Mr. Obama’s family tree — on his mother’s side — stretches back nearly four centuries to a slave in colonial Virginia named John Punch.

In 1640, Mr. Punch, then an indentured servant, escaped from Virginia and went to Maryland. He was captured there and, along with two white servants who had also escaped, was put on trial. His punishment — servitude for life — was harsher than what the white servants received, and it has led some historians to regard him as the first African to be legally sanctioned as a slave, years before Virginia adopted laws allowing slavery.