Ancient Climate and Human Migration

Human beings originated in Africa. Over the course of several million years, a series of dispersal events gradually spread human populations across almost all landmasses on Earth except Antarctica. The earliest known evidence of humans outside Africa dates to about 2.1 million years ago on the Chinese Loess Plateau, and by the end of the Middle Pleistocene, roughly 700,000 years ago, humans had already spread widely across Eurasia.

Despite this broad pattern, the reasons why early humans undertook large-scale migrations, and how these movements interacted with environmental conditions, have long remained poorly understood. To address this question, a study published in 2024 examined climatic changes during the Middle Pleistocene and integrated archaeological evidence to provide a new explanation for the expansion and dispersal of early humans.

Palearctic Realm（圖片來源：carol ，採用 CC BY-SA 3.0 授權。）

Traditionally, the climatic and environmental histories of Europe and Asia have been studied separately. Yet both regions belong to the same biogeographic realm, the Palearctic. This means that vertebrate communities in Europe and Asia were shaped by shared ecological influences. For this reason, research into early human migration must analyze the two regions together in order to reach a more complete and coherent understanding.

In Africa, scientists have long used stable carbon isotopes (δ13C) preserved in soil organic matter and paleosol carbonates to reconstruct ancient vegetation patterns and climate conditions, thereby revealing interactions between early humans and their environments. However, continuous high-resolution carbon isotope records from regions dominated by westerly winds, such as Europe and Central Asia, have been extremely rare since the late Pliocene.

Analysis

To fill this gap, researchers collected two long sequences of loess sediments from Central Asia and northwestern China: one from the western margin of the Pamir Plateau and another from the southern margin of the Tarim Basin. Both areas lie within the influence of the westerlies, and climate variations strongly affect the composition of their sediments.

Samples were obtained from a 180-meter-thick loess deposit in southern Tajikistan and from a 671-meter-deep loess core drilled along the southern margin of the Tarim Basin. Dating results indicate that loess accumulation in these regions began approximately 3.6 million and 2.7 million years ago respectively, and deposition has continued up to the present.

Central Asia and northwestern China represent a crucial corridor through which early humans traveled across Eurasia. At the same time, they mark the easternmost reach of the westerly wind system. Carbon isotope records preserved in these loess deposits therefore provide valuable information about long-term changes in rainfall and vegetation since the late Pliocene. By analyzing shifts in isotope ratios, researchers can reconstruct patterns of humidity and aridity, as well as changes in the distribution of grasslands and forests. These environmental reconstructions help clarify how such conditions influenced the routes and timing of early human dispersal.

Importantly, these loess sequences span the entire Mid-Pleistocene Climate Transition (MPT), a period of major climatic transformation. This allows scientists to examine how dramatic environmental changes during this time contributed to the formation of the modern arid landscapes of Central Asia and shaped the history of human migration.

Results

The results show that plant communities from the late Pliocene through the early Pleistocene remained relatively stable. Around 900,000 years ago, during the Middle Pleistocene, however, a clear shift toward increasing aridity occurred. Vegetation changed dramatically, transitioning from forested landscapes to more open wooded grasslands.

This transformation is believed to have been driven by the expansion of Northern Hemisphere ice sheets during glacial periods. As ice sheets grew, the transport of atmospheric moisture into continental interiors weakened, leading to drier climates. The resulting aridity also promoted the accumulation of loess sediments, gradually transforming large parts of Central Asia into desert environments.

Earlier studies have also observed that during the Middle Pleistocene in Southeast Asia, browsing animals that primarily fed on leaves declined, while grazing animals expanded rapidly. This ecological shift supports the conclusion that the region experienced increased aridity during this period.

Spatio-temporal distribution of archaeological sites（圖片來源：Zan, J et al.(2024). ，採用 CC BY-NC-ND 4.0 授權。）

Because increasing dryness made large areas difficult to inhabit continuously, early humans likely crossed Eurasia only during relatively humid intervals. One example occurred between approximately 621,000 and 374,000 years ago, when warmer and wetter interglacial conditions prevailed. During this time, populations equipped with Acheulean large cutting tools were able to move eastward into East Asia. Archaeological evidence from Central Asia shows that human occupation sites reappear only after the Mid-Pleistocene Climate Transition, suggesting that the region was not suitable for permanent settlement but was inhabited intermittently as climatic conditions fluctuated.

As glacial intensity and climatic instability increased during the MPT, many rivers across Eurasia began to cut deeply into their channels, producing a series of river terraces. These terraces became widespread in regions such as northwestern Europe and northern China. They created new ecological niches and resource zones that were favorable for human settlement and are today important locations for archaeological discoveries.

Spatio-temporal distribution of river terraces（圖片來源：Zan, J et al.(2024). ，採用 CC BY-NC-ND 4.0 授權。）

The distribution of these landscapes corresponds closely with patterns in the fossil and archaeological record. In western and southeastern Europe, the earliest stable human occupations date to roughly 900,000 to 700,000 years ago, with a broader geographic expansion around 800,000 years ago. The populations involved may have belonged to a species capable of tolerating colder climates, such as Homo antecessor.

Fossil of Homo antecessor（圖片來源：Dorieo ，採用 CC BY-SA 4.0 授權。）

Although not all river terraces have survived to the present, their widespread formation during the Middle Pleistocene provided early humans with reliable freshwater resources and abundant gravel suitable for making stone tools. Archaeological evidence shows significant technological improvements across Eurasia between about 900,000 and 600,000 years ago. In southern Spain, Acheulean handaxes appear around 900,000 to 750,000 years ago, and by approximately 450,000 years ago humans were even producing wooden spears for hunting.

Overall, the desertification of Central Asia and northwestern China had a profound impact on the spread of early humans. As food and water became less predictable, human populations had to develop more flexible strategies for exploiting their environments and improving the efficiency of gathering and hunting. As a result, early humans not only began migrating over much larger distances in search of suitable habitats, but also introduced numerous technological innovations in stone-tool production.

River terraces became particularly attractive habitats for these populations. They provided access to freshwater and often contained rocks suitable for tool manufacture, allowing early humans to leave traces of their technologies and lifestyles along their migration routes.

The climatic transition of the Middle Pleistocene transformed Eurasia from a landscape dominated by forests into one characterized by open grasslands and semi-arid environments. Under these environmental pressures, humans developed remarkable adaptability and creativity. These capabilities ultimately laid the foundation for later large-scale migrations and the emergence of complex human societies.

Author: Bai Leng

Reference:

Zan, J., Louys, J., Dennell, R., Petraglia, M., Ning, W., Fang, X., Zhang, W. Hu, Z. (2024). Mid-Pleistocene aridity and landscape shifts promoted Palearctic hominin dispersals. Nature Communications.