Stable isotope chemistry reveals plant-dominant diet among early foragers on the Andean Altiplano, 9.0–6.5 cal. ka

Abstract

Current models of early human subsistence economies suggest a focus on large mammal hunting. To evaluate this hypothesis, we examine human bone stable isotope chemistry of 24 individuals from the early Holocene sites of Wilamaya Patjxa (9.0–8.7 cal. ka) and Soro Mik’aya Patjxa (8.0–6.5 cal. ka) located at 3800 meters above sea level on the Andean Altiplano, Peru. Contrary to expectation, Bayesian mixing models based on the isotope chemistry reveal that plants dominated the diet, comprising 70–95% of the average diet. Paleoethnobotanical data further show that tubers may have been the most prominent subsistence resource. These findings update our understanding of earliest forager economies and the pathway to agricultural economies in the Andean highlands. The findings furthermore suggest that the initial subsistence economies of early human populations adapting to new landscapes may have been more plant oriented than current models suggest.

Full text

RESEARCH ARTICLE
Stable isotope chemistry reveals plant-
dominant diet among early foragers on the
Andean Altiplano, 9.0–6.5 cal. ka
Jennifer C. ChenID
1
, Mark S. Aldenderfer
2
, Jelmer W. Eerkens
3
, BrieAnna S. Langlie
4
,
Carlos Viviano Llave
5
, James T. Watson
6
, Randall HaasID
7
*
1Department of Anthropology, The Pennsylvania State University, State College, PA, United States of
America, 2Department of Anthropology and Heritage Studies, University of California, Merced, CA, United
States of America, 3Department of Anthropology, University of California Davis, Davis, CA, United States of
America, 4Department of Anthropology, Binghamton University, Binghamton, NY, United States of America,
5National Register of Peruvian Archaeologists, Lima, Peru, 6Arizona State Museum and School of
Anthropology, University of Arizona, Tucson, AZ, United States of America, 7Department of Anthropology,
University of Wyoming, Laramie, WY, United States of America
*whaas@uwyo.edu
Abstract
Current models of early human subsistence economies suggest a focus on large mammal
hunting. To evaluate this hypothesis, we examine human bone stable isotope chemistry of
24 individuals from the early Holocene sites of Wilamaya Patjxa (9.0–8.7 cal. ka) and Soro
Mik’aya Patjxa (8.0–6.5 cal. ka) located at 3800 meters above sea level on the Andean Alti-
plano, Peru. Contrary to expectation, Bayesian mixing models based on the isotope chemis-
try reveal that plants dominated the diet, comprising 70–95% of the average diet.
Paleoethnobotanical data further show that tubers may have been the most prominent sub-
sistence resource. These findings update our understanding of earliest forager economies
and the pathway to agricultural economies in the Andean highlands. The findings further-
more suggest that the initial subsistence economies of early human populations adapting to
new landscapes may have been more plant oriented than current models suggest.
Introduction
The extent to which early human subsistence economies relied on meat versus plant foods is
debated [1,2]. Current understanding of the earliest subsistence economies of the Andean
highlands suggest that meat was the major subsistence resource. Early Holocene assemblages,
11–5 cal. ka, consistently reveal abundant camelid and deer remains, projectile points, and
scrapers suggesting hunting-oriented economies [3–11]. For example, Rick [4] concluded that,
“The settlement pattern [of the Junı
´n region] and the faunal collections [from the site of
Pachamachay] strongly support the hypothesis that vicuña, or similar camelids, were the
major food source for puna [ecosystem] hunter-gatherers.” Such observations are furthermore
consistent with diet breadth models, which suggest that early hunter-gatherer populations
would tend to target high-ranked large mammals before resorting to plant foods [12–14].
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OPEN ACCESS
Citation: Chen JC, Aldenderfer MS, Eerkens JW,
Langlie BS, Viviano Llave C, Watson JT, et al.
(2024) Stable isotope chemistry reveals plant-
dominant diet among early foragers on the Andean
Altiplano, 9.0–6.5 cal. ka. PLoS ONE 19(1):
e0296420. https://doi.org/10.1371/journal.
pone.0296420
Editor: Miguel Delgado, Universidad Nacional de la
Plata Facultad de Ciencias Naturales y Museo,
ARGENTINA
Received: July 18, 2023
Accepted: December 12, 2023
Published: January 24, 2024
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0296420
Copyright: ©2024 Chen et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.

Recent analyses of materials from the Archaic Period sites of Soro Mik’aya Patjxa (8.0–6.5
cal. ka) and Wilamaya Patjxa (ca. 8.9 cal. ka), located on the Andean Altiplano (High Plateau),
provide new opportunities to evaluate these economic models for early highland foragers. Sim-
ilar to previous research, an abundance of projectile points, scrapers, and lithic debitage indi-
cate considerable investment in the hunting of large terrestrial mammals, likely camelid and
deer [15–17]. Preliminary zooarchaeological investigations reveal abundant large-mammal
bone, consistent with a hypothesis of large mammal hunting [15,18]. Groundstone artifacts,
though informal and infrequent, suggest some degree of investment in plant resources [15,
18]. Distinctive dental wear patterns on the upper incisors, known as lingual surface attrition
of the maxillary anterior teeth suggest intensive tuber processing [19]. Collectively, studies of
the Soro Mik’aya Patjxa and Wilamaya Patjxa materials indicate diverse diets of large mam-
mals and plants with an emphasis on large-mammal hunting, consistent with previous find-
ings at other Andean highland sites.
