Scholarly article on topic 'Considering the palaeoepidemiological implications of socioeconomic and environmental change in Southeast Asia'

Considering the palaeoepidemiological implications of socioeconomic and environmental change in Southeast Asia Academic research paper on "History and archaeology"

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Abstract of research paper on History and archaeology, author of scientific article — Charlotte L. King, Siân E. Halcrow, Nancy Tayles, Stephanie Shkrum

Abstract Human health change during prehistory is often assumed to be intimately associated with changes to sociopolitical complexity and subsistence regime. It is only possible to theorise about how, when and why populations may have experienced deteriorations in health, however, by taking into account context-specific mechanisms behind changes. In mainland Southeast Asia, for instance, changes to subsistence at the introduction of agricultural resources appear to have been minimal. It is not until agriculture intensifies and staple crops become an indispensable part of the diet that we are likely to see a shift in human health. Here we present a model for envisioning the dynamic interactions between human health, subsistence, social, and environmental change in the Upper Mun River valley, northeast Thailand. For the first time we review evidence from multiple archaeological and bioarchaeological studies in this region to show how current data supports our model of regionally-specific socioeconomic processes affecting host-pathogen dynamics, and therefore palaeoepidemiology and human health. We correlate changes in infant mortality and the emergence of infectious disease with socioeconomic and environmental changes that involved moat-building, corralling, deforestation and increased movement of people. All these factors have consequences for pathogen distribution. We highlight the importance of considering the palaeoepidemiological implications of socioeconomic and environmental change and emphasise the need for future work to evaluate this model both within the region and in the wider Southeast Asian context.

Academic research paper on topic "Considering the palaeoepidemiological implications of socioeconomic and environmental change in Southeast Asia"

Archaeological Research in Asia

Archaeological Research in Asia xxx (xxxx) xxx-xxx

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Archaeological Research in Asia

journal homepage: www.elsevier.com/locate/ara

Considering the palaeoepidemiological implications of socioeconomic and environmental change in Southeast Asia

Charlotte L. King*, Siân E. Halcrow, Nancy Tayles, Stephanie Shkrum

Department of Anatomy, University of Otago, New Zealand

ARTICLEINFO ABSTRACT

Keywords: Modelling

Upper Mun River valley Iron Age Health transition Epidemiology

Human health change during prehistory is often assumed to be intimately associated with changes to sociopolitical complexity and subsistence regime. It is only possible to theorise about how, when and why populations may have experienced deteriorations in health, however, by taking into account context-specific mechanisms behind changes. In mainland Southeast Asia, for instance, changes to subsistence at the introduction of agricultural resources appear to have been minimal. It is not until agriculture intensifies and staple crops become an indispensable part of the diet that we are likely to see a shift in human health. Here we present a model for envisioning the dynamic interactions between human health, subsistence, social, and environmental change in the Upper Mun River valley, northeast Thailand. For the first time we review evidence from multiple archaeological and bioarchaeological studies in this region to show how current data supports our model of regionally-specific socioeconomic processes affecting host-pathogen dynamics, and therefore palaeoepidemiol-ogy and human health. We correlate changes in infant mortality and the emergence of infectious disease with socioeconomic and environmental changes that involved moat-building, corralling, deforestation and increased movement of people. All these factors have consequences for pathogen distribution. We highlight the importance of considering the palaeoepidemiological implications of socioeconomic and environmental change and emphasise the need for future work to evaluate this model both within the region and in the wider Southeast Asian context.

1. Introduction

Most bioarchaeological and archaeological models assume fundamental links between prehistoric subsistence strategy, sociopolitical organisation and human health.1 Reliance on agriculture for subsistence is generally modeled as leading to an increase in social stratification, infectious disease transmission, nutritional deficiencies and a general decline in quality of life (e.g. Diamond, 1998; Johnson and Earle, 2000; Armelagos et al., 2005; Larsen, 2006). These factors are usually coupled with an increase in fertility and therefore net population growth (Bocquet-Appel, 2011). The timing of changes to human health and how this may link to different sociopolitical systems and subsistence choices, however, may vary regionally due to context-specific factors. In this paper we review how the archaeological and environmental context may impact upon population health. We focus on a specific region of Southeast Asia, the Upper Mun River valley of northeast Thailand (Fig. 1), to illustrate how archaeologically observed changes to social organisation, environment and subsistence may have

precipitated negative health effects during the Iron Age. In doing so we put forward a model for understanding the palaeoepidemiological implications of social change which may be tested elsewhere in mainland Southeast Asia.

Evidence from Southeast Asia has, until relatively recently, been under-used in global models of prehistoric health change (cf. Tayles et al., 2000; Tayles et al., 2009). It is, however, a significant region through which hypotheses regarding biosocial change may be explored. It is an area with naturally high biodiversity (White, 2011), which from a palaeoepidemiological standpoint is important as it results in high levels of pathogen speciation. In this context, both natural and human-induced changes to the environment have significant potential to alter pathogen distribution and host-pathogen dynamics (Sattenspiel, 2000). Socioeconomic changes are likely to have variable effects on human health, depending on the environmental contexts in which they occur. Southeast Asia is also interesting as full agricultural reliance and the advent of a hierarchical social structure seem to have developed relatively late in prehistory (White, 2011). While agricultural resources

* Corresponding author.

