Setting and archaeological context

The environment of our study area is one that is, in many ways, intermediate between southern Peru to the north and the central Atacama Desert to the south. The study area is located in the northern section of the Atacama Desert between 18° and 19°S [34]. Unlike the coast, which offers abundant and stable marine resources, the inland valleys of the northern Atacama Desert are comparatively unstable places for human occupation, due to low biological productivity, limited availability of fresh water, periodic droughts, and overall lower predictability of resources [35].

The Lluta and Camarones rivers originate in the highlands (Lluta at 3,900 masl and Camarones at 2,900 masl), draining into the Pacific Ocean. They are the only rivers in northernmost Chile that present permanent runoff throughout the year, but occasional prolonged droughts may cause their runoff to terminate in their lower courses [36]. These rivers are characterized by extremely high concentrations of metals (As, B, Cu, Li, Na, K) [37], which limits the quantity and quality of wild vegetation, restricting it to the riverbed and areas fed by spring water. In fact, the sites under study are strategically located in association with fresh water springs. The limited vegetation coverage and the quantity and quality of water available in the Lluta and Camarones valleys make them unsuitable for supporting large numbers of animals, both wild and domesticated. Pre-Hispanic populations inhabiting these valleys had access to a limited variety of plants, seeds, and fruits. Agriculture generated important complementary resources, although also limited in variety (possibly only maize in pre-Hispanic times).

According to regional paleoclimate records, since 3,000 BP the current hyper-arid conditions have predominated in the Atacama Desert, although there were significant increases in rainfall at various intervals including 1050–600 cal. BP − the Medieval Climatic Anomaly (MCA), which roughly coincides with the Late Intermediate Period and Late Horizon [38–40]. Although climatic conditions were similar to those at present, these rainy events would have been more frequent during the MCA and would have meant a recharge of the fluvial and spring water resources. It is presumed, therefore, that there was greater vegetation coverage and more water available during the periods under study than there is presently.

The Late Intermediate Period and Late Horizon witnessed the development of the Arica Culture as a group of segmented societies that shared a common cultural tradition, although they did not comprise a single political entity. These communities practiced a mixed agro-maritime subsistence and had differential settlement and architectural patterns, as well as interactions with other groups [41–43]. During the Late Horizon, the impact of the Inca in the region is demonstrated by multiple lines of evidence, including typical Inca-style goods, as well as other proxies for changes in local economic activities, settlement patterns, and health conditions [44–46].

The samples analyzed in this study are from five small villages: four from the Lluta Valley (Rosario 2 [LL36], Sora Sur [LL19], Poblado Millune [LL21], and Vinto 1–2 [LL93]), and one from the Camarones Valley (Poblado Huancarane 1) (Fig 1). LL36 was excavated in 1995 by Calogero Santoro [47]. LL19, LL21, and LL93 were excavated in 2011–2012 [48]. Huancarane was excavated in 1978 by Niemeyer and Schiappacasse and its collection was studied in 2015 at the Museo Nacional de Historia Natural (Santiago de Chile). Radiocarbon dates for these sites are summarized in Table 1. All dates listed in Table 1 were calibrated in Calib v7.0.4 [49] using SHCal13 [50].

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Fig 1. Map of the study area.

Relevant toponyms mentioned in the text and study area (box) with archaeological sites including in this study.

https://doi.org/10.1371/journal.pone.0228332.g001

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Table 1. AMS radiocarbon dates obtained from the studied sites.

https://doi.org/10.1371/journal.pone.0228332.t001

The archaeological site LL-36 (Rosario 2) is a hamlet located 16 km from the Pacific coast, composed of totora (reed) walled rectangular structures raised on artificial platforms reinforced with frontal stone walls, to level the steep slope of the valley [43]. Based on stratigraphy, radiocarbon dating, and ceramic components, it has been estimated that the earliest occupation was brief, corresponding to the Late Intermediate Period. More intense occupation occurred during the subsequent Late Horizon, as the inhabited surface of the site became larger, and midden deposits are more dense. The site is also distinguished by the occurrence of typical Inca goods (mainly ceramics, such as Inca Polychrome and Saxamar or Inca-Pacaje types, and some fragments of khipu) and a large number of exotic goods. Domestic remains show consumption of a wide array of marine resources (e.g., mollusks, fish, seaweed, sea stars), terrestrial fauna (e.g., camelids, guinea pigs), wild plants (e.g., molle, totora), crops (e.g., cotton, pumpkin, bean, ch’uño, and corn), mineral copper beads, basketry, weaving tools, and abundant textile remains. Systematic analysis of ceramics indicates important changes related to the Inca influence [46, 47, 49].

LL-19 (Sora Sur), located 48 km from the Pacific coast, is a small village of circular precincts made of stones [48]. An occupation from the Late Intermediate Period and another from the Late Horizon were found during excavation. Generally, there are no significant differences between the material culture and biological remains from these two periods, which consist of wild plant remains (e.g., Schinus molle, Prosopis sp.), crops (Lagenaria sp. and Zea mays), and lithic artifacts. Camelids and other terrestrial fauna such as canids and rodents (e.g., Chaetophractus nationi, Chinchilla sp., Cricetidae) are scarce. Marine fauna is also represented by a small number of mollusks (Choromytilus chorus, Littorina peruviana, Mytílido, Perumytilus purpuratus) and unidentified fish. Some textiles (mainly yarns) were also recovered. Local (e.g., Pocoma, San Miguel, and Gentilar) and highland (e.g., Black on Red and Charcollo) pottery types occur in both strata, with Inca Imperial ceramics limited to the Late Horizon stratum.

