The following correction was printed in the Observer's For the record column, Sunday March 11 2007





It is a question that few thought a man aiming to be America's first black President would ever have to answer: did your family once own slaves?

But that question is now likely to be asked of Senator Barack Obama, who is bidding for the 2008 presidential nomination of the Democratic Party, in part on the appeal of his bi-racial background.

As the son of a black Kenyan father and a white Kansan mother, Obama has seemed to embody a harmonious vision of America's multiracial society. However, recent revelations have thrown up an unexpected twist in the tale.

Obama's ancestors on his white mother's side appear to have been slave owners. William Reitwiesner, an amateur genealogical researcher, has published a history of Obama's mother's family and discovered that her ancestors have a distinctly shadowy past.

Reitwiesner traced Obama's great-great-great-great-grandfather, George Washington Overall, and found that he owned two slaves in Kentucky: a 15-year-old girl and a 25-year-old man. He also found out that Obama's great-great-great-great-great-grandmother, Mary Duvall, also owned a pair of slaves listed in an 1850 census record. They were a 60-year-old man and a 58-year-old woman. In fact, the Duvalls were a wealthy family whose members were descended from a major landowner, Maureen Duvall, whose estate owned at least 18 slaves in the 17th century.

The news comes at a time when Obama is engaged in a fierce battle with Senator Hillary Clinton to woo black voters in their bids to get the Democratic presidential nomination. It also comes ahead of appearances by both Clinton and Obama today in Selma, Alabama, to mark the anniversary of a famous 1965 civil rights march. This is hardly the best time to be exposed as the descendant of slave owners.

Reitwiesner has posted his research, which he warns is a 'first draft', on his website, wargs.com. However, the news is unlikely to be a serious political problem for Obama, despite the fact that some black commentators have accused him of not being a real black American. Nor is he likely to be alone in finding out that his white ancestors once owned the ancestors of his fellow black Americans. America, like Britain, is caught in the grip of a frenzy of genealogical research. Dozens of websites have sprung up, allowing fast and easy access to all sorts of historical records and prompting many Americans to research their family trees.

That can throw up some very surprising results. In fact, last week Obama was not even the only black politician to find out some unusual personal history. The civil rights campaigner, the Reverend Al Sharpton was stunned to discover his slave ancestors were owned by the late politician Strom Thurmond, who once ran for President on a staunchly racist segregationist platform. The pair might even be related. The news prompted Sharpton to issue a statement about his private agony at the revelation. 'Words cannot fully describe the feelings I had when I learned the awful truth. Not only I am the descendant of slaves, but my family had to endure the particular agony of being slaves to the Thurmonds.'

Obama's campaign team have handled the news of his family's slaving past a bit more casually and a lot less emotionally, issuing a statement saying such a family background was 'representative of America'. That is certainly true. Slavery was the economic bedrock of the American economy in the South before the Civil War. It would come as no surprise that anyone tracing their family roots back to the pre-war South would find that his descendants had owned slaves.

But more edifying discoveries can come from looking at the past too. Another of Obama's ancestors, his great-great-great-grandfather, Christopher Columbus Clark, fought for the Union army in the Civil War. As a result Obama can also lay claim to relatives who risked their lives to end slavery. 'While a relative owned slaves, another fought for the Union,' said Obama spokesman Bill Burton in a statement. Perhaps it is just another case of Obama's complex past showing that he can have it both ways.