An African migrant is seen at the CETI, Short Stay Immigrant Center, on Oct. 20, 2005 in the Spanish Enclave of Melilla, Spain.

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How Human Migration Works

Where did humans come from? How did we get to where we are now? Where are we going in the future? Studying the migration patterns of humans gives us a glimpse of the development of human civilization and shows us the patterns of human existence. Studying modern migration helps us understand complex economic systems, and it might even give us a way to ensure the future survival of the human race.

In this article, we'll examine scientific research that shows how the earliest humans spread across the globe, and we'll look at modern migration patterns. Then we'll turn our gaze to space, the future target for human migration.

Human Origins

­No one knows for sure exactly when humans first became humans. Scientists use certain characteristics found in fossil evidence (generally the shape of the skull) to differentiate Homo sapiens from earlier species in the genus Homo, such as Homo erectus. Recently, genetic data has also been used to identify early human populations. Since we're not quite sure when humans evolved in Homo sapiens, we're also not really sure exactly how or when the earliest humans spread across the rest of the world. Paleoanthropologists have several theories based on the best evidence available.

The prevailing theory is the Out of Africa theory. Pre-hu­man hominids probably developed in Africa and spread to Europe and parts of Asia. The first Homo sapiens appeared in Africa roughly 400,000 years ago [source: National Geographic]. This is strongly supported by genetic and fossil data. About 100,000 years ago, they moved north out of Africa into the Middle East, eventually pushing into Europe and Asia. Homo sapiens coexisted with earlier hominids such as Neanderthals. With their greater intelligence and organization, Homo sapiens out-competed other pre-human species for resources, enjoyed greater reproductive success and eventually replaced them [source: Smithsonian Institution].

A competing theory suggests that pre-humans that had already spread throughout Europe and Asia evolved into Homo sapiens. Separate regional Homo sapiens populations interbred, passing the characteristics of modern humans through the entire human population. This theory accounts for some of the regional differences seen between different human populations.

Today, the Out of Africa theory is widely accepted and has the most scientific evidence supporting it. This means that the story of humanity is a story of migration.

An Indian nomadic family undertakes their annual migration. India's 2003 heat wave spurred nomadic families to start their migration early.

AFP/Getty Images

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Why People Migrate

­What drove those first humans to leave Africa? That's best explained by examining the forces that continue to drive humans to migrate even today.

A population of humans living in a given area faces certain pressures. Those pressures depend on the size of the population, the resources available and the community's ability to exploit those resources.

Food - The most basic population pressure, and the one that likely drove the earliest migrations out of Africa, is food. An area of land can only support a certain population with the food produced there. Modern agricultural techniques and technologies can vastly increase food output, but in the African forests and savannas of 100,000 years ago, humans subsisted by hunting and gathering. If the population grew too large, there wouldn't be enough meat or fruit to feed everyone. A portion of the population could simply move a few miles away to find new hunting grounds. Humans may only have moved a few dozen miles per generation, but over tens of thousands of years, this slow but inexorable migration spread humans throughout Europe and Asia.

Space - You can only pack so many humans into a given space. Improvements in medical and sanitation technology make the exact limit enormously variable, and often far higher than the food limit mentioned above, but at some point the population becomes too large for the area. This can lead to outbreaks of violence or the spread of virulent diseases. A general decline in living conditions leads some people to move elsewhere.

Weather and climate - In the short term, weather events can drive a population out of one area. Flooding and severe storms can cause this. Long-term migration patterns have been shaped by climate change. A drought that turns a once-fertile area into a desert will drive the population to find a new home. Changes in sea level can reveal large stretches of coastal land. Massive sections of frozen ocean that occurred during the most recent ice age gave humans access to parts of the world they might not otherwise have reached [source: INSTAAR].

A man carries produce on his bicycle near the entrance to the besieged city of Basra, Iraq as civilians flee due to the war.

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Other Reasons to Migrate

War and politics - One could argue that almost every conflict in human history can be traced to population pressure, which means that war and political oppression might just be symptoms of population pressure. It was oppression that drove English Puritans to settle in North America after first fleeing England for Holland. Today, an estimated two million refugees have fled their home country of Iraq as a result of the war, dispersing themselves throughout the Middle East; another two million have been displaced from their homes within Iraq [source: Human Rights Watch].

Economics - In terms of population pressure, money is a stand-in for food. Ever since the Industrial Revolution, few people grow and harvest the food they eat. We purchase it instead. Now, instead of moving to where the food is, people move to where the money is. These migrations can be slow shifts, such as the decline of population in the northeastern U.S. as the steel industry declined. They can also happen quickly. A major new factory built in a town can draw thousands of workers, plus thousands more who will earn money selling food, clothing and entertainment to the workers.

This illustrates the fact that economic migrations don't follow the same patterns as food migrations. Where food is concerned, higher populations make it harder for everyone to get enough food. Conversely, economies thrive on saturated population levels. More people equals more money.

­The human spirit -­This form of population pressure can't really be measured, but it shouldn't be taken lightly. Humans have an innate desire to explore and colonize new territories. Even when not driven by hunger, politics or economics, humans migrate.