LL-21 (Poblado Millune, 54 km from the Pacific coast) is a complex village of circular precincts made of stones, organized in differentiated sectors: habitation, storage, and funerary [48]. The excavated deposit contained an occupation that was assigned to the Late Horizon. The materials recovered include local (e.g., San Miguel), highland (e.g., Chilpe, Charcollo), and Inca Imperial pottery, lithic and plant artifacts, and textiles (mainly yarns). There is a predominance of plant remains, with both wild (e.g., Prosopis sp., Schinus molle, Equisetum sp., Typha angustifolia) and domesticated (e.g., Cucurbita sp., Gossypium sp., Lagenaria sp., Zea mays) species represented. Faunal remains are relatively scarce, with the assemblage consisting of terrestrial mammals (e.g., Camelidae, Lama sp., Lycalopex sp., Chinchillidae, Cricetidae), mollusks (e.g., Choromytilus chorus, Perumytilus purpuratus), and finfish (Trachurus murphyi).

LL-93 (Vinto 1–2, 57 km from the Pacific coast) is a hamlet composed of residential, funerary, storage, and public use structures (kancha, i.e., enclosure) [48]. Three occupations were identified, dating to: the Late Intermediate Period, Late Horizon, and Colonial Hispano-Indigenous Period. The material culture throughout these periods does not show significant changes in composition and consists of lithic artifacts, and mollusk and plant tools. Ecofacts include gathered plant remains (e.g., Prosopis sp., Phragmites australis, Schinus molle, Tessaria abdinthioides, Equisetum sp., Scirpus sp., Typha angustifolia, Tessaria abdinthioides), crops (Cucurbita sp., Gossypium sp., Zea mays, Capsicum sp.), camelids (Camelidae, Lama sp., Vicugna sp.), rodents (Cavia sp., Caviomorpho, Chinchillidae, Cricetidae, Lagidium sp.), and other unidentified mammals. Coastal resource procurement is evidenced by the presence of mollusks (Choromytilus chorus, Scurria scurra, Tegula spp). Some textiles (mainly yarns) were also recovered. The ceramics include highland types (e.g. Charcollo) and, in late periods, Inca Imperial styles. The colonial occupation of the site was documented by the presence of skeletal remains of Old World taxa and Hispanic pottery.

Huancarane 1 (60 km from the Pacific coast) is a village composed of stone structures, with differentiated areas (habitation, storages, and funerary) architecturally similar to the Lluta villages described above [48]. The archaeological material comes from the excavation of habitation and storage structures, although stratigraphic information is lacking. The evidence recovered from the excavations includes cultivated (Zea mays, Phaseolus sp., Lagenaria sp., Gossypium sp.), and wild plant remains (Prosopis sp.), lithic artifacts, wood, and textiles [52]. Some fiber samples were microscopically analyzed and identified as llama, guanaco, and vizcacha (Lagidium sp.) by Niemeyer and Schiappacasse [52]. The pottery includes LIP Pica-Tarapacá, Arica, Altiplano and Inca components [53]. Marine mollusks (Choromytilus chorus, Oliva peruviana, Cryphiops caementarius) and freshwater crustaceans (Cryphiops caementarius) were recorded, but both were scarce. On the contrary, abundant remains of terrestrial fauna were recovered, including camelids, rodents (Chinchilla sp), and other unidentified mammals.

Isotopic context

The application of isotopic analysis in the study of animal husbandry and the trade in animal products is a rapidly growing area of inquiry. Some isotope systems, such as hydrogen, oxygen, strontium, and lead, record the location in which an animal lived [54], and these techniques have been applied with some regularity in archaeological contexts to address the trade in animal products [55–59]. Stable carbon and nitrogen isotope compositions are not tied to geography in the same way as these other isotope systems, but in certain regions where environmental variation is large over relatively small spatial scales or particular feeding practices that are isotopically unique exist, carbon and nitrogen isotopes can be an effective means of assessing the locality of animals and animal products in the archaeological record [31, 32, 60].

The carbon and nitrogen isotope compositions of a consumer’s tissues reflect the average carbon and nitrogen isotope compositions of the foods consumed during the period of tissue formation [61, 62]. The carbon in bone collagen is predominantly routed from dietary protein [63, 64]. Herbivore tissue δ¹³C and δ¹⁵N therefore reflect the δ¹³C and δ¹⁵N of the plants that they consumed [65, 66], which are in turn sensitive to a number of environmental parameters [67, 68].

