PHILADELPHIA (AP) — They are scarred with uncountable sorrows, these Doors of No Return, which stood as the portals to a slave castle built by British colonizers in what is now Ghana. Inside, 1,500 people at a time awaited the horrific Middle Passage and a life of bondage across the Atlantic.

Now, the doors launch a new black history exhibit, one of the largest ever. Days before the inauguration of Barack Obama, as you walk through the doors and pass hundreds of artifacts — solemn, sad or joyous representations of the past — today's momentous chapter keeps intruding.

Obama's personal copy of his speech on the great American conundrum of race is here, a simple sheaf of paper marking a milestone in his groundbreaking campaign. And the future will unfold here as well, with video from Obama's swearing-in as president to be incorporated into an already stirring video installation.

The multimillion-dollar exhibit, "AmericaIAm: The African-American Imprint," didn't begin with any expectation of such a powerful ending. Two years ago, when the television personality Tavis Smiley conceived the idea for a museum show about blacks' contributions to this nation, an African-American president was all but inconceivable.

Yet the impossible arrives Tuesday. How did this, how could this happen?

Walking through, you realize AmericaIAm can help explain.

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More than 11 million Africans were stolen from their motherland in the slave trade. Most of the survivors landed in South America and the Caribbean. About 500,000 were taken to America, starting in the early 1600s, and they became the economic engine of the world's richest nation.

"We know the narrative of Ellis Island and the immigrants coming to New York," Smiley says. "But there's another story."

That story begins at the Doors of No Return. They feel like a last signpost, pointing back to an invisible past. Who were those people who passed through? How did they feel at this precipice of the unknown?

"We must always remember that, and try to feel the trauma," says James Early, a member of the exhibit's advisory board and Director of Cultural Heritage Policy at the Smithsonian Institution. "But we can't get stuck there. The Doors of No Return are really a way of measuring our progress."

Obama's ancestors were not slaves; his black Kenyan father met his white Kansan mother in Hawaii. But the doors show a stronger connection between Obama, Africa and America than any genealogical chart.

"We involuntarily walked out of the Doors of No Return," Smiley says, "and now we are voluntarily walking into the doors of the White House. A 400-year journey."

Beyond the doors lie grim relics: leg shackles and neck chains; census forms counting blacks as three-fifths of a person; a cotton gin; a matching set of silver ladies' brush, comb, mirror, and small whip.

Then the despair lifts:

"On the 1st day of January, A.D. 1863, all persons held as slaves within any State or designated part of a State the people whereof shall then be in rebellion against the United States shall be then, thenceforward, and forever free..."

The document is a copy of the Emancipation Proclamation. It bears the actual signature of Abraham Lincoln.

Lincoln — the president Obama is most often compared to, whose Bible Obama will use to take the oath of office.

But it's too soon to rejoice. History shows Lincoln cared for the country far more than for blacks. He saw ending slavery as a pragmatic key to healing his fractured nation, which may cast the many comparisons to Obama in a different light.

"I wouldn't say Lincoln believed in equality," says Spencer Crew, an exhibit adviser and history professor from George Mason University.

He did, however, believe in humanity. AmericaIAm shows how.

Frederick Douglass escaped from bondage in Maryland in 1838, began giving anti-slavery speeches, published his best-selling autobiography, and traveled the world as a celebrity. He was a confidant of Lincoln's and visited the White House several times.

Yet some still feared that Douglass could be returned to captivity. Legally, he was still considered property in his home state, a few short miles (kilometers) from where he met with the president. So friends arranged to purchase Douglass' freedom. The original bill of sale is a few feet (a meter) from the Emancipation Proclamation.

The price of an American icon: $100.

Even this, however, was not enough to ensure Douglass' safety. So he was given a sort of letter of recommendation from the Interior Department, dated Aug. 10, 1863. It's folded twice across, as if Douglass carried it in his breast pocket. It says the bearer "is known to us as a loyal free man, and, hence, is entitled to travel unmolested. We trust that he will be recognized anywhere as a free man and a gentleman."

"I concur," reads the small script on the side of the original document. "A. Lincoln."

Once a critic of Lincoln, Douglass came to praise his "exalted character and great works," counting among the latter the decision to allow blacks into the Union army.

The nation's first black brigade was conscripted in Cincinnati. Their flag is in the exhibit, "Black Brigade of Cincinnati" embossed over the faded Star Spangled Banner.

