縄文時代の人口と環境の変動Jomon Era Population Trends and Environmental Shifts

The Jomon period (縄文時代, approximately 15,000 to 2,500 years ago) is a unique historical term specific to Japan, representing what can generally be called the prehistoric era of the Japanese archipelago, characterized by a diverse subsistence lifestyle involving gathering, fishing, hunting, and partial cultivation. Over this extensive period, the population of the Jomon people did not continuously grow; rather, it fluctuated significantly through various phases. Determining the exact number of inhabitants in the Japanese archipelago during this time remains challenging, yet it is widely reported that the population experienced a notable surge from the Early to Middle Jomon period (approximately 7,000 to 4,500 years ago). Particularly, during the Middle Jomon peak, the population across Japan is estimated to have reached more than 250,000 people. These estimates are primarily derived from calculations based on the number of settlement sites and pit dwellings (竪穴住居) found from this era, revealing particularly high population densities in eastern Japan. For example, in parts of the Kanto region, the density reached approximately 300 people per 100 square kilometers, suggesting that society had approached a critical threshold for sustaining traditional hunting-gathering economies.

However, this trend reversed and started to decline after the Middle Jomon period. By the Late Jomon period (approximately 4,000 years ago onward), the population across the archipelago dramatically decreased, marking the onset of significant regional differences. Specifically, in eastern Japan—which had previously been the thriving center of the Jomon era—the population declined substantially, whereas in western Japan, previously less dynamic, the population remained comparatively stable. This shift, characterized by decline in the east and stability in the west, laid the groundwork for the subsequent Yayoi period, during which the population in western Japan surged dramatically. This demographic shift further set the stage for the Kofun period, when Japan’s political and cultural center moved decisively towards western Japan.

Notably, during the Final Jomon period (approximately 3,000 to 2,500 years ago), the population sharply decreased further—a phenomenon now corroborated by DNA analyses. Especially between approximately 2,500 years ago at the end of the Final Jomon and the onset of the Yayoi period, there is strong evidence of significant population decline among both males and females. The major contributing factor to this drastic population reduction is thought to have been the climate cooling and resulting reduction in food resources. For instance, at the Sannai-Maruyama site in Aomori Prefecture—one of the largest settlements throughout the Jomon era—sea surface temperatures around 5,000 years ago, during its period of prosperity, were about 2°C higher than today. However, around 4,200 years ago, rapid cooling led to instability in the cultivation of chestnuts, a major staple for the Jomon people there. This instability severely undermined the settlement’s ability to sustain its population, ultimately leading to its abandonment. Thus, the population trends during the Jomon period were strongly influenced by climatic and environmental changes, and the decline following the Middle Jomon peak is likely attributable to deteriorating natural conditions that reduced food production capabilities and caused resource shortages.

Today, Japan is widely recognized as an isolated island nation separated from the Asian continent. To fully grasp the overall picture of the Jomon period, it helps to briefly look back to the end of the Pleistocene epoch (the Late Paleolithic period), which immediately preceded the Jomon era. At that time, the Japanese archipelago was still connected to the Eurasian continent. Specifically, around 20,000 years ago during the Last Glacial Maximum, sea levels were approximately 120 meters lower than today, connecting Hokkaido to Siberia via Sakhalin in the north, and connecting southern Japan to the Korean Peninsula, creating continuous land bridges to the continent. Needless to say, this geographical context is crucial for understanding the origins of the Jomon people, whose migration into the Japanese archipelago is theorized to have occurred along northern and southern routes. However, around 12,000 years ago, with rapid global warming marking the end of the ice age, rising sea levels gradually submerged these land connections. From that point onward, direct human migration via previously land-connected routes became impossible, creating a geographical condition where any future human movement required the use of boats.

This rapid warming following the end of the Last Glacial Period caused massive glaciers and ice sheets to melt, resulting in rising sea levels—a phenomenon known in Japan as “Kaishin” (海進, marine transgression). Because this was especially pronounced during the Jomon period, it is specifically referred to as the “Jomon Kaishin” (縄文海進). At the beginning of the Incipient Jomon period (approximately 10,000 years ago), the sea level was still about 40 meters lower than today’s levels, yet many land bridges had already submerged, gradually isolating the Japanese archipelago from the continent. Sea levels continued to rise rapidly during the Initial to Early Jomon periods (around 9,000–6,000 years ago), reaching a peak approximately 6,000 years ago. At that time, the shape of the Japanese archipelago was narrower and more elongated than today, coinciding with global temperatures about 2°C warmer than present—an era known globally as the Holocene Climatic Optimum or Hypsithermal period. Although there were regional variations, the sea levels were on average 2–3 meters higher than today, causing coastlines to penetrate deeply inland. Due to this Jomon Kaishin, the Japanese archipelago became geographically isolated, forming islands surrounded entirely by seas.

Later, during the Late Jomon period (around 2000 BCE), climate conditions began shifting again toward cooler temperatures, causing sea levels to gradually decrease. As sea levels receded following the peak of the Jomon Kaishin, coastlines progressively withdrew, and by the Final Jomon period (around 1000 BCE), sea levels had returned approximately to contemporary levels. These climatic shifts protected Jomon communities from external threats, allowing sufficient time—especially in eastern Japan—for the development of distinctive local cultures, aided by the flourishing vegetation resulting from previous warming. However, with the onset of colder climates, survival became increasingly difficult, leading to significant population declines. Notably, during this period of global cooling, important changes occurred on the adjacent continental landmass—events crucial to understanding the end of the roughly ten-thousand-year Jomon era. On the continent, farming-based populations significantly affected by the cooler climate began migrating southward, searching for new agricultural lands. This movement directly led to the arrival of the people known today as the Yayoi, who crossed into the Japanese archipelago.

Jomon life and culture were deeply influenced by these demographic shifts and geographical and environmental transformations. During the warmer and higher sea-level conditions of the Middle Jomon period, abundant forest and marine resources supported more settled lifestyles, leading to the development of large-scale settlements throughout the archipelago. Although hunting and gathering remained the core subsistence methods, the population grew significantly, giving rise to major settlements like the Sannai-Maruyama site, which housed hundreds of inhabitants. In these favorable times, abundant food resources encouraged the extensive and sophisticated production of pottery and clay figurines (Dogu), leading to a flourishing of Jomon culture. However, rapid population growth likely put pressure on living environments and led to tensions with other tribes. Supporting this hypothesis, massive stone circles (kanjo resseki) measuring 30–40 meters in diameter began appearing around 4,000 years ago during the Late Jomon period, especially in prosperous northern Tohoku. It is noteworthy that these structures emerged during a period of population increase.

Due to intensified population density, shortages of food resources and increasingly complex relationships among previously dispersed tribes likely arose, raising tensions as groups formed larger communities. Rather than being the exclusive domain of a single tribe, these stone circles are thought to have functioned as large-scale ceremonial sites where multiple neighboring tribes gathered annually for collective rituals. Such gatherings likely helped to alleviate psychological anxieties and strengthen communal solidarity. In fact, areas in the Tohoku region with stone circles match perfectly the places of concentrated population at that time, supporting the idea that these ceremonial sites served as ritualistic solutions to the newly emerging problems caused by increased populations.

I myself have long researched the Jomon period, frequently visiting the Tohoku region, and despite its current depopulated condition, I consistently sense traces of its former prosperity. What, then, were the Jomon people truly like? This question resonates deeply among many Japanese people today, and understanding our ancestral roots could reveal the unique cultures and customs that make this archipelago globally distinctive.