Photosynthetic pathway is the primary mechanism influencing the δ¹³C values of plants. Excluding plants growing under dense forest canopy, C₃ plants have δ¹³C values that range between −35 and −20 ‰ with a mean of c. −27 ‰; C₄ plants (predominantly tropical grasses) have δ¹³C values that range between −15 and −7 ‰ with a mean of −12 ‰; CAM plants (cacti, succulents, and epiphytes) have δ¹³C values that range between −22 and −10 ‰ [69–71]. C₃ plant δ¹³C values are sensitive to environmental variation through influences on the ratio of ambient to intercellular partial pressure of CO₂ [67, 72]. The δ¹³C values of C₃ plants are correlated with water availability such that plants growing in arid conditions have higher δ¹³C values than those growing in wetter conditions [73–76]. Water availability does not appear to influence C₄ plant δ¹³C [75, 77], which are generally less sensitive to environmental variation and exhibit a much narrower range of δ¹³C values than C₃ plants [72]. The entire region considered for this study is characterized by low water availability, therefore, wild C₃ plants will have δ¹³C values that are higher than the global average. In a series of altitudinal transects between 22°S and 25°S in northern Chile, Quade et al. [78] found the average δ¹³C of C₃ plants to be −23.1 ‰ (c. −21.6 ‰ after accounting for the Suess Effect). Similarly, Rundel et al. [79] found the average δ¹³C of C₃ plants growing in the pre-puna shrubland zone (c. 3,550 masl) at 18°S to be −24.1 ‰ (c. −22.8 ‰ after accounting for the Suess Effect). The δ¹³C of the overall biomass at high altitudes (>3,500 masl) is still lower than at low altitudes because of the rarity of C₄ plants [80, 81]. Despite the rarity of C₄ plants in the arid highland regions of northern Chile, the δ¹³C values of herbivores consuming pure C₃-diets should be relatively high. In In this region, herbivore δ¹³C values of −18 ‰ do not necessarily imply the consumption of any C₄ plants assuming a consumer-diet trophic enrichment factor (Δ¹³C) of c. +5 ‰ [82]. Soil salinity also influences the δ¹³C values of plants, with high values occurring with increasing soil salinity [83–85]. Finally, plants growing under dense forest canopies have lower δ¹³C values than plants growing in open habitats [86–88], although this particular variable is not relevant for this study as the area lacks the necessary tree cover.

Plant tissue δ¹⁵N values are determined primarily by the N source [68], the most important of which are mineralized N (NO₃⁻ and NH₄⁺) and atmospheric N₂ [89, 90]. Some plants in environments with low mineralization rates (typically boreal and arctic environments) also rely to a significant extent on organic N [91]. There is essentially no discrimination against ¹⁵N during the conversion of N₂ to NH₃, a process that is known as biological nitrogen fixation (BNF [92, 93]). Plants that have the capacity to rely on N₂ through symbiotic associations with bacteria (primarily legumes, Fabaceae) therefore tend to have δ¹⁵N close to that of atmospheric N₂ [94–96], which is 0 ‰ [97]. The reliance on N provided through these symbiotic associations is, however, metabolically expensive and if soil N availability is high, legumes and other taxa capable of BNF will rely on mineralized N sources [94].

The types of mycorrhizal associations that plants form are also an important factor in influencing plant δ¹⁵N, with plant values being highest in non-mycorrhizal and arbuscular mycorrhizal (AM) plants, lower in ectomycorrhizal (EcM) plants, and lowest in ericoid (ErM) myccorhizal plants [98, 99]. Generally there is latitudinal variation in the distribution of mycorrhizal types, following trends in temperature and soil N mineralization rates, with AM plants dominating in temperate grasslands and savannahs, EcM plants dominating in temperate forests and boreal taiga, and ErM dominating in high latitude tundra [100, 101]. Altitudinal trends in mycorrhizal associations are less well studied, but it appears that AM abundance declines with altitude, EcM abundance peaks at mid altitudes (1,000 to 2,500 masl), and the abundance of ErM is generally limited by the distribution of their plant partners, the Ericaceae, which tend to occur in acidic soils with low N availability [102].

In agricultural systems, additional N may be added to the soil through fertilization. The use of N derived from fertilizers has the capacity to increase plant δ¹⁵N values by a few ‰ for manure derived from domestic herbivores such as cattle or camelids [103–105] to greater than 20 ‰ for seabird guano [94, 105, 106]. Both types of fertilizer would have been potentially available in the study area and in fact the only convincing evidence of the prehistoric use of seabird guano as a fertilizer to date comes from the Atacama Desert immediately south of our study area [107].

Plant δ¹⁵N tends to be positively correlated with temperature [108, 109] and negatively correlated with water availability [65, 74, 75, 98, 108, 110]. Because average temperature decreases and annual precipitation increases moving from the Pacific coast and into the highlands of Peru and Chile, plants growing at high altitudes have lower δ¹⁵N than those growing at lower altitudes [111]. That said, the high altitude puna of northern Chile is distinct from that which exists in Peru in that it is far more arid. This region does receive more precipitation than the coast, but the region is still arid, with the northern Chilean dry puna receiving 300−350 mm of annual precipitation [112]. Despite the aridity of the puna, studies conducted in the hyperarid salt puna around 23.5°S [113] and in the Argentine dry puna [114] found plant δ¹⁵N values to be comparable to those observed in the highlands of northern Peru (c. +2 to +6 ‰) [111]. Given that herbivores tissue δ¹⁵N values are 3 to 4 ‰ higher than the plants that they consume [115], we would expect camelids that lived in the puna to have tissue δ¹⁵N values between +5 and +9 ‰. This expectation requires testing through the collection of isotopic data from archaeological camelids recovered from sites in the highlands of northern Chile. For animals living in the low altitude coastal river valleys of northern Chile we expect much higher δ¹³C and δ¹⁵N values than those living in the highlands driven primarily by the influence of aridity on plant δ¹⁵N values and the greater abundance of C₄ plants, particularly cultivated maize, which could have been used as fodder for camelids.

