It is the little-noticed force behind the revolutions in the Arab world, the new protests in China and the economic booms in India, Turkey and South America: The largest population shift in human history, currently at its peak, is probably the most significant, and misunderstood, global event of our time.
In Asia, the Indian subcontinent, the Middle East, Africa and Latin America, hundreds of millions of people are rapidly moving from rural areas, where they practiced peasant agriculture, to cities — a shift that makes itself felt in the rough-and-tumble transitional neighborhoods where rural migrants first land, both in their own countries and in places like the United States, where they make up the largest group of immigrants.
We need to pay attention to these neighborhoods and to the huge demographic shift that is shaping them, for they are where either the next great economic opportunity or the next wave of violence and conflict will be born.
Never in human history have so many people changed their locations and lifestyles so quickly. Each month, there are 5 million new city dwellers created through migration or birth in Africa, Asia and the Middle East. China alone has an estimated 200 million "floating" citizens with one foot in a village and the other in a city.
If current trends continue as expected, the urban population of Asia and Africa will double between 2000 and 2030, adding as many city dwellers in one generation as these continents have accumulated during their entire histories. Between now and 2050, the world's cities will add an additional 3.1 billion people.
This will be matched by an almost as dramatic decline in rural population. The United Nations Population Division predicts that the population of the world's villages and rural areas will stop growing around eight years from now and that, by 2050, the rural population will have fallen by 600 million due to migration to cities and urban encroachment on villages.
We need to remember our own history here. This is the same shift that transformed Europe and North America from peasant to urban life in the 18th, 19th and early 20th centuries. That transition gave us both the violent revolutions and teeming slums of that period, but it also triggered, in the West, the end of starvation as a mass phenomenon, a vast rise in living standards and the end of uncontrollable population growth.
The shift began in the developing world during the decades after World War II, and it is now at its peak: The world has gone from being more than 70 percent peasant in 1950 to 50 percent urban today. By 2025, 60 percent of the world will live in cities; by 2050, more than 70 percent; and by century's end, the entire world will almost certainly be as urban as we are in the West.
How is this massive migration being felt? Take a look at the chaotic Cairo neighborhood of Boulaq el-Dakrour, home to 650,000 people, most of them families of rural migrants from Upper Egypt. This slum's frustrated residents formed the first crowd to storm Tahrir Square in January, driving a rebellion that ultimately forced President Hosni Mubarak from power.
Or look at Guangdong, the enormous industrial city in southern China, where thousands of rural-urban migrants rioted for three days this month in anger at their mistreatment by officials in the most serious uprising China has seen in years. China's first-time apartment owners, typically the children of village migrants, have also become political activists, directing their anger not at Beijing but at municipal or neighborhood officials. The ex-peasant is increasingly the most potent political actor in the world.
Or look at Brazil and Turkey, two successful countries that have experienced a decade of democratic stability, open borders and economic growth after parties representing rural-urban migrants came to power.
The "arrival city" neighborhood is increasingly producing political leaders who can unite communities and end divisions: Brazilian President Luiz Inacio Lula da Silva, Turkish Prime Minister Recep Tayyip Erdogan and Los Angeles Mayor Antonio Villaraigosa are all children of rural-urban migrants who grew up in bottom-rung urban neighborhoods.
These neighborhoods want to succeed. They can be the birthplace of a new middle class, as many of America's immigrant neighborhoods have been. But they can also spiral into violent failure and threaten entire countries when barriers are placed in the way of migrants' natural inclination to succeed.
The barriers can be physical: the isolating absence of infrastructure or transportation links.
They can be bureaucratic: the use of zoning, licensing or security regulations to prevent new-immigrant neighborhoods from becoming teeming mixes of industry, commerce, restaurants and homes.
And they can be citizenship barriers. Around the world, there is nothing more damaging to a nation than the presence of a large population of residents who have no pathway to legal citizenship. Without a permanent legal stake, migrants' networks of self-support are forced to become hidden, illegal and often violent, a huge lost opportunity and a threat to a nation's security.
This is a population shift that will affect almost everyone in every country. Never before have so many people reached for the bottom rung of urban success. Our challenge is to make sure there is a second rung waiting for the next wave of brave ex-villagers.
Doug Saunders is the European bureau chief of the Canadian newspaper the Globe and Mail and the author of "Arrival City: How the Largest Migration in History Is Reshaping Our World." He wrote this for the Los Angeles Times.