Varieties of prehistoric human populations in Japan across geographical regions and chronological time periods were investigated in light of dietary habits. Using the new analytical technique, nitrogen isotope analysis of individual amino acid, I successfully answered some of the questions previously raised by archaeological and anthropological studies.

It has been pointed out that this technique has the following advantages over the traditional isotope analysis of bulk materials: (I) Exogenous substances other than the objective compound can be eliminated; (II) Physiological and biochemical processes in which each compound has experienced could be investigated. Owing to the above advantage, the high resolution trophic level analysis of organisms in modern ecosystems and its application to the archaeological human remains based on this technique now provide us more precise and detailed picture of the dietary habits for ancient humans.

In this study, I aimed (i) to explore special utilities of the nitrogen isotope analysis of amino acids in bone for human palaeodietary research and (ii) to reconstruct prehistoric Japanese diets in higher resolution in order to examine several archaeological and anthropological issues. Towards these ends, I focus on four case studies and try to investigate dietary habits of: the coastal Jomon population (ca. 6,000-5,300 cal BP); the Okhotsk Cukture populations (ca. AD. 550-1200); the inland Jomon populations (ca. 9,700-9,100 cal BP and 5,000-3,000 BP); and the late Pleistocene Ishigaki Islanders (ca. 20,000-16,000 BP).

With respect to the methodological aspects in human dietary reconstruction, the new findings in this study are as follows: (1) it is possible to clearly separate marine and terrestrial ecosystems based on the nitrogen isotope composition of phenylalanine, though there could be some exceptions as shown in the case of the Okhotsk Culture sites where unexpectedly large variation was observed in this indicator for faunal species; (2) the nitrogen isotope composition of phenylalanine could serve as an indicator for marine protein consumption by ancient humans; (3) the TLAA (amino acid trophic level) estimates can be applied to archaeological faunal remains both from marine and terrestrial ecosystems; (4) the TLAA estimates could be applied to ancient humans who strongly adapted to terrestrial ecosystems and it is possible to evaluate their extent of carnivory based on this value. The latter three findings could be useful for quantitative evaluation of ancient human diets. In addition to the above findings, (5) archaeological samples with very poorly preserved conditions in this study can show reasonable With respect to the methodological aspects in human dietary reconstruction, the new findings in this study are as follows: (1) it is possible to clearly separate marine and terrestrial ecosystems based on the nitrogen isotope composition of phenylalanine, though there could be some exceptions as shown in the case of the Okhotsk Culture sites where unexpectedly large variation was observed in this indicator for faunal species; (2) the nitrogen isotope composition of phenylalanine could serve as an indicator for marine protein consumption by ancient humans; (3) the TLAA (amino acid trophic level) estimates can be applied to archaeological faunal remains both from marine and terrestrial ecosystems; (4) the TLAA estimates could be applied to ancient humans who strongly adapted to terrestrial ecosystems and it is possible to evaluate their extent of carnivory based on this value. The latter three findings could be useful for quantitative evaluation of ancient human diets. In addition to the above findings, (5) archaeological samples with very poorly preserved conditions in this study can show reasonable TM15N values at least for glutamic acid and phenylalanine. This suggests that the poorly preserved samples in light of established criteria (i.e., the C/N atomic ratio) might be still applicable for palaeodietary study δ15N values at least for glutamic acid and phenylalanine. This suggests that the poorly preserved samples in light of established criteria (i.e., the C/N atomic ratio) might be still applicable for palaeodietary study.

With respect to the archaeological and anthropological questions, the new findings are as follows: (a) some regional differences in dietary habits of the Okhotsk Culture peoples, especially between northern (Rebun Island) and eastern (Abashiri) Hokkaido, were observed for the first time; (b) the Jomon populations in inland central Honshu may have had more carnivorous diets (ca. 54-70% dietary animal protein contribution) than is traditionally assumed; (c) the late Pleistocene human population in Ishigaki Island clearly adapted to terrestrial ecosystem and exploited not much amount of marine food resources (less than 10%). The last finding is not consistent with previously proposed hypothetical models about the adaptations to coastal and island environments for Pleistocene human populations and may require further examinations of these models at least around Japan.

The above new findings suggest that the nitrogen isotope analysis of individual amino acids in bone is quite useful to precisely evaluate dietary habits of prehistoric human populations.

内藤裕一氏によって提出された博士学位申請論文「Dietary reconstruction of prehistoric Japanese populations based on nitrogen isotopic composition of individual amino acids in bone collagen」(骨コラーゲン構成アミノ酸の窒素同位体比を用いた先史日本人集団の食性復元)は、過去の骨資料に残存する有機物に着目し、従来タンパク質(コラーゲン)全体で行われてきた同位体分析を、個別アミノ酸における窒素同位体分析に進化させた画期的な研究であると評価された。海洋生態学であらたに開発されたアミノ酸の窒素同位体比分析の手法を、骨から抽出したコラーゲンに応用するための前処理手法を自ら開発し、その有効性を実際に北黄金貝塚(縄文時代前期)、北村貝塚(縄文時代中期)、栃原洞穴(縄文時代早期)、浜中遺跡(オホーツク文化)、モヨロ貝塚(オホーツク文化)から出土した人骨および各種動物骨から抽出したアミノ酸を分析し、生態学的なニッチの異なる動物種について現在の生態系から期待される関係が、遺跡から出土する動物骨でも観察されることを、とくにグルタミン酸とフェニルアラニンという2つのアミノ酸に着目して示した。そのうえで、先史時代人のアミノ酸における窒素同位体比の分析結果を比較検討することで、従来のコラーゲン全体における分析よりもはるかに高精度で食性を復元できることを示した。

特に、海洋生態系に強く依存している北黄金貝塚から出土した縄文時代前期の人骨群では約8割のタンパク質が海洋生態系から由来するという定量的に評価する事に成功した。また、内陸に位置する栃原洞穴および北村遺跡から出土した縄文時代人骨では、陸上生態系の一部として理解することが可能であり、その栄養段階が2.8程度であり、かなり肉食性が強かった可能性が示された。これは、従来のコラーゲン全体における炭素・窒素同位体比では示すことができなかった結果であり、また考古学的にも検証する事ができなかった新たな情報であり、日本列島の先史時代を理解するために非常に重要な知見である。さらに石垣島の白保竿根田原洞穴遺跡から出土した更新世人骨にもこの方法を応用し、島嶼に生息したにも関わらず、陸上資源を多く利用していた可能性を示した。

申請者によって開発されたアミノ酸における窒素同位体比の分析法は、これまでコラーゲンが分解してしまい食性復元が困難だった更新世人骨なでにも応用することが可能であると期待され、人類が進化の過程でどのような適応戦略をもっていたかを知るための画期的なツールとなると考えられる。

なお、本論文は、米田穣、大河内直彦、ノア・ホンク、力石嘉人、柴田泰之、百々幸雄、石田肇、天野哲也、小野裕子、向井人史との共同研究であるが、論文提出者が主体となって分析及び検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(生命科学)の学位を授与できると認める。