Beyond lifetime averages: tracing life histories through isotopic analysis of different calcified tissues from archaeological human skeletons.

Sealy, Judith ; Armstrong, Richard ; Schrire, Carmel 等

Isotopic measurements on human skeletons

The ratio of 13C to 12C, 15N/14N, 87Sr/86Sr and many other isotopes varies in a patterned fashion in the geosphere and biosphere. The composition of body tissues derives from foods eaten in life, so that isotopic analyses of archaeological human bones offer clues to aspects of diet and lifestyle. There has been a good deal of work on stable carbon and nitrogen isotope ratios in human skeletons (recent reviews include Schwarcz & Schoeninger (1991) and Katzenberg (1992)). Strontium isotope ratio measurements have been less widely applied. Isotope analyses are generally of post-cranial bone, especially ribs, since destructive analysis of ribs involves relatively little loss of morphological information. The most commonly used material for carbon and nitrogen analysis is bone collagen, the structural protein that makes up approximately 20% of fresh bone.

In a study of the isotopic ratios of different skeletal elements in animals raised on controlled diets, the 13C/12C and 15N/14N compositions of femora and humeri were identical (DeNiro & Schoeninger 1983). Schoeninger (1989b) compared carbon and nitrogen isotope ratios in vertebrae and femora of recent human skeletons buried in permafrost, and found no differences within skeletons. Available data also reflect negligible variations in isotope ratios due to sex (DeNiro & Schoeninger 1983; Lovell et al. 1986; Tieszen et al. 1989) and age (Hobson & Schwarcz 1986; Lovell et al. 1986; Tieszen et al. 1989).

The chemical constituents of teeth and bones are similar, but their respective histories of formation are very different. Tooth enamel forms in childhood, and does not undergo subsequent remodelling. Enamel is composed principally of hydroxyapatite, with little protein (LeGeros 1983). Dentine, a major component of teeth, forms mostly during childhood: primary dentine is laid down at the time of initial formation of the tooth. Later in life, a small amount of secondary dentine is added, mostly around the edges of the pulp cavity. Tertiary dentine may also be deposited in response to injury or decay (Hillson 1986). Dentine is rich in collagen, and isotope measurements of dentine collagen reflect diet, principally the diet consumed in childhood.

The first adult teeth to begin forming are first molars, which start to mineralize at birth, and complete their growth between 11 and 12 years of age. Third molars start to form in children of about seven, and the roots continue to grow in individuals in their early 20s (Hillson 1986). Different teeth therefore have isotopic compositions which reflect diet at different times of an individual's early life. Bone tissue, like teeth, forms early in life, but unlike teeth it 'turns over', i.e. it is resorbed and replaced, throughout life. The rate of turnover in healthy individuals is not precisely known. It varies at different times of life, slowing down in later years, and is also affected by nutritional and health status. Bone collagen in adults includes material accumulated over more than 10 years (Libby et al. 1964), so that its isotopic composition reflects a long-term dietary average. The vast majority of published stable isotope values for archaeological human skeletons represent such integrations. Dense post-cranial bone, such as that found in the shafts of the long-bones, remodels relatively slowly, and contains tissue deposited over many years. Spongy or cancellous bone, with its thin bony structures fed by abundant blood vessels, is likely to turn over faster than compact bone. Skeletal elements with a high proportion of cancellous bone, such as ribs, probably contain more material from the later part of an individual's life than do the shafts of long bones (Stenhouse & Baxter 1976). Thus different skeletal elements preserve records of diet and residence patterns at different stages of an individual's life.

Natural distributions of isotopes

Stable carbon isotope ratios (13C/12C) have been used as an index of the relative importance of [C.sub.3] and [C.sub.4] plants in the diet. [C.sub.3] plants include trees, most shrubs, and grasses found in temperate areas. Such plants have low 13C/12C ratios, as a result of discrimination against the heavier 13C isotope during Calvin, or [C.sub.3] photosynthesis. Plants which employ the Hatch-Slack or [C.sub.4] photosynthetic pathway are mainly tropical grasses, with higher 13C/12C ratios. Wheat, barley, oats, rice, and most 'vegetables' are [C.sub.3] plants, whereas maize, millets and sorghum are [C.sub.4]. The isotopic composition of body tissues derives from foods eaten in life, with bone 'collagen' isotope values representing mainly the protein component of diet (Ambrose & Norr 1993). Measurements of 13C/12C ratios in archaeological human bones have thus enabled, among other applications, the tracing of the spread of maize agriculture from central America into the temperate, [C.sub.3] zones of eastern North America (for recent reviews see Schwarcz & Schoeninger 1991; Katzenberg 1992).