Despite general agreement of various lines of evidence, the evidence remains indirect.
Preservation biases that favor projectile points and animal bone compared to plant materi-
als could, to some extent, inflate the hunting signal [20–22]. The biases of previous research-
ers who have generally been males from a culture in which hunting is a distinctly masculine
pursuit could furthermore inflate the hunting signal [23]. It was ostensibly for this reason
that ethnography famously revealed plant foods to play a prominent role in forager econo-
mies—Arctic economies aside—in contrast to earlier models that emphasized hunting [24–
26]. Thus, current archaeological models and evidence leave considerable room for inter-
pretive error.
A more direct but previously unexplored measure of early Andean diets is stable isotope
chemistry of human bone. A study of bone isotope chemisty of six early Holocene (8.2–8.0 cal.
ka) individuals, including four children and two adults, from the Andean highlands of Argen-
tina finds evidence for tuber and herbivore consumption with breastfeeding and environmen-
tal aridity enriching the isotopic values [27]. The current analysis examines the diets of 24
Archaic Period foragers at the highland archaeological sites of Soro Mik’aya Patjxa and Wila-
maya Patjxa using stable carbon and nitrogen isotopes in conjunction with more traditional
zooarchaeological and paleoethnobotanical approaches. These assemblages date to the early
Holocene, collectively spanning 9.0–6.5 cal. ka [16,18]. Given current models of early highland
economies, which point to a mixed diet of animals and plants with an emphasis on large mam-
mals, we should expect to observe the human osteological samples from Soro Mik’aya Patjxa
and Wilamaya Patjxa to exhibit dietary carbon (δ
13
C
diet
) and nitrogen (δ
15
N
diet
) values
between those of local fauna and flora with a bias toward the means of the faunal δ
13
C and
δ
15
N values.
This expectation follows from well established relationships between the isotopic composi-
tion of human bone and the foods that humans consume [28,29]. Stable nitrogen isotopes
(δ
15
N) vary with trophic level. Stable carbon isotopes (δ
13
C) vary with photosynthetic pathway.
Such isotopic values in human bone chemistry can provide insights into major subsistence
resources including C
3
plants, C
4
plants, mammals, and fish. Although stable isotope analysis
does not offer taxonomic specificity beyond those broad categories, by coupling isotopic
insights with zooarchaeological and paleoethnobotanical insights, it may be possible to move
beyond preservation biases to gain more accurate estimates of human diets. Given current
understanding of highland Archaic diets, we expect zooarchaeological analysis of the Archaic
Period sites to reveal an abundance of faunal remains including vicuña, guanaco, or taruca
with few small mammal, fish, or bird remains. Paleoethnobotanical analyses should reveal
abundant wild chenopod seeds or tuber remains.
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Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
Funding: This research was supported by U.S.
National Science Foundation (NSF) grant BCS-
1311626 awarded to RH, NSF grant BCS-9816313
awarded to MA, American Philosophical Society
Lewis and Clark grant awarded to RH, and funding
from the University of California, Davis awarded to
RH. The funders had no role in study design, data
collection and analysis, decision to publish, or
preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.

Materials and methods
Recently discovered human burials and other cultural pit features—possibly roasting or stor-
age pits—at the Early-Late Archaic Period archaeological sites of Soro Mik’aya and Wilamaya
Patjxa afford an opportunity to evaluate models of early subsistence practices on the Andean
Altiplano. A series of radiocarbon dates place Soro Mik’aya Patjxa securely in the Middle to
Late Archaic Periods (8.0–6.5 cal. ka) [16]. Radiocarbon dates and artifact typology broadly
place Wilamaya Patjxa in the Early to Late Archaic Periods (ca. 11–5 cal. ka), with two direct
showing occupation around 9.0 cal. ka [18,30].
Portions of the two sites were systematically excavated with site matrix and feature fill
screened using 6 mm and 1 mm screens, respectively. For each cultural feature, 10-liter bulk
soil samples were taken for flotation analysis unless the feature consisted of less than 10L, in
which case all feature sediment was collected for flotation. Flotation procedures followed
d’Alpoim Guedes et al. [31] and Lennstrom and Hastorf [32] using a modified version of Wat-
son’s [33] flotation machine.
The excavations revealed 18 cultural pit features at Soro Mik’aya Patjxa and 39 at Wilamaya
Patjxa. From these, 16 individuals were discovered at Soro Mik’aya Patjxa and 12 at Wilamaya
Patjxa. Here, we describe the laboratory methods for the three analytical approaches including
isotopic, zooarchaeological and paleoethnobotanical approaches.