E-mail address: charlotte.king@otago.ac.nz (C.L. King). 1 We acknowledge the terminological problems of the word "health" in stress-related bioarchaeological studies. We refer to "health" as a general population-based measure of stress and illness based on a number of studies investigating mortality, oral pathology, infectious disease, and skeletal and dental indicators of stress (Temple and Goodman 2014).

http://dx.doi.org/10.1016/j.ara.2017.05.003

Received 12 October 2016; Received in revised form 2 May 2017; Accepted 24 May 2017

2352-2267/ © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: King, C.L., Archaeological Research in Asia (2017), http://dx.doi.Org/10.1016/j.ara.2017.05.003

Fig. 1. Map of mainland Southeast Asia, with locations of sites and areas mentioned in text. Site abbreviations are as follows: BC = Ban Chiang, BND = Ban Na Di, BNW = Ban Non Wat, BLK = Ban Lum Khao, KWPV = Khao Wong Prachan Valley, KPD = Khok Phanom Di, MS = Muang Sema, NBJ = Non Ban Jak, NUL = Noen U-Loke, PSn = Phum Snay, PSo = Phum Sophy, MB = Man Bac, RN = Rach Nui. Note that a single point represents four of the Upper Mun River valley sites due to their very close proximity. The shaded area represents the Upper Mun River valley.

were introduced in the Neolithic, around 2000 BCE (Fuller, 2007; Fuller et al., 2010; Zhang and Hung, 2010), they were variably incorporated into existing broad-spectrum subsistence regimes rather than forming a major component of the diet (Higham and Thosarat, 1994; King and Norr, 2006; Oxenham et al., 2015).

Similarly, social organisation seems to have remained flexible and heterarchical through much of prehistory (White, 1995; O'Reilly, 2000), and production is argued to have been community-based (White and Pigott, 1996) rather than regionally centred and specialised. It appears that movement towards agriculturally-reliant, hierarchically-structured societies did not begin until the mid to late Iron Age, when it is posited that in some areas a rapid socioeconomic transition involving the intensification of rice agriculture and emergence of elites occurred (O'Reilly, 2008; Higham, 2014; O'Reilly, 2014). This has important implications for human health, and population dynamics. In areas with a slow agricultural transition, changes to human health may not be as marked as in areas with a rapid transition (Halcrow and Tayles, 2011) and the health inequalities associated with hierarchical societies are less likely to appear when social organisation is more flexible (e.g. Levy, 1995).

Our model focuses on the health implications of an Iron Age transition to a hierarchical society with a rice-dominated subsistence

base in the Upper Mun River valley of Thailand. Here, extensive excavation and research projects (Higham and Thosarat, 2005; Higham, 2007; Higham and Kijngam, 2009; Higham et al., 2014) have yielded significant quantities of both archaeological and bioarchaeolo-gical data. As a result, overarching narratives of socioeconomic and environmental change are starting to be developed by archaeological researchers in the area (e.g. Boyd and Chang, 2010; O'Reilly, 2014; Higham, 2015c), most of which identify the Iron Age as a key transitional period.

This model constitutes the first attempt to synthesise both bioarch-aeological and archaeological data from the region. Traditionally these lines of evidence have been viewed separately. It is also the first regional exploration of the palaeoepidemiological implications of archaeologically observable changes to social structure and subsistence. Consideration of the serious human biological ramifications of socioeconomic change is crucial for understanding transitional periods, but is not often integrated into archaeological models in the region. Part of the reason for this has been the paucity of bioarchaeological data with which health-based hypotheses can be tested. Now that this is beginning to be addressed we expect that our model will be testable using emerging archaeological evidence. We emphasise that this model is not intended to fit all regions of Southeast Asia, but instead it will provide a

starting point for considering the context-specific processes which may be affecting disease prevalence and ultimately human health.

2. Archaeological evidence for socioeconomic change in Southeast Asia - the story so far

Current archaeological models of economic change in Southeast Asia, as noted, highlight the probable slow agricultural transition in this area. Archaeozoological, palaeobotanical and isotopic evidence all suggest the retention of broad-spectrum economies beyond the introduction of agricultural resources. For instance, pre-metal coastal sites such as Rach Nui in Vietnam and Khok Phanom Di in Thailand, retained a marine resource reliance but incorporated domesticated rice into their diets (Higham and Thosarat, 1994; Oxenham et al., 2015). Contemporaneously inland sites such as Ban Non Wat in the Upper Mun River valley continued to use wild resources alongside rice and possibly millet (King et al., 2014), and the sites of the Khao Wong Prachan valley chose to take up millet agriculture initially (Weber et al., 2010) rather than rice agriculture, which appears later in the sequence.

The heterogeneity in economic strategy across mainland Southeast Asia seems to have resulted in a lack of large-scale economic and sociopolitical changes after the incorporation of agriculture. The adoption of at least some agricultural contribution to subsistence does correspond with a level of cultural change visible in terms of increased sedentism (Bellwood and Oxenham, 2008), and a corresponding increase in the presence of cemetery sites, as well as the introduction of new ceramic motifs (Rispoli, 2007; Sarjeant, 2014) and items of material culture (Bellwood, 2005; Rispoli, 2007). Local variation in material culture, however, is maintained throughout the metal ages, as is intrasite individuality in mortuary treatment (White, 1995). Social organisation after the adoption of agriculture and throughout the Bronze Age appears to have remained based upon a flexible social hierarchy (White, 1995; O'Reilly, 2000; Higham, 2015b).

In the Iron Age, however, some areas begin to show evidence for rapid intensification of agriculture, social stratification and associated changes to population dynamics. These changes have been best-documented in the Upper Mun River valley of northeast Thailand, the focus of this synthesis, but appear to be mirrored by evidence emerging from northwest Cambodia (O'Reilly et al., 2004; Domett et al., 2011; Newton et al., 2013).

