Slavery was established in parts of the world aeons before the US was even a twinkle in Washington's eye and it was rarely restricted to one race or group. So how, asks David Brion Davis, did chattel slavery come to take on such deep racial connotations?

The word "racism" had not yet been coined in the 1830s, but black abolitionists still saw the concept as their chief challenge. In 1836, Theodore S. Wright told the American Anti-Slavery Society, of which he was a founding member: "The prejudice which exists against the coloured man, the freeman, is like the atmosphere everywhere felt by himI still we are slaves - everywhere we feel the chain galling usI this influence cuts us off from everything; it follows us up from childhood to manhood; it excludes us from all stations of profit, usefulness and honour; takes away from us all motive of pressing forward in enterprises useful and important to the world and to ourselves."

Like other black reformers, Wright recognised that the kind of prejudices they faced had their bases in slavery. But why did slavery become linked to a particular people - to dark-skinned descendants of Africans?

Although by 1836 New World slavery was almost entirely limited to "negroes", 400 years earlier a booming long-distance seaborne trade delivered tens of thousands of "white" slaves from regions around the Black Sea to Mediterranean markets.

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Slaves, regardless of race, ethnicity or time period, have always carried the marks of inferiority that are later ascribed to supposedly inferior races. In 18th and 19th-century Russia, servants, who were from the same ethnic group as their masters, were said to be intrinsically lazy, childlike, licentious and incapable of life without authoritative direction. Russian noblemen even claimed serfs had black bones.

In the time of Plato, there was already an unbridgeable gap between the world of free Greeks and their slaves, who had no hope of rising to citizen status, even if they were manumitted. Plato argued that division between master and slave was part of a vast cosmic scheme, in which irrational nature was ordered and controlled by intelligent and purposeful authority - just as the human body was subordinated to the mind.

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For Aristotle, too, the relationship of master and slave was as natural as other relationships of superior to inferior, such as soul and body, man and wife or father and child. His views were extraordinarily influential and incorporated into Christian theology, especially by St Thomas Aquinas in the 13th century.

Throughout the ancient world, including Asia and pre-conquest South America, slaves were marked off by identifying symbols or icons such as branding, tattooing, collars, hairstyle and clothing, which would not have been necessary if all slaves had shared distinctive physical characteristics.

While all regions and times have chosen to enslave outsiders, in antiquity and later it is clear that enslavement could afflict any racial group. It was often a matter of accident and luck and, until the 16th century, was not associated exclusively with any particular groups or classes of people. It is safe to say that we are all the descendants of slaves and masters.

Nevertheless, in ancient India, slavery was linked with dark-skinned Dravidian people conquered by Aryan invaders from the north. In medieval Europe, words derived from the Latin sclavus denoted captives of Slavic origin, suggesting Slavs were especially suited for bondage.

Religious identities were also significant. When Rome was Christianised, Jews were forbidden to own Christian slaves. Islam, by forbidding forcible enslavement of Muslims, put a premium on enslaving foreign captives in war. For Christians, a turning point came in 1366 when the import and sale of infidel slaves was sanctioned with the explanation that "infidel" meant "all slaves of infidel origin, even if at the time of arrival they belonged to the Catholic faith" and "infidel origin" was to mean "from the land and race of the infidels".

With this decision, Christian leaders bypassed the dilemma of baptism by shifting the basis of slavery from religious difference to ethnic origin. In the 17th century, numerous American colonies took a similar route, affirming that unless a person - meaning an African or Indian - had been Christian in their native country or free in some Christian country before being imported to America, baptism did not affect their status as a slave.

While there has been much recent interest in European images and representations of Africans, there is still a lot to be learned about the influence of Arabic racist literature and Christian views of blackness derived from gnosticism, Manichaeism, alchemy and astrology.

In the late classical age, art depicted fairly realistic portraits of blacks, with a sense of the infinite diversity of a common human nature - blacks as musicians, dancers, jugglers, actors, acrobats, charioteers and soldiers. But there is then a shift to the black demons of early Christianity. The association of blackness with the devil and demons of various kinds and with death, sin, ignorance, idolatry and with Judaism and Islam may have contributed to certain racist predispositions within Christian cultures.