Stable carbon isotope ratios in the sea are, on the whole, enriched in 13C when compared with [C.sub.3] terrestrial ecosystems. In coastal areas where the vegetation on land is [C.sub.3], 13C/12C measurements of human bones tell us about the proportions of marine and terrestrial foods people ate (Chisholm et al. 1982; Hobson & Collier 1984; Keegan & DeNiro 1988; Sealy & van der Merwe 1985; 1986; 1988; Tauber 1981; Walker & DeNiro 1986).

Nitrogen isotope ratios (15N/14N) also differentiate marine and terrestrial diets (Schoeninger et al. 1983; Schoeninger & DeNiro 1984) and are especially useful in areas with [C.sub.4] terrestrial flora, where stable carbon isotopes cannot be used in this way.

A complication arises in arid terrestrial ecosystems, where animal (and perhaps, to a lesser extent, plant) 15N/14N is elevated, blurring or eliminating the distinction between marine and terrestrial values (Heaton et al. 1986; Sealy et al. 1987). In arid coastal areas, nitrogen isotopes cannot be used as a marine/terrestrial indicator, but may reflect past changes in rainfall (Ambrose 1991).

FIGURE 1 summarizes some carbon and nitrogen isotope ratios for archaeological human skeletons from different parts of the world. European Neolithic farmers who grew [C.sub.3] crops in well-watered areas have very negative [Delta]13C and low [Delta]15N values (see the caption to [ILLUSTRATION FOR FIGURE 1 OMITTED] for definitions of these terms). 17th- and 18th-century Dutch whalers also ate predominantly [C.sub.3]-derived diets, with the addition of a good deal of fish, leading to elevated [Delta]15N and slightly increased [Delta]13C values. The salmon-fishers and marine-mammal-hunters are Haida and Tlingit people from northwestern North America, and Alaskan Inuit, who ate very large amounts of seafood indeed, and have extremely high [Delta]15N, together with enriched [Delta]13C measurements. The maize farmers are Havisu, from New Mexico. They have the most positive [Delta]13C values because of their high intake of maize, a [C.sub.4] plant. As this part of New Mexico receives relatively good rainfall, their [Delta]15N values are low.

The set of results in FIGURE 1 encompass almost the entire range of variability in [Delta]13C and [Delta]15N recorded for humans: people with [C.sub.3]-based diets have bone collagen [Delta]13C of around -21 per thousand, [C.sub.4]-based diets give around -7 per thousand. A [C.sub.3]/marine combination results in bone collagen [Delta]13C between -21 per thousand and about -11 per thousand, depending on the amount of marine food consumed. [Delta]15N values vary between about +7 and +20 per thousand.

87Sr/86Sr provides a slightly different kind of isotopic indicator, giving us geological, rather than biological information. 87Sr/86Sr is high in very old rocks and in those with high Rb/Sr. 87Rb decays to 87Sr, while 86Sr is stable, leading to the 87Sr/86Sr ratio increasing through geological time. In the very much smaller window of time which concerns archaeologists, we can, in most parts of the world, take the modern values of 87Sr/86Sr as constant. 87Sr/86Sr in bones is derived from diet, reflecting the geology of the home environment. Marine 87Sr/86Sr has varied through time, but for archaeological purposes we can use the modern value of 0.70915 (DePaolo & Ingram 1985; value normalized to result of 0-71023 for NBS-987 used here). More recently, Lavelle & Armstrong (1993) obtained a value of 0.70912 for southern African waters (similarly normalized). In areas where terrestrial geology has 87Sr/86Sr different from 0.70915, strontium isotope measurements may also be used as a marine/terrestrial indicator. 87Sr/86Sr is not fractionated during biological processes, as is the case for isotopes of the lighter elements carbon and nitrogen, because the single mass unit difference between 87Sr and 86Sr is such a small proportion of total mass. 87Sr/8Sr in bone should therefore be identical to that of the organism's diet. Ericson (1985) suggested that analysis of 87Sr/86Sr in tooth enamel compared with post-cranial bone could be used, in areas with appropriate geology, to study prehistoric residence patterns.