Archaeological evidence for Iron Age change in the Upper Mun River valley has been systematically reviewed elsewhere (Higham, 2011; Higham, 2014; O'Reilly, 2014; Higham, 2015d), with most work highlighting the emergence of moated settlements in both this region (Boyd et al., 1999; McGrath et al., 2008) and northwestern Cambodia (O'Reilly, 2014; O'Reilly and Shewan, 2015). These moat systems are interpreted as being for the purposes of agricultural water management rather than defence (Scott and O'Reilly, 2015). Alongside this there is evidence for the construction of large-scale field systems (Hawken,

2013) and the corralling of domestic animals (Higham, 2011; Higham,

2014).

Most recently Wohlfarth et al. (2016) have used palaeoclimatic records from Lake Kumphawapi (Wohlfarth et al., 2012; Chawchai et al., 2013), and Lake Pa Kho (Chawchai et al., 2015) to the north of the Upper Mun River valley to argue that these social changes, which mostly centre around the mid-late Iron Age (370-450 cal. CE), coincide with a period of faltering of the annual monsoon and decline in available moisture. In an area where monsoonal rains are relied upon for successful rice harvest, periods of drought, or reduced rainfall can have potentially disastrous consequences (Wohlfarth et al., 2016). This environmental change is posited as the impetus for management of water (Boyd and Chang, 2010; O'Reilly, 2014; Scott and O'Reilly, 2015; Wohlfarth et al., 2016), as evidenced in the development of the moats (Boyd and Chang, 2010; Scott and O'Reilly, 2015), and the concurrent greater reliance on rice for subsistence (King et al., 2014).

In the Iron Age we also see the introduction of iron ploughshares,

sickles and other agricultural tools, indicating the intensification of agriculture, and ability to efficiently improve agricultural land (Higham, 2015c). Palaeobotanical evidence also suggests an increase in rice-field weed species in Iron Age layers (Higham et al., 2014), and a series of rapid changes to landscape associated with rice agriculture intensification during the middle Iron Age (Boyd, 2008; Boyd and Chang, 2010). It has been suggested that the inheritance of this improved land would have set the stage for significant socioeconomic change based upon access to and ownership of prime agricultural areas (Higham, 2015a). This inferred economic change is accompanied by the ritualistic use of rice to infill grave cuts and mortuary vessels in the Upper Mun River valley, perhaps indicating that it has taken on a level of symbolic importance (Higham and Thosarat, 2005; Boyd and Chang, 2010; Higham, 2015a).

Several lines of archaeological evidence indicate the emergence of a fixed social hierarchy during the mid-late Iron Age in the Upper Mun River valley, with a rise of social elites based on landscape management, lineage based land ownership, control over labour and differential access to water and exotic commodities (Higham, 2015d). It is during this period that disparities in mortuary wealth within individual cemeteries (Higham, 2011; O'Reilly, 2014) and intrasite differences in residence size begin (Higham et al., 2014). There is increasing evidence for the grouping of burials (Higham and Kijngam, 2012; O'Reilly, 2014), perhaps indicating increasing importance of kinship groups. At sites such as Non Ban Jak, burial within the confines of house structures has been shown, interpreted as a method for strengthening links to the ancestors and evidence for inherited rights to property, wealth and status (Higham et al., 2014).

This intensification of agriculture and shift to hierarchical social organisation occurs alongside an apparent proliferation of trade links (Carter, 2015), evidenced by increasing numbers and variety of exotic goods in sites (Higham et al., 2014), and large-scale production (O'Reilly, 2008). Uniform ceramic traditions emerge at this time, indicating a level of cultural homogeneity (O'Reilly, 2000).

There is also a rise in weaponry in mortuary contexts in the Iron Age of the Upper Mun River valley (Boyd and Chang, 2010). Higham (2002), for instance, notes that arrowheads dramatically increase in frequency in the later Iron Age at Noen U-Loke. Increases in weaponry could indicate an increase in hunting in later periods, but could equally be reflective of human conflict. Though interpersonal violence is certainly not a new thing, craniofacial fractures, for instance, occur in Bronze Age sites in northeast Thailand (Domett and Tayles, 2006a), there do seem to be more instances of violent death in the Iron Age. Some of the only examples of possible violent death in Thailand come from Iron Age Noen U-Loke, in the form of a female with probable sharp force trauma to the cranium and a male buried with a projectile embedded in his spine (Tayles, 2003; Tayles et al., 2007). This trend is also present in Iron Age Cambodia. At the site of Phum Snay, for instance, weapons in mortuary contexts and exceptionally high frequencies of both sharp and blunt force trauma in the skeletal sample have been interpreted as evidence for increased interpersonal violence (Domett et al., 2011).

3. Bioarchaeological evidence for human health change

Though many researchers (Tayles et al., 2000; Pietrusewsky and Douglas, 2002; Domett and Tayles, 2006b; Oxenham et al., 2006; Halcrow et al., 2013; Halcrow et al., 2016) have investigated the possibility of demographic change and deterioration of health after the initial introduction of agricultural resources, no uniform health transition has been identified in mainland Southeast Asia. This is in part due to a paucity of cemetery sites through which large-scale temporal shifts in health status may be identified.

In the Upper Mun River valley, however, recent excavations of sites situated close together on the Mun River floodplain have yielded a skeletal sequence that spans from the Neolithic through to the late Iron

Table 1

The cemetery sites of the Upper Mun River valley, with cemetery dates and demographic information for each cemetery sample. Infants/children classification includes individuals between 0 and 15 years of age (Scheuer and Black, 2000). Table adapted from tables in Halcrow et al. (2016).

