What is the most likely meaning of the phrase “Cook and Tupaia closed the circle” at the end of the following passage:
Two centuries later a global network of geneticists analyzing DNA bread-crumb trails of modern human migration would prove Cook right: Tupaia’s ancestors had colonized the Pacific 2,300 years before. Their improbable migration across the Pacific continued a long eastward march that had begun in Africa 70,000 to 50,000 years earlier. Cook’s journey, meanwhile, continued a westward movement started by his own ancestors, who had left Africa around the same time Tupaia’s ancestors had. In meeting each other, Cook and Tupaia closed the circle, completing a journey their forebears had begun together, so many millennia before.