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The vowel systems of both Izumo and Tōhoku have centralized /i/ and /u/ and raised /e/ and /o/ so that these vowels are all close together and no longer maximally opposed to each other as they are in other Japanese dialects. In central Izumo and in the Tōhoku region (except for a small area far removed from the Japan Sea coast), this has resulted in mergers between vowels (Figure 5). This geographical distribution, too, suggests that an innovation that originated in Izumo was introduced to the northeast by way of the Japan Sea coast.
The similarities between Izumo and Tōhoku (and to a lesser extent the Noto Peninsula) come in shared sets (see Tables S1 and S2; and De Boer, Reference De Boer and Robbeetsforthcoming), which makes parallel independent development unlikely. It suggests movement of people along the coast in at least two different periods: first from Izumo to the area of the Noto peninsula (present-day Ishikawa and Toyama Prefectures), spreading the set of features listed in Table S1; and later from Izumo to the Tōhoku coast, spreading the set of features listed in Table S2.

Izumo in the Late Yayoi period
Although Izumo (Shimane Prefecture) is nowadays relatively poor and sparsely populated, this was not always the case. Izumo was one of the most powerful confederacies of the Mid and Late Yayoi periods. It was the great rival of Yamato and the focal point of a wide maritime trade network that included the Japan Sea coast, the Ryūkyūs, Kyushu, Korea and China (Torrance, Reference Torrance2016). Izumo formed alliances with other adjacent regions along the Japan Sea coast; Watanabe (Reference Watanabe and Takioto1995) speaks of an ‘Izumo cultural zone’ which he places in the Late Yayoi period.

The corner-projected mound burials typical for Izumo in this period are also found on the Noto Peninsula and in Toyama Prefecture, where they stem from 100–250 AD (Maeda, Reference Maeda2007, p. 6). This makes it likely that the presence of the Izumo vowel system on the Noto Peninsula and in Toyama dates back to the Mid to Late Yayoi period, suggested not only by the congruence of burial types (Figure S3) but also the presence of the Izumo-style vowel mergers in Toyama (Figure 5). The Izumo-style tone systems (Gairin A and Gairin B) had apparently not yet developed, as the tone system of the Noto Peninsula does not share these innovations (Figure 4). Some other innovations have occurred in the local tone systems since then, but not the same as those shared by Izumo and Tōhoku.

The fact that the Izumo tonal innovations are today present in the northern Tōhoku region, and in a mixed form along the southwestern Tōhoku coast, means that migrations from Izumo to these areas must have taken place after the Gairin B innovations developed. The presence of both Gairin A and Gairin B in the northeast may indicate migrations in different periods. It may also be related to different points of departure from Izumo, as political and economic prominence in Izumo fluctuated historically between the eastern and western parts (Piggott, Reference Piggott1989; Torrance, Reference Torrance2016). The Gairin B tonal innovation is most advanced in western Izumo (Hirako, Reference Hirako2017), meaning that it most likely started there and only gradually spread to eastern Izumo. In the Tōhoku region too, the Gairin B system was most likely present in a smaller area in earlier times. The tone shifts typical of this system continue to spread to adjacent areas: recent fieldwork (Boiko, Reference Boiko2018) has shown that, by now, the Gairin A area on the Shimokita Peninsula has disappeared, and instead, the Gairin B tone shifts have been adopted.

Looking for the time of the migrations
Exploring the similarity of the Izumo and Tōhoku dialects, it is necessary to examine opportunities for migration of Izumo dialect speakers into the northeast. Although the very first forays of rice farmers from western Japan to the northern tip of Honshū already took place in the Middle Yayoi period, these settlements were later abandoned. The full ‘Yayoi package’ reached the coastal areas of Ishikawa, Toyama and western Niigata as early as 300 BC. It is possible that rice farmers settling along the coast departed from Izumo, but there is no direct evidence for this. Clear evidence of influence from Izumo is more recent: the Izumo-style corner-projected burial mounds in Ishikawa and Toyama date from 100–250 AD. While the full Yayoi ‘package’ appeared briefly in the central mountains (Figure 3: #1, Chūbu district), it also intruded into western Gunma Prefecture at this time. The ceramics accompanying the irrigated-rice culture package derived from the Jōetsu Basin in western Niigata and spread through the river valleys of Nagano Prefecture into western Gunma (Baba, Reference Baba2008).

