What Conditioned First Human Migrations?

Environmental conditions influenced the time and space of the first human migrations according to archaeological, genetic, and climatic data.

By Elisa Huertas Ciorraga

How can we know what led our ancestors to travel throughout our planet? Researchers from the US, Australia, Austria, and France have studied the first human migrations between Europe and Asia and through the American continent—combining archaeological, climatic, and genetic data to understand how the environment influenced the routes they followed.

Temperature, the amount of rainfall, and the type of vegetation conditioned the regions and the time our ancestors chose to expand and migrate to unknown regions.

The environment Homo sapiens liked

According to the recent study published in Nature Communications, when Homo sapiens started expanding between Europe, Asia, and the American continent, they chose to travel through areas they perceived as more productive, with more resources. Those areas were warmer and wetter than other possible pathways, and their landscape consisted of ecosystems of transition or ecotones.

For example, human migration routes with higher temperatures led them from Eurasia to Japan, or in the opposite direction, to Portugal.

Regarding the routes with a higher amount of rainfall, the ones that stand out are the routes from Europe to Japan, from Eurasia to Beringia (a land bridge that once connected Siberia and Alaska), and across the northeast of South America.

Ecotones mentioned in the study consisted of grasslands and forests—highly beneficial areas for humans. First, thanks to the forest, they had access to an important amount of biodiversity (in other words, food). Forests also provided them with benefits such as shelter and wood for fuel. Second, the grassland openings offered higher visibility, which was crucial for hunting and traveling. 

In addition, access to fresh water was very important for human migrations. That’s the reason why migration routes often followed the trajectory of important rivers. For example, the Danube in Europe, the Amazon in South America, and the Yellow River in China.

Researchers found that human movements were frequent and covered short distances in highly productive environments. 

Where and when did human migrations happen?

Nowadays, scientists agree that one or more human migrations moving out of Africa started approximately 177,000 years ago, with human presence in the south of Europe 46,000 years ago and in the north of Europe 32,000 years ago.

According to researchers of the present study, the first humans entered Europe coming from the Fertile Crescent (Middle East), crossing the Caucasus Mountains toward Scandinavia (48,300 years ago), bordering the Black Sea and entering western Europe from the north of the Alps, and other routes coming from the Mediterranean and the Dalmatian coasts (44,000 years ago).

A route coming from the Caspian Sea, between Europe and Asia, was super important for human migrations as it led Homo sapiens to Japan (44,200 years ago) and Beringia (34,700 years ago), the path that connected Eurasia with North America. This route also enabled Homo sapiens to reach Mongolia (47,100 years ago) and the Himalayas (45,800 years ago).

Early human presence in the Americas

The study suggests that the entrance to North America happened 34,700 years ago through Beringia. As most of North America was covered by the Laurentide and the Cordilleran ice sheets, humans started spreading 16,200 years ago via the coast of the Pacific Northwest, reaching South America approximately 14,800 years ago.

There, Homo sapiens followed two different routes, one of them clockwise from the east of Brazil (13,072 years ago) moving downward to the southeast of Argentina, and another route anticlockwise crossing the Andes to the coast of Peru, following the coast downward to Chile and reaching Patagonia 14,400 years ago.

Knowing when Homo sapiens reached each point gives the researchers important information. For example, humans adapted quickly to settle in high elevations shortly after they reached South America.

To spread across North America, humans had to wait 3,000 years for higher temperatures and rainfall that allowed the formation of an ice-free corridor via the Mackenzie River. That information suggests that the ancestors of the first native Americans were locally isolated by ice.

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How did researchers study it?

The research team wanted to know the routes that first humans were more likely to follow to expand within Europe, Asia, and America, and why they chose those routes. To do so, they designed a statistical model that used archaeological and genetic data. 

They had maps with information on where and when certain human groups had been living thanks to paleontological and archaeological records. Also, they gathered information from the DNA of the mitochondria of present-day humans, specifically, from 27,506 individuals from different locations. 

With all that data, the maps with archaeological information, and the mitochondrial DNA, they created an algorithm to develop new maps with the routes that Homo sapiens were more likely to follow between Europe, Asia, and America. 

Then, to compare the more likely and the less likely routes, they analyzed changes in the temperature, changes in rainfall, the type of vegetation, the landscape ruggedness or how accessible the routes were, the distance to the nearest coast, and the distance to the nearest river.

The routes and timing calculated by the algorithm and the statistical model represent how hunter-gatherers expanded, which is different from farmers who are more sedentary and therefore had lower expansion rates.

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Why is it important?

Nowadays, it is still unclear what led the first humans to expand and migrate to unknown regions. Researchers thought that human migrations were determined by important alterations in the climate as a result of changes in the orbit of our planet. 

However, they might have expanded through the continents because of other reasons such as cultural drivers, the structure of their societies, interactions between different groups, or a combination of all of these.

Knowing the environmental conditions that early humans faced helps researchers to understand the technological innovations that Homo sapiens created and used at specific points in time and space, including watercraft, tools, and clothes, allowing us to understand how climate influenced the evolution of our ancestors.

This study was published in the peer-reviewed journal Nature Communications. 

Reference

Saltré, F., Chadœuf, J., Higham, T., … & Bradshaw, C. J. A. (2024) Environmental conditions associated with initial northern expansion of anatomically modern humans. Nature Communications, 15, 4364. https://doi.org/10.1038/s41467-024-48762-8