Sample preparation

Bone collagen was extracted by demineralizing chunks of bone (c. 200 mg) in 0.5 M HCl at room temperature under constant motion (orbital shaker) for 7 days with periodic changing of the acid solution. After demineralization, the samples were rinsed to neutrality with Type I water and then those samples that were darkly colored were treated with 0.1 M NaOH for 20 min under constant motion (orbital shaker). If after 20 min the solution changed color, the solution was removed and fresh NaOH solution was added. Once there was no color change in the solution after 20 min, the samples were rinsed to neutrality with Type I water. The samples were then heated at 75°C for 36 h in 4 ml of 10⁻³ M HCl. After heating the solution containing the water-soluble collagen was transferred to a vial and freeze-dried.

The textile samples were cleaned of any visible particulate matter with a dental pick. The samples were then sonicated in Type I water for 60 min, centrifuged and air-dried. The textiles samples were then sonicated in 2:1 chloroform-methanol (v/v) for 60 min, centrifuged and air-dried.

Isotope ratio mass spectrometry

Carbon and nitrogen isotopic and elemental compositions were determined using a Nu Horizon continuous flow isotope ratio mass spectrometer couple to a EuroEA 3000 elemental analyzer at Trent University. Sample isotopic compositions were calibrated relative to VPDB (δ¹³C) and AIR (δ¹⁵N) using USGS40 and USGS41a or USGS66 [116–118] (S1 Appendix). Elemental compositions were calibrated using USGS40. Analytical uncertainty was assessed using four internal standards interspersed among the samples and 20% of the samples were analyzed in duplicate (S1 Appendix). Standard uncertainty was determined to be ±0.20 ‰ for δ¹³C and ±0.29 ‰ for δ¹⁵N [119].

Sample integrity

Quality criteria are not as well established for ancient keratin as they are for collagen. Boudin et al. [120] found an atomic C:N ratio range of 3.4−3.8 to be reliable within the context of ¹⁴C dating of wool. von Holstein et al. [121] found minimal changes to the carbon and nitrogen isotope compositions of wool textiles undergoing experimental degradation. Our approach to monitor for textile samples with unreliable isotopic compositions altered by post-depositional processes was informed by the approaches of Ambrose [122] and DeNiro [123] for bone collagen. DeNiro defined an acceptable range of 2.9–3.6 for unaltered collagen by examining the elemental and isotopic compositions for different taxa and noting that the collagen samples with C:N ratios outside the range of 2.9–3.6 tended to produce isotopic compositions that were too high or too low (S1 Appendix). Subsequently, others have discussed other quality control measures [122, 124–126], but DeNiro’s range of 2.9–3.6 for atomic C:N ratios remains the most frequently cited quality control measure in isotopic studies of ancient collagen [119]. We have taken a similar approach to DeNiro and compared the isotopic compositions and the atomic C:N ratios of the textiles analyzed in this study, as well as several others from the Andean region (Fig 2).

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Fig 2. Relationship between isotopic and elemental composition of camelid fiber textiles.

Carbon and nitrogen isotopic compositions of textiles analyzed in this study, as well as other camelid fiber samples from previously published papers [26, 31, 32]. Samples are colored according to their atomic C:N ratio.

https://doi.org/10.1371/journal.pone.0228332.g002

The data produced by DeNiro for bone collagen show clear differences in isotopic compositions between samples with C:N ratios within vs. outside of the 2.9–3.6 range (S1 Appendix). Specifically, the δ¹³C values tend to be especially low when the C:N ratios are outside of the 2.9–3.6 range and the δ¹⁵N values are much more variable (may be too high or too low). For archaeological camelid fiber, the samples with relatively high and low C:N ratios were not distinct, unlike those presented for DeNiro’s bone collagen data. This difference may be because highly degraded keratin does not survive in the burial environment while bones containing virtually no residual organic matter are reasonably common.

Although Boudin et al. [120] defined an atomic C:N range of 3.4–3.8 as being acceptable for radiocarbon dating sheep’s wool, modern camelid fiber samples produced a range of atomic C:N ratios between 3.10 and 3.45 (n = 85) [24]. It therefore seems unreasonable to exclude data based on criteria defined for another taxon that produces a distinct type of fiber [127]. For the dataset presented in Fig 2, encompassing 452 analyzed camelid fiber samples from Peru and Chile, a significant number of samples with atomic C:N ratios greater than 4 produced relatively low δ¹³C and high δ¹⁵N values. Fourteen samples produced δ¹³C values less than −16 ‰ and δ¹⁵N values greater than +10 ‰. Of these fourteen, five (36%) had C:N ratios that were greater than 4, while the number of fiber samples in the entire dataset producing C:N ratios greater than 4 was 14 (3% of the total sample). On this basis, we excluded any samples with an atomic C:N ratio over 4 from our analysis.

