Hundreds of years ago, two men named John boarded ships to America to seek opportunity. One worked onboard as a barber; one was an indentured servant. But when they landed in East Coast port cities hundreds of miles apart, their lives abruptly diverged. When John Greene, believed to be an ancestor of Supreme Court Justice Ketanji Brown Jackson, got off a schooner from Trinidad in Charleston, S.C., he was immediately enslaved and dispatched to a plantation, according to family lore. When John Howland, the 10th-great-grandfather of Jackson’s husband, Patrick Jackson, disembarked the Mayflower at Plymouth, Mass., he was given housing...