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Research
Cite this article: Weitzel EM, Codding BF.
2016 Population growth as a driver of initial
domestication in Eastern North America.
R. Soc. open sci. 3: 160319.
http://dx.doi.org/10.1098/rsos.160319
Received: 10 May 2016
Accepted: 30 June 2016
Subject Category:
Biology (whole organism)
Subject Areas:
behaviour/ecology/evolution
Keywords:
origins of agriculture, niche construction,
behavioural ecology, optimal foraging theory,
dates as data
Author for correspondence:
Elic M. Weitzel
e-mail: [email protected]
Population growth as a
driver of initial
domestication in Eastern
North America
Elic M. Weitzel and Brian F. Codding
Department of Anthropology and Archaeological Center, University of Utah,
270 S. 1400 E., Rm. 102, Salt Lake City, UT 84112, USA
EMW, 0000-0001-5214-8451
The transition to agriculture is one of the most significant
events in human prehistory; yet, explaining why people
initially domesticated plants and animals remains a
contentious research problem in archaeology. Two competing
hypotheses dominate current debates. The first draws on niche
construction theory to emphasize how intentional management
of wild resources should lead to domestication regardless of
Malthusian population–resource imbalances. The second
relies on models from behavioural ecology (BE) to highlight
how individuals should only exert selective pressure on wild
resources during times of population–resource imbalance. We
examine these hypotheses to explain the domestication event
which occurred in Eastern North America approximately 5000
years ago. Using radiocarbon date density and site counts
as proxies for human population, we find that populations
increased significantly in the 1000 years prior to initial
domestication. We therefore suggest that high populations
prior to 5000 cal BP may have experienced competition for and
possibly overexploitation of resources, altering the selective
pressures on wild plants thereby producing domesticates.
These findings support the BE hypothesis of domestication
occurring in the context of population–resource imbalances.
Such deficits, driven either by increased populations or
decreased resource abundance, are predicted to characterize
domestication events elsewhere.
1. Introduction
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rsos.160319 or via
http://rsos.royalsocietypublishing.org.
Explaining domestication remains one of the most important
and contentious research problems in archaeology [1–3]. Current
explanations often take one of two approaches. The first
hypothesis draws on the concept of niche construction (NC) to
emphasize that intentional experimentation with and management of wild resources should lead to domestication regardless
2016 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted
use, provided the original author and source are credited.
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2
o ri
ver
Riverton
Cloudsplitter/Newt Kash
Ohi
Phillips Spring
Missis
sippi
river
Cumberland river
Marble Bluff
Ark
ansa
s riv
er
Hayes
Tenn
es
see riv
er
site of early domestication
radiocarbon date location
study area
0
100
200
300
400
km
data sources: ESRI, CARD
Figure 1. Map of the study area bounded by a standard deviational ellipse (α = 0.05) based on the seven sites with the earliest dated
evidence of domesticates in Eastern North America and the locations of radiocarbon dates sampled from the CARD [18].
of Malthusian [4] population–resource imbalances [2,3,5]. The other hypothesis is derived from
behavioural ecology (BE) and, based on predictions from foraging theory models, highlights how
individuals should only exert selective pressure on wild resources during times of Malthusian
population–resource imbalance, which encourage individuals to intensify their economies [6,7] in order
to efficiently acquire more food from the environment [1,8–10]. Thus, the NC framework predicts that
domestication should occur during times of stable populations and high resource abundance, while the
BE framework predicts that domestication should only occur following periods of population growth or
declines in the availability of profitable resources. For simplicity, we refer to these respectively as the NC
and BE hypotheses throughout this paper.
