A health crisis during the Japanese Medieval Period: A new paleodemographic perspective
Introduction
Secular trends in disease and life history parameters provide information about the impact of social developments on the health of past human populations. However, major climate change is also thought to have a negative impact on human health (Steckel, 2004). The historic climate event known as “the Little Ice Age” (˜13th –19th centuries) or LIA is one of those climatic events. Recently published annual tree-ring chronologies and sea surface temperature measurements, based on alkenones (i.e., cold-sensitive phytoplankton ketones) from coastal sedimentary cores, have also demonstrated the occurrence of the LIA in Asia (Cook et al., 2013), including Japan (circa AD 1440–1730) (Kawahata et al., 2017). The LIA climate change in Japan was characterized by low summer and winter temperatures, small precipitation amounts in winter, a prolonged rainy “Baiu” season, and short summer (Yamamoto, 1971; Mizukoshi, 1993; Yoshimura, 1995; Aono and Kazui, 2008). The possible health impacts of LIA global climate change at this time have enabled a hypothesis that encompasses various human groups and provides the opportunity to explain the physical condition of populations in various archaeological settings as well as adaptation strategies to local environments in Japan. This study utilizes a Japanese skeletal series to address these issues. Bioarchaeologists in this country have been interested in secular trends in height (Hiramoto, 1972; Wada, 1999), dental caries (Sakura, 1964), degenerative joint diseases (Suzuki et al., 2016), linear enamel hypoplasia (Yamamoto, 1988), the occurrence of cribra orbitalia (Hirata, 1991), and violence (Nagaoka et al., 2009, 2010; Nagaoka, 2012) in populations that span the prehistoric to modern periods. Although this body of previous research has demonstrated that Medieval populations (i.e., 12th to 16th centuries AD) tend to be short in stature on average and exhibit prevalent weapon-related traumas, an integrative theory which can explain the changes seen in this period has not been postulated due to the paucity of credible evidence. To address this, we have studied a new skeletal series that encompasses time periods that predate and overlap the LIA climate variables of Japan in various regions across Japan; we use this series to reconstruct the age-at-death structure of the skeletal populations as a proxy for health, and test the hypothesis that the global climate change coincided with demographic fluctuations in ancient Japanese populations.
Section snippets
The samples
The data used in this study includes observations of 810 adult human skeletons aged 15 years and older that derive from the Japanese Yayoi (dated to between the 5th century BC and the 3rd century AD), Medieval (dated to between the 12th century AD and the 16th century AD), and Edo (dated to between the 17th century AD and the 19th century AD) periods (Fig. 1; Table 1). All of the data presented in this paper are newly collected, with the exception of that from the Yuigahama Chusei Shudan Bochi
Assumption of uniformitarianism
Demographic estimates based on the skeletal sample used in this study were based on the uniformitarian assumption that biological processes related to aging and sexual dimorphism were the same in the past as they are now (Weiss, 1973; Howell, 1976; Hoppa, 2002; Chamberlain, 2006).
Adult age-at-death estimates
It is generally argued that age-at-death estimates for subadult individuals are more accurate than those for adults (Murray and Murray, 1991;Bocquet-Appel and Masset, 1996 Milner et al., 2000). Complicating the
Results
The data presented in Fig. 2 and Table 2 highlight the number of individuals in each of the Buckberry and Chamberlain (2002) seven auricular surface stages recorded in our samples from the Yayoi Period to the Edo Period. Our use of Fisher’s exact test reveals differences in the composition of stages between chronological ages and between urban and rural areas (Table 3). We are able to conclude that: (1) the Yayoi Period in the northwestern Kyushu was characterized by more deaths in high stages
Secular trends
The results of this study demonstrate that paleodemography is a useful method for understanding demographic phenomena in past human populations. In this case, data show that life length, represented by adult age-at-death structure, varies between the samples surveyed in this research. In the first place, the demographic tendencies detected in our Japanese skeletal series are consistent with earlier studies by the first author because they show that Medieval Period populations are characterized
Ethical standards
The materials and methods used in this study did not include anything which needed to be approved by the ethical committee of our institute.
Conflict of interest
None.
Acknowledgments
We thank Prof. Maria O. Smith for her helpful comments. This study is partially supported by JSPS Kakenhi Grant Numbers 26350377 and 15K07241.
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