On March 8, 1421, the largest fleet the world had ever seen set  sail from China to "proceed all the way to the ends of the earth to  collect tribute from the barbarians beyond the seas." When the fleet  returned home in October 1423, the emperor had fallen, leaving China in  political and economic chaos. The great ships were left to rot at their  moorings and the records of their journeys were destroyed. Lost in the  long, self-imposed isolation that followed was the knowledge that  Chinese ships had reached America seventy years before Columbus and had  circumnavigated the globe a century before Magellan. And they colonized  America before the Europeans, transplanting the principal economic crops  that have since fed and clothed the world.