Site Cemetery dates

(previous bioarchaeological analyses)

Archaeological time periods

Total Sample

Adults

Female

Unknown

Infants/ Children

Ban Lum Khao (Domett, 2001)

1000-500 BCE Mid-late Bronze Age (Higham and Thosarat, 1998)

Ban Non Wat (Tayles and Halcrow, 1750 BCE-500 CE (Higham Neolithic - Iron Age

and Higham, 2009)

300 BCE - 500 CE Iron Age

Noen U-Loke (Tayles and Buckley,

2004; Tayles et al., 2007)

Non Ban Jak (Higham et al., 2014) 250-800 CE (Higham et al.,

Muang Sema (Pureepatpong, 2001; 0-500 CE (Pureepatpong,

Halcrow et al., 2016) 2001)

Mid-late Iron Age Iron Age

110 630 120

32 157 21

> 150 (excavation Demographic analysis ongoing, and currently unpublished ongoing)

53 11 11 8 23

Age (Table 1), allowing researchers to begin to identify changes to population health through time. Here, multiple bioarchaeological lines of evidence point to links between agricultural intensification, changes to sociopolitical organisation and changes to human health and demography during the Iron Age. Isotopic evidence, for instance, highlights close correlations between hierarchical organisation and rice agriculture, showing increases in rice consumption coinciding with the emergence of elite burials in the Bronze Age (King et al., 2013) and entrenchment of a formal hierarchy in the Iron Age (King et al., 2013; King et al., 2014).

Although limited by poor Iron Age skeletal preservation at some sites, the sheer quantity of excavated skeletal material has allowed more extensive bioarchaeological analysis in this region than has been possible elsewhere in mainland Southeast Asia. The bioarchaeological analyses already undertaken in the area reveal a picture of increasing infant mortality, physiological stress and emergence of infectious disease during the Iron Age, combined with potential for a deterioration in oral health associated with reliance on rice (Tayles and Buckley, 2004; Shkrum, 2014; Halcrow et al., 2016).

Possible health deterioration in the Iron Age has been suggested by several lines of palaeopathological evidence, including an increase in non-specific stress indicators in infants (Halcrow et al., 2016), and the first bioarchaeological evidence for systemic infectious diseases. Possible instances of leprosy and tuberculosis, for instance, are reported at Noen U-Loke during this period (Tayles and Buckley, 2004). There also appears to be an increase in physiological stress in the late Bronze Age evidenced by an increase in linear enamel hypoplasia prevalence and decrease in stature in females at Ban Non Wat (Clark et al., 2014), though poor preservation of Iron Age burials at this site has prevented evaluation of whether or not this trend continues into the Iron Age.

Palaeodemographic evidence for Iron Age changes to population sizes and female fertility is more equivocal (Domett and Tayles, 2006b; Tayles et al., 2015; Halcrow et al., 2016), but hints that a shift in population dynamics occurs during this period. Recent studies have highlighted the potential trade-off between offspring survival and female fertility in populations adopting agriculture (Page et al., 2016), with more agriculturally-reliant populations having poorer infant health and higher infant mortality, but an offsetting increase in birthrate. Increases in population size, therefore, should be accompanied by an increasing proportion of infants and children to adults in a cemetery sample. This is both because an increase in births will result in a higher proportion of infants and children in the sample (Johansson and Horowitz, 1986; Bocquet-Appel, 2002), due to increasing mortality in these age groups (Page et al., 2016). This does appear to be the case in the Upper Mun River valley, where the Iron Age sites of Muang Sema and Noen U-Loke have very high proportions of infants and children in their cemetery samples (Halcrow et al., 2016; Table 1). Neonatal (0-1 month old) and infant (0-0.9 years old) mortality is also exceptionally high at Noen U-Loke (Domett and Tayles, 2006b; Tayles et al.,

2007; Halcrow et al., 2016). This aligns with the expected pattern for an expanding population (as modeled by Paine and Harpending, 1998). Palaeodemographic analysis of nearby Ban Non Wat has not yet been published in full, but current results indicate the possibility of a similarly high infant mortality rate in the Iron Age (Tayles et al., 2015).

Dental pathologies have been used as a proxy for increased reliance on a carbohydrate staple, such as rice (Turner, 1979; Lukacs, 1989). There are problems with this applying this approach in Southeast Asia, including the low cariogenic potential of rice, especially in its less processed form (Rugg-Gunn and Nunn, 1999; Sheiham, 2001), and the complex, multifactorial aetiology of caries (Fejerskov, 2004). Recent research has indicated that increased female fertility associated with the agricultural transition may also have consequences for female oral health (Lukacs, 2008). Specifically, physiological and hormonal changes experienced by females during pregnancy may result in a deterioration of oral health, and a discrepancy between male and female oral health measures during times of increased female fertility (Lukacs, 2008; Lukacs, 2010). There is some evidence for sex-based differences in oral health during the Iron Age from the Upper Mun River valley, notably at Ban Non Wat where higher rates of dental caries and tooth infections were observed in females, as well as a higher rate of alveolar resorption, a possible sign of poor periodontal health (Shkrum, 2014). A pattern of general deterioration in oral health is observed in northwest Cambodia where higher rates of dental caries and tooth infections were found at Iron Age Phum Snay and Phum Sophy compared to Bronze Age sites in the region (Newton et al., 2013), though this interpretation is based on small sample sizes. It is possible that development of oral microbiome analyses may contribute to understanding of subsistence changes and their relationship to oral health in the near future (Adler et al., 2013).