Data treatment

When comparisons were made between bone collagen and textile isotopic compositions, the textile δ¹³C values were adjusted by +1.3 ‰ to account for inter-tissue differences in diet-tissue fractionation. The isotopic composition of bone collagen represents the weighted average of foods consumed over a period of several years [128]. While hair grows incrementally and is inert once formed, the nature of textile samples is such that the period of time represented by an isotopic measurement cannot be determined, nor is it certain that a single animal is represented in any particular fiber sample. Therefore, the isotopic compositions of textile samples represent an average dietary intake over an indeterminate amount of time. It is therefore important to generate larger numbers of isotopic measurements of textiles than of bone collagen to adequately capture the variability within a single site or context.

To assess differences in the isotopic compositions between periods or regions, one of the following tests were used: Student’s t-test (normally distributed, equal variances), Welch’s t-test (normally distributed, unequal variances), or Mann Whitney U test (non-normally distributed). Normality was assessed using a Shapiro-Wilk test. Equality of variance was assessed using Levene’s test. The presence of discrete isotopic groups in the textile dataset was assessed using an unweighted pair group method with arithmetic mean cluster analysis with a Euclidean distance function.

The amount of bivariate isotopic variation was estimated using the standard bivariate ellipse in the SIBER package [129]. Ellipse sizes reported in the text are standard ellipse areas corrected for sample size (SEA_c). Comparisons between ellipses were performed with 10⁴ iterative draws (SEA_b) with the results being expressed as the percentage of computed ellipses for Group 1 that are larger than the computed ellipses for Group 2; a value of 0.50 indicates the two ellipses are identical in size, while a value of 1 or 0 would indicate that the ellipses for Group 1 are always larger or smaller than those for Group 2.

The cluster analysis was performed using Past 3.20. The SIBER calculations were performed in R 3.0.3; the scripts are included in the supplementary information (S1 File). All other statistical tests were performed using IBM SPSS 23.

Bone collagen

The bone collagen δ¹³C and δ¹⁵N values are presented in S1 Table and Fig 3. The camelids from the Late Horizon possessed significantly higher δ¹³C values than those dating to the LIP (Welch’s t-test; t = −3.46, df = 8.59, p = 0.008); the δ¹⁵N values did not differ between the two periods (Welch’s t-test; t = −0.46, df = 9.36, p = 0.65). There was no overlap between the standard ellipses generated for the camelid bone collagen dating to the LIP and LH (Fig 3). The bivariate isotopic variation observed for the LH camelids was much greater than for the LIP camelids (SEA_c of 10.4 compared to 3.0), with the modeled ellipses of the LH group being larger than the LIP group in 99% of the comparisons.

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Fig 3. Bone collagen δ¹³C and δ¹⁵N values for camelids from the Lluta Valley along with the standard bivariate ellipses for the two time periods.

Late Intermediate Period samples are indicated by squares and Late Horizon samples are indicated by circles.

https://doi.org/10.1371/journal.pone.0228332.g003

Textiles

The textile carbon and nitrogen isotopic and elemental compositions are presented in S2 Table. There were two distinct groups of textiles, one with lower δ¹³C and δ¹⁵N values and one with higher δ¹³C and δ¹⁵N values (Fig 4); these groupings were confirmed with a cluster analysis (Fig 5). For the purposes of further analysis, each cluster was considered a distinct data set and they are referred to as Textile Group 1 (δ¹³C = −19.77±1.05 ‰, δ¹⁵N = 8.19±2.07 ‰, n = 60) and Textile Group 2 (δ¹³C = −14.44±1.55 ‰, δ¹⁵N = +13.97±1.63 ‰, n = 12). Relative to the overall textile sample, the two groups contained similar proportions of samples from the Lluta (45% of Group 1, 44% of overall sample) and Camarones (55% of Group 1, 56% of overall sample) valleys, as well as the LIP (58% of Group 1, 57% of overall sample) and LH (34% of Group 1, 36% of overall sample). Therefore, the presence of these two textile groups could not be explained by a regional or temporal effect.

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Fig 4. Stable carbon and nitrogen isotope compositions of all of the textile samples analyzed.

https://doi.org/10.1371/journal.pone.0228332.g004

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Fig 5. Results of the cluster analysis for the textile and human hair samples.

https://doi.org/10.1371/journal.pone.0228332.g005

Within Textile Group 1, samples from the LIP and LH did not differ significantly with respect to their δ¹³C (Mann Whitney U test; U = 385.5, p = 0.36) or δ¹⁵N (Student’s t-test; t = 1.70, df = 61, p = 0.09). The textile samples within Group 1 from the Lluta Valley had significantly higher δ¹⁵N values than those from the Camarones Valley (Student’s t-test; t = 3.11, df = 66, p = 0.003), although there were no significant differences in δ¹³C between the two valleys (Mann Whitney U test; U = 541, p = 0.86).

