John Whitman’s paper brings together linguistic and archaeological evidence for the spread of agriculture in Korea and Japan. The image (left) is from Whitman, and summarizes the archaeological picture. He makes a good case that this fits with known linguistic and epigraphic evidence.
Whitman, Northeast Asian Linguistic Ecology and the Advent of Rice Agriculture in Korea and Japan John December 2011, Volume 4, Issue 3-4, pp 149-158 14 Jan 2012 Preview here.
The languages of Northeast Asia show evidence of dispersal from south to north, consistent with the hypothesis that agriculture spread north and east from the vicinity of Liaoning, beginning with the millets approximately 5500 BP. Wet rice agriculture in Korea and Japan results from a later spread, also beginning in Shandong, crossing via the Liaodong peninsula and reaching the Korean peninsula around 1500 BCE. This dispersal is associated with the Mumun archaeological culture after 1500 BCE in the Korean peninsula and the Yayoi culture after 950 BCE in the Japanese archipelago. From a linguistic standpoint, it is associated with the entry of the Japonic language family, first into the Korean peninsula, subsequently into the Japanese archipelago. The arrival of Koreanic is associated with the advent of the Korean-style bronze dagger culture and a temporary hiatus in wet rice agriculture sites around 300 BCE. Both Koreanic and Japonic are relatively shallow language families, with Koreanic the shallower of the two, consistent with the chronology above. The gap between the earliest linguistically motivated dates for these language families and the archaeological events is the result of a linguistic founders effect, providing further evidence for demic diffusion as a source for their distribution.
Another paper that could illuminate, support or tie in with the above theory, is this genetic analysis of the connection between Japan and this cluster of states on a north-south cline.
Jieming, Chen, et al., Genetic Structure of the Han Chinese Population Revealed by Genome-wide SNP Variation
… our study revealed a one-dimensional “north-south” population structure and a close correlation between geography and the genetic structure of the Han Chinese. The north-south population structure is consistent with the historical migration pattern of the Han Chinese population.
… Collectively, these studies support the hypothesis of an initial entry of modern humans into China from southeast Asia, followed by later expansions, chiefly those stemming from the development of agriculture in China’s two main centers of domestication (rice in the south and millet in the north).11,12 Su et al., Chu et al., and the many investigations of Cavalli-Sforza et al. (summarized in 13) have observed largely distinct clusters of southern and northern populations in phylogenetic trees of the Han Chinese sampled throughout the country.6,7
PC4 (Figure S1C) accounts for the small variation between the Chinese and the Japanese…With only the East Asian populations considered, the Japanese (JPT) cluster splits from the Han Chinese cluster in only PC2, which accounts for only 0.18% of the total variation (Figure S2). PC1 suggests a relationship between the Japanese and the Chinese from the northern provinces of China, as geography would suggest.
Restricting our PCA to only the Han Chinese samples, we observed a tendency for the samples from the same province to be clustered together, and various province clusters distributed along PC1 in the following order: Liaoning, Hebei, Shandong, Henan, Jiangsu, Anhui, Hubei, Sichuan, Hunan, and Guangdong (Figure 1A). In comparison to the geographic locations of the ten provinces, the subpopulation structure of the Han Chinese population along PC1 is characterized by a one-dimensional north-south trend (Figure 2)…
There seems to be a loosely defined central region, between the two poles of the very north (red) and south (green), characterized by the mingling of the individuals from Hubei (cyan) and Hunan (dark green), as well as Jiangsu, Anhui, and Sichuan. This one-dimensional structure of the Han Chinese population is clearly characterized by a continuous genetic gradient along a north-south geographical axis, rather than a distinct clustering of northern and southern samples….
With the assumption of two underlying source populations (K = 2), the clusters were anchored by JPT, segregating the more northern provinces of Liaoning, Hebei, Shandong, Henan, Jiangsu, and Anhui from the more southern provinces of Sichuan, Hubei, Hunan, and Guangdong. The very northern provinces of Liaoning, Hebei, Shandong, and Henan were denoted by the sharing of large yellow segments and small brown segments with JPT.
The provinces are arranged from north to south, with JPT on the extreme left, representing the northernmost locality, to Liaoning, the northernmost province of China investigated in this study… In K = 2, the northern provinces are clearly anchored by the JPT, with a huge membership of northern samples (represented by the yellow segment). The northern membership decreases gradually down to the southern provinces, which show a strong membership of southern samples (represented by the brown segment). At K = 3, JPT is clearly separated from the Han Chinese samples. The analysis revealed a demarcation of north-central-south similar to that shown by Figure 2
The inferred north-south pattern in the genetic structure analyses suggests a primary north-south migratory pattern in China. This ties in very well with historical records indicating that the Huaxia tribes in northern China, the ancient ancestors of the Han Chinese, embarked on a long period of continuous southward expansion as a result of war and famine over the past two millenia.29 Furthermore, a broad sampling of the Han Chinese across various regions of China and the employment of genome-wide SNP-variation data allowed the current study to define the population structure at a finer scale. First, our study clearly showed that the one-dimensional north-south structure of the Han Chinese population is characterized by a continuous gradient, instead of distinct subpopulation clusters. This seems to support the hypothesis by Wen et al. for a demic diffusion of the Han Chinese.30 Second, our study was also able to reveal some fine-scale subpopulation structure within local populations of language from the same region.