500 years were more enriched than has been supposed (Sealy, 1986; Sealy & van der Merwe, 1988). The end-point from which we should measure marine food intake is thus not - 21.5%0 or even - 19%0. Humans whose collagen measures - 17%0may have eaten no more marine foods than the local eland or steenbok. Conclusion
To interpret carbon isotope readings in terms of prehistoric diets we need to have (a) an accurate 613C measurement, (b) a knowledge of the diet--collagen spacing, (c) comprehensive measurements on foods available in the past and (d) an understanding of the metabolic processes that turn food signals into measurable bone signals. In the absence of adequate inland, and for some areas coastal, samples from contemporary time frames, the testing of bioarchaeological models of settlement rest on proper interpretations of coastal readings. In current interpretations there are some assumptions that are opaque and implicit, but which may well be wrong. The assumption that the level of protein intake does not affect signal manufacture is not consistent with research results from bone metabolic studies. The assumption that the precolonial vegetation mosaic contained negligible C, components is not consistent with isotope readings on archaeological fauna! remains. Under these circumstances it is premature to claim that isotope readings “contradict subsistence strategy models” (Sealy & van der Merwe, 1986: 143) derived from bioarchaeological remains. Acknowledgements
I am grateful to Judy Sealy, Ed February, John Vogel and Nikolaas van der Merwe for access to unpublished material. I thank Royden Yates, Judy Sealy and Tony Manhire for comments, Professors Stephen Hough, Alistair Stephen and Wieland Gevers for much technical assistance and Andrew Sillen, as well as participants in the Dietary Workshop in Cape Town, for encouragement and editorial help. I thank Professor Eugene Moll for his botanical suggestions. References Avioli, L. V. (1977). Osteoporosis: pathogenesis and therapy. In (L. V. Avioli & S. M. Krane. Eds) Metabolic Bone Disease Vol. I. New York. Academic Press, pp. l-643. Buchanan, W. F. (1986). Sea shells ashore. Unpublished Ph.D. thesis. University of Cape Town. Deacon, H. J. (1972). A review of the Post-Pleistocene in South Africa. South African Archaeological Society Goodwin Series 1,2645. February, E. (1986). An investigation ofprehistoric environments in the southwestern Cape. Honours project. Archaeology Department, University of Cape Town.
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