A scientist searching for hominid fossils at Olorgasailie, the find site for the Homo erectus specimen found by Rick Potts. This specimen is the first hominid fossil found after more than 60 years' work at Olorgasaili.

Kenneth Garrett/National Geographic/Getty Images

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Early Human Migration

No historical record exists that tracks the migratory patterns of the earliest humans. Scientists piece together the story of human migration by examining the tools, art and burial sites they left behind and by tracing genetic patterns. They accomplish this by looking at mitochondrial DNA (mtDNA), which is passed from a mother to her offspring without being blended with the genetic code of the father. We can look at the mtDNA of two people who lived thousands of miles and years apart, and if their mtDNA genetic code is the same, we know they were ancestor and descendant [source: PBS NOVA].

Examining mtDNA is useful for another reason -- it accumulates mutations relatively quickly. Scientists can see how many mutations are present and roughly determine how old that genetic line is. By comparing the number of mtDNA mutations found in people from different locations, we can tell where humans arrived first. The more mutations, the longer humans have lived in that area. All of the mtDNA found in certain parts of Africa has more mutations than any other mtDNA in the world. This evidence strongly supports the Out of Africa theory. However, even with these clues, much about early human migration is uncertain.

Early Migration Routes

When humans first left Africa, they followed the coasts, where resources were abundant. The first wave moved across the Middle East, into southern Asia, and eventually all the way down to Australia [source: National Geographic]. This occurred roughly between 90,000 and 30,000 years ago [source: Haywood]. Additional waves of migration followed. Between 40,000 and 12,000 years ago, humans moved north into Europe. However, their range was limited by an ice sheet that extended into the northern part of continental Europe.

The icy conditions at the time also helped expand early humanity's territory. A massive sheet of ice, combined with lower sea levels, formed a bridge between Siberia and Alaska that we call Beringia. The first humans crossed over 30,000 years ago, moving down the west coast of North America [source: National Geographic]. Other sources suggest a more recent North American migration, starting about 15,000 years ago [source: Haywood]. New evidence seems to keep pushing the date of first North American habitation further and further back. Humans eventually spread into South America and pushed east into what is now the eastern United States and Canada. This theory of North America's settlement is supported by mtDNA evidence and a similarity in the dental structures of Siberian and North American populations of the era.

There have long been competing theories that early humans crossed the Atlantic Ocean, either from Africa to South America or the Caribbean, or from Europe to Greenland to North America. While it may have been possible to make such a trip using available seafaring technology, it is unlikely that a large-scale migration occurred in such a way.

The initial spread of humanity across the Earth was driven primarily by food and climate. Nomadic tribes of up to a few dozen people likely followed the migration patterns of the herd animals they hunted. Climate change opened new areas for hunting, even as technology such as mastery of fire and meat preserving allowed humans to live in less-than-ideal conditions. The human ability to adapt to new circumstances not only gave early humans an advantage over Homo erectus, it also facilitated global expansion.

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Members of Kenya's coastal hunter-gatherer tribe, the Boni, hold up dried-up honeycombs. The Boni, now numbering only about 4,000 members, have maintained a traditional lifestyle of hunting and gathering alongside paltry agriculture.

TONY KARUMBA/AFP/Getty Images

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Technology and Migration

The Agricultural Revolution

Every human group survived as hunter-gatherers for thousands of years. About 10,000 years ago, humans first developed farming technology. This technology didn't develop at one location and then slowly spread throughout the world -- it appeared independently in many different places. Agriculture was successful because it could support greater populations with less land. The end of the Ice Age improved climate conditions in many regions, making farming more lucrative. While many societies maintained a hunter-gatherer existence even into modern times, the success of agriculture effectively ended the widespread constant human migrations that were part of the nomadic hunter lifestyle worldwide. Humans still migrated after the development of farming, but it was no longer the central aspect of their lives.

The migration that did occur was still driven by the same basic reasons -- climate and food. Instead of migrating to follow animal herds, people would migrate to areas of better soil. Without modern farming techniques, early farmers could use up all the nutrients in the soil within a generation or two, forcing migration to unfarmed land. Climate shifts could cause droughts or floods that forced migrations as well.

Migrations tend to follow paths where resources are easy to come by. Coasts and rivers, which provide fish and fertile land, are almost always settled first. Humans didn't push inland or into less hospitable areas until population pressures forced them to.

Agriculture had an enormous effect on humanity. It formed the basis for all modern human civilization. The end of constant migration and the ability to support larger populations lead to the creation of cities, states, governments, organized religions, monetary systems and militaries. None of these would be possible with a nomadic population.

The Industrial Revolution

­The thousands of years which followed the development of agriculture were certainly not migration-free, but the next event that wrought massive changes on the nature of migration was the Industrial Revolution. During the 17th and 18th centuries, the Industrial Revolution mechanized the production of goods and food. It also lead to the ongoing urbanization of the world. Prior to the Industrial Revolution, communities remained relatively small, with decentralized economic centers that served each town. For example, a town might have a mill that processed grain from a few nearby farms and a few small manufacturing facilities run by local craftsmen. Industrialization saw the dawn of factories, massive manufacturing centers that offered hundreds or thousands of jobs. People migrated from rural or semi-rural areas to cities to take advantage of these plentiful jobs.