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It is probable that most Europeans got their first subliminal impressions of "negroes" in their local church or cathedral, ranging from a black "wise man" at the nativity to images of black death squads serving the devil or the devil himself portrayed as an animalistic African.

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Christian Europe long associated Africa with its Muslim enemies. Then, in the Middle Ages, as Christians began their slow reconquest of Spain, light-skinned Arabs and Berbers began exploring and mapping sub-Saharan Africa - really "inventing" Africa and an African people, when Europeans themselves were still classified by Arabs as barbarians. Arab and Persian literature is filled with stereotypes of black Africans as sub-human, oversexed, exuding a terrible smell from their armpits, but very gifted as musicians.

Regardless of occasional tributes to the qualities of black women and regardless of the continual enslavement of white pagans and infidels from Eurasia, medieval Muslims came to associate the most degrading forms of labour with black slaves. The Arabic word for slave, abd, came to mean only a black slave and, in some regions, referred to any black, slave or free.

Winthrop Jordan, in the classic work White Over Black, made the point that the European discovery of the chimpanzee coincided with northern European involvement with Africa and the first significant encounters with sub-Saharan West Africans, who were from roughly the same region. From the late 1600s, there was much pre-Darwinian speculation on the relationship between Africans and apes. But this challenged the orthodox Judeo-Christian view that humans were created in the image of God. So from the Renaissance on, the worst expressions of racism were expounded by eccentrics, heretics and free thinkers. In England and America, mainstream Christians vehemently defended the biblical account of the unity of mankind - there was no room for insinuations that blacks were a separate species.

By the 19th century, this had changed. Was this perhaps caused by Arab prejudice, passed on in some way to later European colonisers?

Like the Hebrew and Christian Bibles, the Koran and Shari'a show no trace or even awareness of racial or colour prejudice. Muslim jurists and theologians continued to reject the popular idea that black Africans were designed by nature to be slaves and insisted that mankind was divided only by faith. All unbelievers, regardless of skin colour or ethnic origin, could lawfully be enslaved in a jihad.

Yet medieval Muslim writers clearly felt the need to respond to a vicious pattern of racial discrimination by specifically describing the pious deeds of particular blacks. For medieval Arabs, as for later Europeans, the blackness of Africans suggested sin, damnation and the devil. And although Arab and Iranian writers usually attributed blacks' physical traits to climatic and environmental forces, they increasingly invoked the biblical curse of Canaan, in which Noah is said to have cursed the descendants of his grandson, Canaan, and condemned them to slavery, to explain their colour.

It seems extremely likely that for most Arabs, the only sustained contact with blacks was as slaves transported far from homelands and that the contempt and dehumanisation of slavery became associated with all black people.

In one of history's bitter ironies, it may have been the Spanish and Portuguese treatment of Jews - or, more specifically, Jews and their descendants who had converted to Christianity - that provided the seedbed for Christian negrophobic racism. There is much evidence that paranoid anxiety over the conversion and intermixing of Christians and Jews in late medieval Spain gave rise to concerns over purity of blood and biological race.

As suspicion of Christians with Jewish ancestry grew, the Inquisition led a witch-hunt to spy on converts, ferreting out evidence of using no lights on Saturday, circumcising male children and so on. By the mid-15th century, fear of religious heresy had been translated into racial terms, with paranoid depictions of Christians with tainted Jewish blood engaged in subtle machinations to gain control of church and state.

It seems highly probable that as Spaniards became familiar with Muslim writings concerning black slaves, they transferred to newly imported blacks the kind of racial concerns originally ignited by conversion and intermixture with Jews.

Whatever borrowings occurred between cultures, it is clear that by the time the Spanish began to conquer and settle the New World, chattel slavery had taken on deep racial connotations. And the dishonour, contempt and dehumanisation always associated with human bondage was transferred to scores of African ethnic groups that Arabs and Europeans perceived as a single race.

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David Brion Davis is professor of history at Yale University.