Historians say there was a trade in human labor, of both whites and blacks, during this period in American history. There were also some free African-Americans. Beginning around 1617, indentured servants were bought and sold, as were debtors, in the Chesapeake Bay region, said Ira Berlin, a University of Maryland professor and expert in the history of slavery. But while those people were in an “unfree condition,” he said, historians cannot pinpoint a date for the beginning of the slave trade.

“What makes the John Punch case interesting is here is a guy who is definitely a slave,” said Professor Berlin, who did not participate in the examination of the president’s ancestors.

The Ancestry.com team used DNA analysis to make the connection, and it also combed through marriage and property records to trace Mr. Obama’s maternal ancestry to the time and place where Mr. Punch lived. The company said records suggested that Mr. Punch fathered children with a white woman, who passed her free status on to those children, giving rise to a family of a slightly different name, the Bunches, that ultimately spawned Mr. Obama’s mother, Stanley Ann Dunham.

The findings come as more and more Americans are discovering their own mixed-race heritage. Elizabeth Shown Mills, a former president of the American Society of Genealogists, said the Internet, coupled with the ease of DNA testing and heightened interest among both amateur and professional genealogists, was helping to reveal the extent of racial intermingling over the centuries.

“It is becoming increasingly common now because people are discovering it,” Ms. Mills said. “In the past, very few records were available. Very few people made the effort to do the research.”