4. Introducing a model - considering how archaeologically-observed changes may affect palaeoepidemiology

In the Upper Mun River valley both the bioarchaeological and archaeological evidence reveal a picture of substantial biological and social change during the Iron Age. Biocultural modelling may be used to combine these two sets of evidence to better understand broad-scale responses to regionally-specific social, cultural and environmental factors. This approach is not new in bioarchaeology (Roberts et al., 1998; Gowland and Knusel, 2006; Halcrow et al., 2008; Oxenham et al., 2008; Zuckerman and Armelagos, 2011; Clark et al., 2014), but is particularly relevant for contexts such as the Southeast Asian Iron Age, as high biodiversity means that human behavioural changes may result in a myriad of possible context-specific implications for human health (Sattenspiel, 2000). All of these possible outcomes need to be considered and evaluated against the bioarchaeological evidence. In Southeast Asia, Domett (2001), Halcrow et al. (2016) and Tayles (1999) have previously highlighted the importance of tropical patho-

Climate

- faltering annual

monsoon

- increasingly dry

conditions

0 Regional Context

Socioeconomic l~3~l context

- Shift to hierarchy - Kinship-based

inheritance

- Rice-agriculture intensification

- Increase in trade

[7] Land Cover

- Forest clearance

- rice paddy creation

- plentiful stagnant

[6~1 Habitat connection

Connectivity of vector and host habitats through proximity of water and corrals to settlement. Increased connectivity of human populations through trade

[§] Impact on vector dynamics

- Changes to seasonal abundance - Vector concentration in areas of standing water

M Host-pathogen dynamics

- Amplification of natural transmission

- Increased host-vector contact

[To] Activity patterns

- Increase in "at risk" behaviour e.g. contact with animals & water sources. - Probable lack of preventive measures/ vector control

0 Disease Transmission

Fig. 2. The relationships between environmental and human socioeconomic factors and disease transmission. Modified from Lambin et al. (2010) to be specific to the northeast Thai context.

gens and their potential impact in prehistory, but a paucity of Iron Age data prevented them from extending their interpretations towards modelling this important transitional period.

In essence, our approach considers how the microenvironments present throughout the Southeast Asian mainland (White, 2011) might have encouraged pathogen spread and virulence. Human populations in the area are likely to have been variably exposed to different pathogen strains and disease burdens. We describe how human modification of landscapes and contact between population groups from different areas may have significantly affected transmission rates between hosts, enabling pathogen transmission to previously unaffected populations or species and causing shifts in the dominant pathogen strains within an area (Daszak et al., 2001).

Looking specifically at the Upper Mun River valley we consider how extensive human modification of the landscape, population expansion, increasing contact between population groups, and natural environmental change in the mid-late Iron Age may have affected the distribution of pathogens, hosts and vectors, and therefore human health and biology (summarised in Fig. 2). We extend the developing archaeological models by exploring how human health and epidemiology is both a product of, and a contributing factor in, the changes that are archaeologically visible. We discuss the main archaeologically visible changes to the natural and social environment, and their implications for pathogen transmission.

4.1. Moats, rice paddies and deforestation

Palynological and phytolith analysis of northeastern Thailand indicates a rapid period of deforestation and replacement with grassland species during the Iron Age (Boyd and McGrath, 2001a; McGrath et al., 2008; Boyd and Chang, 2010). This environmental change corresponds with a decline in moisture availability inferred through carbon isotope ratio changes at La Pa Kho to the north of the Upper Mun River valley (Chawchai et al., 2015), and is accompanied by the building of moats, probably in an effort to conserve water (O'Reilly, 2014; Scott and O'Reilly, 2015). In combination these landscape modifications are likely to have changed transmission rates of pathogens with mosquito vectors, and increased the frequency of diseases associated with aquatic hosts (Fig. 2, boxes 4-7).

The removal of forested areas removes the habitat for shade-loving mosquito species (Patz et al., 2000), but the creation of rice paddies and moat systems increases the available habitats for alternative mosquito species whose preferred habitat is unshaded stagnant water (Sattenspiel, 2000). These new vectors are likely to have brought with them new strains of pathogens resulting in an increase in disease. Modern malaria outbreaks in particular have been strongly associated with changes to human land usage, especially the building of irrigation systems (Livingstone, 1958; Patz et al., 2000; Yasuoka and Levins, 2007). The presence of encircling moats is likely to have affected malarial prevalence in human populations more than the creation of rice paddy systems, as moat systems brought water into much closer proximity to human habitation. This would have allowed vectors such as mosquitos to easily reach human populations during their nighttime feeding hours (Lambin et al., 2010).

The symptomatic burden of malarial infection is primarily borne by infants and children under five years of age, with those between six months and three years of age most vulnerable to mortality (Sachs and Malaney, 2002). Maternal health is also significantly affected by the disease, with pregnancy both increasing malarial susceptibility and the likelihood of complications during pregnancy (White, 2003). As a result of this any prehistoric increase in malarial transmission is likely to have significantly affected population dynamics, and the demography of cemetery samples (Tayles, 1999; Halcrow et al., 2016).

Standing water sources also provide a habitat for other disease vectors and intermediary hosts such as snails and fish. The variety of flukes, helminths and protozoa which may be present in these environments is too great to fully describe here (see Table 2 for detail), but most significant in the tropical Southeast Asian context are parasites such as Schistosoma and Opistorchiidae flukes, causes of schistosomiasis, liver cancer, dysentery and diarrhoea in populations coming into contact with the water (Patz et al., 2000; Steinmann et al., 2006; Lambin et al., 2010). This remains a problem even in modern-day Thailand as the cultural practice of eating raw fish allows the flukes to be readily transferred from infected fish into the human population (Sithithaworn and Haswell-Elkins, 2003). Moats and irrigation systems may also increase the transmission of protozoa such as Giardia and Cryptosporidium, due to increased likelihood of water contamination when sources are stagnant rather than flowing (Caccio et al., 2005; Macpherson, 2005). In a tropical environment without sanitation systems the likelihood of contamination is increased even further, as torrential downpours during the wet season may result in the introduction of faecal-laden rainwaters into moat systems and rice paddies (Sattenspiel, 2000).

