Most scientists believe that human beings first came to America over the Bering Straits about 20,000 years ago. These were the ancestors of the many Native American cultures, which would people the landscape for thousands of years.

The great European migration began with the arrival of Christopher Columbus in the Americas in 1492. The lure of available land and the hope for political and religious freedoms kept the Europeans coming.

In some places, the meeting of Europeans and Native Americans was peaceful. In others, the cultures clashed, leading to violence and disease. Whole tribes were decimated by such newly introduced diseases as small pox, measles, and the plague.

By the end of the 16th century the Spanish were established in St. Augustine, and by the early 17th century thriving communities dotted the landscape: the British in New England and Virginia, the Dutch in New York and New Jersey, and the Swedish in Delaware. By the mid-18th century, the British colonies had become the most prosperous in North America.

But the Europeans weren't the only immigrants in these communities. Slaves from Africa and the Caribbean were brought forcibly into the New World as early as 1619.

In the six years since the United States won the War of Independence, America was becoming, in Thomas Paine's words, "the asylum for the persecuted lovers of civil and religious liberty from every part of Europe."

The first Census counted 3.9 million people counted, with the English being the largest ethnic group. Nearly 20% were of African heritage. German, Scottish and Irish residents were also well represented. Census takers didn't count Native Americans.

The early Congresses could do little to affect immigration; the Constitution gave that power to the states. However, Congress was given the authority to ban the slave trade after 1808, which it did, and the authority to establish rules for naturalization.

In 1790 it passed the first Naturalization Act, which stipulated that "… any alien, being a free white person, may be admitted to become a citizen of the United States...."

In the early years of the republic, immigration was light, 6000 people a year on average. By 1806 the flow of immigration was reduced to a trickle as hostilities between England and Napoleon's France disrupted Atlantic shipping lanes. The War of 1812 between the United States and Britain slowed immigration even further.

With peace re-established in 1814, immigration from Great Britain, Ireland and Western Europe resumed at a record pace. Major port cities of this era (New York, Boston, Philadelphia, and Charleston) were overwhelmed with newcomers, many of them sick or dying from the long journey.

Congress responded with the Steerage Act of 1819, requiring ship captains to keep detailed passenger records and provide more humane conditions for those on board.

With the Industrial Revolution beginning, the slave trade nearing its end, and America pushing westward, thousands of immigrants found work on the trans-continental railroad, settling in towns along the way. Word of the California Gold Rush had spread around the world, drawing immigrants from both Asia and Europe. <

Although many new immigrants came in pursuit of a dream, nearly all the Irish immigrants from the 1840's and 1850's came to escape a nightmare: a devastating famine back home. The Great Hunger would leave 1.5 million dead, and just as many would flee to America.

The Irish weren't the only newcomers. Rapid population growth, changes in land distribution, and industrialization had stripped many European peasants and artisans of their livelihoods. Departing from Liverpool and Hamburg, they came in through the major Eastern ports and New Orleans. Chinese immigrants began to arrive in the 1850's, entering through San Francisco.

As in the past, the immigrants of this period were welcome neighbors while the economy was strong. During the Civil War both the Union and Confederate armies relied on their strength. But during hard times, the immigrants were cast out and accused of stealing jobs from American workers. Some of the loudest protests came from the Know-Nothings - a political party of the 1850's famous for its anti-immigrant and anti-Catholic leanings.

But it was the pro-immigrant voices of this era that would be most influential. The Republican platform of 1864 stated, "Foreign immigration which in the past has added so much to the wealth, resources, and increase of power to the nation … should be fostered and encouraged."

By the 1880's, steam power had shortened the journey to America dramatically. Immigrants poured in from around the world: from the Middle East, the Mediterranean, Southern and Eastern Europe, and down from Canada.

The door was wide open for Europeans. (In the 1880s alone, 9% of the total population of Norway emigrated to America.) After 1892 nearly all immigrants came in through the newly opened Ellis Island. Families often immigrated together during this era, although young men frequently came first to find work. Some of these then sent for their wives, children, and siblings; others returned to their families in Europe with their saved wages.

The experience for Asian immigrants in this period was quite different. In 1882 Congress passed the Chinese Exclusion Act, severely restricting immigration from China. Since earlier laws made it difficult for those Chinese immigrants who were already here to bring over their wives and families, most Chinese communities remained "bachelor societies."

For Mexicans victimized by the Revolution, Jews fleeing the pogroms in Eastern Europe and Russia, and Armenians escaping the massacres in Turkey, America provided refuge. And for millions of immigrants, New York provided opportunity. In Lower New York, one could find the whole world in a single neighborhood.

Between 1880 and 1930 over 27 million people entered the United States, about 20 million through Ellis Island. But after the outbreak of World War I in 1914, American attitudes toward immigration began to shift. Nationalism and suspicion of foreigners were on the rise, and immigrants' loyalties were often called into question. Through the early 20s, a series of laws was passed to limit the flow of immigrants.

The Great Depression, beginning in 1929, left few with the means or incentive to come to the United States. Many recent immigrants returned to their native lands, including hundreds of thousands of Mexicans, many against their will. The restrictive immigration policies of the 1920s persisted.

In the late 1930s, with the Second World War accelerating in Europe, a new kind of immigrant began to challenge the quota system, and the American conscience. A small number of refugees fleeing Nazi persecution arrived under the quota system, but many were turned away.

Once the US declared war against the Axis Powers, German and Italian resident aliens were detained; but for the Japanese, the policies were more extreme: both resident aliens and American-born citizens of Japanese descent were interned. Congress would officially apologize for the Japanese Internment in 1988.

After the war, the refugee crisis continued. President Truman responded: "I urge the Congress to turn its attention to this world problem in an effort to find ways whereby we can fulfill our responsibilities to these thousands of homeless and suffering refugees of all faiths."

Congress answered with the Displaced Persons Act of 1948, offering hundreds of thousands entry into the United States. But millions more were left to seek refuge elsewhere. Between 1956 and 1957, the US admitted 38,000 Hungarians, refugees from a failed uprising against the Soviets. These were among the first of the Cold War refugees.

In this era, for the first time in US history, more women than men entered the country. They were reuniting with their families, joining their GI husbands, taking part in the post war economic boom.

By the early 1960s, calls for immigration reform were growing louder. In 1965, Lyndon Johnson signed the Immigration and Naturalization Act into law. Gone was the quota system favoring Western Europe, replaced by one offering hope to immigrants from all the continents.

The effects of the Immigration and Naturalization Act of 1965 were immediate and significant. Within 5 years, Asian immigration would more than quadruple. This trend was magnified even further by the surge in refugees from the war in South East Asia.

Throughout this period, in a policy that continues to this day, the government has given preferences to professionals like doctors, nurses, scientists, and hi-tech specialists, creating what is often called the "Brain Drain." Many of these skilled workers are women, who are often the first link in a chain of migration, working and saving enough money to bring family members to the US.

At the end of the 20th century, illegal immigration was a constant topic of political debate. Immigrants could enter the country by air, by sea, and by land routes through Canada and Mexico, making it easier than ever to enter the country illegally. In 1986, the government gave amnesty to more than 3 million aliens through the Immigration Reform Act.

These debates continue, as new immigrants arrive on our shores daily, bringing with them their own histories, traditions, and ideas, all of which broaden and enrich our sense of what it means to be an American.