How Evolutionary Forces, Development Constraints And A Changing Climate Influenced Genetic Drift

Reconstruction of Doigahama Yayoi burial site at Doigahama site anthropological Museum. (Credit: Dr. Hirofumi Takamuku)

It has long been suggested that the relationship between Bergmann (1847) and Allen (1877) ecogeographical principles and geography may be applied to humans. For well over a century, these ecogeographical principles have been used by biological anthropologists to test hypotheses about variation in human body shape and size.

As such, we explored variations in body and limb proportions of the Jomon hunter-gatherers (14,000–2,500 BP), the Yayoi agriculturalists (2,500–1,700 BP) of Japan, and the Kumejima Islanders of the Ryukyus (1,600–1,800 AD) in order to understand the complex interaction of genetic adaptation; development and allometric constraints; selection, gene flow and genetic drift with changing cultural factors (i.e., nutrition) and climate. This assessment will illuminate the complex population history in the Japanese archipelago, which has been frequently discussed in the anthropological literature.

First, the Jomon, the Yayoi, and the Kumejima Islanders were compared with 11 geographically diverse skeletal postcranial samples from Africa, Europe, Asia, Australia, and North America. These geographically diverse populations represented different “climatic zones”: tropical, temperate, and arctic populations. And, the Jomon and Yayoi should cluster with cold-adapted populations, and the Kumejima should cluster with warm-adapted populations.

However, the indigenous Jomon people who were the possible descendants of Late Pleistocene modern humans from Northeast Asia showed morphological mismatch: relatively “wide” body breadths which are cold-adapted body proportions with warm-adapted limb proportions. In fact, Jomon’s brachial index was similar to what one would find in people from tropical climates. The Yayoi rice agriculturalists who were the more recent migrants from Northeast Asia, adhered to the Allen-Bergmann expectation of cold-adapted body and limb proportions.

The Kumejima people who lived in the sub-tropical island in the Ryukyu during the Edo Period was a surprise. Postcranial bone measurements indicated short limb lengths, cold-adapted body proportions and cold limb proportions similar to people who live in higher latitudes and colder climates. But, they exhibited smaller body mass which indicated adaptation to a warmer climate.

In order to explore population history and population structure, this study applied Relethford-Blangero analysis as a measure of the magnitude of genetic drift or gene flow. For instance, Jomon and the middle Yayoi experienced relatively little gene flow, and there was little interregional contact among Jomon-derived indigenous people and Yayoi during the middle Yayoi Period. In contrast, Kumejima had gene flow and migration.

During the Jomon Period, the Japanese archipelago went through postglacial warming from 10,000 BP until 4,300 BP when mean annual temperatures were 3°C higher than the mean annual temperatures today in Japan. It is possible that the late Pleistocene ancestors of Jomon who possessed cold-adapted body may have migrated to the Japanese archipelago from colder Northeast Asia, and a change in limb proportions may have occurred after experiencing the warmer environment of Japan over a considerable amount of time.

The recent genomic studies indicate that the later Jomon people (beginning 3,000 BP) might have more allele sharing with the Yayoi agriculturalists. But, the Yayoi display shorter brachial index and crural index compared to Jomon. According to the results of Relethford and Blangero analysis, lower level of phenotypic variation of the Yayoi might be caused by a founder effect at the time of immigration from the small population size of the initial immigrants.

The physical traits of the Kumejima, cold-adapted body proportions and limb, might be a result of gene flow from “the ethnic Japanese” who were descendants of the Yayoi people after the Medieval Period. The difference between body mass and body proportions/limbs might be caused by allometry or climate and/or nutrition. For nutrition, heat-adapted body mass can mimic nutritional response. According to the history of Okinawa, the Kumejima people from 1,700 AD until 2,000 AD had imposed upon them a heavy tax for crops and textiles. Consequently, the Kumejima people might have experienced nutritional stress which may have influenced their body size.

In conclusion, natural selection may have been the primary force in the interaction of genetic drift and gene flow in the late Pleistocene. In the Holocene, however, genetic drift was probably the primary force combined with gene flow with cultural factors (e.g., diet and nutrition). In the fluctuating climate of the late Pleistocene and Holocene East/Northeast Asia, the mismatch in limb and body proportions may have represented the best compromise in the postcranial adaptive complex.

These findings are described in the article entitled An assessment of postcranial indices, ratios, and body mass versus eco-geographical variables of prehistoric Jomon, Yayoi agriculturalists, and Kumejima Islanders of Japan, published in the American Journal of Human Biology. This work was led by Noriko Seguchi (Kyushu University), Conrad B. Quintyn (Bloomsburg University), Shiori Yonemoto (Kyushu University Museum), and Hirofumi Takamuku (Doigahama Site Anthropological Museum).

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