History
Based on human genetics, it is widely believed that prehistoric Africans who left the continent within the past 100,000 years are the ancestors of all non-African humans. But as communities began to form, especially in Egypt and the Middle East, these migrations were greatly reduced because the only land route out of the African continent is through the Sinai Peninsula. After the rise of civilization and the development of sailing, black Africans traveled to the Middle East, Europe, and Asia in a number of occupations.[citation needed] Many of these individuals settled in Europe and Asia and invariably intermarried with the local populations. Today, human genetic research suggests that mitochondrial DNA and Y chromosome haplotypes in Europeans and Asians have distant African ancestry. But these early migrations out of Africa are dwarfed by those associated with the Atlantic and Arab slave trades.
Dispersal through slavery
Much of the African diaspora was dispersed throughout Europe, Asia, and the Americas during the Atlantic and Arab Slave Trades. Beginning in the 9th century, African slaves were taken from the northern and eastern portions of the continent into the Middle East and Asia. Then beginning in the 15th century, Africans were taken from much of the rest of the continent, especially West Africa, to Europe and later to the Americas. Both the Arab and Atlantic slave trades ended in the 19th century.
The dispersal through slave trading represents one of the largest migrations in human history. The economic effect on the African continent was devastating. Some communities created by descendants of Black African slaves in Europe and Asia have survived to the modern day, but in other cases, blacks intermarried with non-blacks and their descendants blended into the local population. In the Americas, the confluence of multiple racial groups from around the world created a widespread mixing bowl effect. In Central and South America most people are descended from European, American Indian, and African ancestry. In Brazil, where in 1888 nearly half the population was descended from African slaves, the variation of physical characteristics extends across a broad range. In the United States, racist Jim Crow and anti-miscegenation laws maintained a distinction between racial groups. The adoption of the one drop rule defined anyone with any discernible African ancestry as African, even though the strictest application of that rule would categorize nearly all Americans as African.
Dispersal through migration
From the very onset of Spanish activity in the Americas, Africans were present both as voluntary expeditionaries and as involuntary colonists. Juan Garrido was one such black conquistador. He crossed the Atlantic as a freedman in the 1510s and participated in the siege of Tenochtitlan.
African immigration has become the primary force in the modern diaspora. It is estimated that the current population of recent African immigrants to the United States alone is over 600,000. Countries with the most immigrants to the U.S. are Nigeria, Ghana, Ethiopia, Eritrea, Egypt, Sierra Leone, Somalia, and South Africa. Some immigrants have come from Angola, Cape Verde, Mozambique, Equatorial Guinea, Kenya, and Cameroon. Immigrants typically congregate in urban areas, moving to suburban areas over time.
Taken from http://en.wikipedia.org/wiki/Black_history.