Although domestic pigs play an important role in traditional food resources in the Ryukyu Islands, southern Japan, the origin of domestic pigs in the Ryukyu Islands is not clear yet. From historical evidence, the oldest date for the introduction of domestic pigs to the Ryukyu Islands was in the 14th century AD. It has been believed that there were no domestic pigs in Ryukyu before this introduction (earlier than 14th century AD), rather people hunted Ryukyu wild boar, one of the subspecies of wild boar that inhabits the Ryukyu Islands. Recent archaeological and zooarchaeological studies in the Ryukyu Islands and surrounding areas, however, suggest that there is a possibility that wild boar or domestic pigs (Sus) may have been introduced to the Ryukyu Islands in the prehistoric times, which is earlier than the 12th century AD according to archaeological chronology. In this thesis, I analyzed tooth samples and ancient DNA (aDNA) derived from bone of Sus excavated from prehistoric sites in the Ryukyu Islands as well as the modern Ryukyu wild boar samples, and investigated morphologically and molecular phylogenetically whether external introduction of Sus into the Ryukyu Islands took place during prehistoric times.
This thesis consists of six chapters.
Chapter one describes previous archaeological and molecular phylogenetic studies concerning the origin and dispersal over the world of various domestic animals including domestic pigs. In this chapter, I also reviewed previous studies on the formation of the Ryukyu Islands as well as prehistoric culture of the Ryukyu.
Chapter two describes archaeological and modern samples, and methods of analyses used in this thesis. Based on geological and archaeological knowledge, the Ryukyu Islands can be divided into three cultural regions, North, Central, and South regions. Of these three cultural regions, for the following analyses, I used archaeological samples from prehistoric sites in Central and South cultural regions. Furthermore, nucleotide sequences of mtDNA D-loop region of modern Ryukyu wild boar were determined to investigate the extent of genetic variation among present population of Ryukyu wild boar.
In Chapter three, I analyzed morphological and molecular phylogenetic characteristics of Sus tooth samples and aDNA from bones excavated from the sites in Central region, including Noguni shell middens (ca.7200 - 4400 years ago) on Okinawa main Island. Measurements of lower third molars from Noguni shell middens were compared with those of Sus remains from later sites in the Okinawa Islands (ca.4800 - 1400 years ago) as well as modern Ryukyu wild boar. Based on measurements of lower third molars, Sus samples from the Noguni shell middens were distinctly smaller than those from modern Ryukyu wild boar and other ancient sites in the Okinawa Islands. In addition to morphological analysis, nucleotide sequences of ancient mtDNA D-loop region from mandibles of the Noguni shell middens were compared with those of Sus in other parts of the world collected from a database. Phylogenetic analysis using aDNA sequence types showed that some sequence types from the Noguni shell middens made a different cluster from modern Ryukyu wild boar, suggesting a presence of the different genetic Sus lineage from modern Ryukyu wild boar at that time.
In Chapter four, aDNA analysis was carried out by using Sus bone samples excavated from Ohtabaru site (ca.4100 - 3800 years ago) and Kanda shell midden (ca.1600 - 900 years ago) in Ishigaki Island, Arafu site (ca.2800 - 800 years ago) and Nagabaka site (ca.1900 - 1400 years ago) in Miyako Island, which belonged to South cultural region. All aDNA sequence types from prehistoric sites in Ishigaki Island were genetically close or identical to those of modern Ryukyu wild boar. However, sequence types from Arafu site and Nagabaka site were in different lineages from modern Ryukyu wild boar but had rather close relationship to other Asian Sus lineages: the similar situation was observed as in Noguni samples.
In Chapter five, I investigated the extent of genetic variation among present population of Ryukyu wild boar to find out whether the different lineage detected from ancient samples still exist among the present populations. Ryukyu wild boar inhabits seven islands in the Ryukyu Islands. Phylogenetic studies based on the mtDNA analysis of 113 Individuals from six of the seven islands show all individuals are genetically close to each other, and no sequence type is either identical or similar to other Asian Sus lineages.
Chapter six discusses the possibility of the external introduction of Sus into the prehistoric Ryukyu Islands. In the present study, some Sus samples from prehistoric sites in the Central (ca.7200 - 800 years ago) and South cultural regions (ca.2000 years ago) had different morphological / genetic characteristics from modern Ryukyu wild boar. Concerning the origin of these Sus population from the prehistoric sites in Ryukyu, I propose two possible hypotheses: first, there were at least two genetic lineages of wild boar inhabited the prehistoric Ryukyu Islands; second, introduction of Sus to the Ryukyu Islands by human took place during prehistoric times. I distinguish these hypotheses as follows.
In the case of former hypothesis, a) It is very likely that some surviving population has different genetic characteristic from those of other habitats (islands). However, multiple lineages of wild boar were not found in 113 individuals from present populations. b) Furthermore, if multiple lineages of wild boar really existed in the prehistoric Ryukyu Islands, they must have migrated from Asian Continent to Ryukyu at the time when land bridge connected these regions, which is earlier than 80,000 years ago. Based on estimation using simulation, probability for coexistence of multiple Sus lineages for the period of 75,000 years was calculated and it reveals to be lower than 1%. These results indicate that it is unlikely that multiple lineages of wild boar coexisted in the prehistoric Ryukyu Islands.
Thus, the latter hypothesis that prehistoric introduction of Sus into the Ryukyu Islands by human was supported by my study. It has been suggested that the South cultural region had no archaeological links with North and Central regions until historic time, ca. 12th century AD. This archaeological evidence infers a possibility of more than one introduction pathways of Sus from outside of the Ryukyu directly to the Central or South Cultural regions during prehistoric times. In the case of Noguni shell middens, some cultural factors of prehistoric Central regions were considered to be related to Jomon culture in Kyushu. This archaeological evidence suggests that Sus population was introduced from main land Japan or from the Asian Continent via Kyushu region. In this case the transported Sus cannot be hunted wild boar because the size of wild boar in both mainland Japan and Asian Continent is much larger than those of Noguni shell middens. There is a possibility that lower third molars of Sus from Noguni shell middens were reduced in size as the consequence of domestication. In contrast, prehistoric culture of South cultural region including Arafu and Nagabaka sites were considered to be related to those of Island Southeast Asia such as the Philippines as well as Micronesia. Since it is revealed that prehistoric human dispersal and peopling in Island Southeast Asia and Oceania was accompanied by domestic pigs and other animals, introduction of Sus population to South cultural region of Ryukyu might be involved in such prehistoric interaction of humans.
In this thesis I conclude multiple pathways of Sus introduction to the Ryukyu Islands existed during prehistoric times. Furthermore, present study indicates the possibility that cultural interaction and movement of prehistoric human took place between the Ryukyu Islands and surrounding areas, accompanied by Sus.
Ancient DNA を用いた先史時代 琉球列島へのイノシシ・家畜ブタ導入に関する動物考古学的研究
運用要項