Causes and effects of human migration

Human migration is the movement of people from one place to another with the intention of settling in the new location. When large numbers of people relocate, historians ask questions about why these people moved and what impacts their movements had.

Broadly speaking, there are two categories of factors that influence people’s decisions to migrate. Push factors occur where someone is currently living and make continuing to live there less attractive. A push factor could be political unrest, a lack of job opportunities, or overcrowding. Pull factors occur in a potential destination and make it an attractive place to migrate to. A pull factor could be better job opportunities or having relatives or friends who have already moved to this location.

In the preindustrial era, environmental factors like droughts, natural disasters, and climate all influenced human decisions about where to migrate. The expansion of Bantu-speaking peoples through Central Africa illustrates this relationship between environment and migration. Before we look at the movement of Bantu people, it is important to note that Bantu does not refer to a single community of people. It is a language family whose speakers also shared many cultural practices. There are several hundred distinct Bantu languages, of which Swahili is most widely spoken today.

People speaking Bantu languages spread from West Africa throughout Central and Southern Africa starting around 2000 BCE—see the first map below, where yellow depicts regions containing predominantly Bantu-speakers. Bantu-speakers migrated to and settled in places where the climate was well-suited to Bantu agricultural practices - see second map below.

The crops raised by Bantu-speaking farmers, such as millet and sorghum, grew best in the tropical savannah that covered much of sub-Saharan Africa. This made these regions the most attractive locations for new settlement. Sometime before 500 BCE, bananas were introduced to mainland Africa. Because bananas could grow in rainforest climates, the adoption of banana cultivation opened even more territory to Bantu expansion.

Bantu-speakers also possessed ironmaking technology, which allowed them to create stronger, more effective tools and weapons. Bantu-speakers had widely-shared agricultural practices and types of tools. This fact makes it easy to follow the spread of Bantu throughout central Africa. The presence of iron tools, for example, allows archaeologists to distinguish Bantu living sites from non-Bantu sites, such as those of the Pygmies in rainforest regions, and the Khoisan in more arid regions.

As Bantu-speakers successfully migrated into a new region, the population increased. This put pressure on local resources and prompted some people to seek new living space. Although estimates of population for sub-Saharan Africa are speculative, they do seem to confirm the trend of increased population growth occurring with the spread of agriculture and iron technology.${}^1$‍

The most important factor limiting how many people can live in a given area is the ability to produce food. On the many small islands of the Pacific Ocean, limited space and limited production capacity helped cause migration. We don’t have population figures for the prehistoric Pacific, but we are able to make some guesses based on other evidence. The general relationship appears to have been that the larger the island, the more people it could support.${}^2$‍

The obvious obstacle to travel in the Pacific was the long stretches of open ocean between islands. Pacific Islanders used a combination of technology—such as catamarans and outrigger canoes—and knowledge of the environment and astronomy to navigate between islands.

Edible plants, such as various palm fruits and root vegetables, occurred naturally on many islands in the Pacific. But, these were not reliable food sources, so people who migrated throughout the region brought along other crops to raise on new islands. Starchy vegetables such as taro, yams, and sweet potatoes were commonly grown by Pacific Island populations because these plants generally grew well in wetter climates.

Pigs, chickens, and dogs were all raised as food sources in the Pacific Islands; they also travelled with humans. Rats tagged along on many of these ocean voyages as well, although they probably were not brought along deliberately. These animals were usually able to survive and reproduce with limited human involvement. However, striking a balance between maintaining the animal population for food and raising and preserving crops was important. For example, many communities constructed storage bins raised on stilts to keep animals out of the food or small fences of reeds or rocks around garden plots.${}^3$‍

In central Africa, the spread of Bantu-speaking people had effects on the environment. Introducing new crops and farming techniques altered the natural landscape. Raising cattle also displaced wild animal species. Agriculture improved the ability of Bantu-speakers to reproduce and expand more quickly. But, agriculture also had more noticeable impacts on the environment than hunting and gathering.

The plants and animals that people spread and cultivated throughout the Pacific allowed them to survive. However, these actions also had impacts on the island environments. The introduction of non-native plants and animals, as well as human activity, altered the ecosystems of the islands they chose to live on.

In the most extreme example of human effects on a Pacific island, the people who settled Easter Island destroyed the environment to the point where it was hardly habitable. Both humans and the rats that had travelled with them hunted and wiped out local bird populations. The Easter Islanders cut down all the large trees, which made agriculture more difficult as fertile topsoil eroded. It also meant they couldn't build boats to leave the island. Because of human actions, the island was no longer able to support a large human population.

Easter Island was the worst case scenario, but it showed what could happen if an island became overpopulated. On most islands, people were able to balance their survival needs with preserving their new homes. We see the effects of these migrations still today. In the Pacific, the people, plants, and animals living on the many islands are the result of earlier migrations. In the case of Bantu-speakers in Africa, there was plenty of land to expand into, but they had to push out other groups already living there. The large number of Bantu-speakers across the continent is evidence of their successful migrations.