Stable isotope analysis of human bone
Human bone samples were excavated and exported under Peruvian Ministry of Culture Per-
mit numbers 064-2013-DGPA-VMPCIC/MC and 138-2015-VMPCIC/MC. Stable isotope
chemistry is performed in four different labs including the University of California Davis Sta-
ble Isotopes Facility (UCDSIFS), University of California Irvine W.M. Keck Carbon Cycle
Accelerator Mass Spectrometer (KCCAMS) Facility, the University of Arizona Accelerator
Mass Spectrometry Lab (AMS), and the Penn State University Laboratory for Isotopes and
Metals in the Environment (LIME). Collagen extraction for the UCDSIFS and LIME submis-
sions is performed at the UC Davis Archaeometry Lab following the protocol of Eerkens et al.
[34]. Collagen extraction for the KCCAMS submissions follows the protocol described by
Haas et al. [18].
To assess the extent of diagenetic alterations to bone collagen, we consider atomic C/N
ratios with the expectation that reliable readings will exhibit C/N ratios in the range of 3.1–3.6
[35]. As an additional quality control measure, we compare the resultant δ
13
C and δ
15
N values
for three of the individuals—SMP9, SMP16, and WMP6—to values previously reported in
radiocarbon analyses performed by The University of Arizona AMS laboratory [16] and the
KCCAMS facility [18].
Baseline δ
13
C and δ
15
N values for candidate subsistence resources, including C
3
plants, C
4
plants, camelid, or freshwater fish, are compiled from published sources [36–40]. To the extent
possible, control samples are restricted to archaeological samples from the central Andean
highlands. For any modern samples included, δ
13
C values are corrected by +1.5‰ for Suess
effects. For any lowland samples included, δ
13
C values are offset +2‰ and δ
15
N values -1.5‰
based on the regression equations of Szpak et al. [38]. δ
13
C bone collagen samples are adjusted
using a -2.4‰ offset if terrestrial [41] and -3.7‰ offset if aquatic [42,43] to adjust for meat-
bone offset.
Bayesian mixing models are used to estimate the dietary composition of the Soro Mik’aya
Patjxa and Wilamaya Patjxa individuals using the δ
13
C and δ
15
N values of the consumers and
potential food resources. Children (n = 4) are excluded from the model to prevent breast-feed-
ing effects from influencing the results. Previous research shows that trophic offsets can vary
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widely due to a variety of environmental and trophic effects [27,44,45]. Given the antiquity of
the system under investigation, the mobility of humans, and the volatility of isoscapes over
time, it is difficult to know which trophic correction factors apply to a dietary regime under
consideration. We therefore consider the range of possible trophic enrichment factors along
with variance terms for both δ
13
C and δ
15
N. For δ
13
C, we use trophic enrichment factors rang-
ing between 4.5–6.0‰, in 0.5‰ increments, with a standard deviation of 0.63‰ [45]. For
δ
15
N, we use trophic enrichment factors ranging between from 3–6‰, in 1‰ increments,
with a standard deviation of 0.74‰ [45]. Considering all possible combinations of trophic
enrichment factors results in 16 models for evaluation.
All models assume uniformed priors given the lack of prior knowledge on the relative die-
tary contributions of the broad resource classes. Although it might be tempting to draw on
zooarchaeological and archaeobotanical data for informative priors, differential preservation
of faunal and floral artifacts precludes this possibility.
Model runs assume both residual and process error, a chain length of 1 million, burn-in of
5000, thinning of 500, and three chains. For dietary estimates, we report median values and
95% credible intervals for each subsistence resource and each model. Model convergence is
assessed using Gelmen-Rubin and Geweke diagnostics, and the models are compared to one
another using leave-one-out cross-validation information criterion (LOOic) and Akaike Infor-
mation Criterion (AIC) weights [46]. All computation is performed using R statistical comput-
ing language [47] with Bayesian mixing modeling performed using MixSIAR package [46].
Although other packages for mixing modeling are available (e.g., FRUITS [48]), we use Mix-
SIAR [49] because of its currency, documentation, integration with R statistical computing
language, and accessibility via open-source Linux operating systems [50]. All code is made
available in S1 Data.
Zooarchaeological analysis
New faunal data are reported for Soro Mik’aya Patjxa. Wilamaya Patjxa data are derived from
a previous investigation by Noe [18]. For newly reported materials, all animal bone is weighed
and counted. Although abundant, the animal bone is highly fragmented, likely due to intensive
processing, making more precise taxonomic identification difficult. The method presented
here serves to broadly distinguish between human, small mammal, large mammal, bird, and
fish bone. Animal bone fragments are distinguished from human bone based on bone macro-
structure where (a) human bone tends to be more porous than animal bone, (b) cortical bone
tends to be thicker relative to bone diameter in animals compared to humans, (c) diaphyseal
trabecula tends to be present in human but absent in animal bone, and (d) human cranial
vaults tend to have thick dipole while animal cranial vaults tend to be more compact [51].
More detailed faunal analysis is ongoing, but the coarse analytical approach taken here is suffi-
cient to address the broad dietary question at hand.