­In 1790, New York City had a population of about 33,000. Fifty years later, it had grown tenfold, to more than 300,000 [source: U.S. Bureau of the Census]. That growth rate far exceeds the growth in the national population [source: U.S. Bureau of the Census]. As of 2005, almost half of the people in the world lived in a city, a number that has risen continually and is predicted to continue rising [source: UN]. Some countries are urbanized at levels up to 80 or even 90 percent [source: World Development Indicators].

Migrants retreat to Mexico during a failed attempt to illegally enter the United States.

David McNew/Getty Images

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Modern Migration

Migration in the modern world has one key difference from earlier forms: national borders. Borders block attempts to migrate, limit migration to certain groups or quotas and restrict migration to certain economic classes. While migration is still driven by the same basic pressures, it is now artificially shaped by political forces as well.

Most modern migration follows economic patterns. People are always seeking better economic opportunities. For decades, this lead to a migratory flow in North America of south-to-north. Northern cities had plenty of industrial jobs and were economic centers. Industrialization had been delayed in the American south and in Mexico, so people there moved north to get jobs. This exact same drive is what fuels migration from Mexico to the United States today.

However, beginning in the 1980s, U.S. migration started moving from north to south. The old industries that had drawn so many southerners in decades past were failing or moving due to pressures from foreign competitors. Meanwhile, cities in the south and west were capitalizing on newer technologies, opening up new industries and offering plentiful jobs. Between 1995 and 2000, the largest movement from one state to the other was from New York to Florida (source: U.S. Bureau of the Census].

In Europe, migration is following a similar pattern. Plentiful jobs in wealthy European nations are drawing an influx of immigrants from Turkey, Pakistan and other areas of the Middle East.

Migrating to Space

­Where will humans migrate to next? Some suggest it is inevitable that we will someday colonize space. There are many reasons people have looked to space as the migration destination. There are resources to be exploited. There is space for people to live, increasingly an issue as the world population grows. For some, survival of the race is the imperative -- if an asteroid or nuclear war wiped out life on Earth, space colonies could carry on our culture and very existence. Some may be motivated like the Pilgrims to make a new start and create a nation that conforms to their vision of a perfect world.

­A lot of work lies between us and the first true space colony. There are numerous problems to be solved, such as overcoming the long-term effects of weightlessness on humans, successfully navigating the vast distances between planets and creating the basic resources necessary for humans to live (water, air and food, specifically). Perhaps the drive that will ultimately lead us to accomplish space colonization is the intangible one we mentioned earlier -- the human spirit.

If you're interested in learning more about humans migrating and related topics, try the links on the next page.

Lots More Information

Related HowStuffWorks ArticlesMore Great LinksSources
  • American Studies at the University of Virginia. "Pilgrims and Puritans: Background." http://xroads.virginia.edu/~CAP/puritan/purhist.html
  • Groleau, Rick. "Tracing Ancestry with MtDNA." NOVA Online. http://www.pbs.org/wgbh/nova/neanderthals/mtdna.html
  • ­Haywood, John, Ph.D. Atlas of World History. Barnes & Noble (2001).
  • Human Rights Watch. "Iraqi Refugees: Summary." http://hrw.org/backgrounder/refugees/iraq0407/1.htm#_Toc164487346
  • The Institute of Arctic and Alpine Research. "Postglacial Flooding of the Bering Land Bridge: A Geospatial Animation." http://instaar.colorado.edu/QGISL/bering_land_bridge/
  • Jewish Virtual Library. "The Tragedy of the S.S. St. Louis." http://www.jewishvirtuallibrary.org/jsource/Holocaust/stlouis.html
  • National Geographic. The Genographic Project. http://­www­.nationalgeographic.com/genographic/atlas.html­
  • Schopf, J. William. Major Events In The History Of Life. Jones & Bartlett, (December 17, 1991).
  • ­Smithsonian Institution. "Theories on Modern Human Origins and Diversity." http://anthropology.si.edu/humanorigins/faq/Encarta/diversity.htm#ooa
  • United Nations Department of Economic and Social Affairs, Population Division. "World Urbanization Prospects: The 2005 Revision." http://www.un.org/esa/population/publications/WUP2005/2005wup.htm
  • U.S. Census Bureau. "Table 2. Population of the 24 Urban Places: 1790." http://www.census.gov/population/documentation/twps0027/tab02.txt
  • U.S. Census Bureau. "Table 7. Population of the 100 Largest Urban Places: 1840." http://www.census.gov/population/documentation/twps0027/tab07.txt
  • U.S. Census Bureau. "Table 2. Population, Housing Units, Area Measurements, and Density: 1790 to 1990." http://www.census.gov/population/censusdata/table-2.pdf
  • United States Holocaust Memorial Museum. "German-Jewish Refugees, 1933-1939." http://devdata.worldbank.org/wdipdfs/table3_10.pdf
  • U.S. Census Bureau. "State-to-State Migration Flows: 1995 to 2000." http://www.census.gov/prod/2003pubs/censr-8.pdf­