In many ways, Marcus Rediker believes, the African slave trade actually created the black and white races.

Before the slave trade to North America took hold strongly in the 1700s, the University of Pittsburgh history professor said, most Africans thought of themselves as members of different tribes and language groups.

And the sailors on sea- going vessels at that time were true "motley crews" of British, American, French, Italian, Spanish, Portuguese, Scandinavian and even some African seamen.

"But then, when the crews got to the west coast of Africa" on slave ships, he said, "they would be known as the white people, even if they weren't technically white. And then, at the same time, a large and diverse and multi-ethnic group of Africans is being thrown together on these ships, and yet at the end of the voyage, they're going to be unloaded as members of a 'Negro race.' "

And so the racial categories that bedevil America to this day, Dr. Rediker said, were largely formed in the fetid bellies of slave ships.

It is a subject that has consumed him for several years now, as he researched and wrote "The Slave Ship: A Human History," being published this month by Viking Penguin.

The new chairman of the Pitt history department is an acknowledged master of maritime life in the 1700s and has written previous books on merchant seamen, naval crewmen and pirates.

But always, the slave trade, which transported more than 12 million people across the Atlantic from the late 1400s to the late 1800s, loomed in his consciousness.

Finally, the intersection of one of his personal passions and his historic interests impelled him to start working on the project.

A longtime campaigner against the death penalty, Dr. Rediker got the idea for the book while visiting prisoners on death row.

As in the slave trade, he said, "I could see that the modern criminal justice system has a great many racial disparities and inequities; and I see capital punishment as a modern form of terror, a form that doesn't especially work, but that did make me want to understand this previous history."

Along the way, he has learned things that will surprise many readers, and he has given voice to human stories that will appall and upset many others.

One fundamental fact that may astonish some people is that the United States accounted for only a small fraction of the slave trade -- probably around 5 percent.

The vast majority of slaves, he said, were shipped to sugar plantations in the West Indies and Brazil, exacerbated by the fact that Brazil didn't outlaw the slave trade until 1888.

Another counterintuitive finding?

Slave rebellions on board the ships were extremely common, and the way that the ships were designed acknowledged that fact.

The frequency of slave uprisings is a fairly recent discovery in historical research, Dr. Rediker said, and helps explain two common features on slave ships -- the netting that surrounded the deck, and the "barricado" -- a wooden wall that was built midway along the top deck.

The netting was there to prevent slaves from leaping overboard, although many of them still managed to do so. The barricado gave the crew protection if the slaves started to rebel, and had holes in it through which they could fire muskets at the insurgents.

The protective measures proved that "even though the slaves were in dire circumstances, they never gave up. They kept fighting even though there was no real chance of winning. In many cases, even if they managed to rise up and kill the crew, they couldn't sail the ships."

Slave resistance took other forms, too.

The refusal to eat was so common, he wrote, that "the Atlantic slave trade was, in many senses, a 400-year hunger strike."

In other cases, he said, "the goal was not to capture the ship but to commit mass suicide to get off that ship."

One reason slaves were willing to throw themselves overboard, even when many could not swim, was the traditional West African religious belief that when they died, they would be transported back to their homeland to live in an ideal Africa. It was called "going home to Guinea," the common term for the African coast.

"Many captains developed specific practices of terror to try to overcome that belief," Dr. Rediker said.

"One captain said, 'If they think they're going home to Guinea, I want to make sure they understand they're not going home in the bodies they inhabited.' "

That captain would pick out a victim, he said, "and sever the limbs of the person, and he would throw those limbs into the areas where the people who were still in chains were forced to live, and would use the dismemberment of corpses as terror to control those who were still on board the ship."

In other cases, captains used a tactic that relied on one of the slave ship's constant companions -- sharks.

In a 1774 history of sharks, British author Oliver Goldsmith described one incident in which a slave ship captain decided to make an example of a woman who had tried to jump overboard.

He tied a rope under her armpits and lowered her into the water, Goldsmith wrote, "and when the poor creature was thus plunged in, and about halfway down, she was heard to give a terrible shriek, which at first was ascribed to her fears of drowning; but soon after, the water appearing red all around her, she was drawn up, and it was found that a shark, which had followed the ship, had bit her off from the middle."

Despite such brutal tactics, slave rebellions never died off, and that is a major reason why the abolition movement against the slave trade finally succeeded 200 years ago, in 1807 in Great Britain and a year later in the United States.

The slaves, he said, "never gave up. They never accepted the reality of slavery. So this is not a book about victimhood.

"Without their resistance, the middle-class abolitionist movement in England and New England wouldn't have gotten anywhere. That's a very important thing to remember."

The other factor that contributed to slave resistance was the fact that captured Africans from different ethnic groups spent months imprisoned together before they ever shipped out on the Middle Passage.

Slaves captured in the interior of Africa often spent one to three months being marched to the coast, and most slave ships were anchored off the coast for six months, buying slaves in groups of two and three, before sailing away.

That gave groups of slaves many weeks to learn each others' languages and build relationships, Dr. Rediker said.

"The slave ship was a kind of factory," he said. "And what did the factory produce? It produced two things. It produced labor power that was crucial to the growth of the modern world economy. And it produced the beginnings of the African-American identity."

There were some African tribes who were more likely to capture slaves and sell them to the Europeans than to become slaves themselves, he noted, especially the Fante, a coastal tribe in what is now Ghana, and the Aro, a warlike trading tribe that called themselves "umuchukwu," or the children of God.

During the abolition debate, slave merchants often argued that they were merely buying slaves that African tribes already had captured.

But the incessant demand for plantation workers in North and South America greatly expanded the scope of slave raids in Africa, he said, so that "in the 18th century, the catchment area for slaves extended hundreds of miles inward from the coast."

Over the long history of the African trade, the mortality figures for slaves were staggering.

In the 1700s alone, he wrote, about 500,000 slaves died before leaving Africa, another 400,000 died during the voyage, and 250,000 died shortly after arriving at their destinations.

Much of the credit for abolishing the slave trade in Britain and the United States goes to one of the most effective mass marketing campaigns ever conceived.

It featured a detailed, cross-sectional drawing of the Liverpool slave ship Brooks. In tiny, exacting detail, the pictures showed how 482 slaves could be packed into the ship's hold, side by side, one loinclothed body after another.

The image was reprinted many times in the late 1700s in Britain and America. It was all the more devastating because it depicted the lesser number of slaves permitted under a new British law, not the more than 600 the Brooks actually carried on previous voyages.

With his book coming out on the bicentennial of the abolition of the slave trade, Dr. Rediker hopes it will generate a serious discussion about the consequences of slavery that still distort society today.

"What we need to do is talk about the role of race and slavery in building this country and producing tremendous amounts of wealth," he said.

He believes "reparations are clearly in order, but at the same time, justice is not simply a matter of money."

And it is not up to him as a historian to suggest the proper remedies, he said. That task belongs to African-Americans, the people most affected by this sad legacy.