The disease burden of these pathogens is likely to have been significant (Table 2) and, although the majority of them do not leave diagnostic traces in human skeletal remains, we may still be able to infer their presence. Gastrointestinal diseases, for instance, are currently one of the leading causes of infant and newborn death (Bryce et al., 2005). Acute diarrhoeal disease may cause death before any trace of it is recorded in the bones (Wood et al., 1992; DeWitte and Stojanowski, 2015), and could be responsible for the very high levels

Summary of major pathogens present in Southeast Asia, their symptomatic burdens and the environmental changes and/or social processes which may have an effect on their distribution.

Pathogen (disease) Transmission Factors affecting presence/ transmission Reason Symptomatic burden

Plasmodium falciparum (malaria) Anopheles sp. (mosquito) Deforestation Moat/rice paddy building Population increase Change in vector habitat means change in species present (Sattenspiel, 2000; Daszak et al., 2001) Increase in breeding grounds and their proximity to human hosts (Hunter, 1992; Carter et al., 2000) More susceptible hosts - ability to sustain a level of endemicity (Dobson and Carper, 1996) Infant and child mortality especially under 5 years (Sachs and Malaney, 2002). Increased susceptibility during pregnancy resulting in infection of the foetus (Lee, 1988), stillbirth, miscarriage, maternal and infant death, and compromised infant health (White, 2003). Chronic anaemia and exacerbation of malnutrition (Nussenblatt and Semba, 2002)

Schistosoma sp. (schistosomiasis) Aquatic snails and fish Moat/rice paddy building Habitat increase; increased contact between humans and water sources Diarrhoea and/or dysentery. Associated malnutrition (Patz et al., 2000; Steinmann et al., 2006)

Opistorchiidae sp. Fish Moat/rice paddy building As above Diarrhoea, weight loss, pain, fatigue, liver cancer (Patz et al., 2000; Steinmann et al., 2006)

Giardia sp. Cryptosporidium sp. Faeco-oral Keeping of livestock in proximity to water sources Poor sanitation Increased contact between water and animal faeces Risk of contamination of water sources (Jobin, 1999) Diarrhoea and associated malnutrition (Caccio et al., 2005).

Shigella, Salmonella, E. coli (infectious diarrhoea) Faeco-oral Poor sanitation High population density Increased contact with human waste Increased contact between infected individuals Diarrhoea, dysentery, malnutrition, infant death.

Strongyloides (Strongyloidiasis) Contaminated soil Poor sanitation High population density Increased contact with human waste Increased contact between infected individuals Abdominal pain, nausea, diarrhoea, weight loss, malnutrition (Olsen et al, 2009)

Necator, Ancylostoma, Ascaris (hookworm/ roundworm) Contaminated food, water, soil Poor sanitation High population density Climate Increased contact with human waste Increased contact between infected individuals Survival of larvae/eggs outside of the host is temperature and humidity dependent (Brooker et al., 2006) Anaemia, malnutrition, B12/folic acid deficiency, gut obstruction Maternal-neonatal complications including premature birth, low birth weight (Jex et al., 2011).

of infant death seen in Iron Age sites of the Upper Mun River valley (Halcrow et al., 2016). Similarly the complications malaria induces during pregnancy can lead to increased neonatal mortality, for which there is strong evidence at Noen U-Loke and in the Iron Age levels of Ban Non Wat (Domett and Tayles, 2006b; Tayles et al., 2007; Tayles et al., 2015; Halcrow et al., 2016).

Malaria has also been associated with haemolytic anaemia (Buckley and Tayles, 2003; Walker et al., 2009), which may result in the skeletal lesions of porotic hyperostosis and cribra orbitalia. Work elsewhere in the world has indicated that intersite differences in cribra orbitalia prevalence may be related to malarial endemicity (Gowland and Western, 2012), although it is also recognised that this bony pathology may be also associated with gastrointestinal infections which cause micronutrient deficiencies (Walker et al., 2009). As noted, in the Upper Mun River valley poor preservation of Iron Age remains has prevented thorough palaeopathological recording in many sites, but there is some evidence for an increase in adult cribra orbitalia in the Iron Age burials at Ban Chiang, further to the north (Pietrusewsky and Douglas, 2002).

High frequencies of cribra orbitalia, porotic hyperostosis and enlarged nutrient foramina of the phalanges have also been linked to genetic forms of anaemia such as thalassemia (Tayles, 1996; Lewis, 2012). This kind of haemoglobinopathy is often found in malarial areas, providing a measure of protection against its effects (Williams, 2006). It is inferred to have been present earlier in the prehistoric sequence at coastal Khok Phanom Di (Tayles, 1996), but is not recorded inland until the Iron Age (Pietrusewsky and Douglas, 2002). It is possible that changes to the distribution of genetic anaemia in the past may reflect changing levels of malaria endemicity due to human landscape modification and environmental changes.

We anticipate that further palaeoenvironmental work in the Upper Mun River valley and beyond will elucidate when and if water control measures and intensive rice agriculture systems were put in place. Recent work is revealing that the presence of moats may be more common than previously thought (Duke et al., 2016), with multiple studies showing that these features date to the Iron Age (Boyd and McGrath, 2001b; Duke, 2009; Kanthilatha et al., 2017).

4.2. Corralling of domestic animals

Though water buffalo were domesticated long before the Iron Age, the large corrals which developed during this period (Higham, 2015c) were likely to have substantially increased risk of pathogen transmission. Particularly important in this context is the potential for diseases that are spread through faecal contamination, such as cryptosporidiosis (Macpherson, 2005), and in Asia there is also evidence for the spread of schistosomiasis through contact with cattle (Jobin, 1999). Archaeological evidence indicates that corrals at Ban Non Wat were located within the moats (Higham, 2015c), a practice which is likely to have exacerbated faecal contamination of drinking water, and increased diarrhoeal disease and parasite transmission (Fig. 2, boxes 6-11).

