Historian Henry Louis Gates can trace the roots of his upcoming PBS documentary about the Reconstruction to his days in school, when he’d hear about the end of slavery during the Civil War, then virtually nothing about race relations until the civil rights movement in the middle of the 20th Century.

“It led me to think, if Lincoln freed the slaves, why did we need a civil rights movement?” the Harvard historian said at a news conference on Saturday.

The answer arrives April 9 with the Gates-produced, four-hour “Reconstruction: America After the Civil War,” which he hopes enlightens people to what he believes is one of the least understood periods of the nation’s history.

Freeing blacks in the South had a brief and dramatic impact on society. Within two years, about 80 percent of freed blacks in the former Confederacy were registered to vote — a greater participation level by percentage than blacks have today, Gates said.