These competing explanations for the origins of domestication are at the centre of an ongoing debate
in Eastern North America where, beginning about 5000 years ago, individuals began to domesticate
the series of plant species that would come to constitute the Eastern Agricultural Complex. Squash
(Cucurbita pepo) was the first of these species to be domesticated, with a date of 5025 cal BP from the
Phillips Spring site in Missouri [11,12]. This date represents the earliest evidence of domestication in
North America. At the Hayes site in Tennessee, domesticated sunflower seeds (Helianthus annus var.
macrocarpus) were dated to 4840 cal BP [11–13]. Marshelder (Iva annua var. macrocarpa) was seemingly
domesticated next, with a date of 4400 cal BP from the Napoleon Hollow site in Illinois [11,12]. Evidence
for domestication from these three sites consists of small, isolated finds of botanical remains, making
discussion of a crop complex difficult until 3800 cal BP, when chenopod (Chenopodium berlandieri), the
last of the four morphologically changed domesticates added to the Eastern Agricultural Complex, was
identified in large quantity from the Riverton site in Illinois, along with remains of squash, sunflower and
marshelder [12]. This date from Riverton just barely edges out what were previously the earliest dates
for domesticated chenopod from the Cloudsplitter and Newt Kash rockshelters in Kentucky, 3700 and
3640 cal BP, respectively [11]. Another site with early evidence of domesticates is the Marble Bluff site
in Arkansas, with substantial amounts of stored squash, marshelder, sunflower and chenopod dating to
3400 cal BP. These latter four sites are argued to represent the beginnings of a domesticated crop complex
due to the representation of all four domesticated plants in considerable quantities [12].
................................................
Napoleon Hollow
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river
Ohio river
uri
Misso
N
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To generate and evaluate the population proxies used in this paper, we undertook a seven-step process
wherein we: (1) defined the study area, (2) queried the radiocarbon record and removed spurious dates,
(3) calibrated the dates, (4) described the temporal distribution of dates following two approaches, (5)
corrected each distribution for taphonomic bias, (6) fitted the resulting data with a statistical model and
(7) evaluated the rate of change in the model fit to identify significant periods of population growth or
decline. Each of these steps is detailed below.
2.1. Defining the study area
The study area for this research was defined according to the seven sites identified by Smith [19] which
provide the earliest dated evidence of domesticates in Eastern North America: Cloudsplitter, KY; Newt
Kash, KY; Hayes, TN; Marble Bluff, AR; Philips Spring, MO; Napoleon Hollow, IL; and Riverton, IL.
A standard deviational ellipse [20,21] was constructed at 2 s.d. (α = 0.05) to define the study area
according to the central tendency, dispersion and directional trend of the distribution of these seven
early sites (figure 1) [21]. The standard deviational ellipse method calculates the mean centre point of the
input data based on the spatial coordinates of these locations. It also determines x- and y-axes based on
the directional trends of the input data, which are then used, along with the mean centre, to calculate
standard deviational distances along these axes for which 95% (at α = 0.05) of the statistical population
from which the sampled input coordinates are drawn should be enclosed within the generated ellipse.
2.2. Querying and cleaning the radiocarbon data
With our study area defined in this manner, we queried the Canadian Archaeological Radiocarbon
Database (CARD) [18] on 25 November 2015 to obtain radiocarbon dates within the bounds of our study
area. While the CARD contains an incomplete and regionally biased sample of radiocarbon dates due
to differential reporting of dates and non-random excavation and dating of sites, radiocarbon dates
throughout most of the study area are numerous and spread across much of the region. To ensure
chronometric hygiene, we made certain that (1) any duplicate dates (based on laboratory numbers) were
removed, (2) all dates originating from palaeobiological and geological contexts were removed from
our dataset and (3) all dates marked as ‘anomalous’ in the CARD were eliminated. Despite well-known
problems with ‘old dates’, dates on different organic materials, and dates with large standard errors,
further radiocarbon dates were not removed based on their age or standard error to ensure transparency,
simplicity and replicability of our results and to avoid making any assertions concerning initial human
colonization of the region. While we cannot provide the data from the CARD as it contains sensitive site
location information, it is publicly accessible to individuals whose credentials have been vetted by the
CARD administrators [18].