In keeping animal species in closer proximity to humans, and increasing interaction with them, there is also increased likelihood of disease vector species adapting to become more anthropophyllic rather than zoophyllic, transmitting diseases to human hosts rather than animals (Patz et al., 2000; Daszak et al., 2001). Many infectious human diseases have zoonotic origins, for instance tuberculosis strains even today continue to jump the gap between bovine and human populations (Ayele et al., 2004). This problem is compounded by the fact that social herd animals, such as the water buffalo and domestic cattle of Southeast Asia, are likely to have harboured pathogens specialised for rapid transmission in larger populations. This makes those pathogens particularly virulent if transmitted to human populations (Wolfe et al., 2007), provided of course that the human population was large enough to sustain them.

4.3. Wealth disparities and sociopolitical setting

Archaeological evidence for a change from flexible hierarchy or heterarchical social structure to a fixed, kinship-based hierarchy in the Iron Age also has significant health implications. Entrenchment of social hierarchy means an increase in social inequality, which is associated with differential access to resources and care (Cohen, 1998; Goodman, 1998; Armelagos et al., 2005). Social position is strongly related to a level of buffering against the nutritional and economic stress experienced by the population. Those who are wealthier tend to be healthier (Wilkinson, 1999), whereas disease and poverty are involved in a negative feedback loop. The poor are more likely to become ill, and when they are sick are more likely to become sicker due to lack of access to resources and healthcare (Armelagos et al., 2005). In addition, the division of labour implicit in hierarchically structured societies means that some individuals (e.g. rice-paddy workers) are more exposed to disease vectors (Fig. 2, boxes 3 and 10), with elites more likely to be removed from the hard labour that would otherwise increase their susceptibility to disease. As social position in hierarchical systems is often passed through lineages, there is also likely to be heritable susceptibility to chronic disease and shared psychosocial stresses between generations, alongside inherited poverty and all the material disadvantages this entails (Gowland, 2015).

All these factors combine to mean that health disparities echo wealth disparities in kinship-based hierarchies. Currently there are too few Southeast Asian Iron Age samples with good skeletal preservation with which to thoroughly extend the testing of this hypothesis. However, the excellent preservation and presence of differential mortuary wealth associated with different residential areas in the site of Non Ban Jak will perhaps provide an opportunity to more fully explore the health implications of social inequality (Higham et al., 2014).

4.4. Increased population sizes and increased trade

There are several areas of mainland Southeast Asia, including the Upper Mun River valley, where dramatic population growth in the mid-late Iron Age appears likely (Welch and McNeill, 1991; Tayles et al., 2007; O'Reilly, 2008; Evans et al., 2016). Increases in both settlement size and density have consequences for disease prevalence as many diseases have population-density thresholds, below which they cannot become established (Harrison et al., 1988). Once the density of humans, hosts and vectors reach critical levels, however, diseases may quickly progress from occasional presence to endemic and ultimately to epidemic behavior (Wilcox and Gubler, 2005). Standard epidemiologi-cal modelling predicts that the size of an epidemic relates directly to population size (Slingenbergh et al., 2004). The more people there are in any given place, the more vulnerable they are to infection, although the number of people required to sustain continuous infection varies depending on the virulence and transmission efficiency of the specific pathogen, in addition to the general health of the population (Dobson and Carper, 1996).

Modelling prehistoric population sizes is very complex and few attempts have been made in Southeast Asia, although Higham (1989) gave a conservative estimate of 1000-2000 individuals in the later Iron Age settlements. This is insufficient to allow more virulent diseases such as measles and smallpox to become endemic, but would certainly have allowed the sustained transmission of mosquito-borne diseases such as malaria (Dobson and Carper, 1996), and diarrhoeal diseases (Fig. 2, boxes 9 and 11). The exact population thresholds for human-human transmission of chronic infectious diseases such as tuberculosis are less well known. There is potential for tuberculosis transmission even in small hunter-gatherer populations (Roberts and Buikstra, 2003), but higher population density during this period is likely to have allowed sustained human-human transmission through droplet infection.

In addition to population growth during the Iron Age, there is also

evidence for increased contact with external populations through both trade and migration. Although isotopic evidence suggests that permanent migration was relatively rare (Bentley et al., 2009; Cox et al., 2011; King et al., 2013), the presence of exotic goods indicates at least some temporary movement of peoples for the purposes of trade was occurring (Carter, 2015). The movement of people from different environments results in the creation of new and unique disease pools (Armelagos et al., 2005) from which pathogens are shared. Trade links not only involve the movement of people, but also microbial traffic (Fig. 2, box 6). They therefore have the potential to transmit diseases endemic in one area to an entirely new habitat, where they may become epidemic in the face of a population with no acquired immunity (Mitchell, 2003). Trade centres are likely to be more adversely affected by these factors than settlements on the periphery of exchange networks. These settlements act as a focus where humans not only interact with each other but also with livestock species and wildlife (Coker et al., 2011). The Upper Mun River valley seems to have played an important role in Iron Age trade networks (Boyd and Chang, 2010; Higham, 2011), and it is therefore not unreasonable to assume that during this time period instances of previously unknown infectious diseases may emerge.

In fact, as noted, the first skeletally diagnosable evidence for specific infectious diseases in Southeast Asia (leprosy and tuberculosis) has been found in the Iron Age Upper Mun River valley site of Noen U-Loke (Tayles and Buckley, 2004). This has been interpreted as evidence supporting increased trade or contact between infected peoples and the previously unexposed inhabitants of the Upper Mun River valley (Tayles and Buckley, 2004). Poor preservation in both the Iron Age and some Bronze Age sites from the area has, however, limited thorough palaeopathological analysis. It is possible that infectious disease was much more prevalent than these isolated case studies would imply, especially as not all of those infected will have developed bony lesions (Wood et al., 1992).