After adjusting for differences in fractionation between tissues, Textile Group 1 had similar isotopic compositions to the camelid bone collagen samples from the sites in the Lluta Valley, falling between the LIP and LH bone collagen ellipses (Fig 6). Textile Group 2 possessed markedly higher δ¹³C and δ¹⁵N values than either the LIP or LH bone collagen datasets from the Lluta Valley (Fig 6).

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Fig 6. Comparison of the camelid bone collagen and textile isotopic compositions.

Comparisons are based on standard ellipse areas for the camelid bone collagen from the LIP and LH in the Lluta Valley and the two textile groups (as determined by the cluster analysis) for the Lluta and Camarones Valleys. The textile δ¹³C values have been adjusted by +1.3 ‰ to make them directly comparable with the bone collagen.

https://doi.org/10.1371/journal.pone.0228332.g006

Unspun fleece samples had higher δ¹³C and δ¹⁵N values (δ¹³C = −16.57±4.52 ‰, δ¹⁵N = +9.33±3.70 ‰, n = 15) than spun yarns (δ¹³C = −19.19±2.40 ‰, δ¹⁵N = +8.54±2.56 ‰, n = 56). This difference was statistically significant for δ¹³C (U = 236, p = 0.01) but not for δ¹⁵N (U = 409, p = 0.88). A higher proportion of the Group 2 textiles consisted of unspun yarns (n = 5 or 50%) relative to Group 1 (n = 10 or 14%).

Camelids originating in the highlands

The bone collagen δ¹³C and δ¹⁵N values of the Lluta Valley camelids are consistent with a highland origin. The LIP and LH camelid bone collagen δ¹³C and δ¹⁵N values are summarized as standard bivariate ellipses and compared to camelids from other relevant sites where the camelids are known to have been raised in the highlands (Fig 7). Among the three comparative sites, there is a pattern of increasing δ¹³C values with increasing latitude, with the lowest values at Chinchawas (~9.5°S), intermediate values at Tiwanaku (~16.5°S), and the highest values for sites on the Tulan transect (~23.5°S). This pattern likely has nothing to do with increasing quantities of C₄ plants in the diet with increasing latitude, but with increasingly high δ¹³C values as the puna environment becomes drier moving north to south, transitioning from moist puna to dry puna to salt puna [112, 130]. In light of their δ¹⁵N values, both the LIP and LH camelids from the Lluta Valley sites lived in areas that were more arid than those at either Tiwanaku or Chinchawas. The northern Chilean puna is the most likely origin for these camelids, but the stark isotopic difference between the LIP and LH camelids from the Lluta Valley sites requires further discussion.

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Fig 7. Interregional comparison of camelid bone collagen isotopic compositions.

Standard bivariate ellipses for the camelid bone collagen from the Lluta Valley and various sites where the camelids are believed to have lived in the highlands: Chinchawas [31], Tiwanaku [131], and Tulan (Late Archaic/Early Formative) [132].

https://doi.org/10.1371/journal.pone.0228332.g007

Changes in geographic origins of camelids during the late horizon

The LH camelids from the Lluta Valley can be divided into two groups, one with lower δ¹³C and δ¹⁵N values, and one with higher δ¹³C and δ¹⁵N values; the latter group is largely responsible for the difference between the two periods (Fig 3). This apparent division among the LH group may be driven by these camelids originating in two distinct geographic regions, one of which is much drier (for the camelids with the higher δ¹³C and δ¹⁵N values) than the other (for the camelids with the lower δ¹³C and δ¹⁵N values). Alternatively, camelids may have been drawn from a more diverse range of environments overall during the LH, but because of the small size of our sample, we have simply failed to capture the full range of this variability. Regardless, our data suggest that there was a shift in the range of environments from which camelids were obtained during the LH relative to the LIP.

During the LIP, we suggest that the inhabitants of the Lluta Valley acquired their camelid meat from the puna located adjacent to the northern valleys. In the northern valleys, the LIP was a time characterized by regionalization in the wake of the Tiwanaku polity. One crucial point of debate has been whether or not there was physical resettlement of groups from the altiplano into the northern valleys following Tiwanaku’s demise [133, 134]. In a recent synthesis, Muñoz et al. [42] conclude that, although there were Tiwanaku influences during this period, there is no archaeological or bioanthropological evidence to support the presence of Tiwanaku colonies in northernmost Chile. The influence of Tiwanaku is mainly expressed in Cabuza and Tiwanaku pottery types. Cabuza was a locally produced and consumed style that attempted to emulate Tiwanaku forms and decoration. The Tiwanaku style reproduces forms and decorations of the expansive Tiwanaku Phase V, but has strong influences from Moquegua, suggesting its provenience in these valley in southern Peru, rather than the altiplano [135, 136]. Recently, radiocarbon dating carried out in Cabuza funerary contexts indicates that most of the dates fall within the LIP, suggesting that the Tiwanaku influence may have been associated with people deserting the Titicaca Basin or colonies in Moquegua following Tiwanaku’s demise [137]. Despite the lack of a strong influence of Tiwanaku on the northern Chilean valleys, the isotopic data from the LIP camelids suggests connections with the altiplano. During the LIP, a variety of altiplanic goods (e.g., feathers, copper, obsidian, sulfur, pottery) have been systematically recorded in lowland settlements possibly as a consequence of social interaction networks, which were intensified during this period [41, 138]. Camelids could certainly have been an important part of these interregional networks of interaction and exchange, particularly in light of the fact that the sites analyzed in this study are 40 to 65 km from the contiguous altiplano.