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2. Material and methods
3
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In line with BE logic, we predict that disequilibrium between populations and resources provided
the incentives for individuals to either intentionally or unintentionally domesticate wild plants in
Eastern North America. To test this prediction, we focus our analysis on population change as a
driver of this disequilibrium. The NC hypothesis of initial domestication explicitly predicts that initial
domestication in Eastern North America occurred in a context of low population density, while ‘evidence
of population growth just prior to or concomitant with the initial appearance of domesticates . . . would
support the [BE] hypothesis’ [2, p. 241]. While relative measures of human populations are difficult to
acquire archaeologically, recent research has made progress using summed probability distributions of
radiocarbon dates as proxies for human population levels [14–17]. Following this approach, we use
radiocarbon date densities and site counts to determine whether or not significant population growth
occurred prior to or during initial domestication in the region surrounding those sites which provide
the earliest evidence of domestication in Eastern North America (figure 1). If the NC hypothesis of
domestication is correct, then there should be evidence of low and unchanging population densities
prior to initial domestication at 5000 cal BP, indicating that individuals domesticated plants through
experimentation in times of relative plenty. If the BE hypothesis is correct, then there should be
evidence of relatively high and increasing population densities before initial domestication, indicating
that individuals domesticated plants out of necessity during times of relative scarcity.
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2.3. Calibration
The first approach describes the distribution of dates through kernel density estimation (KDE). The
second defines the number of occupied (dated) sites per a specified interval of time using a binning
or histogram approach. To implement the first approach, we generated kernel density estimates using
all median calibrated dates following the Sheather–Jones method [24]. The KDE approach is similar to
the summed radiocarbon probability distribution approach [17], but does not incorporate the complete
calibrated distribution. The resulting values were then extracted for every 100 year interval between
15 000 cal BP and the present. To implement the second approach, we determined the number of sites
with dates falling within 100 year intervals between 0 and 15 000 cal BP. Possible redundancies and
inaccuracies in the dataset were eliminated by using only sites with complete Smithsonian Trinomials
listed in the CARD. This provides a semi-independent line of evidence for population changes proposed
by Williams [17].
2.5. Taphonomic correction
To account for the greater probability that older material will be lost, we applied the taphonomic
correction developed by Surovell et al. [25] (electronic supplementary material, figure S1). While this
correction is not universally accepted, it is the best available method to account for the increased
likelihood of loss of sites and materials with increased time since deposition. We also made use of
uncorrected densities and counts of radiocarbon dates to ensure that our results were not produced,
amplified or dampened solely by the taphonomic correction process. To assess the effects of sample
size and overlap between observed and corrected dates, we calculated 95% confidence intervals via a
bootstrapping method, sampling 50% of the database over 1000 iterations (electronic supplementary
material, figure S1). This process resulted in four population proxies: observed and corrected radiocarbon
frequencies described via KDE and observed and corrected number of dated sites described via
histogram. All four were sampled at 100 year intervals for statistical analysis.
2.6. Statistical modelling
To examine whether the variation in these four proxies varies significantly through time, we fit each as
a function of time using generalized linear (GLM) and generalized additive models (GAM; figure 2).
Following Shennan et al. [26], we specified a Poisson distribution and log link relying on quasilikelihood estimation. GAMs allow nonlinear fits between dependent (date density and site counts) and
independent (time) variables using a penalized regression spline approach [27–29]. This allows the data
to ‘speak for itself’ by increasing the degrees of freedom up to an optimal level while still maximizing
parsimony. As a result, GAMs allow the independent variable(s) to explain a greater proportion of the
dependent variable and to account for local variation in the relationship between the two variables.
The optimal degrees of freedom (knots) are defined following generalized cross-validation [28,29].
Model results report the estimated degrees of freedom (edf), r2 - and p-values. A GAM is useful in this
instance because it allows for description of local fluctuations in population that may be missed by more
strictly parsimonious models like a single polynomial GLM, which may better describe ‘global’ trends
in population. Because the model will optimize the degrees of freedom, the fitted trend will also smooth
over what are likely to be spurious fluctuations in the population data.