Paleoethnobotanical analysis
All Soro Mik’aya Patjxa features, which include burial pits and pits of unknown function, are
subject to macrobotanical analysis. Samples are sorted using a stereoscopic light microscope
with 10 to 40X magnification. Due to environmental conditions in the central Altiplano and
the antiquity and exposed nature of the sites, it is highly unlikely that uncarbonized plant
remains would have preserved, so analysis is restricted to carbonized remains. Macrobotanical
remains are sorted into different tissue categories including seeds, wood, and parenchyma.
Parenchyma refers to plant storage tissue. These distinct carbonized tissues with thin-walled
cells are believed to be tuber fragments by paleoethnobotanists working in the Altiplano. All
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specimens are identified to the most specific taxonomic level possible. Paleoethnobotanical
analysis is restricted to Soro Mik’aya Patjxa with Wilamaya Patjxa paleoethnobotanical analy-
sis ongoing.
Results
Isotopic control samples compiled from the Andean literature include 96 large mammals, 84
C
3
plants, 29 C
4
plants, and 10 fish samples. The data reveal strong clustering of carbon and
nitrogen values by category (Fig 1,Table 1,S1 Table) providing an ideal baseline for compari-
son with the human bone samples reported here (Table 2). Twenty-four individuals from Soro
Mik’aya Patxja (SMP) and Wilamaya Patjxa (WMP) show δ
15
N
diet
values ranging from 2.0‰
to 8.3‰ with a mean of 3.4‰ (see Fig 1A) and δ
13
C
diet
values ranging from -24.3‰ to -22.9‰
with a mean of -23.7‰ (see Fig 1B).
Quality control measures indicate that the archaeological stable isotope values reported
here are reliable. Atomic C/N ratios for all but one sample—WMP1—fall within the acceptable
range of 3.1–3.6 (see Table 2), indicating that diagenetic processes have not significantly
altered the collagen [35]. Furthermore, the δ
13
C and δ
15
N values from the four previously
reported radiocarbon dates [16,18,30] are in close agreement with less than 1.3‰ separating
values reported among the three labs (Table 3).
The δ
13
C and δ
15
N values for the human individuals fall closest to the mean of C
3
plants
with slight enrichment from some other set of resources indicating C
3
dominant diets and
excluding the possibilities that mammals, C
4
plants, or fish comprised large portions of the
diets (see Fig 1C). All 16 of the Bayesian mixing models indicate that adult diets were domi-
nated by C
3
plants (Table 4). Median C
3
plant contribution estimates range from 60–95% and
median mammal estimates range from 3–34% for all 16 models. Gelman and Geweke model
diagnostics are consistently zero or near zero, indicating that all models are plausible.
The best-fit Bayesian mixing model indicates that C
3
plants comprised approximately 84%
(73–92%) of the average adult diet with meat comprising just 9% (0–24%), fish 4% (0–13%),
and C
4
plants 2% (0–6%; Fig 2). This model generated the lowest LOOic value and the highest
AIC weight, and is based on a δ
15
N trophic enrichment factor of 6.00±0.74‰ and a δ
13
C tro-
phic enrichment factor of 5.00±0.63‰. However, five other models produced nearly equiva-
lent AIC weights greater than 0.10 suggesting virtually equivalent model performance. Among
these models, median C
3
plant estimates range from 70–95% and mammal estimates range
between 3–23%. These results show that the particular trophic enrichment factors, ranging
from 3.0–6.0‰ for δ
15
N and 4.5–6.0 for δ
13
C, have little effect on the broad dietary estimates.
All models indicate a plant dominant diet with median values for C
3
plants ranging between
70–95% and mammals ranging between 3–23%. Thus all credible subsistence models indicate
that plant foods comprised the majority of individual diets and meat played a secondary role.
These findings are inconsistent with the working hypothesis of a meat-dominant diet and
instead suggest a plant-dominant diet among early forager populations of the Andean Alti-
plano, 9.0–6.5 cal. ka.
Zooarchaeological and paleoethnobotanical observations offer some taxonomic precision
beyond the broad food categories used in the stable isotope analysis. Excavations at Soro
Mik’aya produced 3193 fragments of animal bone (number of individual specimens or NISP)
from across the site. Of the total specimens, the most frequently identified category included
200 large mammal fragments with only trace amounts of small mammal, bird, and fish bones
reported [15]. Most of these large mammal bones were burned (n = 150; 75%). A previously
published assessment of 341 faunal bone elements from Wilamaya Patjxa faunal assemblage
similarly revealed that camelid and deer bone were the most frequently identified taxa with 17
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camelid and 5 deer elements observed [18]. Notably absent, again, are small mammals, birds,
and fish. These data indicate that the slight enrichment observed in the carbon and nitrogen
values in the human bone was likely due to large mammal consumption and not small mam-
mals, fish, or birds.
