In the suburbs of Atlanta and the family histories of 19th-century slave owners, the hunt was on yesterday for the identity of a white man who was Michelle Obama’s great-great-great-grandfather.

Clues to the most enduring mystery of Obama family history have emerged in an investigation identifying one of the First Lady’s ancestors as Melvinia Shields, a girl born into slavery, bequeathed to a white family in Georgia and impregnated as a teenager by an unknown white man on a farm near Atlanta in the 1850s.

The breakthrough in piecing together a journey across five generations from the slave-holding South to the White House came with the discovery of a will written in 1850 by David Patterson, a South Carolina estate-owner. The document listed the “negro girl Melvinia” in an inventory of his property, along with nine other slaves and a miscellany of assets including two tablecloths, three pairs of curtains and a coffee mill.

Then just six years old, Melvinia passed to Mr Patterson’s wife, who according to the will was to inherit “the use and service of the said negro girl, her issue and increase, if any”.

The identity of Melvinia Shields and new information about her descendants provides the first concrete evidence for President Obama’s ringing declaration in his one major speech on race during last year’s campaign that he was “married to a black American who carries within her the blood of slaves and slave owners”.

It will intensify public interest in the highly charged historical debate on miscegenation — the widespread practice of interracial sex — that has permeated American demography for centuries, even though it was illegal in some states until the 1960s.

Rather than hold on to the young slave — who died in 1938 without knowing who her parents were — Mrs Patterson sent her to live with relatives in Georgia. It was there, according to census records studied by The New York Times, that she gave birth to four children, at least one of them fathered by a white man who may have been the Pattersons’ son-in-law, one of his sons, or a visitor to their farm.

Melvinia’s first son’s name was Dolfus Shields. Like thousands in his position, he seized the opportunities offered by emancipation and moved to Birmingham, Alabama, where he is still remembered by parishioners as the founder of the First Ebenezer Baptist Church, and for his light skin.

Dolfus Shields is buried in a neglected black cemetery in Birmingham but in life counted whites among his friends and customers at a carpentry workshop in the white section of the city. His grandson, Purnell Shields, moved to Chicago as part of the Great Migration between the wars.

He married a nurse, Rebecca Jumper. Their daughter, Marian Shields Robinson, is Michelle Obama’s mother and now lives with the First Family in the White House.

Mr Obama has called his wife “the most quintessentially American woman I know”. Her family tree is said to include Native American as well as white ancestors. Obama campaign staff began tracing it back to the slave-holding era last year, when she said on the campaign trail that both the shame and the pride of her family story were parts of “the beauty and the tangled nature of the history of this country”.

Melvinia Shields may have been as young as 15 when she first fell pregnant. Whether she was raped may never be known, but her relationship with Dolfus’s father may have been, or have become, a consensual one since it appears to have continued until after emancipation in the early 1860s.

The search for the identity of Mrs Obama’s white ancestor will gather pace, but it may not be easy. Sex between blacks and whites in the 1860s was “coercive, covert and illegal”, Professor Peter Wood of Duke University, a specialist in the pre-Civil War history of South Carolina, told The Times.

“It’s not like they rushed to baptise the children,” he said. “If it was a slave-owner he would hush it up. If it was an overseer they would just wink at it. But once you have a few names and a few places, you can zero in on a county.”

Legions of genealogists will do just that.

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