FYI: Slavery is despicable. Why did people ever think it was acceptable?

Today marks the 200th anniversary of the signing of the Slave Trade Act by the parliament of Britain, the country that dominated world trade at the time. Pushed through by a passionate group led by William Wilberforce, the act didn't outlaw slavery - that wouldn't happen until 1833 and the Slavery Abolition Act - but it did make the 250-year-old trade in slaves illegal throughout the British Empire. It also represented a significant step towards making the practice abhorrent to the majority of the world's population - not that that means it doesn't exist today: according to a recent United Nations study, 27 million men, women and children are in unpaid servitude around the globe.

In 1865, the 13th amendment to the United Sates Constitution freed the last slaves in North America and provided for the permanent abolition of slavery there. Slavery, however, continues to have far-reaching consequences in the US, not least in the field of politics, where presidential hopeful Barack Obama - who was born to a Kenyan father and a mother from Kansas - is being accused in some quarters of being 'not black enough', in part because his mother's ancestors owned slaves.

Slavery predates written history on almost every continent and was closely tied to warfare. Mesopotamian, Egyptian, Hebrew, Greek, Roman, Persian, Chinese, Aztec and Indian sources abound with references to captured prisoners being forced into manual labour for military, civil engineering or agricultural projects, or to become household servants.

Debt slavery existed in very early times, too. In Africa, it was customary for men to put up wives and children as security for an obligation; if the obligation was not paid, the hostages became slaves. Ancient Egypt made use of slaves but it was the Roman Empire that turned their utilisation into a well-developed operation - at times up to 20 per cent of the population were slaves and the Romans enacted laws to protect them.

Ancient China, too, had its fair share of slaves; the emperor and his court usually owned hundreds, sometimes thousands. Most were born into slavery; others were sold to pay debts or were captured in raids or battle.

The first serious public resistance to slavery can be detected in literature from 12th-century Greece, but no legislation was enacted.