BUNCE ISLAND, Sierra Leone—Twelve American presidents owned slaves, eight while serving in office, and at least 25 presidents count slave owners among their ancestors. But new historical evidence shows that a direct ancestor of George W. and George H.W. Bush was part of a much more appalling group: Thomas Walker was a notorious slave trader active in the late 18^th century along the coast of West Africa.

Walker, George H.W. Bush's great-great-great grandfather, was the captain of, master of, or investor in at least 11 slaving voyages to West Africa between 1784 and 1792....