2.7. Identifying significant rates of change
In order to identify significant periods of change (increase or decrease) in the fitted trend, we relied on
Simpson’s [30–32] approach to estimate the first derivative of the GAM fit based on the finite difference
between points (in this case, 100-year intervals) and then identify significant moments of change where
the 95% confidence interval for the slope of the fit does not overlap with zero. When the first derivative,
or to simplify, the rate of change of the GAM is significantly different from zero, human populations are
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2.4. Summarizing chronological distributions
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The radiocarbon dates resulting from our standard deviational ellipse query and chronometric hygiene
decisions were then calibrated using OXCAL v. 4.2 according to the IntCal13 calibration curve [22]. Using
the median values of these calibrated dates, we adopted the ‘dates as data’ approach [23] formalized
using two methods.
4
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initial
domestication
0.6
0.4
14
0.2
0
initial
domestication
(b)
site count
200
100
50
0
0
5000
10 000
15 000
cal BP
Figure 2. (a) Relative human population density as measured by a calibrated and taphonomically corrected KDE fit by a generalized
additive model (GAM). The plot illustrates the model fit with 95% confidence intervals and is colour coded to indicate significant
(α = 0.01) periods of increase (blue) and decrease (red) based on the first derivative of the model fit. The period of initial domestication is
defined by the earliest occurrences of domestic C. pepo at 5025 cal BP, H. annus var. macrocarpus at 4840 cal BP, I. annua var. macrocarpa at
4400 cal BP, and C. berlandieri at 3800 cal BP. Periods of significant change in the taphonomically uncorrected GAM are shown as coloured
horizontal lines at the base of the panel. (b) Histogram of relative human population levels through time as measured by observed
and corrected archaeological site counts in 100 year intervals. Colour-coded horizontal lines at the base of the panel indicate periods
of significant change in the taphonomically corrected (top) and uncorrected (bottom) GAM fits. As predicted by the model-based BE
framework, human populations increase significantly to a local peak prior to initial domestication in the study area.
either significantly increasing or decreasing. Following Contreras and Meadows [33], we then evaluate
this approach using simulated population data in the electronic supplementary material, text S1.
All analyses were run in the R environment [34]. The complete dataset on corrected and observed
model fits of radiocarbon dates and site counts is available as the electronic supplementary material,
table S1.
3. Results
Sampling the CARD [18] using a standard deviational ellipse (α = 0.05) around the seven sites with the
earliest evidence for domestication (figure 1) returns a total of 3750 dates that pass chronometric hygiene
protocols. The distribution of median calibrated dates ranges from 50 to 15 423 cal BP and is heavily
skewed towards the Late Holocene, with a first and third quartile of 747 and 2812 cal BP, respectively.
3.1. Diachronic trends in date density
Examining the distribution of dates with KDE (electronic supplementary material, figure S1) reveals a
global trend of increasing populations through time (GLM: R2L = 0.81, p < 0.0001). This trend holds when
the KDE is corrected for taphonomic loss (GLM: R2L = 0.65, p < 0.0001). Examining this variation using
a generalized additive model (GAM) shows that local variation in both the observed (GAM: r2 = 0.76,
edf = 8.053, p < 0.0001) and taphonomically corrected (GAM: r2 = 0.73, edf = 8.596, p < 0.0001) trends are
explained by time (figure 2a).
Examining the first derivative of the taphonomically corrected GAM fit reveals long durations of
stasis punctuated by six periods of significant population change (figure 2a). Four represent periods of
significant population increase: 14 100–13 000, 10 400–9100, 6400–5500 and 3400–1500 cal BP; while two
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C date relative frequency
5
significant increase
significant decrease
corrected
observed
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relative human population
(a)
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Examining the distribution of dated site counts shows that these trends are robust across alternative
methods of evaluation (figure 2b). Site counts reveal a global trend of exponential increase (GLM: R2L =
0.80, p < 0.0001), even when corrected for taphonomic loss (GLM: R2L = 0.59, p < 0.0001). Observed (GAM:
r2 = 0.78, edf = 7.77, p < 0.0001) and taphonomically corrected (GAM: r2 = 0.71, edf = 8.274, p < 0.0001)
site counts also vary locally as a function of time (figure 2b).
