Compilation of seismic reflection profiles, rock sample analyses and manned and non manned diving observations revealed that land bridges, connecting the Ryukyu Arc to the Chinese continent, have appeared in two stages since the late Pliocene. The first stage occurred before the deposition of Pleistocene Ryukyu Limestone and ceased in the early stages of deposition, while the second was apparent during the middle and late stages of the Ryukyu Limestone. The land bridge in the first stage may have appeared between 2.0-1.0 Ma and is well identified by the unconformity between Shimajiri and Ryukyu Groups before the deposition of Ryukyu Limestone. The land bridges may have appeared two or three times in the second stage during or after deposition of Ryukyu Limestone between 0.4-0.02 Ma. The last land bridge may have appeared during the Würm glacial stage in the late Pleistocene, connecting the Chinese continent through Taiwan, Okinawa and Amami-Oshima to the Kyushu main land. This supports the hypothesis that the Minatogawa Man migrated to Nansei-shoto Islands passing through the land bridge in the Late Pleistocene.
Precise topographic features at the two major structural boundaries in the Ryukyu Arc, Tokara Gap and Kerama Gap, revealed that there exists a ridge feature of the Ryukyu Arc as a saddle topography. The feature supports the idea recently proposed that a landbridge connecting the Chinese continent, Ryukyu Islands and Japanese Islands, is thought to have been subsided since Late Pleistocene. The saddle is offset by large-scale left-lateral normal faults both of which are related to the opening of the Okinawa Trough. These topographic features and geological structure together with the gravity anomaly and regional tectonics in the Ryukyu area suggest that a north-south tensional stress regime, due to the rolling buck of the Philippine Sea Plate at the Ryukyu Trench, has dominated in this region since last 20, 000 years.
It is of great interest to know how much the land expanded towards the ocean as the result of the sea-level decrease during the Würm glacial stage. The existence of land bridges, that plays an important role in transporting human, animals and plants between the Eurasian Continent and the Japanese Islands during the Last Glacial, is still under debate. Geographical and geological studies have been performed to determine the location of shorelines at the Last Glacial maximum. Mammalian fossils collected from the sea bottom of continental shelf and along island arcs can provide decisive evidence of such expanded land areas, if the ages of those mammalian fossils agree with the accepted 14C age (around 18, 000 yr BP) of the Last Glacial maximum. We have conducted 14C dating, with a Tandetron accelerator mass spectrometer (AMS) at Nagoya University, on carbon from collagen fractions extracted from five mammalian fossil samples : (1) a whale bone collected from the Kerama Gap at a depth of 550 m; (2) a deer bone from the continental shelf in the East China Sea, at a depth of 122 m; (3) a water-buffalo horn core from the continental shelf in the East China Sea, at a depth of 100 m; (4) an elephant molar; and (5) a water-buffalo bone from the Penfu Channel between Taiwan and Continental China, at a depth of around 120140 m. Fossil samples, of 2 to 9 g, were demineralized with 1.2 N HCl in a cellulose tube to extract collagen. After demineralization, HCl was completely removed from the tube by dialysis. Then the solution fraction in the tube was separated by centrifuge, and freeze-dried to get a solution-collagen fraction. Solid remains were put in a Pyrex tube with 20 ml of distilled water and heated at 90 t for 10 hours to extract the water soluble gelatin-collagen fraction. Collagen was oxidized, using CuO to produce CO2, in a Vycor tube at 950 °C for an hour. A small aliquot of the CO2 was analyzed for its' stable carbon isotope ratio, δ13 CPDB, using a triplecollector mass spectrometer (MAT-252). To produce graphite, the remaining CO2 was reduced by hydrogen, with an Fe-powder catalyst, in a Vycor tube at 650 °C for several hours. The sample graphite was analyzed for its' 14C/13C ratio, relative to that of an oxalic acid standard (NBS-SRM-4990). To obtain reliable 14C dates for collagen fractions extracted from the fossil samples, the collagen fractions must be preserved from weathering in nature. However, experimental yields of collagen fractions were very low, except for sample no. 2, ranging from 0.01 to 0.11 %, presumably as a result of weathering. CO2 yields from the collagen fractions ranged from 2.2 to 17.4 %, considerably lower than the 4142 % for gelatin collagen extracted from fresh bone samples. This strongly suggests that the samples have suffered from contamination by other organic materials with low carbon contents and not only from weathering. Thus the 14C dates of 14, 640 to 19, 980 yr BP obtained for samples nos. 3, 4, and 5 should be considered unreliable, though these 14C dates are consistent with the age of the Last Glacial maximum. We suspect the 14C dates for samples nos. 3, 4 and 5 are considerably younger than the actual dates, as a result of sample contamination by younger carbon. The CO2 yield from the mixture of both collagen fractions was only 0.12 mg of carbon for sample no.1, being insufficient for 14C dating even with the Tandetron AMS. The yield of gelatin collagen of 4.38 % and the CO2 yield of 32.8-42.7 % for sample no. 2 were reasonably high, enough to give a reliable 14C date, of 25, 750 ± 220 yr BP.