Several authors have reported isotopic results for a few archaeological human skeletons which are very different from those for most near-by skeletons. Anomalous values may be due to post-depositional collagen degradation, but appropriate laboratory techniques should permit the identification and exclusion of degraded material (Sealy & van der Merwe 1986; Ambrose 1990). Individuals with outlying isotopic values have generally been interpreted as the remains of immigrants into the area. Katzenberg & Kelley (1991) suggested that three skeletons with depleted [Delta]13C values found in southeastern New Mexico may have been Plains Indian immigrants, since Plains Indian communities ate less maize than the maize farmers of the Southwest. Verano & DeNiro (1993) used both craniofacial measurements and stable isotope analysis to study the group affinities of skeletons from the Peruvian site of Pacatnamu, some of whom may have been war captives. The origins of soldiers killed in the War of 1812 between the United States and Great Britain have also been investigated by means of stable isotopes (Katzenberg 1991; Schwarcz et al. 1991).

The studies mentioned above all rely on isotopic measurements of post-cranial bone; usually single measurements on each skeleton. Here, we use 13C/12C, 15N/14N and 87Sr/86Sr measurements on teeth and bones to track dietary and residential changes during life. Analysis of a series of tissue samples from each skeleton, and the application of three isotopic techniques provides much more detailed information than has hitherto been obtained.

Choice of skeletons for this study

Five human skeletons were chosen for this study. Two are prehistoric skeletons from the Cape, South Africa, the remains of Khoisan hunter-gatherers. Both are drawn from the skeletal collection in the Department of Anatomy at the University of Cape Town; their accession numbers are UCT 169 and UCT 248 respectively. Direct radiocarbon determinations on bone collagen yielded a result of 2320[+ or -]50 b.p. for UCT 169 (Pta-5694), and 4730[+ or -]95 b.p. for UCT 248 (GX-13185). Both are adult females, from the Cape Peninsula [ILLUSTRATION FOR FIGURE 2 OMITTED], a region where food production was unknown until the 2nd millennium b.p. These people were coastal foragers who ate shellfish, fish and marine mammals and birds, as well as terrestrial animal and plant foods. They must have searched for food and other commodities, but as far as we know, are unlikely to have travelled great distances.

Three skeletons come from historical archaeological sites at or near the Cape. From the end of the 15th century, European ships on their way to the east stopped at the Cape for fresh food and water, and in 1652 the Dutch East India Company established a permanent refreshment station under the leadership of Jan van Riebeeck. Four days after they arrived, van Riebeeck's men began to build a fort, which served as their base until 1673. During this time, a more permanent structure was erected: the stone castle which still stands in Cape Town. Recent excavations at the site of the fort revealed a burial (UCT 457): an adult male lying extended on his back, with his arms folded across his chest. There was no sign of a coffin. The body was buried into grey sand mixed with quartz grit, which the excavator believes characteristic of the courtyard inside the fort (Abrahams 1993). The overlying level includes 17th- and 18th-century debris, so that it is difficult to estimate the precise date of the burial. If it is associated with the period of occupation of the fort, this man is likely to have been a European soldier or sailor who died at the Cape during the second half of the 17th century.