The most abundant plant-food specimens in the Soro Mik’aya Patjxa paleoethnobotanical
assemblage is parenchyma tissue, identified as tuber fragments with 52 specimens found in 9
Fig 1. Carbon and nitrogen plots for control samples and 25 human bone samples from Soro Mik’aya Patjxa and Wilamaya Patjxa,
indicating a plant-dominant diet. a) δ
13
C
diet
values are consistent with those of C
3
plants with slight enrichment from some other resource types.
b) δ
15
N
diet
values are most consistent with those of plants. c) Biplot of δ
13
C and δ
15
N values are consistent with a mixed diet principally based on
C
3
plants with low levels of enrichment from some other resource. Dietary values assume δ
13
C TEF = 5.0‰, δ
15
N TEF = 6.0‰ based on mixing
model results (see Table 4). Dots = individual samples, ellipses = 95% variance ranges for each category, and crosshairs = mean values by category
(see Tables 1and 2).
https://doi.org/10.1371/journal.pone.0296420.g001
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of 15 features (Fig 3,Table 5). Chenopod seeds are nearly absent with just three wild specimens
observed among three features. The most abundant paleoethnobotanical samples are non-food
resources including wood fragments (n = 448) and grass (Poaceae) seeds (n = 161) observed in
Table 1. Summary statistics for stable isotopic control data for high-altitude Andean food resources and archaeological samples from Soro Mik’aya Patjxa (SMP)
and Wilamaya Patjxa (WMP). See S1 Table for sample data.
population n δ
13
C mean (‰) δ
13
C sd (‰) δ
15
N mean (‰) δ
15
N sd (‰) references
C
3
plants 84 -24.7 2.1 2.9 4.6 [36,40]
C
4
plants 29 -9.0 2.7 2.9 4.9 [36,38,40]
freshwater fish 10 -17.1 3.2 7.1 2.2 [40]
large terrestrial mammal 96 -21.2 2.3 6.5 2.4 [39,52–54]
SMP/WMP
diet
*24 -23.7 0.4 3.4*0.9 NA
*Dietary values assume δ
13
C TEF = 5.0‰, δ
15
N TEF = 6.0‰ based on mixing model results (see Table 4).
https://doi.org/10.1371/journal.pone.0296420.t001
Table 2. Human bone collagen isotopic results for Soro Mik’aya Patjxa and Wilamaya Patjxa individuals. See S2 Table for additional metadata.
burial age class
a
element lab
b
δ
13
C
raw
(‰) δ
15
N
raw
(‰) δ
13
C
dietc
(‰) δ
15
N
dietc
(‰) atomic C/N date (95% cal. BP)
d
SMP 1 child parietal (squama) UCDSIF -18.1 10.8 -23.1 4.8 3.2 n.d.
SMP 2 adult temporal (petrous portion) UCDSIF -18.9 9.9 -23.9 3.9 3.3 n.d.
SMP 3 adult rib 10 (left) UCDSIF -18.9 9.2 -23.9 3.2 3.2 7565–7177 [16]
SMP 4 adolescent rib 1 (left) UCDSIF -19.3 8.2 -24.3 2.2 3.2 7565–7177 [16]
SMP 5 adult hand mid phalanx (right) UCDSIF -18.7 11.3 -23.7 5.3 3.2 6856–6569 [16]
SMP 6 adult metatarsal 5 (left) UCDSIF -19.3 9.2 -24.3 3.2 3.3 7153–6756 [16]
SMP 7 adolescent hand proximal phalanx (right) UCDSIF -19.1 8 -24.1 2 3.2 6780–6510 [16]
SMP 8 adult hand proximal phalanx (right) UCDSIF -19.3 8.5 -24.3 2.5 3.2 7160–6885 [16]
SMP 9 adult hand proximal phalanx (left) UCDSIF -18.5 8.6 -23.5 2.6 3.2 7465–7317 [16]
SMP 10 adult hand proximal phalanx (left) UCDSIF -18.2 9.7 -23.2 3.7 3.2 6907–6574 [16]
SMP 11 adult rib (right) UCDSIF -18.9 9.4 -23.9 3.4 3.2 6883–6669 [16]
SMP 12 adult rib (left) UCDSIF -19.1 9.1 -24.1 3.1 3.2 n.d.
SMP 13 child temporal (petrous portion) UCDSIF -17.9 11 -22.9 5 3.3 6883–6669 [16]
SMP 15 adult hand proximal phalanx (left) UCDSIF -18.8 8.7 -23.8 2.7 3.2 n.d.
SMP 16 adult mandible (right) UCDSIF -18.8 10.1 -23.8 4.1 3.3 7247–7009 [16]
WMP 1 adult tibia frag (side indeterminate) UCDSIF -18.5 9.5 -23.5 3.5 3.9 8990–8650 [30]
WMP 2 adult long bone diaphysis fragment KCCAMS -18.6 8.7 -23.6 2.7 3.3 n.d.
WMP 3 adult bone frag UCDSIF -18.6 9.9 -23.6 3.9 3.4 n.d.
WMP 5 adult left petrous portion UCDSIF -17.9 10 -22.9 4 3.4 n.d.
WMP 6 adolescent indeterminate bone fragment UCDSIF -19.1 9.2 -24.1 3.2 3.6 8992–8651 [18]
WMP 7 adult left scapula KCCAMS -19.1 8.4 -24.1 2.4 3.2 n.d.