Examining the first derivative of the GAM fit describing the taphonomically corrected number of sites
reveals that populations increase significantly over three periods (10 700–9400, 6500–5600 and 3400–1700
cal BP) and decrease over two periods (8100–7900 and 1200–0 cal BP). The period of significant decrease
between 8100 and 7900 cal BP does not appear in the model of observed site counts (figure 2b) and
may therefore be an artefact of the taphonomic correction process. Importantly, the period of population
increase prior to initial domestication remains significant across both corrected and uncorrected site
counts (figure 2b; electronic supplementary material, figures S4 and S5).
The number of sites present in the CARD through time shows the same trends as the radiocarbon KDE
model. The first derivative of the fit across all four proxies exhibit a significant increase in population
preceding initial domestication, supporting the BE hypothesis of domestication.
4. Discussion
4.1. Increasing human populations prior to initial domestication
The observed trends suggest that populations in the study area were relatively high throughout the
Middle Holocene and increased significantly to a local peak concurrent with the earliest date of initial
domestication (5000 cal BP). Not only were populations at this time higher than ever before, but also they
had been significantly increasing for approximately 1000 years, likely placing strong pressure on local
resources. Between 5000 cal BP and the earliest dates for the full adoption of a crop complex at 3800 cal
BP, populations then experienced a period of stasis, suggesting sustained pressure on the resource base
through this time period. While the evidence does not indicate whether this population growth occurred
in situ or was the result of immigration from outside the study area, both possibilities would result in the
same outcome: population pressure leading to resource deficits that encourage domestication.
This pattern of high human populations that increased prior to 5000 cal BP aligns with the predictions
of the model-based BE hypothesis of initial domestication. High population levels likely led to local
declines in foraging return rates, to which individuals responded optimally by expanding diet breadth in
order to take lower profitability resources and eventually manipulate these resources. With broader diets
that included more abundant but lower profitability resources, populations should grow at the same
rate, but up to a higher carrying capacity provided by the broader resource base that allows for more
individuals within a given area [35–38]. Given broader diets, lower foraging returns, and more people,
we expect individuals would have placed selective pressure on certain plant resources. While this may
have been unintentional at first, individuals may have eventually begun to intentionally alter plants in
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3.2. Diachronic trends in site counts
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represent periods of significant population decrease: 8200–7300 and 1100–0 cal BP. Most importantly for
the focus of this paper, and as predicted by the BE hypothesis, the period of initial domestication is
preceded by a period of significant growth from 6400 to 5500 cal BP. From the trough in the model fit
at approximately 6900 cal BP to the peak at 5200 cal BP, relative human populations roughly double in
the region. An examination of the model residuals (electronic supplementary material, figure S2) shows
that the GAM does well predicting population change throughout most of the record except for during
the final two periods of significant change, where model predictions are less precise due to the high
rate of change in population from 0 to 3000 cal BP, including the sharp peak at 700 cal BP (electronic
supplementary material, figures S1 and S2) which is not adequately captured in the model fit.
Four of these six periods of significant change, as well as the broader trends, are similarly shown in
the observed radiocarbon data when not taphonomically corrected (figure 2a; electronic supplementary
material, figure S3). This demonstrates that the significant increase prior to initial domestication is
not a result of the taphonomic correction process, but is an innate feature of the radiocarbon record.
However, the significant increase in population from 14 100 to 13 000 cal BP and the significant decline
in population from 8200 to 7300 cal BP shown in the taphonomically corrected model (figure 2) may
be due to taphonomic correction, as they do not appear in the uncorrected model (figure 2a; electronic
supplementary material, figure S3).
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While our analysis reveals significant population increases in Eastern North America that may have led
to declines in per capita foraging return rates through increased competition [41] or resource depression
[47], it is important to note that population growth is a sufficient, but not a necessary condition
for disequilibrium between populations and resources that may precipitate initial domestication. It is
entirely possible that, holding population constant, local ecological changes that reduce the abundance
of profitable resources could lead to the same population–resource imbalance, as evidenced in other
domestication events around the world.