In the Quaternary stratigraphy, nannofossils serve superior transoceanic datum planes in open sea and fall tephra give the most reliable marker horizons in and around near shore area. Especially, island arcs in the western margin of the Pacific are suitable area to constract the combined stratigraphy which mediates both datum planes. In the Osaka bay area, a new airport plan was started in 1981. On the course of geological survey, some long cores were taken from the area. In the cores Brs. 56-9, 57-5, 57-9, 57-30 and H-6 are very interesting for the present study. Nakaseko et al. (1984 b) defined the late Pleistocene sequence of off Senshu as the Senshu-oki Group and devided it the lower, Senshu-oki and the upper, Kuhko-shima Formations. Then, they separated the Kuhko-shima Formation to eleven members based on their lithofacies and some kind of microfossil occurrences. Concerned to nannofossil biostratigraphy, three nannofossil events were detected in the sequence and they were the last occurrence (LO) of Pseudoemiliania lacunosa (Kamptner) Gartner, the first appearance (FA) of Emiliania huxleyi (Lohmann) Hay and Mohler and the start of acme stage (s.a.) of Emiliania huxleyi in ascending order. In the transoceanic correlation, LO of Pseudoemiliania lacunosa is estimated at 440 ka, FA of Emiliania huxleyi 270 ka and s.a. of Emiliania huxleyi 70 ka (Gartner, 1977). On the other hand six conspicuous marker tephra are defined in the Kuhko-shima Formation based on volcanic glass major chemical composition analyzed with an energy dispersive microspectrometry (EDS). They are Hatchoike, Sakura, Sugawara, V-7, AT and Ah in ascending order. These tephra are found as a volcanic glass-rich layers and spread widely in the core samples in and around the Osaka Bay. Hatchoike is in Ma5 and Sakura is in the lower part of Ma7 clay beds of the group. Both tephra are reliable markers in the Upper Formation of the group which were defined in land sequence of Kinki. The fall ages of Hatcho-ike and Sakura are estimated at ca. 500 ka and 400 ka respectively. Machida and Arai (1992) correlated V-7 to Ata which ejected in 90-110 ka from Ata caldera of southern Kyushu. Aira (AT) and Akahoya (Ah) are reported by many authors and their fall age are estimated at 24 ka and 6.3 ka respectively. In the present area, following nannofossil events and tephra falls are listed in ascending order : LO of Pseudoemiliania lacunosa, falls of V-15, Sakura, V-14, V-10, Sugawara, V-8, FA of Emiliania huxleyi, s.a. of Emiliania huxleyi, falls of AT and Ah. In general age estimation of these events are correspond with international correlation and established tephrostratigraphy in Japan.
Paleoclimatic and vegetational changes during the Pleistocene and Holocene in the Ryukyu Islands were inferred from pollen assemblages. Pollen analyses of some early Pleistocene to Holocene sections in the Ryukyu Islands show several drastic vegetational changes with climatically meaningful variations. A mixed pollen assemblage containing the warm and cool temperate trees such as Cryptomeria, Abies and Tsuga, and the tropical-subtropical trees including Dacrydium, Liquidambar, , Lagerstroemia and Sapium occurred in the upper Pliocene to lower Pleistocene Shinzato Formation and the basal part of the lower Pleistocene Kunigami gravel formation. These pollen assemblages strongly support that there were high mountains over 1, 000 to 1, 500 m above the sea-level in the Ryukyu Islands in the late Pliocene to early Pleistocene. The warm and cool temperate forests in the mountainous areas were disappeared possibly by the subsidence of the basement rocks of the Islands in relation with the opening of the Okinawa Trough. Invasion of evergreen broad-leaved trees to the Ryukyu Islands in the early Pleistocene was indicated by the first occurrence of evergreen broad-leaved tree pollen in the lower Pleistocene Kunigami Gravel Formation. The pollen assemblages in the last Glacial period (ca. 22, 000 y.B.P) are dominated by the coniferous trees such as Pinus and Podocarpus, indicating that an arid climate was prevalent in the Ryukyu Islands. This evidence is consistent with the wide development of ancient dunes and paleosoil of aeolian sand origin in the islands during the last Glacial period. The pollen sequence of the uppermost Pleistocene to Holocene reveals the change of vegetation and climate from the pine forests of the late Glacial period to the climax forests of evergreen broad-leaved trees of the Climatic Optimum. It is suggested that the climatic condition in subtropical Ryukyu Islands around 20, 000 years B.P. was not so much cool but arid than the climate of the present day.