The fact that the LIP and LH camelids from the Lluta Valley were characterized by distinct isotopic compositions suggests either a change in the geographical origin of the camelids or a change in the way in which they were managed. These could be a consequence of the Inca presence in the lowland valleys of northern Chile. Although no investment in state infrastructure is present, significant changes in the ways of life of the local communities, reorganization of the economy and alterations in the political systems have been recorded as result of the Inca control over this area. These changes are expressed in the incorporation of conspicuous Inca prestige goods (e.g., aribalos, khipu, unku, tumi), the concentration of populations in larger villages, alterations of health conditions (intestinal parasites) as a result of overcrowded residences, and the intensification of maize and textile production, which demonstrate clear alterations to the local way of life, although without a visible impact on the architecture [46, 139].

During the LH, rights over resources and lands were claimed as state property by Tawantinsuyu (the Inca Empire) [140]. Camelids were strictly controlled and regulated by the Inca, with three different types of herds: state, church, and community [12]. There were also exclusive regulations for the use of hunting grounds of wild camelids, especially vicuña [141]. Additionally, the Inca state employed redistribution as a socio-political and ideological mechanism to control local populations, although a diverse range of economic structures almost certainly existed within Tawantinsuyu [142]. Among the goods distributed to local communities, wild and domesticated camelids are frequently mentioned in the chronicles [143]. Murra [144] specifies that the camelids redistributed by the state did not come from the community herds, but from the state herds. Chroniclers agree that relatively little camelid meat was consumed during Inca times, except at feasts and ceremonies, although this assertion requires explicit testing with archaeological data [145]. In some Andean regions an increase in the production and/or consumption of camelids during the LH has been identified, typically interpreted within a framework of access to meat and livestock regulated and centralized by the Inca state, sometimes in the form of state-sponsored feasts [140, 145–150].

Consequently, if the Inca state controlled the flow of camelids or their meat among the conquered provinces through state redistribution networks, then the camelids consumed in the Lluta valley may have had a different geographical origin relative to those of the LIP because they were the result of new animal distribution circuits associated with the redistribution networks controlled by the Inca state. During the Inca occupation of Tiwanaku (Pumapunku complex), a pastoralist area par excellence, large quantities of camelids were consumed at state-sponsored feasts. Of the basis of isotopic analyses, most of these camelids had a local origin but some of them originated outside of the altiplano in the Titicaca Basin [151], possibly because of the diverse origins of camelids from state herds. Although it cannot be established if the camelid bone samples included in this study were associated with state-sponsored commensal meals, some of them could have arrived in the Lluta Valley via redistributive networks.

Origins of camelid fiber

The fact that the isotopic patterns observed for camelid skeletal remains and fiber from textiles are distinct from one another (Fig 6) suggests that animals raised for different purposes (i.e., meat vs. fiber) originated in different geographic areas, consumed different foods, or a combination of these. During the Late Horizon, camelid herds were segregated according to their function: fiber, meat, and cargo [146, 149]. This is consistent with our results, but that fact that there is no difference in the fiber isotopic compositions between the LIP and LH suggests that the Inca did not influence the geographic regions from which fiber was being obtained in the Lluta and Camarones Valleys. Alternatively, there may have been significant changes in the regions of fiber production and the movement of these goods but if these changes occurred within an isotopically homogenous highland environment, it would not be detectable with the methods applied in this study. For example, if fiber production intensified during the Late Horizon (more fiber produced from the same area of land being grazed [152]), possibly driven by the extraction of tribute from some communities and the development of new categories of specialized weavers [11, 44], this scenario may not have left traces in the isotopic compositions of the textiles. On the other hand, if fiber production extensified (more fiber produced from camelids being raised in a larger number of areas), we might expect to see isotopic evidence of localized camelid husbandry, or at least non-altiplanic camelid husbandry, exclusively in the Late Horizon. Some chronicles indicate that while for local textile needs fiber from the community was used, for the mit’a obligations, in contrast, the Inca state provided the fiber, which came from state deposits [143, 144], likely located in the highlands. The presence of weaving tools in households in the valleys of northern Chile suggest that textiles were being manufactured locally even if the fiber was being imported from the highlands.