The threat of Confederate forces just across the Ohio River in Kentucky forced the hand of locals who contended blacks were not citizens, according to an account in the Ohio Historical Center Archives. The first all-black unit of the United States military went to work, digging trenches, clearing roads and building forts to protect the city.

The first steps of a long march that leads, on Tuesday, to a black man becoming commander in chief.

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Moving on through the AmericaIAm exhibit, which fills 14,000 square feet (1,300 square meters) in the National Constitution Center and will travel to 10 cities over four years, you come upon a simple but profound object.

The ballot box is plain, about a foot long and 150 years old, from Franklin County, Ohio. Its only feature is a slot in the top. Its power is in its promise.

"Voting is for all Americans an index of existence," Early says. "It's a physical expression that says, 'I am.'"

The ballot also frames a century of black struggle after the Civil War. This period is presented in the exhibit through artifacts that can be more frightening than slave whips or chains.

A full Ku Klux Klan robe hangs high inside a glass case. With it are seven carved wooden figurines, about four 4 inches (10 centimeters) tall, perhaps children's toys, depicting tiny hooded Klansmen.

Who wore that robe, played with those figurines? Are they alive today? If so, how do they feel at the precipice of the unknown?

This was one of the hardest parts of the exhibit to assemble, according to AmericaIAm's executive producer, John Fleming. "During slavery, at least blacks were valuable property, so they were somewhat protected," he says. "But during Jim Crow, you could just be attacked or beaten or even killed at any time."

That very brutality, in time, would help destroy legalized racism in America.

You move on. Behind the gray jail bars are a bench, a key, and a stool. The bench is where Martin Luther King sat before an Alabama judge. The key locked him into solitary confinement. The stool is where he composed the "Letter from a Birmingham Jail."

This was 1963. King's strategy of peacefully defying segregation laws had begun to provoke violent police responses. Images of children being bitten by dogs and blasted by fire hoses were beamed around the world, to America's shame.

Critics, including black ones, called King impatient and reckless. After being jailed in Birmingham, eight local white clergymen admonished him in a letter to the newspaper and urged local blacks to shun his protests.

King's response, written piecemeal on smuggled scraps of paper, is now a legendary manifesto, its essence captured in a single line: "Injustice anywhere is a threat to justice everywhere."

Lawrence J. Pijeaux Jr. is president and CEO of the Birmingham Civil Rights Institute, which loaned the items to AmericaIAm. He remembers riding in segregated buses, drinking out of water fountains stamped "Colored."

"When I was growing up, I could not just walk into a museum," says Pijeaux, 64. "I'm the first in my family to vote. When you think about growing up not having the right to vote, then living long enough to see an African-American elected president, you're talking about a major, major transformation in this country."

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The transformation accelerates when you continue past King's cell, into the era of black stardom.

Muhammad Ali's robe from the Rumble in the Jungle. Ray Charles' tuxedo and sunglasses. The first manuscript page of Toni Morrison's first novel, "The Bluest Eye," which was the only page to survive a fire at her home. The guitar Prince played in the rain at the Super Bowl halftime show, millions praying he wouldn't get electrocuted. A uniform Michael Jordan sweated in while winning a championship. Dozens more examples of game-changing genius, ability and accomplishment.

The dread and degradation are gone now, ushered off stage by advancing equality.

"I think most people going into the exhibit will have the attitude that American history and culture is European-based," Fleming says. "What we show is that Africa and Europe came together to create a new nation and a new culture that never existed before."

Africa and Europe — the direct lineage of Barack Obama.

How could he not end the exhibit? His image closes a thrilling video presentation, which will be edited to include scenes from the inauguration. And Obama's personal copy of his speech on race is the final artifact of the show.

He was forced into making this speech, the same way African-Americans have been forced into so much of their history. He was forced into it by his former pastor's harsh views about America, and by fear of whether Obama shared them.

Standing at a podium in the same Constitution Center that houses the AmericaIAm exhibit, Obama spoke of understanding, of unity, of the "original sin of slavery."

He urged blacks to embrace "the burdens of our past without becoming victims of our past," and recognized the frustration of hard-working whites "who don't feel they have been particularly privileged because of their race."

After the speech was done, it was left behind on the podium, along with a sense that there is much more of this story to be written.