The second historical skeleton comes from Oudepost I, a Dutch East India Company outpost about 100 km north of the main settlement at the Cape of Good Hope. Oudepost I, founded in order to ward off a threat by French ships to colonise Saldanha Bay, subsequently served as a frontier post and trading station where the Company's soldiers bartered with local Khoikhoi or 'Hottentot' pastoralists for sheep and cattle (Schrire 1988; 1995). It was occupied between 1669 and 1732, with a break between 1673 and 1685 due to the massacre of the garrison by local Khoi. Excavations at the site revealed a grave containing the skeleton of an adult male, 45 to 60 years old. The grave fill included colonial artefacts, which means that it was dug after a colonial presence had established itself at the post, but we cannot be sure whether the burial occurred during the occupation of the post, during its temporary abandonment, or after it was finally abandoned. Some support for an earlier, rather than a later, date comes from the fact that the skeleton was found in a gabled coffin, similar to those recorded at the 17th-century North American site of Martin's Hundred (Schrire et al. 1990). The remains may be those of a soldier in the service of the Dutch East India Company who served at Oudepost and died there in the late 17th or early 18th century. They may also be those of a man who died on board ship, and was buried at Oudepost.

The fifth skeleton was found at Vergelegen, an estate approximately 50 km east of Cape Town, established at the beginning of the 18th century as the country residence of the then Governor of the Cape, Willem Adriaan van der Stel (Markell 1993). Buildings on the estate included a slave lodge, and recent excavations inside the ruined lodge uncovered a grave cut through the floor surface, but not through the debris resulting from the destruction of the lodge. The grave contained a makeshift coffin with the skeleton of a woman in her 50s. Was she a slave, buried under the building in which she lived? The surfaces of many of the bones were eroded by acids produced by the decaying coffin wood, making detailed physical anthropological analysis difficult (Sealy et al. 1993). Isotopic analyses were undertaken to try to glean more information about her life and identity.

Analytical methods

Samples of tooth and bone tissue were taken from each skeleton for isotopic analysis, in each case: a tooth formed in early childhood; a third molar; the shaft of a long-bone; and a rib. Pathological lesions were avoided, as were teeth showing evidence of non-dietary wear. The prehistoric skeletons are incomplete, so that it was not possible to select exactly the same bones from each skeleton. Specimens chosen for analysis nevertheless contain, in each case, tissue laid down at different stages of the individual's life.

Teeth were dissected to separate enamel and dentine, and each bone and tooth divided into sub-samples, in order to isolate different components required for various analyses. No attempt was made to separate primary and secondary dentine. Small chunks of bone and dentine were superficially cleaned, then soaked in dilute (1-5%) hydrochloric acid in order to extract the acid-insoluble protein residue, loosely referred to as 'collagen'. (There may be small amounts of non-collagenous material present.) These residues were further treated with 0.1M sodium hydroxide to remove humic acids, then rinsed to neutral pH in distilled water and freeze-dried. Five to ten milligrams of the resultant bone 'collagen' were weighed into quartz glass tubes containing copper oxide, copper metal and a small piece of silver foil. Oxygen from the copper oxide combines with carbon in the sample to form carbon dioxide gas; copper metal acts as a reducing agent so that all nitrogen is converted to nitrogen gas, rather than oxides of nitrogen. Silver metal is a catalyst and reacts with halides and sulphur. The tubes were evacuated to less than [10.sup.-2] Torr, sealed with a glass-blowers' torch and baked at 800 [degrees] C for at least six hours. The oven was then allowed to cool slowly. Carbon dioxide and nitrogen gases were purified by cryogenic distillation, and 13C/12C and 15N/14N ratios measured on a Micromass 602E light isotope ratio mass spectrometer. Measurements were made against laboratory reference gases; in the case of carbon dioxide the reference gas has been related to the PDB standard by calibration against six National Bureau of Standards isotopic reference materials, NBS 16, 17, 18, 19, 20 and 21. Results reported here are relative to PDB. For nitrogen, the laboratory reference gas has been calibrated against atmospheric nitrogen and International Atomic Energy Agency standards N-1 and N-2. Results reported here are relative to atmospheric nitrogen, where [Delta]15[N.sub.atm] = 0 per thousand. For both carbon and nitrogen, the reproducibility of measurements on homogenized samples is better than 0.2 per thousand.