WMP 8 adolescent 3
rd
molar LIME -18.7 9.8 -23.7 3.8 3.2 n.d.
WMP 9 child cranial fragment KCCAMS -18.5 8.7 -23.5 2.7 3.2 n.d.
WMP 10 child cranial fragment KCCAMS -18.3 10.2 -23.3 4.2 3.2 n.d.
mean -18.7 9.4 -23.7 3.4
standard deviation 0.4 0.9 0.4 0.9
*age classes as defined by Buikstra and Ubelaker [55]
b
UCDSIF = UC Davis Stable Isotope Facility; KCCAMS = UC Irvine Keck Carbon Cycle Accelerator Mass Spectrometry facility; LIME = Penn State University
Laboratory for Isotopes and Metals in the Environment
c
Dietary values assume δ
13
C TEF = 5.0‰, δ
15
N TEF = 6.0‰ based on mixing model results (see Table 4).
d
calibrated using Southern Hemisphere Calibration Curve 2020 [56].
https://doi.org/10.1371/journal.pone.0296420.t002
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nearly every feature. The wood most likely reflects use as fuel. Similarly the small grass seeds
likely reflect fuel use whether from burning grasses or the dung of camelids [57]. The small
grass seeds are typical of the region, and none of the taxa are suitable for human consumption
but are excellent forage for camelids. This finding is consistent with a model of early wild
tuber consumption and other lines of evidence that suggest intensive tuber use in the region
during the Archaic Period [19,58,59].
Discussion
The stable isotope, faunal, and paleoethnobotanical evidence from the sites of Soro Mik’aya
Patjxa and Wilamaya Patjxa converge to indicate that C
3
plants, likely wild tubers, comprised
the major component of early forager diets on the Andean Altiplano and that meat, including
Table 3. Inter-laboratory comparison of isotopic results showing consistent results and acceptable atomic C/N ratios. UCDSIFS compared to AMS and KCCAMS.
individual δ
13
C (‰) δ
15
N (‰)
14
C comparisons
lab lab ID
14
C age (B.P.) C/N
atomic
δ
13
C (‰) δ
15
N (‰)
SMP 9 -18.5 8.6 AMS AA107345 [16] 6529±41 3.2 -19.2 7.9
SMP 16 -18.8 10.1 AMS AA107490 [16] 6259±38 3.3 -18.8 9.7
WMP 1 -18.5 9.5 KCCAMS UCIAMS 259854 [30] 8010±25 3.3 -18.4 8.6
WMP 6 -19.11 9.2 KCCAMS UCIAMS 212748 [18] 8035±20 3.2 -18.8 8.2
KCCAMS UCIAMS 212749 [18] 7965±25 3.3 -19.0 8.0
https://doi.org/10.1371/journal.pone.0296420.t003
Table 4. Bayesian mixing model comparison considering different trophic enrichment factors.
model TEF
a
estimated dietary contribution (%)
b
Gelman diagnostic
c
Geweke diagnostic
d
LOOic
e
Akaike weight
f
δ
13
Cδ
15
N C
3
plants C
4
plants fish mammals
1 4.5 3.0 60(42–76) 0(0–3) 4(0–47) 34(2–52) 0,0,0 0,0,1 -38.5 0.00
2 5.0 3.0 75(59–88) 1(0–3) 4(0–22) 19(1–36) 2,0,0 0,0,1 -36.3 0.00
3 5.5 3.0 86(75–95) 0(0–3) 3(0–12) 9(0–22) 1,0,0 0,3,2 -34.9 0.00
4 6.0 3.0 94(86–98) 0(0–2) 1(0–6) 4(0–12) 1,0,0 1,2,2 -35.5 0.00
5 4.5 4.0 62(50–78) 1(0–3) 4(0–21) 32(6–48) 0,0,0 0,0,1 -49.8 0.03
6 5.0 4.0 75(63–87) 1(0–3) 4(0–16) 20(2–36) 0,0,0 0,0,1 -48.7 0.02
7 5.5 4.0 86(75–95) 1(0–3) 3(0–11) 9(0–23) 0,0,0 0,6,0 -48.0 0.01
8 6.0 4.0 94(85–98) 0(0–2) 1(0–6) 4(0–13) 0,0,0 1,6,0 -47.6 0.01
9 4.5 5.0 70(57–83) 1(0–5) 5(0–19) 23(3–40) 0,0,0 1,9,0 -52.6 0.13
10 5.0 5.0 78(67–89) 1(0–4) 4(0–14) 16(1–31) 0,0,0 1,5,1 -52.2 0.11
11 5.5 5.0 88(77–95) 1(0–3) 3(0–10) 8(0–21) 0,0,0 0,1,2 -51.7 0.09
12 6.0 5.0 94(86–98) 0(0–2) 1(0–6) 3(0–12) 0,0,0 1,3,0 -51.1 0.06
13 4.5 6.0 79(67–88) 3(0–8) 6(0–17) 10(1–28) 0,0,0 1,10,0 -52.6 0.13
*14 5.0 6.0 84(73–92) 2(0–6) 4(0–13) 9(0–24) 0,0,0 2,0,0 -52.7 0.14
15 5.5 6.0 90(81–96) 1(0–4) 2(0–9) 6(0–17) 0,0,0 0,9,1 -52.6 0.13
16 6.0 6.0 95(88–98) 0(0–2) 1(0–6) 3(0–10) 0,0,0 0,0,9 -52.4 0.12
a
TEF = trophic enrichment factor
b
posterior probability median (95% range)
c
variables >1.01, variables >1.05, variables >1.1 (27 variables)
d
number of variables outside 95% confidence level for each of three chains (should be <2, or 5% of 27 variables)
e
leave-one-out cross validation information criterion (LOOic) for assessing model efficacy. Smaller values indicate more powerful models.
