In the 18th Century, metal bracelets called manilas were brought to West Africa.
There are, shockingly, more people in slavery today than at any time in human history - but campaigners think the world is close to a turning point and that slavery may be eradicated in the next 30 years. The estimated number of people in slavery - 27 million - is more than double the total number believed to have been taken from Africa during the transatlantic slave trade. Ship records make it possible to estimate the number of slaves transported from Africa to the Americas and the Caribbean, from the 16th Century until the trade was banned in 1807 - and the figure is about 12.5 million people.
The figure of 27 million slaves today comes from researcher Kevin Bales, of Free the Slaves - who blames the huge figure on rapid population growth, poverty and government corruption. Many people still think of slavery as a thing of the past, but it exists in many forms, on every continent - ranging from sex and labour trafficking, to debt bondage where people are forced to work off small loans.
"I often think about a quarry slave from North India," says investigative journalist Ben Skinner, who has travelled all over the world documenting cases of slavery. "I could go in at night and interview him, so I asked him why he didn't run away. It was because he feared the extraordinary violence of the quarry contractor who held him to a minuscule debt. "In his world, the contractor was god. He was not only the taker of life but also the giver of sustenance.
When we look at why slavery has persisted we have to look at breaking those cycles of dependence." Not all slaves were from Africa. Slavery existed among some Native American groups and in some Asian countries, and Europeans were sometimes enslaved by the Ottoman Empire. Russian serfdom can be seen as a variant of slavery. At the time of emancipation in 1861, there were more than 22 million serfs. Slaves were common in the ancient world - the Greeks, Romans and, Egyptians all held slaves. As well as being transported out of Africa during the transatlantic slave trade, slaves were also captured and sold within Africa.
Skinner says that many of the slaves he met in India had never known a free life. They came from extremely isolated communities, and were not aware of their basic universal rights. But while developing countries have the highest number of slave labourers, developed countries with strong human rights laws "fail to resource the law enforcement to deal with the problem in comparison to virtually any other law", says Bales. Barack Obama recently painted a portrait of contemporary slavery.
"It's the migrant worker unable to pay off the debt to his trafficker," he said. "The man, lured here with the promise of a job, his documents then taken, and forced to work endless hours in a kitchen. The teenage girl, beaten, forced to walk the streets."
The US government spends billions on tackling homicide, Bales argues, but only a fraction is spent on slavery "even though we know there are many more slaves than homicides in the US". In Europe too, victims of slavery cannot always rely on the law to protect them. Anti-trafficking charity, Stop the Traffik, cites a case where a girl was returned to Hungary after being trafficked abroad. Upon her return to supposed safety, she was raped and returned to her traffickers.
Brazil has an official "dirty list" of employers using slave labour. 2,700 charcoal camp workers have been freed from slave-like conditions in recent years, according to Greenpeace. There are thought to be 3.3 million child labourers in Pakistan. Many children and adults in India become bonded labourers because of family poverty. The conditions of employment mean workers can't pay off debts so can be stuck for life.
But the International Labour Organization (ILO) - whose figure of 20.9 million people worldwide in forced labour does not include bonded labour - believes slavery can be completely eradicated. The momentum has been growing for the last 10 years, says the ILO's Beate Andreas, pointing to a "growing movement and growing leadership on the part of key countries to take action".
The British Empire outlawed slavery in 1833, but it never really went away, emerging in different forms such as child labour, says Dr Richard Huzzey from the University of Liverpool's Centre for International Slavery. "In the early 1840s, the British declared slavery in India was over as part of the 'kindness' of British rule, but what in fact happened, was that definitions changed - people were called labourers or servants rather than slaves." He compares this struggle to the battle against HIV, where it took a number of years to generate the momentum and the commitment needed to overcome the epidemic.
Slavery is already illegal in every country in the world. "We have not quite reached the tipping point, but it's much more difficult for countries and companies to get away with forced labour nowadays," Andreas says. "There is reason to be optimistic. We have seen a sweeping change in recent years in terms of legislation and better regulation.
"There's a clear sign that more companies are becoming aware, and more governments are willing to take action. If we have the critical mass of leaders ready to take action, then it can be eradicated."
Bales says there was a time when law enforcement agencies knew how to deal with a truck full of drugs, but lacked clear procedures for dealing with a truck full of people. This is changing, he says. The UN's anti-trafficking protocol talks about the "three Ps" - prosecution, protection and prevention. How much is a human life worth? An average slave today costs around $90 (£56) according to Free the Slaves . They calculated it would cost $11bn (£6.8bn) over a 25 - 30 year period to get everyone out of slavery and keep them "slave proof". In 1850 a slave cost the equivalent of $40,000 (£25,000) of today's money .
How much is a human life worth?
The pressure group Stop the Traffik focuses on prevention. In Kyrgyzstan for example, it works with street children to teach them to recognize the warning signs so they can avoid being recruited to beg and steal. Businesses are also playing a role in prevention, by boycotting goods produced by forced or slave labour. A number of major retailers have stopped buying cotton produced by forced labour in Uzbekistan, and last year a trade deal with Uzbekistan was rejected by the European Parliament because of its use of child slaves.
In Brazil, a nationwide anti-slavery plan set out in in 2003 introduced changes in regulation and labour inspection laws that have resulted in the freeing of thousands of slave workers. Employers are put on an official "dirty list" if they are found to use slave labour. This currently includes nearly 300 companies and individuals. The ILO also works to help other countries also spot the "invisible signs" of forced labour.
In 2008 the state of Niger was found guilty by a West African court of failing to protect a former domestic slave, and the government ordered to pay compensation. Dr Aidan McQuade, director of Anti-Slavery International, says that following the verdict, the news spread and large numbers who were in forced labour simply walked away from their situation, something they would have been too afraid to do before. These are some of the factors that make slavery "a solvable problem within our generation". Bales argues - 25 to 30 years.
"The best estimates suggest that slavery puts about $40bn (£24.9bn) in the global economy. While that's a lot of money it's also by far the tiniest fraction of the global economy ever represented by slave output. Twenty-seven million slaves is a large number, but its also the tiniest fraction of the global population to ever be in slavery. "Slavery is standing on the edge of its own extinction - if we give it a hard push. Certainly, we need governments to work together and to enforce their own laws. But that's do-able."