Whole bone, dentine and enamel were retained for strontium isotope analysis. Samples were powdered and washed repeatedly in acetic acid/sodium acetate buffer to remove post-depositional contamination. This 'solubility profile' method has been described elsewhere (Sillen 1986). Sr/Ca in the washes usually peaks early in the profile, representing contaminated bone mineral. Later washes have consistent Sr/Ca, as expected for biogenic apatite with its tightly-controlled chemical composition. The technique may not remove all contaminants (Tuross et al. 1989, but see also Sillen 1990); however, it certainly removes some (Sealy et al. 1991). Ca/P of 2[center dot]1, or thereabouts, for the later washes provides an additional check on the integrity of the mineral (Sillen 1989). Residual contamination from strontium in the soil will reduce variability in 87Sr/86Sr within a skeleton, so that any intra-skeletal differences are minimum estimates. In this study, 87Sr/86Sr analyses are of washes from the later, plateau part of the curve. Measurements were performed on a VG Sector mass spectrometer normalized to 86Sr/88Sr = 0[center dot]1194. Strontium isotope results reported here have been normalized to a value of 0.71023 for the NBS standard SRM 987.

Results

FIGURE 3 summarizes the results of the analyses of each of the five skeletons. The prehistoric skeletons, UCT 248 and UCT 169, both have similar isotopic values in all tissues. In each skeleton, dentine from the incisor has slightly depleted [Delta]13C and lower [Delta]15N compared with the third molar. Compact and cancellous bone are intermediate between the two. This pattern may result from slightly different diets at different stages of life, or it may simply represent normal variation. Studies of additional skeletons will help to distinguish between these two possibilities. The carbon and nitrogen isotope values are consistent with a much larger set of results for coastal-dwellers from this area (Sealy et al. 1987; Sealy & van der Merwe 1988), and indicate a diet which included considerable amounts of seafood, together with [C.sub.3]-based terrestrial foods. Strontium isotope results also fit this picture: 87Sr/86Sr of 0[center dot]709 reflects marine and coastal terrestrial foods (Sealy et al. 1991). For UCT 248, all the strontium isotope results are within analytical uncertainty of one another: there is no difference between them. For UCT 169, the enamel of the first incisor has the highest 87Sr/86Sr value (0[center dot]70970[+ or -]0[center dot]00002), while the femur has the lowest (0[center dot]70938[+ or -]0[center dot]00002). The mean values differ by 0[center dot]00032.

87Sr/86Sr results are given to five decimal places, and variations in even the fifth decimal place can be meaningful to geochemists. Although there is no biological fractionation of 87Sr/86Sr, bone is a heterogeneous material, and most diets incorporate a mix of different elements. We may, therefore, see more variation in 87Sr/86Sr in bones than the geochemical literature might lead one to expect. 87Sr/86Sr for a seal, a cormorant and a gannet, all fish-eating animals, ranged from 0[center dot]70926 to 0[center dot]70933 (Sealy et al. 1991). These values are slightly different from that of sea-water, at 0[center dot]70915 (DePaolo & Ingram 1985) or 0[center dot]70912 (Lavelle & Armstrong 1993). 87Sr/86Sr within the marine food-web seems to vary in at least the fourth decimal place. The mixing processes that occur in the sea do not take place on land, so terrestrial food-webs may be more variable.

87Sr/86Sr variation within a bone is poorly understood. Duplicate analyses of the femur of the Vergelegen skeleton (see below) gave values of 0[center dot]71916 and 0[center dot]71947 respectively. More work is needed in order to establish the range of 87Sr/86Sr we can expect within bones, but it is certainly greater than the analytical variability expressed by the standard deviation. In the meantime, we should be cautious in our interpretation of small differences in strontium isotope ratios in archaeological specimens.

The variation of 0[center dot]00032 seen between the tissues of UCT 169 is almost identical to that seen within a single bone from the Vergelegen skeleton. We conclude that this degree of variation is to be expected in biological samples. These results are consistent with our expectation that prehistoric hunter-gatherers are likely to have eaten the same kind of diet, and lived in more or less the same area throughout their lives. They furnish a base-line against which to evaluate measurements of historical skeletons.