The richness of fauna diversity is seen on the Japanese Islands, which range through 3, 000 km. When and from where have theanimals immigrated to Japan? Also, for understanding natural history of the Japanese Islands, it is very importantto investigate evolutinary history and origin of animals distributed on each island. While the evolutionary questions have been paleontologically and geologically examined so far, some of them are still unclear. Recent development of molecular phylogenetic study provides a reliable sight to understanding evolutionary history and origin of species. Advantageously, molecular study can analyze phylogeny of living species without fossil data. In Japan, two wildcats, the Iriomote cat and the Tsushima cat, live on the Iriomote Island and the Tsushima Island, respectively, where land bridges between the Asian continent and the Japanese Islands were present in the past time. To know evolution of these wildcats gives us useful information for better understanding of natural history of the Japanese Islands. Moreover, because both the wildcats are now endangered, it is now strongly needed to understand phylogenetic status of them for conservation and management. For the reason, we investigated molecular phylongeny of the two wildcats based on mitochondrial DNA sequences, and revealed that both the wildcats are very closely related to the leopard cat Felis bengalensis, which is widespread throughout Asia. Furthermore, from the DNA data, the Iriomote cat and the Tsushima cat were estimated to have diverged from the continental leopard cat approximately 200, 000 and 100, 000 years before present, respectively. The dates estimated by molecular data were in concordance with formation dates of the Ryukyu Arc and the Tsushima Island, respectively. These results suggest that geographic barrier has led fixation of some unique morphological characters into each cat population, while the two Japanese wildcats are still genetically close to the continental leopard cat.
Is it possible that modern Ryukyuans are the descendants of Palaeolithic Minatogawa Man, and the people of the Shellmound Period, which is partially equivalentto the Jomon Period ? There is a blank period of about 10, 000 years between the Minatogawa population and the population of the Okinawan Shellmound Period. Sakishima prehistoric culture was one of the Southeast Asian prehistoric cultures until the beginning of the Heian Period. A dramatic cultural change occurred in Okinawa after the Shellmound Period, in the Gusuku Period, which began in the 10th to 11th centuries as a result of culturalinfluence from the Mediaeval Period of Kyushu. Darling 13th to 14th centuries the culture of the Gusuku Period also expanded into the Ryukyu Islands from Amami Islands to the Sakishima Islands, and there was a general rapid population increase. Modern Ryukyuans are descended from the populations of the Gusuku Period. Some geneticists and anthropologists insists that modern Ryukyuans possess hereditary blood factors found in northern Asian populations. It is suggested that those factors flowed into the Ryukyuan population in the Gusuku Period.
The author examined fossilized deer bones with forks at one or both ends which were excavated from the Gohezu cave of Ie-jima, Okinawa Prefecture, and whichhave been considered them as the result of shaping by hominids. He decided that they were not artificial, but in a much more advanced stage of bone damage due to chewing by deer himself as a result of osteophagia. So, at present, any archaeological evidences in palaeolithic age are not recognized inOkinawa Prefecture. Recently an amorphous flake industry with wedge shaped tools has been found in the layer, contained AT-tephra dated more than 20, 000 years B.P., at the Yaaya and Kisikawa, Amamioshima Island, Kagoshima Prefecture. The undated same industry, too, is reported from the Amangusuku, Tokunoshima Island, Kagoshima Prefecture. The author asserts that the flake industry in final Upper Pleistocene should been widely distributed throughout the Nansei (Southeastern) Islands of Okinawa and Kagoshima Prefecture. Further, the same flake industry with wedgeshaped tools exists at the Baxian-dong caves (5, 000-15, 000ys. B.P.) in Eastern Taiwan, Lang Rongrien rockshelter (27, 000-37, 000 ys. B.P.) in Southwestern Thailand, and Devil's Lair cave (12, 000-31, 000 ys. B.P.) in Southwestern Australia. The author thinks that the prehistoric people with the flake industry had adapted themselves to coastal environment somewhere around Southeast Asia about 30, 000 years ago, because of findingsof fish bone sand fishing tools from the sites. They had known about the technology how to sail across the sea, and dispersed on each islands northward along the Nansei Islands from the cradle of flake industry in Southeast Asia until 20, 000 years ago.
The present paper describes the physical characteristics of Ocean Mongoloid peoples who are indigenous in the Oceania proper, analyses variation patterns among them, and reconstructs the processes that they colonised and settled the oceanic world, following a brief introduction to the human population history in Oceania. Based on the analysis, their remote origins are discussed. The best example of Ocean Mongoloids is the Polynesian population. Polynesians are very homogenous in physique, language and culture. Linguistic and archaeological evidences so far gathered suggest they differentiated into island groups in the past 3, 000 years from a Lapita-associated ancestoral population. Anthropological data show their uniqueness for the large, muscular and robust body form as well as a hyperostotic growth pattern from Homo sapiens standards, and their Asiatic nature in many genetic characters. Accordingly they should be considered “the unique Asian in South Pacific”, and their origins should be sought in Asia. It is highly probable that the Polynesian phenotype was developed during their dispersals into Oceania as the result of adaptation to the specific oceanic habitats humans ever encountered. The speculation here suggests that the remote origins of Ocean Mongoloids were somewhere in circum-China Sea areas, and that some group associated with Jomon people in Japan should be a candidate for their ancestors.