It is unlikely that the rice farmers who moved inland from western Niigata spoke a dialect with the features typical of Izumo. Linguistic influence from Izumo in the form of migration was not only later, it also may have been initially limited to Ishikawa (i.e. the Noto Peninsula) and Toyama, as Izumo-style mound burials have not been found in Niigata. If there was already some Izumo influence on the dialect of Niigata at the time when rice farmers moved inland to Gunma, it would have been on the vowel system only. The tonal innovations in the Izumo dialect had not yet occurred, judging from the fact that these are not present in the dialects of Ishikawa and Toyama. What applies to the dialect of Niigata also applies to Nagano (Chūbu) and Gunma (West Kantō), which was settled from Niigata, and to Tochigi and Ibaraki (East Kantō), which were all settled overland. If the Tōhoku region was entirely settled overland from these areas, the similarities with the Izumo dialect remain unexplained.

Can migration of Izumo dialect speakers into Tōhoku in later periods explain the similarities between the dialects? Piggott (Reference Piggott1989) examined the history of Izumo throughout the Late Yayoi and Kofun periods, distinguishing between eastern and western Izumo. Izumo's unique mound-burial culture of Late Yayoi gave way to cultural and political inroads from Kibi to the south and Yamato itself, first into western Izumo. The chieftains of eastern Izumo maintained trade relations with northern Koshi (western Niigata) into the sixth century, but by the 540s the Izumo chieftains had all allied with Yamato (Piggott, Reference Piggott1989, pp. 59–60). Torrance (Reference Torrance2016, p. 4), on the other hand, argues that Izumo remained an important and independent presence along the Japan Sea coast, at least until the late sixth or early seventh century.

Under the circumstances it is possible that groups from Izumo, avoiding Yamato pressure, would migrate northward along the Japan Sea coast. Diffusion into Tōhoku through mountain basins and over passes could have taken place, but were the numbers enough for linguistic change throughout Tōhoku? The mixed nature (and collapse) of the tone systems in south and central Tōhoku suggest that there was some influence but not enough to determine the dialect. The linguistic influence in northern Tōhoku, however, was much stronger.

Hudson (Reference Hudson, Finlayson and Warren2017) mentions that until the fifth century the pottery types of the northern Tōhoku region and Hokkaidō were identical. From the fifth to the seventh centuries, however, this pottery type starts to disappear from northern Tōhoku, and is from then on restricted to Hokkaidō only. This development has been taken to mean that the Epi-Jōmon population among the emishi who, from the evidence of place-names, must have spoken an Ainu-related language, moved away from northern Honshū into Hokkaidō in that period. According to Matsumoto (Reference Matsumoto2018, p. 158), after the late fifth century there are no traces of habitation in the northern Tōhoku for about a century, after which a new population arrives in the late sixth century. The new arrivals were archeologically indistinguishable from Kofun cultures elsewhere in Japan (Matsumoto, Reference Matsumoto2018, p. 159).

If the people arriving in the late sixth century were from Izumo, then northern Tōhoku would have been sparsely populated at that time, as the Epi-Jōmon population had for a large part moved away. That means that the new population could rapidly spread out over the entire area, which would explain why the Izumo-style tone system was preserved well there, in contrast to the situation in south and central Tōhoku. In south and central Tōhoku there was interference not only from other dialects but most likely also from languages of the local Epi-Jomon population, who were no longer hunter–gatherers but settled farmers by then. The relatively low internal diversity of the Tōhoku dialects may be attributed to the overall later spread of Japanese to the northeast compared with other areas of mainland Japan (Inoue, Reference Inoue1992).

Migration of people from Izumo to the northeast may even have left a genetic trace. Saitō (Reference Saitō2017, p. 127) uses a principal component analysis (PCA), in which nuclear DNA from modern Tōhoku individuals is compared with nuclear DNA from modern Ainu, Ryūkyūan and mainland Japanese individuals (Figure S4). A PCA simplifies complex datasets, creating summaries that emphasize the components explaining the maximum amount of variance. In population genetics, this method is often a quick and useful tool to examine population relationships, as the maximum amount of variance is often attributed to shared population history. However, it is not a formal test of shared ancestry.

Saitō concludes that, in contrast to Ryūkyūans, the Tōhoku population has very low traces of Ainu-related ancestry, which was used as a proxy for Jōmon-related ancestry. He also includes a PCA showing the position of Izumo individuals relative to Okinawan and Kantō individuals (Saitō, Reference Saitō2017, p. 155), and he remarks upon the fact that these individuals occupy the same relative position as the Tōhoku individuals (Figure S4), namely they are to the lower right of the average Mainland Japanese individual.

It is better to be cautious in drawing conclusions until more detailed comparisons from different areas of Japan are available, but the initial results are definitely interesting in light of the dialectal similarities between Izumo and the Tōhoku region.