Illustrating the effects of population increases on foraging efficiency, domestication in Southwest
Asia follows a long period of ‘broad spectrum’ foraging [48] resulting from population increases and
subsequent reductions in encounters with high-ranked prey [5,49–53]. On the opposite side of this
imbalance, domestication events in the Neotropical lowlands of Central and South America [54] and
Northern China [55,56] do not reveal evidence for high or increasing human populations prior to
domestication; yet, these events are still due to deficits in the food supply relative to demand. In the
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4.2. Population–resource imbalance as a global driver of domestication
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ways that led to increased yields and reduced processing costs, ultimately resulting in domestication.
The prediction of the NC hypothesis, that there should be low populations and no population increases
before the time period of initial domestication, is not supported by these data.
These findings are consistent not only with predictions we derive from foraging theory, but also with
other archaeological data. Anderson [39] evaluated changes in the number and location of archaeological
sites through time in the Southeastern USA and identified an increase in site numbers from the Middle to
the Late Holocene. In the Duck River Valley of Tennessee, near the Hayes site, Miller [40] makes use of the
number of archaeological sites within physiographic sections of the river valley to show that populations
increased from the terminal Pleistocene through the Late Holocene. In line with predictions from an
ideal free distribution model [41], he argues that infilling of people occurred in this region through
time, suggesting that populations were increasing prior to initial domestication. Lending support to
our findings, Miller interprets his results as indicating that a warming climate in the Middle Holocene
increased the abundance of shellfish, oak, hickory and deer. Miller argues that foragers narrowed their
diets to focus on these resources, but once these positive climatic effects ended in the Late Holocene and
the abundance of these resources declined, individuals living at high population densities at this time
were driven to intensively exploit the low-return plants that would become the Eastern Agricultural
Complex.
Based on the observed high and increasing populations, we predict low foraging efficiency prior to
initial domestication, potentially as a result of increased competition or overexploitation of high-return
resources, but future work is needed to evaluate this. Styles & Klippel [42] document a broad trend of
declining white-tailed deer and increasing fish exploitation through time in the region which may be
suggestive of declining foraging efficiency and increasing intensification; however, the exact relationship
between such changes and initial domestication remains unanalysed. It must be emphasized that our
results do not demonstrate low foraging efficiency anywhere in the study area, only the increased human
population levels that may precede and cause such a phenomenon out of increased resource exploitation
and competition. An analysis of faunal and botanical data from sites in the study area, specifically those
seven from which the earliest domesticates have been recovered, may provide the data needed to address
this question.
In addition to an absolute increase in human impact on the environment, it is also possible that the
increased population levels documented here brought about decreased mobility and smaller foraging
territories, thereby amplifying the per capita impact on local resources leading to initial domestication in
the region [43], though this remains to be assessed as well. Given this possibility, settlement pattern data
may elucidate trends in population packing that could have led to population–resource imbalances.
Research is also needed to determine the environmental setting in which this predicted process of
resource depression and domestication occurs. The sites of earliest domestication are predominately
located in lowland river valleys, with the exception of the Marble Bluff, Cloudsplitter and Newt Kash
rockshelters. This observation has been used to argue that domestication first occurred in floodplain
settings in North America [19]. Others, however, claim that an upland origin is just as likely [43,44].
Ongoing work illustrating where these observed periods of population growth are concentrated, coupled
with detailed analyses of subsistence data, will help determine whether the initial domestication events
occurred through experimentation in floodplain settings [19,45], or out of necessity in upland habitats
[43,46].
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For simplicity, we refer to the two competing explanations of initial domestication in this paper as the
NC and BE hypotheses, but for the sake of clarity it is important to note that the specific predictions
tested here are two of many that could potentially be drawn from these larger bodies of theory to
explain domestication. Support or falsification of one of these hypotheses therefore does not necessarily
constitute support or refutation of NC and BE as overarching bodies of theory, but of the specific
predictions derived from these frameworks.