f
Akaike weight for model selection. Larger values indicate more powerful models.
*best approximating model based on LOOic and Akaike weight.
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vicuña and taruca, played a secondary role. C
4
plants, small mammals, fish, and birds appear
to have played negligible roles in these early subsistence economies. The findings presented
here depart from current thinking about early Andean highland diets and force a reconsidera-
tion of existing economic models.
One possible explanation for the unexpected emphasis on plant foods among this early
highland population may be that large mammal populations had been severely reduced by 9
cal. ka. Although the Wilamaya Patjxa assemblage includes the earliest archaeological period
of the region—the Early Archaic Period, 11.0–9.0 cal. ka—it is restricted to the latter end of the
Fig 2. Bayesian mixing model results for the six best-fit models showing that C
3
plants comprised the majority of the diet and mammals
played a secondary role in the subsistence economies of Soro Mik’aya Patjxa and Wilamaya Patjxa. a. model 14, b. model 9, c. model 13, d.
model 15, e. model 16, f. model 10 (see Table 4).
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period [18]. It is currently unclear when human populations first arrived in the region [60],
but if they arrived as early as 11 cal. ka., then humans would have been hunting the region for
2,000 years prior to the earliest individuals under investigation. This would certainly have
been enough time to decimate the region’s animal population in the absence of animal conser-
vation strategies [4]. A second possibility is that the earliest populations simply did not engage
in hunting to the extent previously thought. Previous research suggests that prey choice mod-
els may over-estimate the dietary values of large mammals whether due to physical risk [61],
prey behavior [62], or economic risks associated with long encounter intervals [63]. A third
explanatory possibility is that early highland populations relied heavily on hunting large mam-
mals but incorporated animal digesta into their diets [64], which could simultaneously account
for both the archaeological signatures of hunting—animal bone and projectile points—and the
depleted nitrogen values in human bone chemistry observed here. Evaluating these hypotheses
will ultimately require investigation of earlier archaeological assemblages.
These findings furthermore hold implications for our understanding of domestication in
the high Andes. Pearsall [65] and Kuznar [66] proposed that plant management commenced
after camelids were managed in corrals. In this view, camelids transported plant seeds to cor-
rals where they thrived in soils fertilized by camelid dung, creating a mutually beneficial
Fig 3. Results of paleoethnobotanical analysis from features at Soro Mik’aya Patjxa. Ubiquity is measured as the
proportion of archaeological features containing each taxon. Values based on 15 features, 300L of flotation, and 688
paleoethnobotanical artifacts (see Table 5).
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relationship between camelids, plants, and human communities. Versions of this model sug-
gest that Chenopodium spp. (including the crops quinoa and kañawa) and the tuber maca
(Lepidium mevenii Walp.) were domesticated in this way. These coevolutionary processes also
likely led to the domestication of potatoes (Solanum tuberosum L.) and up to 15 other species
of roots and tubers in the Andes [67]. Consistent with this model, chenopod seeds, maca
tubers, and managed camelids appeared after about 4,000 years ago at the rock shelter site of
Panalauca where the sizes of chenopod and maca specimens increased through time [65].
Recent research into domestication mutualism supports the Andean version of the camp fol-
lower hypothesis showing that chenopods, tubers, and camelids were likely domesticated in
tandem as complementary foods [68].
While current models suggest initial co-evolutionary processes involving maca, chenopod,
and camelid intensification in the highlands, the paleoethnobotanical evidence presented here
fails to find strong evidence of early intensive chenopod use on the Altiplano during the
Archaic period. This may reflect preservation biases given the small and delicate nature of wild
chenopod seeds. Alternatively, it may be that chenopods did not become economically impor-
tant in the region until sometime after 6.5 cal. ka. A later incorporation of chenopods into the
diet would be consistent with prey choice models given the low post-encounter return rates of
small seeds relative to tubers [69].
The finding of tuber fragments at Soro Mik’aya Patjxa is consistent with the role of tubers
in the early stages of the co-evolutionary process. However, the tuber fragments are unlikely to
be maca, which is a more northerly taxon. Several tuber species could potentially account for
the tubers observed at Soro Mik’aya Patjxa, but they are most likely associated with wild potato
species, which are concentrated in the region and were likely domesticated there [59,70–72].