UCT 457, from the fort in Cape Town, also shows little variation in stable isotopes within the skeleton. The dentine of the first molar has marginally more negative [Delta]13C than the third, but distinctly lower [Delta]15N, in the same pattern as seen above for UCT 248 and 169. The rib and the shaft of the humerus are intermediate. 87Sr/86Sr for bone and dentine varies from 0[center dot]71047 for the dentine of the third molar, to 0[center dot]71067 for the humerus. Once again, the difference is only 0[center dot]0002, and we cannot be certain that it results from anything other than normal biological variation. 87Sr/86Sr for the enamel of the first molar, however, is 0[center dot]71206. This difference may be sufficiently large that it reflects a change of environment from childhood to adulthood. The first molar forms in the first decade of life, and its enamel preserves a reliable indication of 87Sr/86Sr in the area in which this man spent his childhood. He was in his 50s when he died (Fourshe pers. comm.). It would be interesting to know exactly when he moved away from his childhood environment, and analysis of enamel from other teeth which form later in life should provide clues. There is probably substantial addition of secondary dentine to the first molar, to give 87Sr/86Sr of 0[center dot]71061. His move away from his childhood home did not happen shortly before his death; he may have spent much of his life as a soldier or sailor, serving the Dutch East India Company and/or other employers, before he died at the Cape.

Isotopic results for the skeleton from the Dutch East India Company station at Oudepost show a different pattern from those described above. The spread of values, for both [Delta]13C and [Delta]15N, is greater than that seen in any of the previous skeletons. There is a consistent trend towards less negative [Delta]13C values and enriched [Delta]15N values as one moves from the dentine of the first incisor to the third molar, and thence to the femur and rib. This sequence of skeletal elements comprises tissues with increasing proportions of material laid down in later life. Adult diet was enriched in both 13C and 15N compared with childhood diet, a pattern probably due to increased consumption of sea-food in adulthood. Faunal remains at Oudepost include a great many fish bones, as well as the remains of domestic and wild animals. 87Sr/ 86Sr for all tissues analysed are very similar. Results for dentine from the first incisor and the rib are almost identical, as are those for the third molar dentine and the femur. First incisor enamel has slightly lower 87Sr/86Sr, but as the difference occurs, once again, only in the fourth decimal place we cannot be sure that there is anything here that we need to explain in terms of human behaviour. The dietary shift apparent in the carbon and nitrogen isotope values is not reflected in strontium isotope ratios. This man came from a region where the terrestrial geology has 87Sr/86Sr of 0[center dot]709, perhaps a coastal area covered with recent sands of marine origin, true of many coast-lines. 87Sr/86Sr of 0[center dot]709 for the femur and rib of the Oudepost skeleton are consistent with values for the coastal fringes of the southwestern Cape (Sealy et al. 1991), and a diet which probably incorporated a large proportion of marine foods in later life. His childhood home, however, was different from that of the man whose skeleton was uncovered at the fort, with first molar enamel 87Sr/86Sr of 0[center dot]712.

Results for the skeleton from the governor's estate, Vergelegen, are especially striking. Again, there is a great deal of variation in the stable isotopes. Dentine samples from both the second incisor and third molar have low [Delta]15N and relatively positive [Delta]13C. Such a combination can only result from the consumption of [C.sub.4] grains in a well-watered environment, probably a tropical or sub-tropical area, since [C.sub.4] plants grow best in regions of summer rainfall. This diet was continued into early adulthood, since the results for the third molar are virtually identical to those for the incisor. Her diet in later adulthood, however, had substantially higher [Delta]15N, reflected in the femur and, to a greater extent, in the rib. As for the Oudepost skeleton, this pattern is consistent with our prediction that ribs, with their greater proportion of cancellous bone, reflect changes in diet during later life to a greater extent than slow-remodelling compact bone. Vergelegen receives more than 600 mm of rain per annum, so the increased [Delta]15N is unlikely to be due to aridity. It is probably due to sea-food.