5. Summary of model

Our model highlights the fact that there are multiple processes occurring in the Iron Age Upper Mun River valley which are likely to negatively impact on population health on the whole, and result in more negative effects for those lower in the social hierarchy in particular. With these processes acting in concert, it seems probable that we will observe a health transition during this period. Indeed, the limited bioarchaeological evidence we currently have supports our model, showing an increase in infectious disease prevalence, infant mortality and evidence for systematic stress in the Mun River populations. As new skeletal data from ongoing excavations in the area emerges we anticipate more thorough evaluation of the model will be possible.

6. The applicability of the model beyond the Upper Mun River valley

We have focused our modelling on the probable impacts of socioeconomic change inferred in the Upper Mun River valley. As detailed, many of the observable changes in this area correlate with those seen in the Cambodian Iron Age (O'Reilly et al., 2004; O'Reilly et al., 2006; Wohlfarth et al., 2016). It is important to note, however, that there remained regional variation in both subsistence strategy and social structure across Southeast Asia throughout the Iron Age (King and Norr, 2006; O'Reilly et al., 2006; White, 2011; King et al., 2014). Similarly, the climate change thought to have precipitated changes to social structure in the Upper Mun River valley will have differentially affected different regions of the Southeast Asian mainland (White, 2011). This is likely to result in variable health outcomes in different areas.

For instance, although isotopic evidence in the Upper Mun River valley indicates increasing rice consumption in the Iron Age, the

opposite trend is apparent in isotopic analyses from the more northern Sakhon Nakhon basin (King and Norr, 2006; King et al., 2014). Here, at the sites of Ban Chiang and Ban Na Di, a decline in rice consumption is recorded in the Iron Age, interpreted by King and Norr (2006) as a return to supplementation of the diet with wild resources. Similarly, archaeozoological analysis at Phum Snay in Cambodia shows the retention of a broad-spectrum foraging economy even in the Iron Age (O'Reilly et al., 2006). This hints that social stratification and rice reliance may not be as intrinsically linked as our model using the Upper Mun River valley data suggests, and merits investigation as further excavation is undertaken in these areas.

In northern Vietnam attempts at modelling health changes have tended not to treat Bronze and Iron Age samples separately because of limited sample sizes but nonetheless they note an observable increase in evidence for both general physiological stress and specific infectious diseases during the metal Ages compared with the pre-metal phases (Oxenham et al., 2005). Differential involvement in trade and exchange is a possible causative factor in this. Areas such as northern Vietnam traded regularly with and were colonised in part by Han Chinese (Oxenham et al., 2005). Areas such as the Thai-Malay peninsula with its extensive trade links with India and beyond (Bellina-Pryce and Silapanth, 2006) are also likely to have been exposed to much higher levels of previously unknown infectious diseases.

Much more data is needed in order to fully evaluate the potential links between human health and population growth. Currently palaeo-pathological evidence for health deterioration, and changes to both population size and female fertility, reveals few clear-cut patterns elsewhere in mainland Southeast Asia. Research from Vietnam has suggested a potential deterioration in female oral health associated with increased female fertility during the adoption of agriculture in the Neolithic, rather than during the Iron Age. Specifically, Willis and Oxenham (2013) argue that the increases in fertility associated with the Neolithic demographic transition contributed to poor oral health for females because of physiological changes that occur during pregnancy (Lukacs, 2008; Lukacs, 2010).

Palaeodemographic analysis of cemetery samples has not been extensive in Southeast Asia, and the evidence which does exist is difficult to interpret. Some palaeodemographic measures from Ban Chiang further to the north of the Upper Mun River valley, for instance, suggest an increase in fertility levels, and corresponding population growth in Iron Age layers, while other indicators suggest little in the way of fertility change (Pietrusewsky and Douglas, 2002). Others have used palaeodemographic evidence from mainland Southeast Asia on the whole to suggest a Neolithic change in population dynamics, rather than an Iron Age shift (Bellwood and Oxenham, 2008).

7. Conclusion

Negative health outcomes are often assumed in models of prehistoric socioeconomic change. There are, however, clear and context-specific implications for host-vector dynamics as socioeconomic changes occur. In this paper we have explored how archaeologically-observed changes to social structure, and the intensification of rice agriculture may have affected population health in a specific region of mainland Southeast Asia, the Upper Mun River valley. In the Iron Age Upper Mun River valley we infer that natural changes to the environment acted in concert with human landscape modification to create habitats for newly introduced disease vectors. Population growth and increased trade links would have further contributed to disease transmission, and increases in social inequality are likely to have resulted in health disparities. As a result, our model predicts large-scale negative impacts on human health at this time. Currently there is limited bioarchaeological data from the Iron Age with which to evaluate the model, but what little exists supports it, with evidence for increased infant mortality and the incoming of infectious disease during this period.

While this model is not necessarily uniformly applicable across mainland Southeast Asia it is intended as an example of the value of considering the palaeoepidemiological implications of socioeconomic and environmental change when building bioarchaeological models of human health change. Current excavations in mainland Southeast Asia are doing much to address past issues of a lack of bioarchaeological data, allowing broad-scale models to be built for the first time. By considering how different environmental and social conditions may affect human health and adaptation we will be able to reconceptualise major socioeconomic transitions in this area.

Acknowledgements

This research relies heavily on evidence generated through the Origins of Angkor Project, which would not have been possible without support from the University of Otago, Marsden Fund (e.g. grant numbers 09-UOO-012, 15-UOO-018), and Earthwatch Institute and its Research Corps. We are also indebted to Charles Higham for his comments on the manuscript draft. Several anonymous reviewers have made comments on earlier versions of the manuscript which have helped us to refine our arguments and shaped the current manuscript.

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