It is furthermore important to recognize the inconsistencies between the use of NC in biology and in
anthropology. While the biological literature is also rife with disagreements between practitioners of BE
and advocates of NC theory [60], these debates are more focused on whether NC is a unique evolutionary
process or simply part of standard evolutionary theory [61,62]. Unlike in anthropology [2,3,5,45],
researchers on both sides of the debate in biology agree that NC ‘does not make an unambiguous and
clear prediction about the natural world’ [60]. This stands in contrast to studies of initial domestication,
in which researchers oppose simple models that help generate predictions about why individuals would
benefit from niche constructing behaviour. In order to explain why an organism would modify their
environment, researchers therefore need to draw on evolutionary theory that can make predictions about
how individuals are expected to behave in particular environmental circumstances [10].
As such, we contend that the BE-derived hypothesis of initial domestication is not only better
supported by the available archaeological data, but is also better equipped to generate theoretically
grounded and testable predictions than is the NC hypothesis. This does not mean, however, that there
is no need to pay attention to NC. Explaining variation in human behaviour also certainly requires
understanding the dynamic processes of habitat modification and ecosystem engineering [63] that are the
focus of NC theory. For example, evidence of anthropogenic NC in the Late Holocene of Eastern North
America [64,65] requires explaining why people engaged in such niche constructing behaviour and what
the effects of that behaviour were on subsequent generations. These patterns of habitat modification
probably occur for reasons best predicted by foraging theory models derived from BE, which provide a
framework in which researchers can generate testable predictions based on logic from economic and
evolutionary theory [10]. But the long-term impacts of this anthropogenic habitat modification also
require understanding that subsequent generations inherited an altered environment that would change
subsistence decisions. We conclude, as have others [66], that NC can and should be integrated with
foraging theory under the umbrella of BE in order to best understand the evolutionary motivations
behind human behaviours.
4.4. Potential sources of error
Given the potential risk of drawing spurious conclusions from this dataset, an exploration of possible
confounding factors is necessary. Here, we briefly discuss bias that may be introduced from the CARD,
differential preservation, modelling and researcher bias [23].
As stated previously, we obtained only unique radiocarbon dates from the CARD, eliminated any date
from geological and palaeobiological contexts, and did not include any dates marked as ‘anomalous’
in the database. Our dataset is therefore restricted to those dates associated with human activity and
consistent with other contextual information (stratigraphic, typological, etc.) as determined by the
original researcher who supplied a given date to the CARD. While these decisions reduce the risk of
inaccurate analyses, other problems remain. Radiocarbon dates from several decades ago are listed in
the CARD as are dates with large standard errors. The error inherent in these kinds of dates could
introduce inaccuracies in our model. However, the periods of significant change we observe in human
populations through time are almost all several centuries or millennia in duration. The decadal-scale
error that may result from imprecise dates would be unlikely to substantially alter our model to the
point where thousand-year-long periods of significant change were impacted.
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4.3. Integrating behavioural ecological models and niche construction
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Neotropical case, environmental changes at the Pleistocene/Holocene transition are argued to have
lowered encounter rates with high-ranked prey items and thus lowered foraging return rates [57,58].
In Northern China, hunter–gatherer populations prior to domestication were low and highly mobile, but
still experienced resource shortages that encouraged intensified foraging leading to the manipulation of
wild millet [55,59]. Such environmental changes in resource availability result in population–resource
imbalances just as an increase in population would, though by affecting the resource base instead of
population levels.
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The results of this analysis align with the predictions derived from the BE hypothesis for initial
domestication in Eastern North America: populations were high and increased significantly prior to
the beginnings of domestication at 5000 cal BP, doubling over the course of the preceding 1700 years.
This suggests that imbalance between populations and resources, manifested as low foraging efficiency,
drove the shift towards domestication. It is possible that wild foods were not able to sustain high
human populations in the Middle and Late Holocene, making domestication an optimal response to
low foraging profitability. It is also possible that increasing populations led to decreased territory size
[43], excluding foragers from profitable patches and leading to low foraging efficiency.