87Sr/86Sr for the dentine of both the canine and third molar is 0[center dot]732. Canine enamel has 87Sr/86Sr of 0[center dot]73507, and third molar enamel of 0[center dot]73904. These are very high values. High 87Sr/86Sr have been reported for ancient rocks in the southwestern Cape (Allsopp & Kolbe 1965), but are limited to restricted geological exposures. It is unlikely that humans eating mixed diets would inherit the elevated ratios. Analyses of animal bones, and of local prehistoric skeletons from areas of Cambrian and pre-Cambrian geology have so far yielded only values below 0[center dot]72 (Sealy et al. 1991). Such elevated strontium isotope ratios strongly suggest that this woman grew up somewhere other than at the Cape. In combination with the [C.sub.4] diet reflected in the stable isotope ratios, this is compelling evidence for a foreign childhood. 87Sr/86Sr of the femur is 0[center dot]719, and of the rib 0[center dot]717. Once again, the shift is more marked in the rib. Adult remodelling of these bones incorporated strontium of lower 87Sr/86Sr, perhaps from sea-food with 87Sr/86Sr of 0[center dot]709, and/or from terrestrial foods. A sheep bone and a cow bone from Vergelegen gave 87Sr/86Sr of 0[center dot]71283 and 0[center dot]71373 respectively. The nitrogen isotope results, however, indicate that marine foods made at least a partial contribution to lowered adult 87Sr/86Sr.

Comparison of enamel 87Sr/86Sr with that of dentine for the same tooth is consistent with this. 87Sr/86Sr for canine enamel is 0[center dot]73507, compared with 0[center dot]73267 for dentine. 87Sr/86Sr for enamel from the third molar is 0[center dot]73904, compared with 0[center dot]73298 for dentine. The lower values for dentine indicate addition of secondary dentine in adulthood with 87Sr/86Sr lower than the childhood diet.

All the isotope results for the Vergelegen woman, therefore, taken together, indicate a childhood in which her diet was rich in tropical grains and poor in sea-food. In early adulthood (but probably not until she had reached her 20s, because her third molars were already formed) her diet changed radically, to include a large proportion of sea-food. The shift occurred early in adult life, in order to allow time for substantial remodelling of even the dense bone of the shaft of the femur, before death in her 50s. The change was probably coincident with her move to the Cape. Considered in combination with the placement and nature of the grave, we may deduce that this major event is likely to have been her removal, as a slave, to the Cape. Historical records indicate that Cape slaves were often fed on fish, a freely-available protein source. In the first half of the 18th century, slaves were brought to the Cape from the Indonesian Archipelago, India, Malaysia, Madagascar and, in small numbers, from Cape Verde and the Guinea coast of West Africa. Further strontium isotope measurements from these areas, for comparison with the results for tooth enamel, may help to identify her likely homeland.

Conclusions

We have shown that measurements of [Delta]13C, [Delta]15N and 87Sr/86Sr in tooth enamel and dentine, compact and cancellous bone from the same skeleton reflect diet and place of residence at different times in an individual's life. For people who travelled considerable distances, changed their diets and/or crossed major geological boundaries, this approach offers a powerful method of tracing some aspects of life history from skeletal remains. All the skeletons described here come from unknown individuals. It is sincerely to hoped that, in the future, work such as this will have access to named individuals whose historically attested dietary histories may be checked against the chemical findings. There are many archaeological questions which can be answered in this way, and the technique may be useful also in forensic anthropology.

Acknowledgements. We thank Gabeba Abrahams for making the skeleton from the Fort available for analysis, and Kiersten Fourshe for carrying out the physical anthropological study of the skeleton. Alan Morris allowed us to take samples for analysis from the collection of skeletons in his care. Ann Markell excavated the burial from Vergelegen, and provided a great deal of valuable information on its context, etc. These excavations were funded by Anglo-American's Chairman's Educational Trust. Schrire's research at Oudepost was funded by the National Science Foundation (BNS85-08990), the Mauerberger Foundation, the John Simon Guggenheim Foundation, the Rutgers University Research Council and the Human Sciences Research Council. The Foundation for Research Development and the University of Cape Town provided financial support for the laboratory work.

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Judith Sealy, Department of Archaeology, University of Cape Town, Private Bag, Rondebosch 7700, South Africa. Richard Armstrong, Research School of Earth Sciences, Australian National University, Canberra 0200 ACT, Australia. Carmel Schrire, Department of Anthropology, Douglass College, Rutgers University, New Brunswick NJ 08903, USA.