Additional sites on the Altiplano should be examined with particular attention to contempora-
neous sites that would assess replicability of the current findings and to non-contemporaneous
sites that would afford diachronic comparisons.
Table 5. Carbonized macrobotanical materials from Soro Mik’aya Patjxa flotation samples.
feature flotation volume (L) human food camelid food fuel total
Chenopodium sp. parenchyma Ruppia sp. Malvaceae Potamogeton sp. Poaceae wood
2 27 0 25 0 2 0 3 57 87
3 23 0 1 1 0 0 8 20 30
4 31 1 4 0 12 0 57 75 149
5 25 0 8 0 2 0 29 33 72
6 18 0 1 0 1 1 4 35 42
7 1 0 0 0 0 0 1 0 1
8 5 0 0 0 0 0 0 2 2
9 10 0 3 0 0 0 5 7 15
10 25 1 0 0 2 0 10 91 104
13 33 0 1 0 0 0 2 19 22
14 12 0 6 0 0 0 1 49 56
14/15*13 0 2 0 0 0 4 41 47
15 10 0 0 0 0 0 6 8 14
16 48 1 1 0 3 0 28 0 33
17 9 0 0 0 0 0 0 0 0
18 10 0 0 0 0 0 3 11 14
total 300 3 52 1 22 1 161 448 688
*sample contexts mixed. Excluded from ubiquity calculations.
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In sum, the results presented here are consistent with a model of human-tuber-camelid co-
evolutionary dynamics beginning approximately 9,000 years ago on the Andean Altiplano.
These findings support an updated model of Archaic Period subsistence practices in the cen-
tral high Andes in which forager subsistence economies 9.0–6.5 cal. ka. emphasized plant for-
aging with lesser attention to large mammal hunting and a virtual absence of small animal
hunting and fishing. This resource base may have catalyzed potato and camelid domestication
in the subsequent Terminal Archaic Period after 5 cal. ka [58].
These findings further highlight the need to re-evaluate anthropological understanding of
early forager diets more generally. Current perspectives vary with some models emphasizing
the primacy of plants and others of animals [1,2] with plant foraging becoming increasingly
important relatively late in time on the eve of agriculture [73]. This may still be so, but the cur-
rent analysis suggests that the shift to plant-foraging economies may have happened relatively
rapidly, evidently having transpired in less than 2,000 years in the Andean case. This observa-
tion resonates with recent archaeological theory and findings that reveal a prominent role for
plant foods in early forager diets [31,74,75] and ethnographic findings of the 1960s when,
contrary to dominant thinking of the time, many subsistence economies once thought to be
meat-dominant were shown to be plant-dominant [24,25]. Stable isotope chemistry gives
archaeologists the opportunity to reliably extend such investigations into the deep past. The
current study arrives at a similar place to the earlier ethnographic findings—plant foods were
central to early human economies.
Supporting information
S1 Data. Bayesian mixing model code in R language.
(R)
S1 Table. Isotopic control data.
(CSV)
S2 Table. Isotopic data for Soro Mik’aya Patjxa and Wilamaya Patjxa individuals.
(CSV)
Acknowledgments
Field support was provided by Collasuyo Archaeological Research Institute, and the communi-
ties of Mulla Fasiri and Totorani, Peru. Bryna Hull (UC Davis) provided support for stable iso-
tope analysis. Samples of human remains were excavated and exported under Peruvian
Ministry of Culture Permit numbers 064-2013-DGPA-VMPCIC/MC and 138-2015-
VMPCIC/MC.
Author Contributions
Conceptualization: Jennifer C. Chen, Randall Haas.
Data curation: Jennifer C. Chen, Mark S. Aldenderfer, Carlos Viviano Llave, Randall Haas.
Formal analysis: Jennifer C. Chen, BrieAnna S. Langlie, James T. Watson, Randall Haas.
Funding acquisition: Mark S. Aldenderfer, Randall Haas.
Investigation: Jennifer C. Chen, Mark S. Aldenderfer, BrieAnna S. Langlie, Carlos Viviano
Llave, James T. Watson, Randall Haas.
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Isotope chemistry reveals plant-dominant diet among early foragers on the Andean Altiplano
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Methodology: Jennifer C. Chen, Jelmer W. Eerkens, BrieAnna S. Langlie, James T. Watson,
Randall Haas.
Project administration: Carlos Viviano Llave, Randall Haas.
Resources: Mark S. Aldenderfer, Jelmer W. Eerkens, BrieAnna S. Langlie, Randall Haas.
Supervision: Jelmer W. Eerkens, Randall Haas.
Validation: Jennifer C. Chen, Jelmer W. Eerkens, Randall Haas.
Visualization: Jennifer C. Chen, Randall Haas.
Writing – original draft: Jennifer C. Chen, Randall Haas.
Writing – review & editing: Jennifer C. Chen, Mark S. Aldenderfer, Jelmer W. Eerkens,
BrieAnna S. Langlie, Carlos Viviano Llave, James T. Watson, Randall Haas.
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