The earliest predecessor of the game is chaturanga, which originated in India in the 6th century and formed the basis for all chess variants. By the 8th or 9th century, the game had reached China. Sometime in the 10th to 12th centuries, Chinese chess, xiangqi, was brought to Japan, where it spawned a number of variants. Shogi in its present form was played as early as the 16th century, while a direct ancestor without the "drop rule" was recorded from 1210 in a historical document Nichūreki, an edited copy of Shōchūreki and Kaichūreki from the late Heian period (c. 1120).
According to The Chess Variant Pages :
Perhaps the enduring popularity of Shogi can be attributed to its 'drop rule'; it was the first chess variant wherein captured pieces could be returned to the board to be used as one's own. David Pritchard credits the drop rule to the practice of 16th century mercenaries who switched loyalties when captured—no doubt as an alternative to execution.