Within this framework, declining foraging returns are likely to be the most immediate motivation
to initiate the intensification processes that precedes domestication. Regardless of whether populations
increase or resource availability decreases, such deficits should either unintentionally alter the selective
pressures humans place on wild plants or encourage the intentional manipulation of plants in order to
reduce processing time and increase yields. In Eastern North America, it is the case that populations
increased prior to domestication, but we do not expect this to be a universal trend. What we do predict
to be universal, however, is the disequilibrium between supply and demand. Humans are unlikely to put
significant enough pressure on wild resources unless there is a need to do so. Profitable wild foods must
................................................
5. Conclusion
9
rsos.royalsocietypublishing.org R. Soc. open sci. 3: 160319
Differential preservation of organic materials may also impact the frequencies of radiocarbon dates
through time. Certain contexts such as shell middens, rockshelters and caves create conditions more
amenable to the preservation of organic remains. More abundant radiocarbon dates for a given time
period may therefore result not from increased human populations, but from increased dating of
abundant sites with a high degree of organic preservation. Of these three site types, shell middens are
particularly abundant around the time of initial domestication in Eastern North America, particularly
in the Southeast. At least in certain parts of the region though, shellfish exploitation actually peaked
several millennia after initial domestication occurred [67]. If our radiocarbon or site-based models of
human population were heavily impacted by the abundance of shell middens preserving more organic
remains, the GAM should not peak locally at 5000 cal BP as it does, but probably between 2000 and 4000
cal BP. The greater degree of organic preservation afforded by shell middens is therefore not likely to
substantially affect our results.
Error in interpretation may also result from the way population proxies are analysed. Here, we
undertook a novel approach using GAMs to fit diachronic population trends, and the first derivatives of
these fits to evaluate significant periods of change. This method is useful in that it balances parsimony
and goodness of fit, but there is a possibility that these models could identify spurious periods of change
that result not from differences in human populations, but from differences in the sample of radiocarbon
dates [33]. While simulations show that this method can produce spurious results using site density,
false positives are extremely rare using histograms of counts (see the electronic supplementary material,
text S1). Given that our empirical results are consistent across both the KDE method examining date
density and the histogram method examining site counts, we find the conclusions to be real and robust.
An additional problem with the use of radiocarbon dates as a proxy for human population levels
may be found in the form of researcher bias [23]. Certain time periods, geographical regions or site types
may be preferentially investigated by archaeologists, leading to differential dating through time and
space that reflects the actions of scientists, not past human population levels. This problem continues to
plague radiocarbon-based population measures. While some solutions have been proposed to control for
researcher bias in geographical sampling intensity [68], a way to control for temporal biases in sampling
has yet to be proposed. We therefore acknowledge that it is possible that our results may be impacted
by differential investigation of various time periods, geographical regions, site types and archaeological
contexts. With 3750 radiocarbon dates from a large region and lengthy time span, we expect such biases
to be minimal, but cannot rule them out.
Finally, these methods may also be capturing false periods of change that result from peaks or plateaus
in the calibration curve [69]. If periods of inferred population change covary with periods of time in
the calibration curve, models may show inaccurate fluctuations or periods of stasis that result from the
calibration curve, not from past human population change or stability. However, Brown [69] has shown
that such systematic error does not typically lead to false peaks in sampled radiocarbon records, and
no problematic peaks and troughs exist in the calibration curve during the millennia surrounding initial
domestication at 5000 cal BP [22].
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Data accessibility. All data from the models presented in this paper are available as the electronic supplementary material
(dataset S1). Radiocarbon dates and site counts were obtained from the CARD on 25 November 2015 and are available
to researchers with permission from the administrators of the CARD.
Authors’ contributions. E.M.W. and B.F.C. conceived of the study, designed the study, carried out the data analysis and
drafted the manuscript. Both authors gave final approval for publication.
Competing interests. We have no competing interests.
Funding. No funding has been received for this article.
Acknowledgements. We thank Loukas Barton, Simon Brewer, Stephen Carmody, Shane Miller, Erick Robinson, Michael
Weight, the University of Utah Archaeological Center Lab Group, and three anonymous reviewers for their helpful
suggestions and comments on previous versions of this paper—all of which greatly strengthened the final product. We
are also grateful for access to the Canadian Archaeological Radiocarbon Database, which made this research possible.
10
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