In 1965, the Rev. Martin Luther King Jr. delivered a speech on Human Rights Day in New York. In his call for opposition to apartheid in South Africa, he spoke about the common history of the United States and Africa. He addressed "one of the blackest pages of our history" — our participation in "the infamous African slave trade."

He decried "the rape of Africa" for the profits that accrued to the United States, Britain and Spain, and the atrocities of the trade: "There are few parallels in human history of the period in which Africans were seized and branded like animals, packed into ships' holds like cargo and transported into chattel slavery."

He emphasized the scale of suffering and linked it to other human catastrophes: "Millions suffered agonizing death in the middle passage in a holocaust reminiscent of the Nazi slaughter of Jews, Poles and others." He solemnly added, "We have an obligation of atonement that is not canceled by the passage of time."

This month marks the 200th anniversary of the abolition of the slave trade by the U.S. government. King, were he here today, might ask, what have we learned about this shameful part of history? And what have we done about it?

We know vastly more about the "infamous African slave trade" than we did when King uttered those words. A generation of scholarship has taught us much about the origins, timing, flows, scale and profits of the slave trade, how history's greatest forced migration "underdeveloped" Africa as it facilitated Europe's commercial revolution, its building of plantations and global empires, its development of capitalism and eventually its industrialization.

Over almost four centuries, from roughly 1500 to 1870, 12 million to 13 million Africans were forced onto slave ships and sailed to New World plantations (although only a modest number, about 1 in 20, came to what would become the United States). We know that during the middle passage, about 1.8 million of these enslaved men, women and children died, their bodies thrown overboard to the sharks that usually trailed the vessels. We know that the survivors and their offspring would, over several generations, produce astonishing wealth for their masters, their societies and their governments.

King deserves some of the credit for the advance in our knowledge, having led and inspired social movements for civil rights, black power and progressive change, all of which demanded a new, more inclusive, more democratic American history.

But have we as a society accepted the obligation of what King called atonement? Have we righted the wrongs of slavery and overcome a historic injustice?

We have made progress, to be sure, but we have not faced the darker side of our history. It is one thing to admit that the slave trade and slavery existed; it is another to acknowledge that calculated, indeed, spectacular violence underwrote both for centuries, making possible profits, fortunes and no small part of our national prosperity.

My research for a book about the human — and inhuman — relationships among captains, sailors and the slaves in the trade led me to conclude that if European, African and American societies are haunted by the legacies of race, class and slavery, the slave ship is the ghost ship of our modern consciousness.

How shall we, as a nation, face the black pages of our history? Should we follow the South African model and establish a Truth and Reconciliation Commission about our past? Or should we instead support the bill introduced in Congress — and defeated — every year since 1989 by Rep. John Conyers Jr. of Michigan, in which he seeks acknowledgment of "the fundamental injustice and inhumanity of slavery" and proposes a commission to study slavery and its effects over the generations and to make recommendations for redress?

King would have reminded us that there can be no reconciliation without justice, and that justice cannot be reduced to money. A bigger reckoning, both moral and material, is called for, and broad public education must be central to it.

King challenged us to live up to our own professed ideals of freedom, justice and equality. As we celebrate his birth and mark the 200th anniversary of the abolition of the U.S. slave trade, King's words remind us that we still have "an obligation that is not canceled by the passage of time."

Marcus Rediker is the author of "The Slave Ship: A Human History." He wrote this commentary for the Los Angeles Times.