Considering the separation of herds for fiber and herds for meat as well as the diverse fiber used for local needs versus for mit’a, the scenario outlined in the preceding paragraph could suggest different circuits of distribution for meat and fiber during the Late Horizon. Textile production for local consumption, which constitutes the majority of textile samples recovered from these archaeological sites, continued to be dependent on the highlands for fiber, as was during case during the LIP. On the contrary, for the textiles manufactured through mit’a labor, we expect that the raw material would have originated in state fiber deposits. Moreover, if the mit’a used either state or community fiber, it would likely not be possible to recognize this distinction in the archaeological record because these textiles should have been redirected from local households for state purposes and would not have remained in the area where the fiber was produced [11]. Therefore, mit’a textiles should have low archaeological visibility in domestic contexts such as those sampled in this study. In some domestic contexts of the Lluta Valley where important transformations of the Inca state have been verified, including the relocation of populations and the presence of several khipu, a remarkable increase in spinning and weaving tools has been identified, which is interpreted as a possible result of textile mit’a obligations [44]. Isotopic analyses from contexts such as this may shed light on any changes in fiber production during the Late Horizon.

Fiber originating outside of the highlands

A subset of the textile samples from both the LIP and LH have isotopic compositions that are inconsistent with camelids that lived in the highlands (Figs 4 and 5). Given the greater importance of llamas for meat and their adaptability to a wider range of environments than alpacas [153], which were primarily fiber producers [20], it is surprising that the textiles, rather than camelid bone samples, appear to have non-highland origins. The issue of raising camelids in the lowlands has not been studied systematically in northern Chile, either through the application of isotopic analysis or traditional archaeological methodologies. Archaeological evidence for the maintenance of camelid herds in the lowlands (although not their permanent presence) includes corral-type structures found in villages [45, 52, 154–156] and concentrations of camelid dung [45, 52, 157–160]. For northern Chile, sixteenth century ethnohistorical accounts describe the maintenance of camelid herds in the lowlands and the tribute of livestock and manufactured textiles to the Spanish Crown by local indigenous groups from Ilo, Tacna, Arica, and Tarapacá [143, 161, 162]. If there were local herds oriented towards fiber production, we would expect to see a significant quantity of unspun fleece with local isotopic signatures. Consistent with this expectations, only 14% of the camelid fibers analyzed had isotopic compositions consistent with a local origin but a disproportionately high number of these (50%) were unspun fleece rather than spun yarns.

In addition to the textiles manufactured from camelid fiber, three textile fragments from Huancarane were made of human hair. These three samples possess δ¹³C and δ¹⁵N values that are very similar to the group identified as non-highland camelids (Fig 4). These human hair isotopic compositions are comparable to other LIP and LH data from the region, although humans from northern Chile possess highly variable tissue isotopic compositions [163–167]. The human δ¹³C and δ¹⁵N values are consistent with a substantial amount of maize in the diet rather than marine foods as marine organisms from northern Chile possess extremely high δ¹⁵N values due to strong upwelling and denitrification in this region [168, 169]. Human populations in this region that relied on marine foods to a significant degree tend to the highest tissue δ¹⁵N values among any human group, frequently in excess of +20 ‰ or even +25 ‰ [164, 170], substantially higher than those observed in this study: δ¹⁵N = +13.9 ‰, +15.0 ‰, and +15.4 ‰.

The humans from which the hair was obtained were almost certainly locals and the fact that the non-highland textile groups possesses very similar isotopic compositions suggests that these animals may have consumed a similar range of agricultural plants to the humans living in the Camarones Valley. If camelids were kept in these valleys, they would have needed to consume significant quantities of agricultural plants because there is scarce wild vegetation on which they could feed, although there may have been some fields composed of halophytic grasses (Distichlis spicata) growing close to the coast at the river mouth based on the presence of this plant today [171]. Distichlis spicata is a C₄ plant and given its association with salty environments, it might be expected to possess relatively high δ¹⁵N values [172] but this has not been observed for northern Chile (δ¹⁵N = +4.6±3.1 ‰) [173] or northern Peru (−3.2 ‰) [111]. The similarity in both δ¹³C and δ¹⁵N values between the camelid fibers and the human hairs are consistent with both humans and camelids consuming a similar range of agricultural plants, suggesting that some animals were kept locally. An alternative possibility is that the non-highland camelid hair was derived from wild guanacos living at these lower altitudes. If, however, guanacos could consume a sufficient quantity of local plants, it begs the question as to why this would not have been equally plausible for llamas.

The absence of any non-highland isotopic compositions in the camelid bone collagen similar to that observed in the textiles may simply be a product of sample size, or it may imply that the primary motivation for keeping these animals locally was for fiber production rather than for meat or transport, underscoring the quintessential importance of textiles and camelid fiber in the Andean region [19, 174–176]. Niemeyer and Schiappacasse [52] estimated that given the low number of juvenile camelids identified in faunal assemblages, animal husbandry was oriented towards fiber production and/or transport, rather than for meat. However, they believed that the fiber was obtained from the adjacent altiplano because the environment near Huancarane would not be suitable for pastoral activities. They envisioned two strategies being employed by the local people: (1) possession of pastoral lands and herds in the highlands, and (2) the acquisition of livestock through exchange with llamas caravanners. These actions were also observed during the 1970s by these authors in this sector of the Camarones Valley. Our data suggest that by at least the LIP, groups living in the lower Camarones and Lluta Valleys kept small numbers of animals locally, but acquired the majority of their fiber through exchange with groups in the highlands. The reliance on imported fiber may have been driven by the limitations of the environment for camelid husbandry, necessitating an external supply.